Engine Principles(Hyundai )
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Engine Principles...
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Engine Principles
Engine Principles
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Engine Principles
Chapter 1. What is the Engine? 1. What is the Engine?............................... 2. Kinds of Engine..................................... 3. Engine layout........................................ 4. Expansive force & Inertia Force............... 5. Reciprocal Engine.................................. 6. Exhaust & Intake Stroke........................ 7. Compression & Combustion Stroke........ 8. Engine structure.................................... 9. Diesel Engine........................................ 10. Internal Combustion & Motor................ 11. Lean Burn Engine................................ Chapter 2. Cylinder block and Moving parts 1. Cylinder Block....................................... 2. Cylinder Liner........................................ 3. Water Jacket......................................... 4. Piston................................................... 5. Piston Ring........................................... 6. Connecting Rod..................................... 7. Crankshaft............................................ 8. Crank Case........................................... 9. Journal Bearing..................................... 10. Flywheel.............................................. 11. Balance shaft, Balancer for secondary inertial force....................................... Chapter 3. Cylinder Head 1. Cylinder Head........................................ 2. Cam & Camshaft................................... 3. Driving the Camshaft............................. 4. Intake & Exhaust Valve.......................... 5. Valve Driving System............................. 6. Valve Timing.......................................... 7. Variable Valve Timing............................. 8. Malfunction of Valve.............................. 9. Overrun & Red Zone.............................. Chapter 4. Intake System 1. Enhance the Volume Efficiency............... 2. Intake Inertia Effect & Pulsation Effect....
3. Variable Intake System.......................... 4. Intake System....................................... 5. Throttle Valve & Manifold....................... Chapter 5. Exhaust System 1. Exhaust System.................................... 2. Exhaust Inertia Effect & Pulsation Effect.. 3. Component of Exhaust Gas.................... 4. Air-Fuel Ratio & Exhaust Component....... 5. Exhaust Purification System................... 6. Blow-by Gas Recirculation Device............ Chapter 6. Charger 1. The Kind of Charger............................... 2. Turbocharger......................................... 3. Boost Pressure & Compression Ratio..... 4. Turbo Lag............................................. 5. Supercharging System & Heat................ 6. Supercharger........................................ Chapter 7. Lubrication System 1. Role of Engine Oil.................................. 2. Lubrication Method................................ 3. Parts of Lubrication System.................... 4. Engine Oil............................................. Chapter 8. Cooling System 1. Cooling System..................................... 2. Radiator................................................ 3. Cooling of the Cylinder Head.................. 4. Over Heat............................................. Chapter 9. Fuel System 1. Carburetor............................................ 2. Mechanical Fuel Injection System........... 3. Electrical Fuel Injection System.............. 4. Fuel Supplying System........................... Chapter 10. Ignition System 1. Point type Ignition.................................. 2. Full Transistor type Ignition..................... 3. Distributor-less Ignition.......................... 2
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Engine Principles 4. Spark Plug............................................ Chapter 11. Combustion and Combustion Chamber 1. Combustion Process.............................. 2. A/F ratio & Flame Velocity...................... 3. Ignition Timing....................................... 4. Swirl Effect........................................... 5. Knocking............................................... 6. Abnormal Combustion............................ 7. Shape of Combustion Chamber.............. 8. Intake-Exhaust Valve & Combustion Chamber................................................... 9. Piston & Combustion Chamber............... Chapter 12. Performance, Fuel consumption, Noise, and Vibration 1. Required Performance.......................... 2. What is Output?................................... 3. Representing Method for Power.............. 4. What is Torque?.................................... 5. To enhance Power................................. 6. S/B ratio & Output................................ 7. Compression Ratio & Output.................. 8. Enhancing Output by High rpm................ 9. Transient Characteristic & Response...... 10. Cylinder Array & Performance............... 11. Fuel Consumption Ratio....................... 12. Output & Fuel Efficiency....................... 13. Fuel Efficiency of Vehicle...................... 14. Vibration of the Engine......................... 15. Noise of the Engine.......................................
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Engine Principles
Chapter 1. What is the engine? 1. What is the engine? This book introduces about the automobile engine especially the gasoline engine. However, it is very complicated to define about the engine, so called, what the engine is. In general concept, the engine is 『the devices driving something by changing the energy in the natural source such as fire, wind or electric material to the mechanical energy continuously』. There are many types of the engine and they are driven in different ways. Therefore, we can define the gasoline engine, as a kind of combustion device in other word, the 『device changing the heat acquired by combusting the gasoline to the mechanical force for driving the vehicles』. How is the heat energy changed into the mechanical energy? For example, the bottle or pot. As they are heated, the covers move with a noise.
The heat acquired from the gas or electric energy boils the water so as to make the water vapor pushing up the cover of the bottle or pot.
There is an important thing. The moving force is not come from the heat energy, but the hot air or vapor by the heat works.
That is, the media is need for changing the energy form. This media is the working fluid in the technical terminology. The working fluid for 4 Chonan Technical Service Training Center
Engine Principles the gasoline engine is the air in-taken with the gasoline into the engine and then combusted and exhausted.
Fuel
transforming the energy, there should be a lot of loss of energy, at any case. Therefore, how much energy of the heat can be transformed into the mechanical energy, the efficiency, is an important factor in the engine. 2. Kinds of engine
Combustion
Heat Generation
Air Expansion
There are many kinds of the engines. The engines can be classified by the number of cylinder, the array style of the cylinder or the equipped method at the vehicle, and so on. As you know, the engine makes the driving force by the reciprocal movement of the piston in the cylinder so that the power is decided by the number of the cylinder.
P ressure Generation
P iston Movement The procedure for transmitting the fuel to the mechanical energy in the vehicle engine will be explained in the following sections. In this process, the working fluid is the air. If there is no working fluid, the energy transformation shall not be performed. In contrary, think about the transforming the mechanical energy to the heat energy. In the vehicle, the brake is the represented example. The principle is the friction heat coming from the rubbing or striking the two materials. We can warm our hands by rubbing each other, that is, the moving force(rubbing) can easily transform to the heat energy(warming hands). At this time, there is no working fluid. The force is changed into heat directly. However, when the heat energy is transformed into the mechanical energy, there must be a working fluid. Being the media for the
Therefore, the engine is mainly classified by the number of the cylinder. The commercial vehicles are classified into the 2, 3, 4, 5, 6, 8, and 12 cylinders. More displacement volume has the more cylinders. According to the array of the cylinder, there are three types including the In-line type with serial arraying of the cylinder, the V type with Vshaped arraying of the cylinder and the opposed type in which the cylinders are arrayed facing each other. According to the engine installation type, there are two types; the one is the lengthwise type and the breadthwise type. When the engines are arrayed in length direction of the vehicle is called as the lengthwise type, when the engines are arrayed in width direction of the 5
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Engine Principles vehicle is called as the breadthwise type.
Lengthwise type
Breadthwise type
For example, the FR type car having the engine at front and driving the rear wheels has the lengthwise type engine. The reason is that the propeller shaft transmitting the driving force to the rear wheels shall be run underneath the floor. In the case of FF type car, the most small car, having the engine at front and driving the front wheels, the engine is breadthwise type because that the rotation axis of the engine and the driving axis rotating the wheels should be are parallel. However, when the FF type car has the 6-cylinder engine, if the serial engine is installed in width direction, then the width of the car is too wide. Therefore, in that case, the engine rather is the V-type. In this manner, considered with the displacement volume and vehicle type, the engine array type and the installation type are selected for the best combination in design.
installed at the front of the car. However, the all vehicles do not have their engine at front side. In 1770, the origin of the car, the engine of the steam car of Cugnot was located at the front ends of the body. In 1885, the first car equipped the gasoline engine, the engine of the Daimler’s car is located at the under the seat and the front of the rear wheel axis. In the history, many researches had been performed to find the best condition where the engine was installed. In 1891, the FR (front engine rear drive) car was in France. The FR type is that the engine was installed at the front side and the rear wheels were driven. After that, this type would be the set as the standard layout of the engine. Until now, the most large passenger cars and the sports cars accept this type. The typical characteristics of the FR type car is that the cabin space is located to rear side, the steering is controlling the front wheels and the driving is contributed to the rear wheels so that the operation and weight load are evenly divided to the front side and rear side and the movement performance and passenger convenience are balanced as well as the vibration and noise are less than other types.
3. Engine layout
FF : Front Engine Front Drive FR : Front Engine Rear Drive MR : Midship Engine Rear Drive RR : Rear Engine Rear Drive It is possible to assume that the engine will be 6
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Engine Principles
Engine Layout & Vehicle Features
In the layout of the relatively small public car, there had been two big developments for about hundred years. The first one is the Volkswagen in 1936 having the engine at the rear side for driving the rear wheel. After the World War II, this type was leading the worldwide design of the mainstream the passenger car of rear engine and rear drive (RR) type. The second one is the Mini of England in 1959. The Mini accepted the front engine and front drive (FF) type having the breadthwise type engine at front side for driving the front wheels. Nowadays, this type is applied to the small cars as well as the middle sized sedans.
at rear side, it is called the rear engine type. 4. Expansive Force and Inertia Force Almost of the commercial gasoline engines for the vehicles are the Recipro-engines except for the rotary engine of which working principle is different. The word “Recipro” comes from the “Reciprocating”. The reciprocation is the mechanical movement of go and back so that the Recipro-engine is the device transforming the reciprocal movement to the rotational movement using the crank which is the device having the prominence and depression shape.
The FF type car has the engine and driving devices at the front side so that it should not applied to the car of which weight is concentrated to the front side. The defect of this type is that it is not easy to steer the car. However, it has the relatively large space of indoor and trunk, and the safety is better than other types. Therefore, it is the best design for the layout of the utility cars. The midship engine and rear drive (MR) type is focused on the performance rather than the convenience of the passenger so that it is mainly applied to the sports cars. When the main parts of the engine are located at front side than the rear wheel type, it is called the midship type. When the main parts are located 7
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Engine Principles
The cross sectional view of the Recipro-engine shows that there is a piston reciprocal moving in the cylinder at the upper side and there is a crankshaft at the lower side and the piston and the crankshaft are connected by the connecting rod. The gasoline engine uses the air as the media of working fluid for transforming the heat energy to the mechanical energy. The air is mixed with the mist of gasoline in the cylinder. When the mixture of air and gasoline are compressed using the piston and combusted, then the expansive gas will press the piston. The expansive force pressing the piston shall drive the car. At this time, except for the expansive force, there is an inertia force which we are considering. This force is somewhat strange because it is not generated intentionally; however, this force is followed in the moving part of the engine naturally. Therefore, it can affect to the performance of engine or is the source of the vibration or noise.
Considering the reciprocal movement of the piston, the piston starts moving from the halt state at the highest position of the stroke, has the maximum speed at the middle of the stroke, After that, the speed is reduced and stops to the lowest position of the stroke, and then goes back to the highest position again. During performing these movements, the inertia forces shall be generated when the speed of movements are changed. For example, from the highest position of the piston to the middle of the stroke, there is an ascending inertia force. After that there is a descending inertia force from the middle of the stroke to the lowest position of the piston. When this inertia force is resonated with the other inertia forces from the other pistons, there may be vibrations or noises. 5. Reciprocal Engine
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Engine Principles this time, the exhaust valve shall be open to take out the burnt gas [Exhaust Stroke]. These strokes are repeated. Among these 4 strokes, only at the combustion stroke, the engine makes the power for work. Therefore, there are needed the additional forces, for intake and exhausting the gas at the intake and exhaust stroke, and for compressing the mixture at the compression stroke. To do so, a flywheel is installed to the crankshaft to make these additional forces using the inertia force to ensure the continuous rotational movement. The Recipro-Engines are classified into two types, the 2-cycle type and the 4-cycle type. After developed by Daimler of Germany in 1883, the 4-cycle type which had been completed its standard system until 1900 is the mainly used in the most of vehicles. The principle of the reciprocal engine is that the mixture of the air and the gasoline are injected into the cylinder, the mixture is combusted using the electric spark to explode, the combustion force drives the piston in reciprocal movement, and the reciprocal movement is changed to the rotational movement by the crankshaft.
The 2-cycle engine comprises of the two strokes. The operation of this engine comprises the four components such as the intake, the compression, the combustion, and the exhaust as the 4-cycle engine.
The operation of the 4-cycle engine is like that. When the piston is at the highest position, the intake valve is opened. Being down the piston, the mixtures of fuel gas is injected into the cylinder and then the intake valve is closed [Intake Stroke]. Next, the piston will go to upward to compress the mixtures of gas [Compression Stroke]. Then, the compressed mixtures of gas will be burnt by an electrical ignition [Combustion Stroke]. The burnt gas having the high pressure and temperature will press the piston downward. At
Compression and Expansion 9
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Engine Principles
Exhaust and Intake However, before and After the piston is located at the highest position, the compression and combustion operations are performed, and before and After the piston is located at the lowest position, the exhaust and intake operation at the same time. So, through the two strokes, the one cycle of engine operation is completed. The 4-cycle engine performs the combustion stroke at one time per two cycles of the crankshaft, but, the 2-cycle engine performs the combustion stroke at every cycle of crankshaft. Therefore, the 2-cycle has higher efficiency.
The exhaust gas is exhausted by the entering new mixtures of gas when the piston is located at the lowest position. Therefore, some new mixtures will be mixed the combusted gas and unburned gases will be exhausted. It makes the air pollutions and the large fuel consumptions. 6. Exhaust & Intake Stroke This content will explain about the engine on focusing the 4-cycle gasoline engine, the most used type. To understand the 4 strokes, the intake, the compression, the combustion, and the exhaust, it is helpful to refer the figures, the indicator diagram (P-V diagram), showing the process of the engine operations.
Scavenging Additionally, it has no intake and exhaust valves so that it has simple structure and low cost. However, this merit can be a defect.
It looks somewhat complicated, but it is easy to understand just following the figures.
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Engine Principles
Exhaust stroke
The indicator diagram is the graph consisting of the horizontal axis representing the pressure of the chamber and the vertical axis representing the volume of the chamber. At the left ends of the graph, C and F, the piston is located at the highest position of the cylinder, and at the right ends of the graph, G and H, the piston is located at the lowest position of the cylinder. By comparing the lines of the graph to the 4 strokes, the line of A-B is the intake stroke, the line of B-C is the compression stroke, the line of C-D is the combustion stroke and the line of D-E is the exhaust stroke. In act, the cycle of the strokes is starting from the intake stroke. To understand the engine operation, it is easy to start from the exhaust stroke. To inhale more air as possible, the engine uses also the force from the exhausted gas outgoing to the outside of the engine through the exhaust port.
However, actually, the exhaust valve prefers to be opened before the piston reaches to the lowest position, that is, at the position of D in the drawing. Remaining the pressing force of the burent gas, the exhausting the used gas is more effective by opening the exhaust valve in advance. After that, the piston will push out the remained used gas thoroughly to finish the exhaust stroke. At the intake stroke, the intake valve is opened, and the piston goes down from the highest position to the lowest position so that the mixtures of fuel gas and the air are inhaled into the cylinder from the intake port. At that time, the intake valve shall be opened just before the piston reaches the highest position, that is, the E in the drawing. Doing so, the intake operation is enhanced somewhat because of the exhausting force of the used gas will pull the intake gases.
The exhaust stroke is for pushing the combusted gas to outside of the chamber by moving the piston from the lowest position to the highest position with being opened exhaust valve. In principle, it is think that the exhaust valve will be opened when the piston is reaching at the lowest position. Intake stroke 11
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Engine Principles With the same manner, the intake valve shall be closed when the piston is at the B. Doing so, more mixtures of gas shall be inhaled into the cylinder by the inertia force of the intake gas. To inhale air into the engine is performed by the difference of air pressure. When the piston goes down, the air pressure in the cylinder is lower than that of the outside of the cylinder, so that the air around the intake valve shall be inhaled to the cylinder. The opening timing of the valve is different from the stroke position is to use this force to inhale more air as possible 7. Compression and Combustion Stroke At the compression stroke, the mixtures of gas are compressed by the piston, so that the pressure is increased and the temperature is high by adiabatic compression. Therefore, the gasoline is vaporized by the compressive heat of the air, ready to be combusted. The injected gasoline in the cylinder like the misty with the air is vaporized to be gas state by the heat from the adiabatic compression. Then it is ready to be combusted easily. This space for combustion is called the combustion chamber.
is to mix more gasoline with the air. Another problem is that; when the gasoline is vaporized, the ambient heat is used to the vaporization, so the temperature of the chamber is lowered somewhat. It may decrease the fuel efficiency of the engine. To prevent from lowering the fuel efficiency, the amount of the gasoline is reduced. However, to do so, the temperature of the chamber is so high that the combustion may be performed prior to the ignition, so called the abnormal combustion. The most important fact from the intake stroke to the compression stroke is the flow of the mixtures of the gasoline and the air. It is not proper that the flow is too strong to be ignited. The little particles of gasoline should be mixed with air to be the mixtures. Therefore, many manufacturers research and develop the shape of the intake port and flow pattern of the mixtures so that the strength and the pattern of the mixture flow are maintained until the combustion stroke for the best efficiency of the engine.
Compression stroke
The reason of that it is hard to start the engine in winter is that it is hard for the gasoline to be vaporized. To solve this problem, one method
As processing the compression stroke, the piston reaches the highest position. When the piston is at the C of the drawing, the ignition will be performed by the electric spark generated from the spark plug. The timing to make the spark is very important. The mixture 12
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Engine Principles is not fully combusted at the ignition time, but the combustion is started from the ignition. It is needed some time interval between the time of the ignition and the time to be maximum pressure of the chamber.
Combustion stroke The ignition timing is determined by considering that the combustion shall be completed between the highest position and the almost half position of the combustion chamber. Additionally, this combustion speed is proportion to the rotational speed of the engine so that the ignition timing should be adjusted with the engine speed. Beginning the combustion, the vaporized mixtures are combusted in a short time so that the pressure and the temperature are increased. At this time the expanded gas by the combustion shall press the piston. This pressing force should be stronger as possible. The time period of combustion is preferable to be short to strength the force. If the time period for combustion is longer, then the combustion force is not leading the pressing the piston but following the piston. Therefore, the engine efficient is worst. The combustion time period is affected by flow of the mixtures defined by the size and shape of the combustion chamber and the component of the
mixture and so on. 8. Engine structure
The gasoline engine is a complicated machine having comprising of parts. Look into how the engine is structured. The engine is similar with the 3 floors building. The first floor is the crank case including the crankshaft transforming the reciprocal movement to the rotational movement. The second floor is the cylinder block including the cylinder of which a piston is moving with reciprocal movement. The third layer is the cylinder head. In this structure, the moving parts of the first layer and the second layer are called as the main moving part. It includes the piston, the crankshaft and the connecting rod. In the third layer, there are the valves controlling the intake and exhaust of the mixtures gas and the used gas and the camshaft operating the valves. These are called the cylinder head system. On the cylinder head, there are intake manifold sending the gasoline and the air to the cylinder and the exhaust manifold taking out the burnt 13
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Engine Principles gas. These are called the intake-exhaust system. The manifold consists the “many” and the “fold”, that is, many materials are combined. In actual, they are many branched pipes distributing the air and gasoline to each cylinder or merging the exhausted gases in one place. There is the fuel system including the fuel pump taking the gasoline from the fuel tank and the carburetor or fuel injector to make a air fuel mixture. There is the lubrication system including the oil pump supplying the oil for reducing the frictions and the oil filter for filtering the oil. Also there is the cooling system including the radiator and water pump for maintaining the temperature of the engine properly. To drive the engine, the electric power is needed. There are electric devices including igniting spark plug, the alternator generating the electric power and the start motor supplying the initial movement to the engine. Additionally, there are the auxiliaries such as the oil pump for the power steering the air-con compressor and so on. 9. Diesel Engine The diesel engine has similar shape and structure with those of the gasoline engine. The different point is the ignition method. The gasoline engine ignites the mixture of fuel with the electric spark. Contrary, in diesel engine, the fuel is injected into the compressed air having the high temperature. When the air is compressed, the temperature of compressed air is increased. The gasoline engine compresses the mixture of fuel gas up to 1/10 of the initial volume.
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Engine Principles In the gasoline engine, the ignition is performed by the electric spark to the mixture so the combustion time period is very short. However, in the diesel engine, the fuel diesel is sprayed into the compressed air, so it needs some time period to be vaporized. Therefore, the maximum speed of engine is limited relatively lower and the output shall be lower than the gasoline engine. The diesel engine compresses the air about 1/20 of the initial volume to increase the temperature of the air over 600℃, and injects the fuel compressed with over 100 atm at the injection pump during 1/1000∼2/1000 seconds. The output shall be controlled by the amount of injected mixtures of fuel and air for the gasoline engine. On the other hand, the output of the diesel engine can be controlled by the amount of the injected fuel without controlling the air (the fixed amount of the air).
Compared with the gasoline engine, the expansion force and the inertia force of the moving part are bigger so it makes bigger noise and more vibration. It has merits of easy maintenance resulted from not having the delicate parts such as the ignition system and of good fuel efficiency so that it is used for commercial or business purpose rather than passenger’s car.
To combust the fuel perfectly by increasing the temperature of the air, the compression ratio shall be increased. However, doing so, the expansion power will be increased also. Therefore, the engine should be stronger to endure the increased force. Additionally, the high quality of fuel injecting pump may be needed. Then the engine is heavier and the cost is expensive. So diesel engine is not proper to apply to the passenger’s car.
The force of the 4-cycle gasoline engine, as one of the internal combustion, is changed according to the RPM (Revolutions Per Minute) of the engine compared by the electric motor or the steam engine. So, it is impossible to drive with the lower revolutions than certain RPM value. Therefore, the clutch and the transmission should be equipped when the gasoline engine is used for vehicles.
In the diesel engine, because almost constant amount of the air about the volume of the cylinder is inhaled, the load applying to the engine is relatively light. When the fuel amount is small at the low speed, the fuel shall be almost perfectly combusted. However, with the full load, the diesel engine needs more amount of fuel so that the air amount is respectively small. Therefore, it may exhaust a lot of black smoke.
10. Internal Combustion & Motor
For the 4-cycle engine, using the four strokes, it makes moving force by combusting the mixture of fuel and air in the cylinder. It is very different with the electric motor used in electric vehicles which can start just by applying the electric power. If the mixtures are not supplied into the cylinder at idle condition, the engine can not continue to run. In order that the engine should be operating continuously when the vehicle is stopped, the device for connecting or disconnecting the moving force of the engine to 15
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Engine Principles the wheels such as clutch should be equipped.
11. Lean Burn Engine
In general, the vehicles needs larger power when it is started or accelerated, but when it is driven in constant speed, it does not need larger power. For the motor, it outputs larger force when it rotates with lower RPM, and when the RPM is increased, the output will be lower. Therefore, the electric motor can be applied to the engine of the vehicles without any transmitting device.
The exhaust purification system using the 3way catalysts has the characteristics of maintaining the actual air-fuel ratio to the ideal valve to perform the oxidation and reduction of the harmful compounds simultaneously. To do so, the purification of the exhausted gas will be limited, and the fuel amount used for the engine is decided by the engine driving status. Therefore, the engine shall not be developed no more to get higher driving force with less amount of the fuel.
However, for the gasoline engine, the power is determined according to the RPM of the engine. The range of the RPM is limited within certain ranges. For example, the RPM of the gasoline engine is about 700∼7000 revolutions per minute, and the RPM for getting the maximum power (torque) is about 4000 revolutions. Therefore, when the vehicles are running with various speeds, it is necessary to control the speed and power of the vehicle by inserting a transmission between the engine and the wheels. At simple sight, the motor may be the best engine for the vehicles. The important thing is the fuel, the source of power. The gasoline is easy to store during operating the engine, but it is hard for the motor to store the electric power effectively. To develop the electric vehicles, it is essential to develop the batteries having the high efficiency for charging and maintaining the rechargeable batteries in equivalent state. Many companies are trying developing the method for maintaining the rechargeable batteries. Even though the basic performance has been developed in field test, the manufacturing cost is very high. However there are being suggested some patents and technologies for utilizing.
The lean burn system is developed for enhancing the fuel efficiency with the good purification of exhaust gas. To enhance the fuel efficiency is most important point for future. The lean burn engine is one of the most attracted public attention technologies. With the high A/F ratio by reducing the gasoline in the mixture, how are the three major harmful materials, carbon monoxide, hydrogen carbon, and nitrogen oxide. The oxygen is more plentiful than fuel, so the amount of carbon monoxide will be less or the most carbon monoxide will be changed into carbon dioxide, harmless gas. The hydro carbon will also be completely combusted and changed into carbon dioxide and water. Now we will concern only the last one, nitrogen oxide. If the A/F ratio is higher, then the temperature will be increase by the plentiful of the oxygen and the amount of the nitrogen oxide will be 16
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Engine Principles increased. At about 16 of A/F ratio, the nitrogen oxide will be maximized. If the A/F ratio is higher than 16, then the combustion temperature will be down so the torque is also down. If the A/F ratio is higher and higher, the combustion is not stable, and torque is very unstable, finally combustion will not be performed. A manufacturer focused on the torque variations according to the lean burn. By adapting the combustion pressure sensor detecting the combustion pressure in the cylinder, the engine is operated with the A/F ratio just before the torque variation is occurred. Therefore, they can make next generation lean burn engine having the low fuel consumption and the less amount of nitrogen oxide. In that system, the lean burn is performed at the condition in which the driving is not hindered by the low torque at low load. When the vehicle in accelerating or high load, the combustion is performed with the theoretical A/F ratio and the exhaust gas is purified by 3way catalyst. Many manufacturers continue to research for enhancing the fuel consumption by focusing on the intake system and combustion chamber with 16∼20 of A/F ratio. Many new engines satisfying this combustion requirement and having less exhaust gas problem are shown more and more.
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Engine Principles
Chapter 2. Cylinder Block & Moving parts 1. Cylinder block
The cylinder block is the basic part of the engine. It is made of cast iron or aluminum. It comprises of the cylinder in which the piston shall be moving reciprocally, the water jacket for circulating the cooling water maintaining the temperature of the cylinder, and the crankshaft installed underneath. The role of cylinder is for guiding the reciprocal movement of the piston accepting the force and high temperature from the combustion of the mixtures, for cooling the cylinder properly, and for supporting the crankshaft. As the basis of the engine, it should have enough strength for enduring the supporting the attached all parts of engine. For these purposes, the cylinder is generally made of cast iron because that the iron is easy to be processed mechanically and has the characteristics of good resistance against the ware and corrosion. Recently, instead of the cast iron, the aluminum alloy is more popular. The aluminum is lighter and transmitting the heat easier than steel so that it is deemed as the ideal material
for engine. It is not easy to apply the aluminum to the engine because it has different heat expansion coefficient with the steel, the main material of other parts and it is complicated to design the engine structures, as well as it is more expansive than steel. For the passenger’s car, the weight of the engine is about 10∼15% of the total weight of the car. The 15∼20% of the engine weight is come from the cylinder block. It is very important to be light maintaining the strength of it as possible. Therefore, the skeleton structure of the cylinder block has different thickness such that the thickness is thicker of the portion applied heavy force or having possibility of deformation and the thickness is thinner of the other portions. To design the cylinder block regarding these factors, the structure analysis is performed by the finite element method in which the engine is divided into triangular or rectangular cells and the each element is established in simultaneous equations to calculate by numerical analysis using computer. In side of the block, there should be the water jacket for circulating the cooling water so it should be precisely manufactured for the complicate structure. To prevent from cracking at the bottle neck point of different thickness or to enhance the resistance against wear, it should be heat treated. 2. Cylinder Liner The inside wall of the cylinder block is the frictional face with the piston with a lubricant oil there-between. Therefore, it satisfies the strict requirements that it endures at the high temperature and wear, that its changes of the dimension by the heat expansion coefficient shall be within the tolerance, and that it should 18
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Engine Principles not be adhesive with each other by the high temperature.
Generally, when the block material is the steel, this part is made by polishing the cast iron cylinder, so called as the linerless type. When the block material is the aluminum alloy, the inside wall of the cylinder having a cylinder liner made of cast ion for preventing the ware of the side wall. The liner is the thing which is attached the inside of the cylinder. The cylinder liner may be made with the cylinder block or separately and joined after that with the cylinder. For the aluminum cylinder block, the cast iron is used. It is heavier than the aluminum alloy as well as it has the lower heat transmission ratio than the aluminum. Therefore, for the engine of racing car or high efficiencies, the special liner made of the silicon alloy based on the aluminum or having special treatment on the aluminum surfaces are utilized. These special liners are so expensive and hard to manufacture. Also, there are some tries to develop the linerless cylinder with aluminum alloy cylinder block. Even though the linerless cylinder is more expensive, the engine can be lighter and compacted so that it is mainly accepted to the high performance engines. The gap between the cylinder liner and the piston is depended on the material. When the liner is the cast iron and the piston is the aluminum alloy, considering that the heat expansion ratio of the aluminum is almost
twice than that of the steel so that the gap will be reduce at the high temperature of the engine, the gap shall be 30∼40 microns (0.03∼0.04mm) at the room temperature. If the liner and the piston are all the aluminum, then the gap shall be 10 microns because there is no difference of the heat expansion between them. The around of the cylinder liner is formed as a shape of path for cooling water, the water jacket, to maintain the temperature of the engine to certain value by absorbing the heat energy come from the remained energy of the combustion. 3. Water Jacket When casting the cylinder block, the cylinder is surrounded by the core made of sand to form vacant spaces. These spaces are the water jacket for circulating the cooling water to take down the temperature of the cylinder head and cylinder to the proper temperature for operating. The water circulating inside the water jacket goes into the engine from the lower outlet port of the radiator cooling the heated water. The water flows from the lower part of the engine to the upper part of the engine. After cooling the cylinder head, the heated water is taken out from the engine and goes into the upper inlet port of the radiator. During circulating inside the water jacket, it is important to cool down the each cylinder equivalently. The design of the water jacket is focused on the flow method to spread the water smoothly over the all parts with smaller volume of the water as possible. The heated water is cooled in the radiator and then return to the water jacket again. In winter, the heated water selectively flow into the another radiator for heating the cabin. 19
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Engine Principles 4. Piston
Generally, the water jacket is surrounding the cylinder thoroughly. In order to reduce the path length along to the cylinder array, the water jacket is surrounding the out side of the cylinder so that the water does not flow into the adjacent space the cylinders. This type is called the Siamese type. Like the Siamese twins, some portions of the jacket surrounding the each cylinder are merged into one body. The conventional jacket is called the full jacket type.
For the engine having the liner, the type is divided into two kinds by whether the water is contacting with the liner or not. When the cylinder liner is surrounded by the wall of the cylinder block so that the outside of the liner can not contact with the cooling water, it is called the dry type liner. When the most portions of the liner contact with the cooling water directly, it is called the wet type liner. The wet liner has better cooling efficiency. It should be sealed with an O-ring between the liner and block to prevent leaking the cooling water. In HMC, the most engines having the liner are equipped with the dry type liner because HMC have not any problem come from the heat adhesive of the engine yet and we worry about the leaking the cooing water.
The piston moving inside the cylinder reciprocally transmits the weight force of 3~4 tons (5 tons for diesel engine) according to the combustion of the fuel mixture gas having over temperature of 2000℃ at the combustion stroke to the connecting rod. The first thing to be considered in design of the piston is that the piston should be made of light materials to reduce the inertia force of the reciprocal movement. The next point is that its material should have the strength enough to endure the combustion force. And then, the material of the piston shall have the good heat-trance and not be distorted or deformed by the high temperature.
At first, the aluminum or aluminum alloy can be considered for lightening and strengthening. Then, for enhancing the heat resistance to prevent from changing in dimension, the heat treatment shall be performed. 20
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Engine Principles The upper part of the piston is called as the piston head or the piston crown. It is very important part forming the combustion chamber between the cylinder head. To enhance the combustion efficiency by combusting the fuel mixture instantly, the shape of piston head prefer to be flat. To enhance the compression ratio, the middle portion may be upraised or there are some recessed positions, the valve recess, to take the intake and exhaust valves not to touch with the piston. The under portion of the piston is the piston skirt stabilizing the reciprocal movement of the piston. The some front portions of the skirt looks like being cut out because that the balance weight is passing these areas when the piston goes down. There is a gap between the piston and the cylinder. This gap shall be sealed with the piston ring. When the piston is moving in reciprocal, some portions of the skirt may be touching the cylinder wall. To reduce this touch, the shape of the skirt shall be changed. The shorter length of the skirt has fewer noises from the friction with the piston and lighter weight. However, it is preferable for designing the skirt to be balanced with the size of the piston. The piston is connected with the connecting rod by a piston pin. So, the most forces of combustion are applied to this pin. As the piston pin is the shape of hollow cylindrical structure, the larger of outer diameter, in a same weight, is the more strength against the bending force. However, when the diameter of the piston pin is enlarged, the piston pin boss shall be also enlarged. Therefore, the compression height, the length from the pin to the piston head, is also longed so the weight of the engine is heavier. So, the diameter should be designed by considering the balance with the piston size.
5. Piston Ring The main roles of the piston ring, the wheel shaped steel surrounding the head part of the piston, are to prevent from leaking the gas by sealing between the piston and the cylinder, to prevent from remaining the lubricant oil in the combustion chamber by gathering the oil down from the cylinder wall and to prevent from transmitting the heat from the piston to the cylinder. Generally, the piston ring comprises of three rings. The two rings near to the piston head are called the compression rings, and the one ring near to the skirt is the oil ring. The top ring of the compression rings is used for sealing the gas, the oil ring is used for removing the lubricant oil, and the second ring of the compression rings is used for helping the sealing and for controlling the thickness of the lubricant oil film.
Some piston comprises of the two rings, the compression ring and the oil ring. In this case, the roles of rings are somewhat loss, but the fuel efficiency can be enhanced by reducing the loss of force from the friction between the piston ring and cylinder wall. Some racing cars accept the two ring system for shortening the piston height to reduce the engine weight. 21
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Engine Principles The compression ring is made of the spring steel by casting iron, and the surface should be heat treated to reduce the friction and to enhance the lubricant of the piston. To insert the ring into the grooved portion of the piston and to ensure the tensile force for compressing to the cylinder, one portion of the ring shall be opened. This open portion is called the end gap. The combusted gas is leaked out through this end gap a little. This blow-by gas is returned to the combustion chamber by the returning device not to leak out. The grooved portion of piston for the compression ring has slightly lager then the width of the rings. When the piston is moving up and down, the rings are rotating to prevent the end gap of the three rings from being aligned each other. If the rings have not enough strength, the rings are fluttering within the grooves at the high speed of the engine so it cannot seal the gas properly.
The cross section of the oil ring has the shape of reversed “C”. The gathered oils by the rings are returned to the inside of the piston through the hole located at the lower portion of the Cshaped ring. When the engine has high speed, the ring can not gather the oil only with the its tensile force, so an additional spring, the expander, shall be attached to enforce the compressing force of the ring to the cylinder.
the piston and the crankshaft. It transfers the reciprocal movement to the rotation movement. The connecting rod moves very complicatedly with swing movement about the piston pin and the linear movement up and down. So, there is a balance weight to control the inertia force generated by the complicating movements.
The contribution ratio of the connecting rod weight to the inertia force is about 2 to the one reciprocal movement. To lightening the load to the bearing and the vibration by reducing the inertia force, the connecting rod should be light as possible. However, it has enough strength to transmit the combustion force to the crankshaft. The connecting rod is made of the special steel by casting or forging. The forging is preferably used for ensuring the strength. For the racing cars, the expensive but very light and strong material, titanium alloy, is used also.
6. Connecting Rod The connecting rod is the rod for connecting
The types of the rod can be divided into two types according to the cross sectional shape of 22 Chonan Technical Service Training Center
Engine Principles the rod, I type and H type.
7. Crankshaft
If the strength is same, then the I-type is lighter than H-type. Therefore, general cars accept the I-type rod. The H-type has stronger structure against the bending force to the direction of the pin axis. As the Connecting rod is longer, the lateral vibration is smaller. The reason is that, considering the force applied to the piston at the rotation of the crank by divided into lateral direction and longitudinal direction, the longer connecting rod can reduce the ratio of the force to the lateral direction than the shorter connecting rod so that the vibration and friction also shall be reduced. However, if the connecting rod is so long, the engine weight is heavier so it is not preferable. Generally, the length from the center of the piston pin to the crank pin, is about twice than the length of the stroke.
The end portion of the connecting rod to the piston side is called as the small end, and the end portion to the crank pin side is called as the big end. The small end is connected to the piston with the piston pin, and the big end is attached to the crank pin by inserting a bearing.
The crank means the 『bended handle』 as the transfer from the reciprocal movement to the rotational movement as have mentioned until now. At the early time of the vehicles history, the engine is started with the crank. After the electric motor was used for the starting the engine, until 1950s, Some cars had have a crank at the front of the engine for emergency device at the malfunction of the motor.
The crankshaft connects the cranks of each cylinder. The main shaft is called the crank journal and the attaching part to the big end of the connecting rod with the crank is called the crank pin. The other side, the attaching part to the small end of the connecting rod with the piston is called the piston pin. The connector connecting the crank journal and the crank pin is called the crank arm. The sector formed pendulum at the front of the crank arm is called the counter weight or the balancing weight.
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Engine Principles counter weight should be small as possible within the requirement load range of the journal in order to reduce the weight of the crankshaft. 8. Crank Case The crankcase is the part covering from the cylinder of the cylinder block to the crankshaft.
The reason of the shape of the counter weight being spreading from the center (root) to the circumferential portion (outer portion) is that it can have larger inertial force when it rotates about the root part; even the counter weight has the same weight density. In the reciprocal engine, the piston is press the crank journal with the connecting rod at every combustion stroke. The crankshaft affected by the complicated bending and distorting force. Therefore, the crank journal shall have strength enough to endure these forces so it is made of the casting or forging steel. For the high performance engine or the racing car engine, the forging steel is most used for ensuring the strength. For the commercial or general purpose vehicle, the casting steel is used because the forging process is more expensive. Even though the casting steel has less strength than forging steel, it is not so critical point because that it is possible to manufacture the counter weight precisely.
In the crankcase, there are some auxiliary devices such as the alternator (the alternative current generator), generating the electric power, the compressor of the air conditioner and the oil pump for the power steering. And the engine mount brackets installing the engine to the vehicle body are also attached to the crankcase. As the crankcase is one part of the cylinder block, it is always vibrated by the reciprocal movement of the piston and the rotational movement of the crankshaft. Therefore, the material of the crankshaft should satisfy the requirement of the resistant against the shocking force and vibration. The types of the crankcase are divided into two types according to the covering range over the crankshaft, the half skirt type and the deep skirt type. In the half skirt type, the front portion of the crankcase is covering to the center of the crankshaft. In the deep skirt type, the crankcase is covering over the bearing cap.
The counter weight balances the weights force between from the reciprocal movement of the piston and from the rotational movement of the crankshaft. Simply think, to balance the weight is to match the inertia forces from the piston and the counter weight as the ratio of 1:1. The 24
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Engine Principles
9. Journal Bearing The bearing is for helping the smooth rotation of the rotating axis and supporting the rotation axis. There are various types of the bearing including the plain bearing supporting the axis with the flat and wide side, and the bearing supporting the axis and the around of the axis with balls or rollers. Generally, for the crankshaft of the engine, the plain bearing is more used.
Because the half skirt type has the short length, it is possible for the block to make be light. However, the joint strength shall be weaker than the deep skirt type, because the joining area is small when the transmission is attached to the engine. It is easy to make some vibrations so that it is necessary to be assisted by supporters. Additionally, the space for attaching the auxiliary devices shall be smaller. To secure the crankshaft to the cylinder block and to reinforce the strength of the block, a supporting device might be formed with the bearing of the crankshaft at the lower part of the crankcase. According to the type of this supporting device, there are the ladder frame style and bearing beam style. At the lower part of the cylinder block, an oil pan is attached also. This is for gathering the oil completing the lubricating and cooling role. It is made of a pressed steel sheet and attached by rubber packing like the head cover. The oil pan is easy to make a noise so that it is made of the vibration resistance steel plate. The vibration steel plate is manufactured by inserting a resin plate between the two steel plates to prevent from vibrating.
The reason that the roller bearing type is not applied to the crankshaft is that the load can be concentrated at the contacting portions of the ball or roller in a point or linear type. In the plain bearing the load is applied on the lubricated side, the larger contacting area than the ball or roller bearing so that the plain bearing can support large force. As the plain bearing is also called as a sliding bearing, the shaft is sliding on the bearing with the lubricant oil. Even if the surface of the solid metal body is applied the smoothing surface treatment precisely and carefully, it should have roughness somewhat. Therefore, when the two solid bodies are directly contacted, they 25
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Engine Principles should be worn. The lubricant oil inserting between the plain bearing and the axis can make the rough surface of these two solid bodies to be smoothly. The two solid bodies are not contacted directly even they are so closed. The thickness of the oil film, that is the gap with the bearing, is changed by the load or heat expansion. When it is so small, it may be adhered by friction heat, otherwise, when it is so big, it may make vibrations and noise. The bearing is made by welding the bearing alloy having light weight and good fatigue resistance such as the copper or aluminum. On the surface, a special metal basis on the lead is coated. The bearing has the oil hole and oil groove for supply the lubricant oil to lubricate the contact portion between the connecting rod and the crank pin and between the crankshaft and the crankcase. The crank journal, the rotational axis of the crankshaft, is attached at the lower part of the cylinder block by the bearing cap with the plain bearing. For the serial engine, this bearing should be attached at the front side and rear side of the cylinder. If it is the 4-cylinder, it has 5 bearings and if it is the 6-cylinder, it has 7 bearings, that is, it called as 5-bearing and the 7-bearing, respectively. Certain old style engine of 4-cylinder might have 3-bearing structure. This type is not used because the crankshaft is easy to be bent and make vibrations. 10. Flywheel
The flywheel is equipped to the transmission side of the crankshaft to maintain the smooth rotation using inertia force and to reduce the irregularity of the rotational force. The crankshaft is rotated twice per one of the combustion. At the other strokes, the reversed directional force shall be needed for the compression, the intake and exhaust. If there is no flywheel, then the rotational force of the crankshaft shall be reduced at these strokes. Therefore, when the intervals of the each combustion stroke are long like in the idling state, the engine may be stopped. Around the flywheel, a ring gear is attached to 26
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Engine Principles rotate the crankshaft by being teethed with the pinion gear. The clutch disk can attach to the flat side of the flywheel by the spring to transmit the driving force to the transmission.
inertia mass of the flywheel, the rotation of the engine can not be changed easily. It is hard to increase the engine rotation by pressing the accelerator, or to take the engine brake by releasing the accelerator. That is, the engine response will be worse. So, the fuel efficiency shall be worst too. Some engines use the 30% of the torque generated from the engine to increase the rotation of engine itself when the accelerating is performed with the lower speed shift. The size and the weight of the flywheel are decided by the purpose of the vehicles. For example, the engine for the racing car uses small size one, and that of the family car uses large size one. For the general purpose, the flywheel is made of the cast iron, and for the special purpose such as the racing car, it is made by cutting the steel material having high strength. 11. Balance shaft, Balancer for the secondary inertial force
The magnitude of the torque is calculated by multiple the magnitude of the force to the distance between the center of the axis to the point at which the force is applied. The magnitude of the force is proportional to the inertia mass so that if the fly wheel is heavy and the outer diameter is large, or if the outer portion is heavy, then the force of the flywheel might be large. In general engine, the half of the total inertia mass is distributed at the flywheel. Therefore, when the rotation of the engine is low or when the engine is in the idling state, the inertia mass of the flywheel should be large to rotate the engine regularly. However, with the large
The piston, the Connecting rod and the crank make inertia force according to the reciprocal and rotational movement. For this reason, if the one-cylinder engine has not the counter weight for balancing between the inertial force and the weight of the piston, the connecting rod and the crank, then the engine may be severely vibrated by the unbalancing. For the serial 4-cylinder engine, the four pistons are connected to the crankshaft with being paired the first, forth and the second , third, in facing each other. When the crankshaft is rotating, the inertial forces are offset so that the counter weight might be not needed. In actual movement structure of the pistoncrank system of the 4-cylinder engine, the inertia force shall not be offset. This is come form the structure in which the piston in 27
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Engine Principles reciprocal movement is connected to the crank in rotational movement with the connecting rod. For example, in the half rotation of the crankshaft when the piston moves from the highest point (TDC, Top Dead Center) to the lowest point (BDC, Bottom Dead Center), the piston has the maximum speed at the near of the highest point of the stroke rather than at the middle of the stroke. The rotation of the crank is regular so that the inertia force of the crank of each cylinder (the first inertia force) is easily offset. However, the inertial force of the piston is not. For example, the upper inertia force generated when the first and forth pistons are moving from the highest point to the lowest point is larger than the lower inertia force generated when the second and third pistons are moving from the lowest point to the highest point.
By representing these relationship on the graph with the inertia force at the vertical axis and the rotation angle of the crankshaft at the horizontal axis, when the upper inertia force of the first and fourth pistons is maximum value, the lower inertia force of the second and third pistons is minimum value, and vice versa after the crankshaft with 180°. From this relationship, we know that the inertia force is generated with the ratio of 2 times per one rotation of the crankshaft. This inertia force is called as the secondary inertia force. It is easy to be generated when the engine is in the idling state. The four-cylinder engine is equipped in the small passenger car generally. For the convenience of the passengers, a balance shaft having the half circle shape in the cross sectional view shall be attached at the both side of the engine to reduce the vibration from the secondary inertia force. This balance shaft is designed to rotate with two times of speed in reverse direction against the crankshaft. The additional inertia force generated from the balance shaft will offset the vibration from the secondary inertia force.
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Engine Principles
Chapter 3. Cylinder Head 1. Cylinder Head
The cylinder head is attached on the cylinder block with being inserting a gasket to prevent from leaking the combusted gas. The bottom of the cylinder head is also the roof of the combustion chamber. Therefore, the shape of the cylinder head is very complicated. The rectangular box shaped portion located upper position has the valve driving system inhalation the mixture of fuel to the engine and exhausting the combusted gas, and the ignition plug so that the shape and operation of this part can decide the engine performance such as the combustion of the mixture of fuel. The structure of the cylinder head is various according to the type of engine. The most cylinder head has the common structure as this. At the upper part, there is the valve driving system in which the intake port inhaling the mixture of fuel into the combustion chamber and exhaust port taking out the combusted gas in the longitudinal direction. Inside the cylinder head, there is a water jacket circulating
the cooling water from the cylinder block. The combustion chamber is very important part to decide the engine performance. So the shape and the size are the important factors. If the combustion chamber is big, the time interval for combusting the mixture of fuel is long even the mixture can be compressed enough. So the bigger power cannot be ensured. Therefore, it is prefer for the sized of the combustion chamber to be compact. Additionally, the shape of the combustion chamber has less uneven surfaces as possible to enhance the combustion of the mixtures. If the shape of the combustion chamber is complicated, then the heat from the combustion shall be easily lost because the surface of the chamber is too large about the volume of it. So, the force pressing the piston shall be lowered. The intake port is also important part because that the flow of the mixture is defined by the size and the shape of it. Considering just about the flow, the smoother inner surface is better for reducing the resistance against the flow and the straight shape of port is the better. However, the shape of the port is helpful for the inhaling mixture into the cylinder to be form the swirled flow in order to be combusted at the combustion stroke as well as possible. The water jacket will absorb the remained heat after the combustion until finishing the exhaust stroke as fast as possible to prevent from increasing the temperature of the next inhalation mixtures. Especially, the around portions having the high possibility of increasing the temperature such as the exhaust valve and spark plug should be cooled mainly to prevent from making a trouble by the over heat. At the cylinder head, there is bearing for 29
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Engine Principles supporting the valve driving system including the cam shaft. The bearing is lubricated and cooled by the engine oil. 2. Cam and Camshaft
The cam drives the valves operation opening and closing the intake port for inhaling the mixture of fuel into the combustion chamber and the exhaust port for taking out the combusted gas. For the OHC or DOHC engine, the cam is attached at the camshaft installed at the middle part of the cylinder head. The camshaft has the cams with the same number of the valves for intake and exhaust which are arranged with angles according to the timing of the opening and closing. For the 4cycle engine, the opening ratio of the intake and exhaust valves is one about the two revolutions of the crankshaft. Therefore, the camshaft revolves with the ratio of one turn about the two revolutions of the crankshaft. The extrusion portion of the cam is called as the cam nose or the cam robe. The height is called cam lift. The “lift” means that the cams lift the valve so that the opening status is determined by the cam profile. The opening and closing timing of the valves are determined by the operation angle, the angle from the start point to the end point of the nose.
When the valve is closed to the valve seat, it is preferred that the impact shall be little as possible, so that the cross sectional shape of the cam is the egg shape. The valve is always applied a force in the closing direction by the valve spring. By pressing the spring with the cam nose, the valves will be opened. If the cam velocity becomes fast to enlarge the inertia force of the valve, the reciprocal movement of the valve is not balanced with the rotational movement of the cam. The engine speed generated by this limited speed of the valve opening and closing is the maximum speed of the engine. So the cam profile is very important. The cam nose should be treated special surface treatment to ensure the strength for enduring from the frictions with the valve lift and rocker arm installed at the valve. To do so, the camshaft is made of cast iron and the cam nose should be treated with a cooling treatment such as the chilling method to strength the textures of the surface, when it is cast.
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Engine Principles Small cam lift
Big cam lift
There are two methods for supplying the lubricant oil to the cam journal supporting the cam nose and the camshaft, from outside and from inside. In the outside supplying method, the oil will be supplied from the journal. In the inside supplying method, the camshaft has a hole for supplying the lubricant oil from the middle portion of the journal. In addition, the camshaft is made by hollow tube type and the lubricant oil may be supplied through the hollow tube. 3. Driving the Camshaft For the OHC engine, the crankshaft is located under the cylinder block and the camshaft is located on the cylinder head, so there should be a chain or belt for transforming the rotational movement of the crankshaft to the camshaft. As the driving for the intake and exhaust valves should be exactly timed with the rotation of the crankshaft, for some case such as racing car, a gear is used for transforming the rotational movement exactly.
In the chain system for transforming, the tooth wheel for the chain is called the sprocket. The one attached at the crankshaft is the crankshaft sprocket, and the other attached at the camshaft is the camshaft sprocket. In the system in which the camshaft is driven by the chain, the ratio the teeth numbers between the crankshaft sprocket and the camshaft sprocket is 1:2. In order to maintain the tension of the chain, a chain tensioner is attached, and the chain guide prevents from shaking the chain during rotating. If this method is applied to the DOHC engine, the sprocket shall have the large diameter according to the number of teeth ratio. Therefore, the gap of camshaft and gap of valves of intake and exhaust will be enlarged. Consequently, this method is not applied to the compact type engines. To solve this problem, an additional sprocket is inserted there between to transmit the rotational movement to the camshaft sprocket. The timing belt type uses belt having toothed surface and pulley instead of the chain and the sprocket, respectively. In this type, the 31
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Engine Principles sprocket and pulley attached at the end of the camshaft have the timing mark indicating the opening and closing timing of the valve. So we use the term of “timing” to the name of the parts. The pulley attached at the crankshaft is the crankshaft timing pulley and the one attached at the camshaft is the camshaft timing pulley. As well as the chain type, in this type, there are direct reduction type and double reduction type. Even though the OHC engine can accept the long chain to drive the camshaft, the belt driving method is mainly used. The reason is that the long chain can make mismatching the timing and big noises, and the chain system needs the lubricating equipment. However, the belt is made of fiber and rubber so it can be easily broken by the heat or oil. It is prefer that it should be exchanged at every 90,000km running. 4. Intake and Exhaust Valve
The cylinder head includes the intake port inhaling the mixture of fuel to the cylinder and the exhaust port taking out the burnt gas. The valves at the ports are the intake valve and the exhaust valve, respectively. According to the valve shape of mushroom, we call poppet valve.
The poppet valves consist of the valve head and the valve stem. The valve stem supports the valve guide and the valve spring. The valve is opened by the pressing operation of the cam nose, and closed by the elastic force of the valve spring. The combustion temperature is over 2000℃, and the temperature of the burnt gas passing through the valve is over 1000℃. Therefore, the temperature of the exhaust valve is over 800℃ and that of the intake valve is over 300℃. So the material of the valve should be heat resistance steel.
The valve size is represented by the diameter of the head portion. The intake valve is larger than the exhaust valve. Representing by the area of the head portion, when the intake valve is 100, the exhaust valve is about 75∼85. This difference in their sizes is for balancing of the gas flow. The intake is performed by the decreased pressure resulting from the downing 32
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Engine Principles of the piston, on the other hand, the exhaust is performed by the high pressure resulting from the combustion. To balance the flow of the intake and exhaust, the intake valve size may be larger than exhaust valve. The valve stem is designed, considering the flow, such as that of the intake valve is thinner as possible to reduce the flow resistance and that of the exhaust valve is thicker as possible to transmit the heat from the extrude portion to the stem. The heat is transferred through the path of the valve stem → the valve guide → the cylinder head → the cooling water. Some high performance engine uses the hollow stem securing sodium for enhancing the valve cooling. The portion of the port contacting with the valve face is called as the valve seat. If the cylinder head is casting iron, then the valve seat has double structure. If the cylinder head is made of aluminum alloy, then the seat is made of heat resistance steel. The valve spring always press the valve to attach to the cam so that the valve spring is preferable to be soft to reduce the frictional resistance generated when the cam nose presses the valve. Additionally, in order to enlarge the amount of intake and exhaust gas, the valve may be enlarged and the lift of the cam may be extended as well as the valve spring may be softened to operate faster. Then, it may make the surging problem and it is hard to be balanced. 5. Valve Driving System As the valve controls the gases for inhaling into and the taking out from the cylinder, its driving method can give an important affects on the engine performances. There are various type of valve driving system. It has been developed
from the side valve type, OHV, OHC to the DOHC type.
Side valve type In the side valve type, the camshaft installed near the camshaft presses the long valve system to open and close the valve. The combustion chamber is large and it has long time interval for combusting the mixture of fuel so that it can not make high output power. Nowadays this type is not used.
OHV (Over head valve) type In the Over Head Valve (OHV) type, the valve like in the side valve type is attached on the cylinder to open and close the valve using a long rod, the push rod. The shape and structure is similar with that of engines used now to enhance the performance.
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Engine Principles we see from the name, there are two camshafts so we call it as the twin cam system also. Additionally, in the V-type engine, it has two cylinder head so the camshaft shall be four.
OHV (Over Head Camshaft) type After that, the OHC (Over Head Camshaft) type is developed. As know from the name, the camshaft is located over the head, exactly to say, in the middle of the cylinder head. In the OHC type, there are two types, the one is the in-line type in which the intake valve and the exhaust valve are alternatively arrayed along the linear direction, and the other is the Vshaped arrange type in which the intake valve and the exhaust valve are faced each other in opposite side to form the V-shape. The later has more enhanced efficiencies and higher performance.
Direct type Swing arm type Rocker arm type The driving method for the intake and exhaust valves is divided into the direct type, in which the cam directly drives the valves, and the rocker arm type, in which the cam drives the valve using a lever. The rocker arm means the lever connecting the leverage point and the cam. Using the lever, it can drive the valve faster than the cam lift. The direct type has less components and high strength. Using the oil pressure for the hydraulic tappet, the valve can trace the cam profile always. 6. Valve Timing
DOHC (Double Over Head Camshaft) type
The valve timing is the time at the opening and closing the intake and exhaust valve. Each time indicating when the valve starts opening and when the valve finishes closing is represented by the rotational angle of the crankshaft about the highest point or the lowest point of the piston as the standard time.
Developing this V-shaped arrange type more and more, the DOHC (Double Over Head Camshaft) type, in which the intake valve and the exhaust valve are independently driven by the different camshaft, is mainly used nowadays for the high performance engine. As 34
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Engine Principles lowest point, so that the mixture of fuel can be inhaled enough.
Intake valve timing As simply saying about the opening and closing the valves, the exhaust valve will open when the piston is at the lowest point. After taking out the gas, when the piston is at the highest point, the exhaust valve will be closed. At the same time, the intake valve will be opened to take in the mixture of fuel. When the piston is at the lowest point, the intake valve will be closed. However, this is just concept for the operation of valves. The mixture of fuel and the used gas have mass so the flow of the mixture or gas is not completed at a moment but need some time intervals. Furthermore, the valves can not open and close at an instant moment, too. For example, the intake valve need some time interval to open fully, and the mixture is not inhaled at a moment into the cylinder but inhaled during some time interval by the inertia of flow. Therefore, the intake valve shall open prior that the piston is reached at the highest point. When the piston starts to go down, then the valve is already little opened to take into the mixture of fuel into the cylinder. By that the intake open little early, the valve will be fully opened when the piston is reached at the
When the piston is passed the lowest point, the intake valve is not closed thoroughly. To do so, the mixture will be more inhaled into the cylinder by the inertia of the flow of the mixture. At the end of the combustion stroke, the exhaust valve will be open just before the piston is reached at the lowest point(BDC).
Exhaust valve timing It is for exhausting the combusted gas as fast as possible by the remained expansion force in the cylinder. In the same manner of the intake valve, even the piston is passed the highest point(TDC), the valve is still opened to exhaust the burnt gas fully using the inertial of exhausting flow. 35
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Engine Principles
Variable camshaft angle type According to these operating process of the valves, there is some period in which the intake valve and the exhaust valve are opened at the same time, because the exhaust valve is closed after passing through the highest point(TDC) and the intake valve is opened before reaching to the highest point(TDC). At this time, the vacuum inertia force generated from the exhausted gas can accelerate the inhalation of the fuel mixtures. This period is called the valve overlap.
Being overlapped in the opening time of intake and exhaust valve, the enhanced intake efficiency of the mixtures is as high as the enhanced exhaust efficiency of the used gas, that is, the rotation of the engine. On the other hand, when the engine is in the rotating state with low speed such as the idling state, the engine efficiency may be lowered by the lowered gas flow. Especially, for some high performance engine having large overlap, in the low speed, the intake valve will open with large amount of burnt gas so that the burnt gas will be adhered to the intake port. Therefore, the combustion will be unstable or incompletely combusted. In the 4-valve engine, if the valve overlap is too large, then the engine will be easily unstable at the idling state. So, the overlap time for the 4valve engine shall be very short, or some cases have the zero overlap time, that is, the intake valve is opened according that the exhaust valve is closed.
7. Variable Valve Timing
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Engine Principles that the engine output shall be enlarged. This system also called variable valve timing system but this system controls the valve lift also. It is more advanced system. 8. Malfunction of the Valve
Selective camshaft-lobe type As we mentioned, the valve timing is different according to the rotation speed of the engine. That is, the intake valve should be opened with little lately at the lower rotation, and with little early at the higher rotation. So, an additional switch operated by oil pressure is installed at the intake cam sprocket in order that the camshaft can be rotated somewhat when the engine is rotating over certain RPM so that the cam presses the intake valve early. This is the variable valve timing system. In the variable valve timing system, the cam shape is not changed, so the valve shall be also closed early when it is opened early. Early closing the valve, the amount of the inhaled mixture is reduced. Therefore, the choice of the valve timing is decided by not only the rotation speed of the engine but also the load of the engine.
The valve is opened by the cam but closed by the spring. Actually, the valve attached by the spring to the valve seat is opened with the pressing by the cam nose. At that time, the force rotating the camshaft is rather small as possible. So the soft spring is more proper. However, for some high performance engine having large size of valve or lift, the spring shall be harder and the balance of the spring strength is important factor. Even not occurring in the normal driving situation, the hardness of the spring with the weight and the strength of the valve can make an abnormal operation such as the valve jump, the valve bounce or the valve surge, when the engine is rotating over the limitation rpm. The valve jump is that the inertial force of the valve is too large so that the cam can not press the spring and then the valve is jumped up from the cam nose when the camshaft is rotating with high speed. The valve can return back to the original position but the other dynamic components including cam, rocker arm, valve lifter, etc are damaged by friction each other.
Consequently, the cam system will include the two kinds of cams, the one for the low speed of the engine and the other for the high speed of the engine. For the low speed cam, the valve will be opened lately and closed early, and the lift will be small and then the inhaled mixture of fuel will be reduced so that the fuel efficiency can be enhanced. For the high speed cam, the valve will be opened early and closed lately and the lift will be large to inhaled more mixtures so 37
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Engine Principles The valve bounce is that the valve face is not contacted to the valve seat (contacted portion with the valve) but bounced from the valve seat when the valve is closed by the spring. The dynamic components shall be damaged by this bouncing. As increasing the rotation of the engine, the bouncing is occurred over than limited rpm. This limited rpm is called as the crush speed or limit rpm of the engine.
The valve surge is the abnormal vibration of the spring. As the natural frequency of the spring is corresponding with to the elastic timing by the cam, the spring can make a high movement by the self-excited vibration. When the engine is rotated in force. If it is continued then the spring shall be broken. These abnormal operations of valve can be easily made when the valve is heavy and the lift is large. When the most used engine was the 2valve OHC engine, they were main problems in engine. After the 4-valve DOHC engine is mainly used, these problems are scarcely occurred. As the 2-valve system became to the 4-valve system, the valve area would be enlarged so the intake and exhaust flow would be smoother. Therefore, the lift has not to be enlarged. Furthermore, the valve is to be lightened so that the spring has not to be strengthened even if the rpm is increased. 9. Overrun and Red Zone In the tachometer of the engine, there is certain rpm range colored in red called as the red zone. Some meter has the yellow zone just
before the red zone.
The rpm at the beginning of the red zone is the maximum allowable rpm decided by the characteristics and endurance requirement of the dynamic components including valve and valve spring and the main system components including piston and connecting rod when the engine is operated with the maximum speed having the maximum output. To operate the engine over the maximum allowable rpm is called as the overrun or overrevo means the revolution. The over-revo may occur when the shift is down to the lower speed gear at the driving with high speed. When the engine is in the idling state, if the rpm is increased in force, then the engine will be overrun state. When the engine is overrun, the valve shall be abnormally operated such as the valve surge, jump or bounce. In this case, the valve and spring may be damaged or in some cases, the piston may be damaged by bumping with the head of the piston and the valve. In order to prevent bumping between the piston and valve, a recessed portion is made at the piston. However, if the piston is jumped up over the recessed portion, then the piston will be bumped to the valve. If the average speed of piston movement is abnormally fasted by the overrun, the gap between the piston ring and the cylinder will be 38
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Engine Principles damaged and the oil film of the bearing at the piston pin and the crankshaft will be broken so that the temperature will be increased or some parts will be adhered by this heat. When the engine is operating with high speed, the combustion speed is also fast and the around temperature of the combustion chamber will be increased. So, these problems might be easily occurred, so it needs to be careful. Additionally, by the overrun, the engine has the increased inertia force being results of the abnormal vibration. The parts shall be broken or cracked. The maximum allowable rpm is set with 300~1300 rpm higher than the maximum output rpm. In some engine, in the red zone the fuel injection will be cut to prevent the rpm from increasing over the maximum rpm and to prevent from making problems by the overrun.
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Engine Principles
Chapter 4. Intake System 1. Enhance the Volume Efficiency
The pressure of the fuel mixture at the end of the intake stroke is lower than the ambient pressure because of the resistance against the flow of the air cleaner and duct. And the temperature of the fuel mixture is high because that it contacts with the hot valve and cylinder wall when it is inhaled. As the density of the air becomes low when the pressure becomes lower or the temperature becomes higher. The volumetric efficiency is used for indicating the intake ability of the mixture. As indicating the amount of the inhaled air in rate about the engine displacement, the volumetric efficiency is calculated from dividing the weight of the inhaled air by the weight of the air amount with the displacement at the same temperature and the same pressure. In another method for indicating the efficiency of the intake stroke, the charging efficiency can be used also. This is the volume efficiency when the temperature and the pressure is in the standard condition (25℃, 99kPa).
high as possible. The method for enhancing the volume efficiency is like the followings. ⑴ The ambient air shall be inhaled into the manifold with not high temperature as possible. For the engine having the turbocharger, it should be equipped with the intercooler to prevent from increasing the temperature of the inhaled air.
⑵ Reduce the flow resistance of the inhaled gas by increasing the number of valves, and enlarging the size and the bending radius of the duct and manifold.
⑶ Enlarge the diameter of the valve and the height of the cam lift, and balance the valve timing properly.
In order to enlarge the maximum output of the engine, this volume efficiency should be as 40
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Engine Principles
⑷ Choose a long intake manifold at the lower speed , and choose a short one at the higher speed to use the inertia and pulsation effect of the air.
⑸ Enlarge the intake gas pressure by installing the turbocharger.
possible in order to enhance the volume efficiency. It is possible to apply the (2) ~ (4) of above mentioned methods applied to the intake system except (1) and (5) to the exhaust system. For example, the “flow resistance of the inhaled gas” of (2) is took place with the “flow resistance of the exhausted gas”, and the “inertia energy of air” of (3) is took place with the “exhausting inertia”. Furthermore, the exhaust interference shall be minimized as possible. Additionally, a turbocharger shall increase the intake resistance. By tuning up the engine, it can be effective for increasing output even it is very difficult to tune up the engine. 2. Intake Inertia Effect and Pulsation Effect In order to inhaling air having high density to the engine, to use the inertia of the air flow is called the inertia effect, and to use the characteristics of the longitudinal wave such as sound wave according to the density of the air is called the pulsation effect. In the inertia effect, as the high density air is inhaled into the engine using a inertia energy of air. It is called as the inertia supercharging. The air into the engine has the flow inertia as a gas and is a media for transmitting the pressure wave. The air flow in the intake manifold is periodically intercepted by the valve, so the manifold pressure has variations from the difference of pressures between high density portion and low density portion.
Until now, we mentioned about the enhancing method for volume efficiency related to the intake stroke. It is important to exhaust perfectly the burnt gas at the exhaust stroke as
Valve open (Inhaled air)
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Engine Principles
Valve close (High density air at valve)
Valve close (High density air at valve)
Valve open (Inhaled high density air) Therefore, the intake inertia effect and the pulsation effect could be made. When this pressure variation affects to the intake stroke at the cycle generating the wave directly, it is called as the inertia effect. When the pressure variation is not reduced and then affects to the next cycle, it is called as the pulsation effect. However, it is not distinguished between them exactly. We, here, will call as the inertia effect when the air flow inertia is mainly governed and as the pulsation effect when the pressure wave is mainly governed. As the first example, assume that the intake valve is closed during the fuel mixture is inhaled into the cylinder. As the fuel mixture has the flow inertia, the mixture flow in the intake manifold can not stop instantly just at the closing the valve, but pretends to flow continuously. Therefore, the air just before the valve will be pressed by inertia energy of intake air. Consequently, the air density at the port portion will be increased. At that time, if the valve is opened in time, then the air of high density can be inhaled into the cylinder. This is the inertia effect.
Density of following air is low & pressure wave are reflected by surge tank
High density air by reflection of pressure wave is inhaled When the air density near the port is increased, the density of the following air is low respectively. So, the boundary portion makes the pressure variations, that is, noise. This air density variation passes through the manifold with the speed of sound. It reflects to the end of the manifold, and then it returns to the port. When the high density air is back to the port, if the port is opened, then the high density air can be injected into the cylinder. This is the pulsation effect. As these effects are combined, it is hard to separate from each other. However, in order to maximize the effect, it is prefer to make the pressure wave in the manifold to make high air density near the port when the valve is opened. 42
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Engine Principles To do so, the diameter and length the intake manifold and the shape of the intake port shall be controlled. 3. Variable Intake System
At low speed
At high speed The air flow in the intake manifold is not uniformed but variable according to the engine speed. When the high density air flow reaches at the port, if, ideally, the intake speed is maximum just before the closing the valve, then the intake inertia effect will be maximized. The air pulsation frequency is decided by the diameter and the length of the manifold. When the diameter is same, the frequency of the long length manifold is small. It is the same that the sound has the lower frequency when the distance between the hole and the mouth piece of the recorder is larger.
Generally, the length of the manifold is already decided, so if the engine is running with certain velocity, the intake inertia is effective. However, if it is rotating with variable speed, then the lower density air can reach at the port when the valve is opened so, the charging of air may be worst. Consequently, the method for varying the length of manifold is developed according to the rpm of the engine. When the engine has high rpm in which the valve is frequently opened and closed within the same time interval, the short length manifold is chosen to make the cycle be short. Contrarily, when the rpm is low, the long manifold is chosen to make the cycle be long. So, it is possible to get the intake inertia effect 43
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Engine Principles in wide range of rpm. As being the variable intake system, it is called as the variable inertia charging system or the variable intake control system. There are many types to control the length of the intake manifold. Mainly, the two types are used. The one type is that the separated two manifold groups are connected together. When the engine is high speed, the path is divided each other, and when the engine is low speed, the two manifold groups are linked each other to elongate the length of the total manifold.
separated manifold. In this case, the inertia charging effect can not be expected even at the high speed. This phenomenon can be inhibited by enlarging the volume of the intake collector connecting to the manifold. When the resonance is occurred, at the middle and low speed, the inertia supercharging effect becomes high so the charging efficiency will be increased. This is called as the resonance supercharging effect. 4. Intake System The intake system takes the air to mix with the gasoline and inhales the mixtures into the cylinder. Generally, the intake system comprises the air cleaner filtering the dust in the inhaled air, the carburetor mixing the air and the gasoline, and the intake manifold (or inlet manifold) inhaling the mixture into the cylinder, at the head portion of the cylinder. Nowadays, the electrical controlled unit for fuel injecting to the intake manifold directly is widely used, so the design of the intake system is changed very much.
The other is to attach a bypass at the manifold system in which the air pass through the bypass when the engine is low speed, and the path to the bypass is closed to reduce the length of manifold at the high speed.
Carburetor intake system
In case of linking some manifolds, the resonance may be occurred between the manifolds. This comes from the pressure vibration having the same frequency in the 44
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Engine Principles
MPI intake system At first, the air inlet port which was near the cylinder head is moved to the front grill to intake the ambient air having lower temperature than the air around the engine room. As being low temperature air, the air has high density, therefore, the much amount of oxygen will be contained into the inlet air. With the same volume of the inlet air, the lower temperature of the air is the more prefer. For example, in summer of 30℃ temperature, when the air conditioner is working at the low speed such as at the rush hour in the city, the temperature of the engine room could be over 80℃. In this case, by calculation, the amount of the oxygen in the air around engine room is 15% less than that of the ambient air.
The resonance chamber is a small box branched from the duct as the device for reducing the intake noise, and it is called as the regenerator chamber or the side branch. According to the opening and closing the intake valve, the vibration of the air inside the air cleaner box or the duct can make a large intake noise or hinder the intake operation. Using the resonance effect by installing a resonance device, this vibration will be terminated. 5. Throttle Valve and Manifold
The inlet air to the front grill is inhaled into the intake manifold through the long duct via the air cleaner, resonance chamber and throttle body. In the carburetor system, the dish type air cleaner was looking on the carburetor, but nowadays, the box type air cleaner is installed at one corner of engine room. The air cleaner not only cleans the air going into the cylinder but also reduces the noise from the intake operating. The air cleaner element should be maintained periodically.
Butterfly type, slide type throttle valve To rise up the engine rpm, we press the accelerator pedal, and to lower down the rpm, we release the accelerator pedal. As the accelerator pedal is linked to the throttle valve by the wire and linkage, when the pedal is pressed the throttle valve will be opened to intake the air into the cylinder. That is, the 45
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Engine Principles carburetor or the electronic control system for fuel injection automatically checks the amount of the air to supply the amount of gasoline proper to the driving situation. In the carburetor system, the throttle valve is equipped with the carburetor. In the electronic control system, it is installed at the middle of the throttle body (throttle chamber) separately installed in the intake system and being with the air flow sensor detecting the air flow amount and the throttle position sensor checking the status of the throttle valve opening.
manifold or from the cooling water for the engine, the intake manifold should be heated up. The method for heating the intake system using the exhaust temperature is only used for the counter flow type engine in which both the intake and the exhaust manifolds are installed at the same side of the engine. The method for heating the intake system using the cooling water is accepted in the cross flow type engine in which the manifolds are installed at the opposite side of engine each other.
In the throttle valve type, there are a butterfly valve in which disk plate having the shape of the butterfly wing is attached at the axis inside the pipe to control the air amount by rotating the axis, and a slide type in which an aluminum plate controls the amount the air without any hindrance at opening the throttle valve, especially for racing engine. The air passing through the throttle body and the mixture mixed with gasoline at the carburetor are distributed into the cylinder by the intake manifold. The fuel injection is performed before distributing the air at the manifold, or at each cylinder as the mixture. The important thing is that the intake manifold should inhale the mixture into the cylinder as smoothly as possible, so that the manifold should have less bent portions and smooth inside faces. The mixed gasoline in the carburetor is inhaled into the cylinder as the foggy state in the air. When the temperature is low such as just before starting the engine, this foggy particle of the fuel can attach to the manifold wall during being inhaled. Therefore, the mixture is leaned so the combustion is not enough. To solve this problem, using the heat from the exhaust 46
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Engine Principles
Chapter 5. Exhaust System 1. Exhaust System
By opening exhaust port of the cylinder head, the combusted gas is exhausted through the exhaust manifold, the exhaust pipe gathering the combusted gas from each cylinder, the catalyst converter purifying the used gas and the silencer (muffler) reducing combustion noise. The most important thing in the exhaust system is the smooth path also. The exhausted gases from each cylinder are bumped at the exhaust pipe gathering the each exhaust manifolds. So, it is important to prevent from hindering each flow or to increase the exhaust efficiency using the exhaust inertia effect having the same principle in the intake inertia effect. However, it is hard to balance between the increasing the exhaust efficiency and the enhancing the engine performance, because there are many weak points from the engine to the muffler. The exhaust manifold is made of the casting iron having high heat resistance, or the aluminum alloy. Also the exhaust gas temperature is high. So, it is need to make the exhaust system with the material having better heat resistance or to cool the exhaust system with the wind.
The catalyst converter is used for purification of the exhausted gases. There are a manifold catalyst installed near the manifold and a under catalyst installed under the floor. The manifold catalyst is more effective because the exhaust temperature is higher than the other system. However, it can be easily aged by the high temperature so usually two pieces are used. The under catalyst is not easily degraded, however it has high performance of purification.
The muffler is equipped for reducing the temperature and pressure of the exhausted gas as well as the combustion and exhausting sound. Generally, the muffler has many sector walls to make the inside space into maze path for flowing the exhaust gas, so it called as the maze type. There is also a straight muffler type in which a tube having many holes on the surface and silencer such as glass wool inside the tube. The maze type muffler has more effective sound absorption ability but has larger 47
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Engine Principles flow resistance. The straight muffler has more enhanced output but has louder sound. In some cases, these two muffler types are simultaneously equipped using separated two pipes so that the maze type muffler is only used when the engine is working in low speed, and the straight type muffler will be additionally used when the engine is working in high speed and load. 2. Exhaust Inertia Effect and Pulsation Effect
The most important thing determining the performance of the exhaust manifold is the smoothness in the exhausting. The exhaust interference is the main problem hindering the smoothing exhaust. From the each cylinder, the combusted gas is exhausted according to the order of ignition. As they are merged into the manifold, if the exhaust system is not well arranged and combined, then one exhausted gas passing through the manifold can be collided with another exhausted gas from another cylinder or the pressure in the manifold will be increased, so that the exhaust gas can not be exhausted well.
The exhaust interference can be overcome by elongating the distance between the exhaust valves to the collection portion of each cylinder gas or be making the angle of collecting portion into an obtuse angle to ensure the well flow. As increasing the number of cylinder, it is hard to ensure the well exhaust flow by combining the increased manifolds. In the serial 6-cylinder turbo engine, with not being fully closed the first exhaust valve, the fifth exhaust valve is opened, and with not being fully closed the fifth exhaust valve, the third valve is opened. So, if the all 6 exhaust manifolds are gathered into one collecting pipe, then the sequentially exhausted gases can be collided each other. Therefore, the manifolds are divided into two groups; the one includes the first, the second and the third manifolds and the other includes the fourth, the fifth and the sixth manifolds. Each group has own turbo charger to eliminate the exhaust interference and to enhance the engine output. The inertia effect and pulsation effect are utilized to exhaust the remained combustion gases in the combustion chamber. As being the same with the intake system, when the exhaust valve is closed, the gas density around the valve shall be reduced to accelerate the exhausting gases from the chamber effectively. When the exhaust valve is open, the combusted gas having the high pressure is exhausted through the exhaust valve and the 48 Chonan Technical Service Training Center
Engine Principles remained gases will be exhausted by the following compressing pressure of the piston at the exhaust stroke and then the exhaust valve is closed. Therefore, the exhausted gas flow has high density portion and low density portion in the manifold. As we mentioned in above chapter, when a gas flow has a difference in density, then pressure wave is generated. Therefore, this difference is transmitted during the manifold with sound velocity. This is called the exhaust pulsation. Just before the exhaust valve is closed, if it is possible that the density around the valve is lower than other portions, then it is accelerated that the remained gas in the chamber shall be took out as well as the fuel mixture shall be inhaled from the intake valve. 3. Component of the Exhaust Gas The exhaust gas from engine and fuel system to atmosphere comprises of the burnt gas from the exhaust pipe, the blow by gas from the crank room, and the vapor gas from the fuel tank by the hot weather and hot temperature of the working engine. As these gases include harmful materials contaminating the atmosphere, a purification system should be equipped.
Especially, the exhaust gas is the most important gas.
If the fuel is combusted perfectly, then any harmful material is not included in the exhaust gases. The fuel, gasoline, consists of hydrocarbon, the compound of the carbon and the hydrogen. In the chamber, the fuel is changed into the carbon dioxide (CO2) and water (H2O) with making heat energy. In actual chemical reaction, the hydrocarbon and the oxygen are not changed at a moment into the carbon gas and water vapor. This chemical reaction is very complicated. For example, the hydrocarbon would be divided into a small unstable material by the heat and reacts with the oxygen, or the results particles are reacting each other, and so on. Among the gases generated during this complicated reaction, the carbon monoxide, hydrocarbon gas and the nitrogen oxide are the major harmful materials. The carbon monoxide (CO) is the unstable material having one carbon and one oxygen so that it can be easily changed into the carbon dioxide, the stable and harmless material, if additional oxygen and heat are supplied. If we breath the carbon monoxide, then it will catch the oxygen delivered by the hemoglobin in the blood to be carbon dioxide, more stable material. So, our body is lag of oxygen.
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Engine Principles oxide will be less; however, the combustion efficiency will be worse. So, the nitrogen oxide shall be treated at the exhaust system. 4. Air-Fuel Ratio and Exhaust Composition
The hydrocarbon gas (Hm Cn : here m, n are integer number) is come from the fuel not combusted or the intermediated material during the chemical process of combustion. It is come from the blow by gas or the vaporized fuel from the fuel tank. If this gas is revealed at the atmosphere, then it will react with the oxygen and hydrogen and change into the aldehyde, the harmful material having strong stimulus.
The nitrogen oxide (NOx) is come from the reaction between the nitrogen (78% of the air) and the oxygen in the air by the high temperature of 2000℃ in the chamber. As the nitrogen oxide is made by the difference mechanism with the carbon monoxide or hydrocarbon gas, it will be increased when the carbon monoxide and hydrocarbon are reduced by almost perfect combustion. When the combustion temperature is low, the nitrogen
As the harmful material in the exhaust gas is the product from the combustion, one of chemical reactions, the amount of it is decided by the air-fuel ratio, e.g. the ratio between the amount of air and the gasoline, the combusting temperature and the gas flow status. When the air-fuel ratio is lower than theoretical value (Stoichiometric), that is rich fuel, the combustion is not completely performed, so that more hydrocarbon gas and carbon are made. Contrarily, if the air-fuel ratio is higher than the theoretical value (Stoichiometric), that is lean fuel, then the gasoline shall be combusted completely. So, the amount of the carbon monoxide and the hydrocarbon gas will be less, however, the amount of the nitrogen oxide will be increase because the combustion temperature is high. Especially, when the temperature is over 2000℃, the nitrogen oxide will be abruptly increase. Even the nitrogen and the oxygen are not react in the room temperature, in the high temperature, they are changed into the nitrogen monoxide and then changed into nitrogen dioxide After exited from the exhaust system. 50
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Engine Principles The amount of the nitrogen oxide will be maximized at about 16 air-fuel ratio higher than the theoretical ratio (14.7). With lower ratios than 16, the nitrogen oxide amount will be reduced because the combustion temperature is lowered. When the air-fuel ratio is over 18 with less fuel, the fuel can not combusted properly so that the hydrocarbon will be increased. To reduce the harmful material, it is important how to decide the air-fuel ratio. In some cases, the air-fuel ratio can be controlled by inhaling the combusted gas into the mixtures; it is called the exhaust gas recirculation device (EGR).
The exhaust gas recirculation device is called as EGR as an abbreviation. It is the device for returning some amount of the exhaust gas back to the cylinder. Doing so, the actual amount of fuel is reduced and the combustion speed is slow, and then the maximum temperature of the combusting chamber will be lowered and the amount of the nitrogen oxide will be also reduced. But, if the amount of the re-circulated exhaust gas is too much, then the engine output and fuel efficiency will be worse, so it is important to control the amount of EGR.
optimizing the vehicle status by the sensing and calculating with the temperature of mixture and cooling water, the vehicle speed, and the load. 5. Exhaust Purification System Devices for reducing the harmful material from the exhaust gas are the exhaust oxidation device combusting the carbon monoxide and carbon and the 3way catalysts device treating the exhaust gas using oxidation and deoxidation reaction by the three catalysts for carbon monoxide, hydrocarbon, and nitrogen oxide. As the carbon monoxide and the hydrocarbon gas are come from the incomplete combustion of the hydrocarbon and oxygen, the oxidation device supply additional air to the exhaust port to make oxidation the incomplete combusted gas included in the exhaust gas. So it is called as the secondary air device. In some system, in the middle of the exhaust pipe, a oxidation catalyst coveter, the vessel including the oxidation catalyst may be equipped for converting the carbon monoxide and the hydrocarbon into the carbon dioxide and water, respectively.
In the carburetor system, the amount of the recirculated exhaust gas is controlled by the reverse pressure of the intake manifold. In the ECM system, the amount will be determined in 51
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Engine Principles complete combustion is 14.7. Because the 3way catalyst is not work properly when the oxygen is remained, it is necessary to maintain the air-fuel ratio with the theoretical value. To do so, an oxygen sensor is used for detecting the oxygen. If oxygen is detected in the exhaust gas, then the computer will calculate the amount of intake air and EGR gas to maintain the fuel ratio to the theoretical value.
The electronic control engine uses the 3way catalysts device, generally. In the nitrogen oxidation, there are nitrogen monoxide consisting of one nitrogen and one oxygen, and nitrogen dioxide consisting of one more oxygen. If the oxygen is removed from the nitrogen oxidation, e.g. de-oxidation reaction, then it becomes nitrogen gas. If the acquired oxygen from de-oxidation reaction of the nitrogen oxidation is supplied to the carbon monoxide and hydrocarbon to oxide them, then the three harmful gases can be simultaneously purified. From this idea, the chamical action is performed the de-oxidation reaction on the nitrogen oxide and the oxidation reaction on the carbon monoxide and the hydrocarbon by controlling air-fuel ratio to eliminate the oxygen in the combusted gas completely. The catalyst is the material accelerating certain chemical reaction. The catalyst used in this purification is called the 3way catalysts. There are the pellet type covering a film of platinum and rhodium on the particle alumina and the honey comb type. As the theoretical air-fuel ratio, the ratio of
6. Blow-by Gas Recirculation Device The blow-by gas is the leaked gas to the crankcase through the end gap of piston ring at the combustion stroke. It includes vaporized engine oil, too. The old type engine or some racing engine exhausts this gas into the atmosphere. When you stand near the racing car, you can smell like some oil burning result from the blow-by gas.
The components of this gas are the flammable gas of 75∼80% and the combusted gas of 20∼25%. As they are the main causes of air pollution, the perfect combustion system by recirculation of this gas should be equipped by law. This device is called as the blow-by gas recirculation device or the positive crankcase ventilation, PCV in abbreviation. In the 1ℓ of blow-by gas, 0.04∼0.05g of the 52
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Engine Principles strong acid moistures are included, so that the inside of the engine can be easily corroded and the engine oil can be easily oxidized. Therefore, it is important to treat the blow-by gas for maintenance the engine. The blow-by gas amount will be increased as the pressure difference between the cylinder and the crankcase is large. The pressure inside the crankcase is not changed so much even the engine is running with high speed. So, when the engine speed and load is increased, the blow-by gas will be increased. The blow-by gas treating should be performed in two steps according to the engine load.
The blow-by gas recirculation device comprise of hoses, one is connected between the rocker cover and the surge tank, the other is connected between the rocker cover and the intake duct before throttle body for fresh air. Also there is an air passage between the crankcase and the rocker cover. When the engine is working, the pressure in the intake manifold is always negative pressure so that the blow-by gas will be flow from the crankcase to the manifold. The blow-by gas in the intake manifold is inhaled into the cylinder. The blow-by gas will be treated by these methods.
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Engine Principles
Chapter 6. Charger 1. Kinds of charger
Turbocharger
The basic of increasing the engine power and torque is 『How more oxygen can be inhaled into the engine』. For example, if the density and amount of inlet air is high, then the power and torque shall be high.
Supercharger
Additionally, by developing the intake system and combustion chamber, it is possible to intake more air. One of possible methods is 『to compress the air and to inhale』, that is, to use auxiliary device, the charger.
The Turbocharger is, as the “Turbine driving charger”, a system compressing air by the compressor rotating a turbine using the exhaust gas flow. It is possible to get high power output using a small device. However, when the engine is rotating with low speed, the turbine can not rotate with high speed, so the compressing power is not enough and the acceleration will be delayed.
There are some types in the chargers. Typically, there are a Turbocharger type in which the charger is driven by the exhaust turbine, and a Supercharger type in which the supercharger is driven in mechanically using driving force of some part (such as crankshaft rotation). Therefore, even the accelerator is pressed the engine rotation may not be response 54
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Engine Principles immediately, that is Turbo Lag. The lag means the time delay. As driven by the interlocking mechanism with the crankshaft, the Supercharger, the mechanical supercharger, has the well response. However, in the low engine speed, the engine efficiency will be worse by increasing leak air through the gap between the rotor and the housing. In the high engine speed, the driven force loss may be increased. To recover these defects, the structure of supercharger system may be changed or a the Turbocharger and the Supercharger may be combined to new system. The engine without such supercharger is called as the naturally aspirate engine or NA engine. 2. Turbocharger
As the turbocharger is a combined word of turbo (turbine) and charger, it comprises of a turbine and a compressor directly linked, so that the turbine wheel is rotated by the exhaust gas energy and the inlet air will be compressed by the rotation of the turbine wheel. The body of turbocharger comprises of turbine wheel, compressor wheel and axis, and attached near the exhaust manifolds.
The heated air by the compressing is cooled by the intercooler and supplied into the engine via the throttle valve. The exhaust gas passes to the turbo charger to rotate the turbine wheel. In order to prevent the over boosting at high speed, when the boost pressure is over the predetermined pressure, a waste gate valve (exhaust bypass valve) of the WGT (waste gate turbocharger) will be opened.
The turbine wheel is rotated 100,000 rpm ~ 160,000 rpm with hot exhaust gas about 900℃ at high speed, the turbine wheel is made of light material having high heat resistance such as ceramics. The smaller and lighter turbocharger is better for the engine response such as acceleration and deceleration but worse power at high speed. On the contrary, the bigger turbo charger has a high power at high speed but 55
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Engine Principles slow response. Therefore, it is important to select the size of turbo wheel according to the displacement volume of the engine. Generally, the compressor wheel is made of aluminum to make lighter turbocharger. For the supporting of the high speed rotor shaft, lots of the engine oil is supplied to the shaft for the lubrication and the cooling. If the engine is stopped from the high speed abruptly, the turbocharger is rotated without the oil by inertia force of turbocharger until self stopping. So, the rotor shaft may be adhered. For this reason, the turbo engine should be stopped after idling. 3. Boost Pressure and Compression Ratio The air pressure which is pressurized by the turbocharger is the Boost pressure. If the boost pressure is increased, the intake air amount into the cylinder is also increased and the power output will be increased. However, the boost pressure can not be increased without limitation. As increasing the boost pressure, the actual compression ratio is also increased, so knocking shall be occurred at high compression pressure. The actual compression ratio represents how the inlet air into the cylinder compressed actually is. So, the boost pressure is regulated by waste gate valve. The knocking is the self ignition phenomena while the flame is spreading after the ignition of the spark plug, the un-burn mixture can be easily self-ignited at high temperature condition by high compression. So, the knocking is occurred as much as the actual compression ratio high.
By this reason, the compression ratio of the turbo engine in specification is smaller than that of the NA engine. For example, if the engine having the represented compression ratio of 10 is supercharged with 1 atm, then the air amount will be 2 times and the actual compression ratio will be 20 and then knocking will suddenly be occurred. Generally, compression ratio of the commercial turbo engine is set lower than that of the NA engine balancing with the power, torque and fuel efficiency. The knocking can be prevented by controlling the ignition timing in the NA engine but the knocking control by the ignition timing in trubo engine is not easy because the knocking is affected by the boost pressure. The maximum power and fuel efficiency is made just before the knocking is occurred 56
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Engine Principles because the combustion speed is most fast at this situation. By sensing the knocking noise, the ignition timing can be fully advanced by ECM (electronic control module) until the knocking is occurred. The sensor for detecting the shock noise is the knock sensor.
The knock sensor converts the vibration about 7 kHz into an electrical signal. By attaching at the cylinder block, this signal is treated by computer with the engine rpm, crank angle, and the intake air amount to control the ignition timing for preventing the knocking 4. Turbo Lag The turbo charger is an air compressor for 『how much air is supplied』. By using the negative pressure acquired when the piston goes down and the flow inertia for intake air, the NA engine can make the charging efficiency of 65∼95%. In the turbo charger, charging amount is 1.2∼1.5 times higher than NA engine if the displacement volumes are same. Therefore, it is possible to make smaller engine than NA engine if the power and the torque are same. However, it has a defect, so called the turbo lag. The turbo lag is the time delay from starting the acceleration to the actual rpm increasing of the engine. Especially, it is occurred when the car is started, accelerated
suddenly, or speed up from the low speed.
The process is like that; at first, the throttle valve is opened, then the air amount is increased, then the combusted gas is increased and the exhaust gas temperature is increased. After that, the rpm of the turbine is increased by the increasing exhaust gas, so that the supplied air amount by the compressor is increased. By this process, the intake air amount is increased more. According to this process, the engine acceleration is delayed from the starting of acceleration to the actual rpm increasing of the engine. To minimize the turbo lag, there are many researches and developments. For example, as the simplest method, there is a method for increasing the exhaust speed pushed into the turbine wheel. By reducing the diameter of nozzle of exhaust pipe, as the exhaust speed can be increased with the same displacement volume, the turbo lag can be minimized. However, in this case, the maximum power is limited. To attach small two turbo instead of large one, it is possible to reduce the turbo lag. For example, in the 6-cylinder engine, one turbo is attached at each 3-cylinder. Doing so, it is possible to prevent the exhaust interference as well as to increase the power. This method is called as the twin turbo type. On the other hand, the two way twin turbo type also accepts the two turbo, but just one turbo will be driven at the low speed to maintain better response and two turbo will be driven at the 57
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Engine Principles high speed for enhancing torque.
Hybrid turbo For another example, there is a hybrid turbo type in which the supercharger is used for the low speed, and the turbocharger is used for high speed. In the electronic control engine, VGT (Variable Geometry Turbocharger) is used to reduce the turbolag and to increase the torque of the engine. The VGT uses a set of adjustable vanes, or nozzles, to direct flow into the turbocharger turbine. When the vanes are closed, flow is directed tangentially into the turbine wheel. This imparts maximum energy into the turbine, causing the turbocharger to spin faster. Conversely, when the vanes are opened, they direct flow into the turbine in a more radial direction.
VGT (Variable Geometry Turbocharger) This reduces the angular momentum of the flow going into the wheel, producing less turbine work and ultimately slowing down the turbocharger. 5. Supercharging System and Heat The turbine shaft of turbocharger is supplied lots of engine oil to cool and to lubricate. Therefore, engine oil in the turbo engine is deteriorated easier than NA engine. As combusting more mixture, the charging system can not avoid increasing temperature of combustion chamber. As the turbocharger performs the boosting using the exhaust gas energy, the boosting efficiency will be better at the higher temperature of exhaust. For example, by adopting the heat resistance material to the parts such as the exhaust valve with natrium and the stainless exhaust manifold, and making the supercharger of high heat resistance material, the performance shall be enhanced. The inlet air should have as lower temperature as possible. Because the air density is lowered as the temperature is higher, the actual 58
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Engine Principles compression ratio will be decreased as the temperature is increased with the same volume and the oxygen amount will also be decreased. If the inlet air is hot, the compress mixture at the compressing stroke has higher temperature easy to make a knocking. When the air is compressed the temperature is increased. This is the same in the charging process. So, the boosting effect is reduced as the amount of increased temperature. Therefore, the heated air should be cooled using a radiator before it reaches at the throttle valve. This cooling device is the intercooler.
running of the vehicle. The intercooler is attached at the front or side of the radiator. The structure of intercooler is similar with the radiator, but the flow is not the water but the compressed air by the charger. The water type intercooler cools the compressed hot air using an additional cooling water separated from the engine cooling water. The water has higher heat capacity than air, so the water type is more effective than air type, but it has high cost for parts and maintenance. 6. Supercharger
There are two types in the intercooler, air cooling and water cooling system.
Air cooling type
The supercharger drives the blower and compressor to boost using the engine power. It can make higher torque and not make any response delay such as the turbo lag. However, the driving force for supercharger is from the rotation of the crankshaft, so the engine power shall be consumed. The maximum power output is lower than turbocharger. There are some kinds in the supercharger, typically, the Roots Blower and the Lysholm Compressor.
Water cooling type The air type intercooler cools the inlet air temperature using the wind get from the
The Roots Blower has been used in vehicle engine. The boost control is performed by computer. It is operated when the high output is needed such as for accelerating and driving in high speed. 59
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Engine Principles The Lysholm compressor had been used in industrial field not in vehicle engine. The structure is that two rotors including 3 and 5 screw blades respectively are combined in the elliptical shaped housing. It is driven by a V belt linked with the engine. The rotor is made of aluminum alloy coated with resin of teflon. When it is rotating, airs supplied from one side is accumulated and transmitted to other side so the air is compressed. Consequently, the intake air is compressed up to 2 times. In the supercharger, the roots blower is not a compressor but a fan as know from the name. In the NA engine, the air is charged by the negative pressure generated when the piston goes down, but the supercharger is additionally equipped the blower for enhancing the charging efficiency by positively sending the air. The Lysholm compressor is a compressor so that it can send the compressed air like the turbocharger.
Roots blower The structure of the roots blower is, to send the air from one side to other side by rotating two elliptical shaped aluminum rotors coated with special resin, in the oval housing. If the boost pressure is over charged, the valve is opened to return some amount of the charged air.
Lysholm compressor 60
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Chapter 7. Lubrication System
explain the other roles of the oil. The first is the sealing role in which the engine oil between the piston ring and cylinder prevents the leaking the compressed air or combustion air.
1. The Role of Engine Oil
The main role of the engine oil is to move the dynamic system such as the piston in the cylinder and the crankshaft smoothly. To do so, the oil forms a oil film at the metal surface to reduce the friction between the metal surfaces. At the same time, the engine oil can secure the combusted gas not to leak to the crankcase, cools the piston and valve, and reduce the shock transmitted from the piston to the crankshaft as well as clean the engine inside.
Oil jet The cylinder head and the piston crown directly contacts with the hot combusted gas. The cylinder head is cooled not only by the cooling water but also by the oil at the dynamic system. The heat of piston crown is transmitted to the cylinder wall via the piston ring. Also the piston cools down by spraying the engine oil. Another important role of the oil is to reduce the shock. The combustion force is very strong up to several tons weight force. This force is transmitted from the piston to piston pin, to Connecting rod, to crank pin and to crankshaft. In that process, the oil absorbs shock at the piston pin and the crank pin.
As the lubrication is already mentioned in the section of 『Journal Bearing』, here, we will 61
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Engine Principles pump to eliminate any tiny foreign materials. There is a oil gallery in the engine block, it is a oil passage for each dynamic parts such as crankshaft, cylinder head, connecting rod and cylinder wall. Except that, the engine oil can wipe out the foreign materials such as carbon particle generated by burned oil and metal particles from the wear of metal surfaces.
Wet-sump type
The oil also prevents inside of the engine from being rusted by chemicals from the combustion. 2. Lubrication Method In old style engine, the lubrication method was so called the Splash type in which the big end of connecting rod hit and sprayed the oil contained in the oil pan under the connecting rod. Nowadays, the lubrication oil is sent to necessary portion by the oil pump and collects to the oil pan. According to the oil circulation method, there are the dry-sump type and the wet-sump type. The lubrication device consists of the oil pan containing oil, the oil filter purifying the oil, the oil pump sending the oil to each portion. The wet-sump is equipped in most car. The oil contained in the oil pan is filtered big foreign materials using the oil strainer consisting of steel meshes and sent to the oil filter by oil
Dry-sump type The oil is returned to the oil pan from the piston, connecting rod, crankshaft and cylinder head. When the car is turned rapidly or accelerated or decelerated abruptly, the oil in the oil pan is leaned one side so it can not be pumped well. Some engines have a separator, a kind of partition in the oil pan to prevent the oil from leaning. For the special engine for racing car, the Scavenging pump takes the oil 62
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Engine Principles and air together, and the oil and air are separated by a separator and then oil is contained to the separated oil tank. This is the dry-sump type. As the dry-sump method does not contain the oil in the pan, the oil pan can be thinner. So, the lower part of engine is smaller and the engine is designed to have lower weight center. However, the device should be complexed. It is applied only to the special case for equipping the opposed engine. Some engine equips a semi-dry-sump type having the oil pump for collecting the oil in the oil pan. 3. Parts of Lubricant System
The 3 main parts consisting of the lubricant system are oil pump, oil filter and the oil cooler cooling the heated oil. There are many kinds in the oil pump taking up the oil in the oil pan. The passenger car uses a gear pump combined with toothed wheel and linked to the crankshaft directly. The gear pump comprises of a driven gear in the pump body and the drive gear linked with the tooth of the driven gear. When the drive gear is rotating, the driven gear is also rotating. However, the centers of each gear are departing each other so the oil between them is pumped from the inlet port to outlet port. According to the tooth shape, there is an
Involute type, the Trochoidal type, and so on.
Gear type oil pump The oil sending amount by the oil pump is proportional to the engine rpm. At the high rotation of the engine, the oil pressure is too high. At the low temperature, the degree of viscosity of oil is increased so the oil pressure is too high. The device for maintaining the oil pressure should be equipped. It is called the pressure regulator or the relief valve.
The oil filter purifying the oil from the carbon or metal particles. A folded filter paper is inserted into the cleaner case. There are two types for the replacement the element type and the cartridge type. The cartridge type is replaced the filter with case, the element type is replaced only filter element when the filter is replaced. The proper working temperature of the engine oil is about 80℃. If the temperature is too low, the friction becomes high because of high viscosity. If the temperature is too high, the oil pressure is lowered so lubricating ability will be degraded and also the oil in the high performance engine can be easily deteriorated. 63 Chonan Technical Service Training Center
Engine Principles So the oil cooler is required for the high performance engine.
1ℓ, 4ℓ or 20ℓ. At the case, there are name of manufacturer, brand name and oil name with the viscosity class and the quality class.
The kinds of oil cooler are divided into the water type and the air type. The water type oil cooler maintains the oil temperature using the engine cooling water, and the air type oil cooler uses the running winds. The air type has simpler device, but the cooling efficiency is lower than the water type. The water type is more complicated, but it ensures the more stable cooling efficiency. 4. Engine Oil As we have mentioned, the engine oil works for reducing the wear, cooling the piston and the cylinder head, sealing the gap between the piston and cylinder, releasing the shock, cleaning the engine inside, preventing the knocking and so on. The required characteristics of the engine oil are as follows, - Proper viscosity at working condition - Good lubricant performance - High heat and corrosion resistances - Anti-bubble The most important characteristic is the viscosity. Therefore, the engine oil is classified by two aspects, the viscosity or the quality. The commercial engine oil is sold in case of
In the viscosity classification, according to the standard by SAE(Society Automotive Engineers), the lower viscosity has lower number and higher viscosity has higher number. For cold weather, letter “W” is added. For example, certain classification like that the number 30 is for general purpose, and the number 20 is for winter, is the single grade using one number system only. Another classification like that a range is represented such as 5W-30 or 10W-30, is the multi grade. In this case, by comparing the 5W-30 with the 10W-30, the 5W-30 has lower viscosity than 10W-30 at low temperature, but has higher viscosity at the high temperature. Generally, when the temperature is increased, the viscosity of oil will be decreased. To indicate how the viscosity is changed, the viscosity index is used. If the viscosity is not easily changed, then the viscosity index of the oil is high. The higher viscosity index is easier to use. In the quality classification, the standard by the API(American Petroleum Institute) is used. 64
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Engine Principles For gasoline engine, the letter “S” followed from other alphabet letters is assigned. For the diesel engine, the letter “C” followed from other alphabet letters is assigned. For example, letters from SD to SG are assigned for the gasoline engine. The kind of engine oil and replacing period are decided by type of engine, driving condition and ambient temperature, so please refer to the manual carefully to select the engine oil. The replacing running time is about 10,000km for SD, and 15,000km for SE, and SF 15,000km for gasoline engine roughly. For turbo engine, the engine oil should be replaced at every 5,000km running time because the driving condition is very tough. The maintenance intervals for each engines are varies, refer the manuals for the each engine.
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Chapter 8. Cooling System 1. Cooling system Of the total heat energy generated by the combustion of fuel mixture in gasoline engine, about 30% is converted into the kinetic energy to push the piston, another about 30% is wasted with exhaust gas, another about 30% is transmitted to coolant through the piston and the other about 10% is lost by friction. Among them, if the heat transmitted to the combustion chamber wall is not eliminated as quickly as possible, the piston or the cylinder will be deformed by this heat or the film of lubricant oil will be broken. If this heat is cooled excessively, the much heat energy will be transmitted to the coolant so that the heat efficiency will be degraded. Therefore, cooling system should be controlled to maintain the proper temperature according to the driving situation.
The cooling system for vehicle engine is classified into the water-cooling system and the air-cooling system. The air-cooling system is hard to cool uniformly and easy to make a loud noise, so nowadays almost this system is not used in the vehicle. According to the flow type of water cooling system, there are the U-turn flow type flowing from one side of the engine to the same side, and the cross flow type flowing from one side to the opposite side. Additionally, according to the flow direction, it can be classified into the longitudinal flowing type flowing along to the longitudinal direction of the engine, and the lateral flowing type along to the lateral direction of the engine. In the water-cooling system, the cooling water is circulated by water pump from lower portion of the water jacket to the radiator. During running, strong winds can cool the radiator, however, when the car is stopped or slowly driven, a fan should send winds in force. The cooled water shall be returned to the water jacket by the pump. By installing a thermostat between the water jacket and the radiator for sensing the cooling water temperature, if the cooling water has too low temperature, the thermostat blocks the water passage. The most used thermostat is the wax type in which 66
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Engine Principles a wax is secured between capsules, and the wax opens and closes the valve using the expansion and shrinking by heat. The thermostat working temperature is about 85℃.
2. Radiator
Down flow type
The radiator is a body radiating heat, in this case, it is a device for radiating the heat into the atmosphere. As the heater is called also as radiator, the cooling device in a car is called as radiator; especially this is called as heat core.
Cross flow type
To enhance the efficiency of radiator, the area should be large as possible. Generally, the radiator comprises a radiator core manufactured by welding many metal fins around a tube flowing cooling water therein and two tanks containing the cooling water at both sides of the core. The tanks are attached at the upper portion and lower portion of the heat core to flow hot water to upper tank and cold water to lower tank using the convection principle.
U-turn flow type
It is the most used type so called the down flow type. There is the cross flow type in which the tanks are attached at the left and right side to flow in horizontal. In the cross flow type, the height of radiator can be lowered and the 67
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Engine Principles design of front grill can be freely changed. However, it has larger flow resistance. For another example, there is an U-turn flow type in which the radiator core is divided in up and down at the center portion, and the cooling water flows into the one side of the upper core and returned from the lower core side. The aluminum material is more used in both the tube flowing water and the fin colliding with wind. To make be lighter, the resin tank including nylon with glass fiber instead of brass or aluminum is more used.
higher so the difference with the outer temperature is larger. Therefore, the cooling effect shall be increased. At the pressurized radiator cap, a pressure valve and a vacuum valve are attached, When the cooling water is about 110∼120℃ and the inside pressure is high, then the pressure valve will be open to take out the cooling water; when the temperature is low and the pressure is lowered, then the vacuum valve will be open to suck the cooling water to the radiator. So the pressure of cooling water is maintained uniformly. 3. Cooling of the Cylinder Head
High temperature
Low temperature The radiator is equipped with a radiator cap to supplement the cooling water. Conventionally, this cap is just a cover so the cooling water can contact outer atmosphere directly. Nowadays, the cap secures the inside of the radiator that is the pressurized radiator cap. In the atmospheric pressure, the water is boiling at 100℃ and not increasing the boiling point. By pressuring the cooling water, the pressure is increased and the boiling point of water will be
The most heated part of the engine is the combustion chamber. The cylinder and the cylinder head are cooled by the cooling water, the piston is cooled by the engine oil. This heat affects to the temperature of fuel mixture, the combustion condition, and the temperature of exhaust gas, that is, to the engine performances. How to cool the cylinder head is the most important thing. The most cylinder head parts are made of aluminum alloys because it has high heat conductivity and light weight. As the intake port having the similar temperature of outer atmosphere and the exhaust port taking out the 68
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Engine Principles so hot exhaust gas are nearly installed, they may deformed by the difference of the heat expansion. Therefore, it is very important to cool with balancing the each part temperature. The cooling water is took into the cylinder head from the water jacket at the cylinder block, and flown around the exhaust port heated by the exhaust gas, and then took out via the intake port. Doing so, the temperature difference among the cylinder head, the exhaust port and the intake port can be minimized. It is possible to prevent the engine parts from being deformed or distorted by the difference of the heat expansion. Additionally, the each temperature around each cylinder head in the series of cylinder should not have big difference. So the temperature of each cylinder should be similar as possible. For example, if the cylinders are sequentially cooled from the front to the end, then the later cylinder can not be effectively cooled because the cooling water is already heated. In some cases, the cooling water is distributed to each cylinder at the beginning of the cooling to be cooled equivalently. The oil can be used for cooling the cylinder head. However, the portions of the cooling effect by the oil is about 20%, the most cooling is performed by the cooling water.
engine will be stuck. If the engine is normal and the thermometer is varying, then it means that overheat will be occurred. The main reasons are four; one is that the wind amount passing the radiator is too small; another is that wind temperature is too high; the third is that amount of cooling water is too small and the last is that the engine is working in hard situation continuously. When a car has aero parts or large fog lamp so the wind passing is hindered, or when the car runs unpaved road so the radiator covers with dirty thereon, then the cooling water can be easily overheated. In other case, if the fan belt is not tightened or broken then the cooling water can be easily overheated because the wind amount sent to the radiator is reduced. To tune up the turbo engine, when a large intercooler is installed in front of the radiator, then, then wind amount will be reduced and the cooling water may be overheated. If the water pipe is old so the cooling water is leaked, or the belt for driving the water pump is loosened, then the cooling water may be easily overheated because the cooling water is deficiency.
4. Overheat The water temperature of the engine is decided by the balance between the generated heat capacity from engine and the radiated heat capacity from radiator. If the cooling is not enough, then the cooling water can be boiled. Consequently, water vapor can be ejected from the radiator cap. This is called overheat. If the car is going with overheat state, then the performance will be degraded, at last the 69
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Engine Principles
Chapter 9. Fuel System 1. Carburetor In the gasoline engine, the ECM is used for mixing the most 4-cylinder engine uses only some small engines use
carburetor or the air and fuel. The the ECM system, the carburetor.
Carburator fuel system
Electronic fuel system The carburetor uses spraying principle for mixing the gasoline and the air. That is, a fuel spraying hole is made at the venturi and the air flow at the venturi is high. So the pressure at the fuel spraying hole is decreased. So the gasoline will be following the air flow in the fog form and it is mixed with air.
This phenomena is called as the Venturi effect named by the inventor. To enhance this effect, the ventury is made in narrow as possible. This inside diameter is called the main bore size indicating the size of the carburetor. By define the diameter of the venturi about the main bore size, the gasoline amount can be controlled according to the air flow, so the air-fuel ratio can be maintained uniformly. The pipe for supplying the gasoline is the main jet (fuel spraying hole). By selecting proper jet according to the driving state, it is possible to cover the wide range of rpm. However, when the engine is rotating with low speed such as idling state, the pressure in venturi is lowered so the proper amount of gasoline is not injected. When the engine is rapidly accelerated, the amount gasoline should be larger than normal state. However, it is not so easy to control these variations. To overcome these hindrances, various carburetors have been developed. Especially, in the motor sports, as the displacement volume is determined, the carburetor is most important device for enhancing the engine performance. The operation of carburetor is like that; at first, the gasoline from the fuel tank is contained into the floating chamber; when the amount of gasoline in the float chamber is reduced then the float goes down and the more gasoline is supplied from the fuel tank. When a driver presses the accelerator pedal, the throttle valve is opened and air flow passing the venturi part so that the gasoline will be taking out from the main jet. The carburetor has this simple structure, so the cost is low and it has fewer defects. However, for the modern engines which need to enhance the emission, the fuel efficiency and the power output, the carburetor has many problems. 70
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2. Mechanical Fuel Injecting Device Basically, as regarding the negative pressure as the air amount injected, the carburetor supplies proper amount of gasoline using only mechanical device. Therefore, it can not maintain the air-fuel ratio uniformly. It is need to control the injected fuel amount exactly using the predetermined the air-fuel ratio to prevent from wasting the fuel or to enhance the engine response. A new system is developed in which the inhaled air amount is measured directly, and the gasoline is sprayed to the intake manifold with the optimized the air-fuel ratio using the mechanical device. This is the K-jetronic invented by Bosch. By comparing with the carburetor, the fuel injecting device is injecting the fuel to the manifold directly. Therefore, it has good response at the start and at accelerating and decelerating. It is hard for this device to make the vapor lock in which flow of fuel is broken by vaporizing the fuel in the middle of fuel pipe. Additionally, it does not make icing that the carburetor is frozen. The main feature of the K-jetronic is that installing a circular plate named sensor plate in front of the throttle valve, the gasoline amount is controlled using that the opening state of this plate is changed by the air amount. When the throttle valve is open, the air presses the sensor plate installing in the air flow meter. The lever supporting this plate is connected to the device for controlling the injected fuel amount, and the gasoline is injected to the fuel injecting device by a response to the plate operating.
of carburetor and controls the fuel amount mechanically. Therefore, it is hard to control the mixture ratio precisely. In some cases, by accepting an ECM to a portion of the system, the KE-jetronic supplementing these week points is developed. However, step by step, the full ECM system is replacing these mechanical systems in order to regulate the exhaust gas and to enhance the fuel efficiency. 3. Electrical Fuel Injection System The main part of the electrical fuel injecting system is the fuel injecting device deciding the amount of fuel to make proper mixing ratio by measuring the inlet air amount. This system comprises of the device of measuring the air amount, the device of injecting the fuel, and the device of controlling theses operations. The representative mechanical fuel injecting device, the K-jetronic, uses the sensor plate for device measuring air amount, and transmits the movement of sensor plate to the valve controlling fuel supplying. Contrarily, the electrical fuel injecting system the measured air amount by the air flow sensor is sent to computer as an electric signal to decide the fuel amount with the result from the sensor for checking the engine status. By these devices, it is possible to control the air-fuel ratio precisely.
As a system replacing the carburetor, the Kjetronic has better reliance. However, it sometimes injects the fuel in the same manner 71
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The structure of the fuel injector is differed according to the manufacturer and the applied engine type. Except the air flow sensor, most parts are similar. Here, we will explain about the mass air flow type using the air flow meter as the air flow sensor. The filtered air through the air cleaner is measured its amount at the air flow meter, and taken into the surge tank (intake collector) via the throttle body including throttle valve connecting to the accelerator pedal. This air is distributed to the intake manifold of each cylinder, and inhaled into the cylinder with the gasoline injected from the fuel injecting valve (injector) to the manifold or intake port. At this time, a controller decides the gasoline amount optimized to the driving status and running condition of the vehicle. This is called as the ECM, Electronic Control Module. The driving status is the electrical signal from the water temperature sensor, the intake air temperature sensor, and the throttle position sensor, and the running condition is the signal from the speed sensor and the signal indicating the air conditioning working status. These signals are sent to the ECM.
memorizing the data concerning to some combinations of these signals and the control method which indicate what amount of gasoline is injected according to the combination to the computer installed into the ECM, the computer can inject proper amount fuel decided by the computer according to the accelerator operating, from the injector. 4. Fuel Supplying System
The gasoline, the fuel, is contained in the fuel tank and sent to the fuel injection device after eliminating dust and water by filter. 72
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Engine Principles The fuel tank is made of galvanized steel to prevent rust, as well as the plastic tank is more used. To protect the fuel from leaning one side, there are some partitions, called separator, and level gauge is attached. The fuel pump has various types. The carburetor uses mechanical pump, ECM system uses electrical pump using electric motor mostly.
A regulator, pressure controller, is attached for sending the gasoline to the injector after controlling the gasoline pressure within the specific range.
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For injecting the gasoline, an injector is used. The injector has a needle valve closing the front of the nozzle so the valve is open by flowing an electric current to the solenoid to injecting gasoline. In the injecting method, there are a Single Point Injection (SPI) injecting gasoline to the collecting portions of manifold, and a Multi Point Injection (MPI) injecting gasoline to each manifold corresponding with each cylinder. The SPI has the injector at the same place as the carburetor does, but this makes the mixture more effectively than carburetor does. The MPI can be classified to point injection, group injection and bank injection according to the injecting timing. Each manifold has injector.
The point injection performs injecting operation at the intake stroke of each cylinder according to the engine rotation. The group injection performs the injecting operation with a group of cylinders which have the sequential intake stroke. Of course, the point injection can inject the gasoline with most optimized timing and amount. However, the electrical circuit for driving the injector is more complicated. So, some commercial car accepts the group injection. Simplifying the group injection is the bank injection. At the intake stroke and the combustion stroke in which the piston goes down, the needed gasoline is injected after dividing into two turns and taken into the cylinder after gathering at the intake stroke. Due to the simple structure, it ensures the 74
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Engine Principles most efficiency in the gasoline injection. So it is the most used method in the gasoline engine.
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Engine Principles
Chapter 10. Ignition System 1. Point type Ignition The ignition system is to fire the mixture compressed in the combustion chamber making a flame with a spark plug. When the plus and minus poles of the 12V battery is shorted, a spark can be made. However, it is too weak to ignite the mixture. Therefore, this voltage should be boosted up to 10,000∼30,000V. The ignition system consists of a device for boosting the voltage, a device for distributing the ignition timing and a series of spark plug. To boosting the voltage, an ignition coil, an electric inducer, is used. The ignition coil comprises of a iron core shaped of rod, the secondary coil wound with about 20,000~ 30,000 turns using a hairy thin copper wire around the core and the primary coil wound with 150~300 turns using copper wire of 0.5∼1mm diameter over the secondary coil. When a current is flown at the primary coil, the iron core will be an electromagnet. And at time when the current at the primary coil is broken, a boosted high voltage is induced at the secondary coil. The distributor has a device for controlling the current flowing at the primary coil and a cam having the same number of extruded portions in the middle of it. When this cam presses the arm by rotating, the point attached in front of the arm is opened to cut the current of the primary coil (contact breaker). And the distributor has also a device, overlapping with the contact breaker, for distributing the boosted voltage of the secondary coil to each plugs.
At the cam of the contact breaker, an advancing device for controlling the ignition timing at the variations of engine speed to send the boosted high voltage to the spark plug in time. As the cam should rotate with 1/2 of the engine rpm to match the ignition timing of the plug, it should be attached at the end of camshaft having the same rotation speed. The advancing device shall be explained at the section of 『Ignition Timing』 in detail. After that, the high voltage generated by the ignition coil is sent to each spark plug through a series of High-Tension Cord. This ignition system is called contacting or point type. If the driving for the contact breaker is performed by a transistor, then it is called the full transistor type. Furthermore, developing the system, the distributor is driven by a computer in the distributor-less ignition device. 2. Full Transistor type Ignition In the point type ignition device, the point is opened by the cam, and high voltage is induced at the secondary coil when the current at the primary coil is cut. Similarly, when the switch is turn off, a small unwanted spark will be made at the contact point. As the same of mechanical inertia, the electrical current has the inertia. This unwanted spark will be prevented by inserting a condenser within the ignition circuit. Furthermore, the point would be burned or not work normally at the high speed. Therefore, for intermitting the current of the primary coil, a transistor is developed instead of the mechanical point. As being many kinds in the transistor, the application method is various also. When it is used for a switch, NPN type transistor is used. This consists of a base made of P type 76
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Engine Principles semiconductor and two N type semiconductors at the both side, one is a collector, and the other is emitter. In normal state, the current is not transmitted from the emitter to the collector of NPN type transistor. However, if a little current (base current) is applied between the emitter and the base, then an amplified current is induced from the emitter to the collector. Using this characteristic, it can be utilized as a switch. Instead of the cam and point in the distributor, the signal generator is equipped for detecting the ignition timing, the electrical signal is sent to the igniter including a transistor. Then the base current made by the signal generator is applied to between the emitter and the base, and then an amplified current is generated from the emitter to the collector. Using this current amplifying, the intermittent current is applied to the primary coil, so a boosted voltage can be generated at the secondary coil. The signal generator consists of a rotor having the same number of extruded portions with that of the cylinder, a permanent magnet, and a pick-up coil detecting the change of magnetic flux. As the signal rotor rotates with the 1/2 speed of the engine speed, the extruded portions passes through the gap between the pick-up coil and the permanent magnet corresponding with the ignition timing. The magnetic flux generated by the permanent magnet will be changed, so an induced current will be flown at the coil. This current is used for the base current of the transistor. Doing so, the trouble from the contact breaker can be resolved.
In the full transistor ignition system, the primary coil current switching is performed by the signal generator and the transistor, while the advancing and distributing of secondary current is performed by the same device used in point ignition system. The distributor-less ignition system is that the advancing of ignition timing is performed by a computer using an electrical signal from the sensor for ignition timing, and the ignition is performed using generated secondary current from the ignition coil installed near the ignition coil. The main feature of this ignition device is the high-tension cable connecting the ignition coil – the distributor – the spark plug. As this cable will be short only for connecting the spark plug and the coil just nearby or this cord is not needed in some cases, the troubles from the electromagnetic wave interference from the high current of the high-tension cable or the ignition faults from the electrical resistance of the cable can be prevented. Additionally, the electrical advance device is more compact than the mechanical one.
3. Distributor-less Ignition
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Inductive type crankshaft position sensor
Individual ignition system
Optical type crankshaft position sensor In the sensor for deciding the ignition timing, the crankshaft position sensor or the camshaft position sensor, one type comprises of a timing rotor driven by the camshaft and a pick-up detecting the position of the rotor electrically. Anther type comprises of a LED attached at the rotor blade driven by the camshaft and a photo diode to decide the ignition timing. In the ignition method, there are the individual ignition method and the bank ignition method.
Bank ignition system In the Individual ignition method, the ignition coil is installed at each cylinder to ignite sequentially according to the order decided by the ECM. In the bank ignition method, the ignition spark is occurred at two cylinders at the same time by one ignition coil. In this time, one cylinder is in the compression stroke and the other cylinder is in the exhaust stroke. The ignition spark at the compression stroke is effectively used, but the spark at the exhaust stroke is meaningless. In this method, the used amount of the transistors and coils is half of individual ignition method, so the cost will be down.
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Engine Principles 4. Spark Plug
The spark plug ignites at the compressed mixture to fire it according to the spark arc by the high voltage generated from the ignition coil. The spark plug contacts, at first, with the mixture having the similar temperature of outer atmosphere in the intake stroke, After that, it contacts with the exhaust gas over 2000℃ and generates high voltage of 20,000V. Therefore, it is the device working in most severe condition. The spark plug is used for many kinds of engine in common, and manufactured with a standardizing specification internationally. There are many kinds according to the dimension, structure, performance and characteristic (especially, heat characteristic). This is distinguished by the alpha numerical indicator. This indicating character differs from the manufacturer, so be careful when replacing them. Generally, spark plug is classified as 14mm, 12mm, and 10mm according to the size of attaching screw. To make the combustion chamber compact, the smaller plug is the better. However, the smaller can be the easier to be affected by heat. It is very important to inspect the temperature situation of the engine in order to use the spark plug. The spark plug, during driving, is affected by the various temperatures. The most affecting temperature is the combusted amount of fuel
mixture per time. As the engine is high speed, the temperature of plug will be high. Even in the same driving condition, the temperature differs according to the heat range of the plug.
The heat range is the degrees of which the spark plug radiates the heat acquired from the chamber. As the plug easily radiates the heat, the heat range is high. In racing engine, the cold type having high heat range should be used to prevent the temperature from increasing easily. Contrarily, if the engine is usually used in low speed, the hot type having low heat range should be used to prevent from being cold easily. The heat range is represented with numeric character on the plug. It differs with the manufacturer. It should be selected according to the used standard plug. If the plug not corresponding with the engine features and driving conditions, for example, if it is used in the low temperature, then carbon slug will be deposited on the end portion of the plug, so it doesn’t work. If the temperature is excessively high, then the mixture is exploded before the ignition is worked, that is pre-ignition will be occurred.
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Chapter 11. Combustion Chamber
and
Combustion
1. Combustion Process
To get high output and to enhance the fuel efficiency, it is necessary to combust the mixture of fuel and air perfectly as soon as possible while the combustion process. Therefore, to enhance the engine performance is to learn the relationship with the combustion and to study how to increase the fuel efficiency. The mixture of fuel and air by the carburetor and injector is, at first, inhaled into the cylinder through the intake valve with the swirling flow. And then it is compressed with the swirl flow by the piston going up from the BDC (bottom dead center). At this time, the fog state fuel is converted into a vapor state by the heat from the chamber wall and adiabatic compression, and the strong flow of the mixture. Some components may change to the flammable gas. When a flame is applied to the gas with high temperature, then a flame kernel is made between the electrodes of the plug. This flame kernel is a combusting gas unit having high temperature made from the reaction the fuel gas and the oxygen in the air.
This combusting gas unit immediately heats the mixture there-around. The more mixture surrounding the kernel reacts with more oxygen by this heat and then converts into larger combusting gas unit. Within a short time, this sequence is widely spread so the whole mixture is converted into the combusting gas. This is the combusting process of the mixture. As the time for being the spark is only about 2/1000 seconds (2 mille seconds: ms), if the temperature around the flame kernel is low or the kernel is blown out by the swirl of the mixture, then the mixture can not be combusted. This phenomenon is called the misfire. In the process of the combustion, the boundary between the combusting gas and the combusted gas is called the flame surface. The expansion velocity of the flame surface is the flame velocity. The flame velocity is the same mixing the combustion velocity which is the speed of flame developing with statistic fuel gas, the expansion velocity which is the speed of gas expansion by the combusting heat, and the velocity of the gas flow. The combustion velocity is changed by the component of the fuel and air-fuel ratio which is the weight ratio between the fuel and air. However, it is very slow, i.e. several cm per second, As adding the gas expansion velocity 80
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Engine Principles and flow velocity to the combustion velocity, the flame velocity is about 15∼20m per second, even it can be 30m per second. Therefore, the flow of mixture is very important. 2. Air-fuel ratio and Flame Velocity
The gasoline is a liquid consisting of 4∼12 carbon atom in chain link and various molecules including hydrogen atom. If the component ratio is changed or a material is added to accelerate the combustion, then the combustion velocity and the gas expansion velocity shall be faster. The mixing ratio is a number representing the ratio of fuel amount and the air amount. It can affect to the combustion velocity. So it can be represented by the three indicating number such as the air-fuel ratio (or A/F ratio), the excess air ratio, and the equivalency ratio.
To enhance the engine performance, the flame velocity should be fast and the amount heat energy which will be converted into the kinetic energy should be as large as possible. The flame velocity is decided by the three main elements including the combustion velocity, the gas expansion velocity, and the mixture flow velocity. To combust the mixture fast, these elements should be maintained in the best condition. Considering the combustion velocity and gas expansion velocity, the flame velocity is decided by the mixture ratio which is the ratio between the fuel and air and the temperature and the pressure of the mixture. The temperature and the pressure are decided by the temperature of chamber and compression ratio. To consider the temperature and pressure is very complicated, so here, we assume these conditions are constant. We focus on the fuel component and the mixing ratio.
The air-fuel ratio is the value calculated by which weight of air included into the mixture is divided by the weight of fuel included into the mixture. It is called the AIR/FUEL RATIO, or A/F ratio. When the air and fuel are mixed, the A/F ratio for complete combustion theoretically is called the theoretic A/F ratio. The theoretic A/F ratio of the regular gasoline is about 14.7. If the actual A/F ratio is less than the theoretic A/F ratio, then the amount of the gasoline is more than the theoretic A/F ratio so it is indicated as 『RICH』, otherwise, as 『LEAN』. For the mixture is combusted in the best condition and for the flame velocity is fastest, the A/F ratio is little smaller than the theoretic A/F ratio, that is 13.5∼14. This means that when the fuel is little more than air, the combustion is better. The combustion velocity has the maximum value at the A/F ratio of 12∼13, with more gasoline amount. Therefore, the engine power output will be 81
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Engine Principles maximum at the A/F ratio of 12∼13. Otherwise, the output will be reduced. In the aspect of fuel consumption ratio, the consumption ratio will be minimum value about the A/F ratio of 16, that is, little lean state has the best fuel efficiency. After combusted, if any oxygen is not remained, then the gasoline is not completely combusted. 3. Ignition Timing The ignition timing is when the compressed mixture is fired, that is the timing for making a electrical flame at the spark plug. Generally, it can be thought when the mixture is fully compressed and the piston reaches at the TDC (top dead center) is the best timing for the ignition. However, it is too late. The reason is that the combustion velocity of the mixture is changed by the gas flow velocity. As the engine speed is increased, the gas flow will be faster and faster. Therefore, the flame velocity will be faster. So, to ignite when the piston is at the highest point is too late. The best timing is when the piston is almost at the highest point that is, when the area of flame surface is almost half of the combustion chamber. The ignition timing is represented by the rotation angle of the crankshaft about the TDC of the piston. In terms of the angle, if the ignition timing is set to 40∼30° before the TDC, then the combustion chamber has the maximum pressure at the 15∼20° after the TDC. If the ignition timing is too early performed, then the combustion is occurred before the piston reaches at the highest point. In this case, the combustion force will press the up rising piston, so the force will be reduced. If the ignition timing is too late, then combustion force will press the downing piston. So the
combustion force will not be work effectively. As the flame velocity is as fast as the engine speed, the ignition timing should be corresponded with the engine speed in order to maximize the pressure of combustion chamber at the TDC of the piston. This operation is to advance the angle of the ignition in considering of the crankshaft rotating angle, so it is called the advance angle. In the system for performing the advance angle, there are the mechanical type and the electrical type. The mechanical advance angel device is assembled between the distributors applying currents to the spark plug. By detecting the engine speed mechanically, the timing for applying current is controlled according to the engine speed to advance the ignition timing of the spark plug. For example, in the vacuum advance angle device, the advance angle is performed by the operation proportional to the negative pressure of the device connected to the carburetor with pipe using the phenomenon in which the negative pressure in the intake port is increased according to the engine speed. The electrical advance angle device is that the engine speed and the intake air pressure are detected by the sensor, and the best ignition timing is decided by the computer. 4. Swirl Effect As the flame velocity is fast, more heat energy can be converted into the kinetic energy. Ideally, the mixture should be exploded when the piston just passes the highest point to transmit the expansion force of the combusted gas to the piston most effectively. For full combustion, in terms of crankshaft rotation angle, the time of 40∼60° rotation should be needed. So, the actual situation differs from the ideal situation. 82
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Engine Principles To ensure the fast combusting, the gasoline should be mixed with air well to be enable to perform the chemical react between hydrogen carbon and oxygen. To do so, the gasoline particle from the injector should be tiny and easy to be vaporized as possible. And the injector orifice should face to the intake valve in order not to adhere the gasoline particles to the intake port wall. For some racing engines, two injectors may be attached at each cylinder. Additionally, in order to be make the flame velocity be fast, the flow velocity of gas should be faster. When the engine is rotating in slow speed, the flow velocity of the mixture is very important element. When the engine is rotating in high speed, the flow of mixture is high, so the mixing is well and the flame velocity is enough fast. However, when the engine speed starts to be decelerated, the piston downing speed is low, so the mixture flow velocity is lowered and the gasoline fog within the mixture can not be easily vaporized.
Therefore, some researches and developments for direction of intake port, for reducing the size of intake port and for using two intake ports in which one intake port is closed to flow in whirl when the engine works in low speed, to mix the fuel with air enough. The flow of whirl is divided into the swirl of which direction is in horizontal and the tumble of which direction is vertical. The important thing in the swirl is that the swirl generated at the intake stroke should be remained even should be much stronger in the ignition-combustion stroke.
Swirl
Tumble
To do so, one method is that a little gap called squish area is made between the most far position from the plug and the end portion of the piston crown, to blow the mixture by squish area when the piston is near the highest point. 5. Knocking
Even it is rarely occurred in nowadays, the engine makes a noise when the car is 83
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Engine Principles accelerated in high load condition. This is the typical knocking. This comes from that the combustion is not started from the flame kernel of the spark plug and expansion of the flame surface, but from the early combusting of the mixture in the end zone which will be combusted at last. As the flame surface is a boundary, inside of the surface is filled with the combusted gas and outside of the surface is filled with un-burn gas. That is the combustion is spreading from the flame surface. Before this flame surface is not reached, the un-burn gas is self combusted by the pressure of the gas expansion. This gas with the high pressure and high temperature knocks the cylinder head and piston, so the engine has harmful damages. The knocking is occurred at once, then the piston and cylinder have the abnormally high temperature, so the sequential knockings can be easily following. Because that the knocking is generated at the end zone of the combusting chamber, the bore will be enlarged by the SHORT STROKE and it is easily generated in the engine having longer flame spread distance. Therefore, the modern engine is equipped with the spark plug, especially the center plug, at the center of the chamber or with a squish area enhancing the mixture flow by making the end zone be narrower.
Nowadays most car doesn’t make any knocking during driving. The engine is developed to prevent from knocking. On the other hand, there is a research for enhancing the engine performance using the knocking. The knocking, as the firstly concerned, is occurred at the low engine speed in which the combustion of mixture is lag behind of the abnormal combustion. Generally, it is occurred at the ignition timing is advanced when the compression ratio is increase or the flame velocity is high. Therefore, by detecting the knocking, if the engine is run with maximum advance of ignition timing, the best combusting condition can be made. 6. Abnormal Combustion The all combustion contrary with the normal combustion in which the combustion starts from the spark plug and spread over all chamber are called abnormal combustions. The knocking is the representative example. There are also other types of abnormal combustions. ⑴ PRE-IGNITION & POST-IGNITION As the PRE is “before” and the POST is “After”, these ignition means that the mixture can be combusted by the other flame before or after the normal ignition is occurred. The PRE-IGNITION is occurred at the 84
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Engine Principles compression stroke by any reason such as remaining at the carbon slug attached at the plug, chamber wall, piston or valves. The POST-IGNITION is that the mixtures not combusted at the normal flame period by misfire, un-burned gas is combusted at the combustion stroke. Both of them are very similar with the knocking, so they can make a great affects at the parts around the chamber. ⑵ RUN ON As being also called as dieseling, this is the phenomenon that the engine is still working even the ignition switch is off. Very similar with the PRE-IGNITION, the carbon slug works as a flame seed. This is generally occurred when the key is off with the overheated carburetor engine. This is named from that the diesel engine combusts without ignition.
⑷ BACK FIRE In the state that the almost of the combusted gas is taken out at the exhaust stroke, there are some amount of remained gas. The remained gas with high temperature make a ignition the air/fuel mixture at the beginning of intake stroke. In some cases, the fire can reach back to the air cleaner. This is mainly occurred at the carburetor system. These abnormal combustion is not often occurred in normal driving situations, however, be careful to maintain the engine. 7. Shape of Combustion Chamber
⑶ AFTER FIRE This is also called as the AFTER BURN. This is that the incompletely combusted gas is exploded at the exhaust system with a big combustion sound. When the accelerator is turn to open or close abruptly, the exceeded gasoline is exhausted into the chamber and then the incompletely combusted mixture is exploded at the catalyst converter or at the muffler. This can make damage to the exhaust system.
According to the combustion method, the engine performance shall differ. Then, which shape of the combustion chamber is the best for engine performance. It may be true that the faster flame velocity is the better in order to increase the engine output. With the same gasoline and A/F ratio, we can consider the following five items for the 85 Chonan Technical Service Training Center
Engine Principles engine power. ⑴ The amount of the inhaled mixture shall be plentiful (More fuel, more heat)
⑸ Combustion chamber should be compact size to prevent heat from losing. (to ensure the heat energy converted into kinematical energy) ⑵ The flow just before the ignition shall be proper. (The faster is the better, however, too fast makes a misfire)
First of all, concerning the inhalation amount of mixture in ⑴, this is decided by the attaching angle, number, size, lift and shape of the intake valve. It is explained in the intakeexhaust valve section in detail.
⑶ The ignition plug should be installed at the center of the combustion chamber (to ensure fast combustion of mixture)
In the mixture flow in ⑵, here, how the mixture is taken into the cylinder is the most important point. Even the mixture flow is well, if the shape of valve inside and piston crown are complicated, then the gas will not be expanded smoothly, so it should have the simple shape as possible. The plug position in ⑶ is decided by the number and position of the intake-exhaust valves. In the 4-valve engine most used nowadays, the plug shall be installed at the center of the combustion chamber, ideally.
⑷ The compression ratio should be as high as possible (With high compression, heat efficiency is good)
As the compression ratio mentioned in ⑷ is higher, the combustion will be faster because the temperature and pressure of the combustion chamber just before ignition is high. However, if the combustion is too fast, 86
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Engine Principles then the combustion is performed abnormally. So the chamber can be damaged by this abnormal combustion such as knocking. To make that the heat can not be lost easily as mentioned in ⑸, consider that; as the inside area of the combustion chamber is bigger, the heat loss when the exploded gas presses the piston will be higher, that is, the heat energy which will be converted into the force energy will be lost. With the same volume of the combustion chamber, as the inside surface area of the chamber is smaller; the heat converting ratio will be higher.
Therefore, when the ratio between the SURFACE and VOLUME of the combustion chamber is the S/V ratio, this ratio represents the combustion efficiency. The smaller S/V ratio is better for the combustion efficiency. 8. Intake-Exhaust Chamber
Valve
&
Combustion
and shape of the intake port is important as well as the attaching angle, diameter and number of the valves should be appropriate to enhance the volume efficiency. The larger diameter of valve is the better. If the valve is too large, it is heavy so it has large inertia force when it is open and close. Therefore, it will hinder the engine from rotating with high speed. The size of valve should be optimized. The 4-valve engine having two set of intake-exhaust valve is more applied recently than the 2-valve engine having one set of intake-exhaust valve. The three-valve engine having two intake valves and one exhaust valve was noticed. However, the plug was not installed at the center of chamber, and the exhaust valve was too large so the two intake valve system is worse than 4 valve system. The chamber types of 4-valve engine are the PENT ROOF type having the roof shaped cylinder head and the Poly-spherical type having the some overlapped spheres. In the both types, a pair of intake-exhaust valve is facing with each other, and the spark plug is located at the center. It satisfies the requirement condition for the excellent volume efficiency.
Big valve angle
To get better volume efficiency, more amount of intake air is needed, and the flow of intakeexhaust should be made smoother. The size
Small valve angle
The valve inclined angle is the angle of intakeexhaust valve about the center line of the cylinder. The valve angle is the angle between the center lines of each valve. These angles make an important affect to the chamber 87
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Engine Principles shape, the S/V ratio, the compression ratio, and the shape of intake-exhaust ports. If the valve angle is to be larger, then the valve diameter can be made widely, and the intakeexhaust gas will be flown more smoothly. However, the chamber is to be larger also, so it has demerits such that the compression ratio will be reduced, and the S/V ratio is to be large. New type engine has the compact combustion chamber of which valve angle is smaller than ever. The five-valve engine having the three intake valves and two exhaust valves is for high performance by enlarging the cross area of the valve and lightening the valve weight. However, it has more complicated chamber so the S/V ratio will be larger as well as the mechanism around the valve will be more complicated. 9. Piston & Combustion Chamber The piston head forms the combustion chamber by facing the intake-exhaust valves portion of cylinder head. To combust the mixture fast, the inside surface of the chamber should have fewer extruded or recessed portions to flow the mixture smoothly, and the S/V ratio should be small as possible. Therefore the piston head should be flattened.
extruded, highly. Furthermore, if the engine has high compression ratio, then the gap between the cylinder head and piston head should be narrow so it need to make the valve recess be larger to prevent the valve from abnormally operating. With these limitations in the mechanism, there are many researches for better combustion. The piston has an important role to transmit the combustion force to the connecting rod effectively, so the other portions except the piston head should be precisely designed. The combusted gas is sealed with the piston ring. To ensure the sealing, the gap between the piston and cylinder (piston clearance) should be small as possible. The piston will be cooled by the lubricant oil and the heat will be radiated through the piston ring. The thermal expansion coefficient of the aluminum, the main material of the piston, is 23 relatively higher than steel of which thermal expansion coefficient is 12~15, which is the main material of the cylinder. Therefore it is hard to match the piston size to the cylinder size. For example, as the back side of the piston head is reinforced, it is made little smaller than the skirt part and the piston diameter along to the inserting axis of piston pin is little smaller than the perpendicular axis. As the connecting rod rotates the crankshaft, the piston will press the connecting rod with inclined direction. Therefore, the piston may be trembling along the lateral direction so the skirt will strike the cylinder wall. This is called the piston slap or the sides knock. This is the cause of the noise or power loss by friction.
In actual, considering other elements such as the valve angle, the cylinder head shall have the recessed shape. Therefore, to increase the compression ratio, the piston head should be
To minimize this slap, the center of the piston pin is offset about 1∼2.5mm along to the movement direction of the connecting rod. Doing so, the force pressing the piston to the 88
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Engine Principles lateral direction will be reduced. It is called the offset piston.
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Chapter 12. Performance, Fuel consumption, Noise, Vibration
exhaust gas. The carbon monoxide and the hydrogen carbon among the three major harmful materials in the exhaust gas may not be exist if the gasoline can be completely combusted, in ideal. The one, the nitrogen oxide is also one of the important problems.
1. Required Performances
Various performances are required for the engine. Each performance is related with each other in complex manner, so these affect to the performance of the vehicle. Furthermore, as time is going, which performance is more important has been changed. However, the output power is the most important item because the purpose of the engine is developed making a power for the vehicle. Conventionally, to operate engine for more work should need more fuel to be used. Recently, by enhancing the engine efficiency, better fuel efficiency and better output can be acquired.
To develop the combustion efficiency of the engine is concerned to the purification of the
Considering that the engine weight is 10∼15% of whole weigh of the vehicle, another method for acquiring better output and fuel efficiency at the same time is to make engine be compact and light. With the same output, the power of vehicle having lighter engine will be higher. The lighter and compact engine enhances the fuel consumption. Also, to ensure the good steering, the vehicle should be light and the weigh balance is distributed 50:50 at the front and the rear or similarly.
Engine also should have the features to be driven by the driver’s willingness. For example, when the driver presses the accelerator pedal, if the engine output is too high, then it is not so good for the safety. The response, how to act 90
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Engine Principles according to the pressed amount of the accelerator pedal, can make a big affect to the drive condition.
As the engine get the driving force by the combustion of the fuel, the noise and vibration can not be avoided. It is important to prevent these noises and vibrations from transmitting to the passenger.
The engine is the device converting the heat energy to the force. The basic performance is represented by the three major factors such as the fuel consumption indicating how much gasoline is combusted, the torque indicating how much force is generated thereby, and the power indicating how much work is performed per unit time. Among them, the fuel consumption is easy to be notified because it is easily detected by measuring used fuel amount. Then, what are the others, the power and the torque?
Additionally, as the engine is one mechanical part of the vehicle, how to maintain the engine is also an important factors. 2. What is Output?
As we have explained frequently, the working principle of the gasoline engine is that the expansion force by the combustion of the gasoline is converted into the force pressing the piston to rotate the crankshaft. When ascending up to the inclined road, we press the accelerator pedal to get high power. When running constantly in the even road, we just press the accelerator a little. As connecting to the throttle valve controlling the amount of intake air, the pressed depth of the accelerator pedal directly affects to the open amount of the throttle valve. The fuel injection amount is decided by the amount of the intake air. When the air is 91
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Engine Principles inhaled with little amount by little pressing the pedal, the gasoline injection amount will be small. When the air amount is large by pressing deeply, then the fuel injection amount will be increased. That is, the ratio between the air amount and the fuel amount is already preset, so the accelerator pedal controls only the inhale air amount. So, pressing the pedal little, the mixture amount is small and the power of engine, while pressing the pedal more, the power of engine will be increased because the mixture amount is increased. Generally, the engine performance is decided by the engine force. The force is represented in unit of kg. In the vehicle, as the ultimate force is the rotating force driving the wheel, it is prefer to represent in unit of torque, i.e. kg·m, acquired by multiple the magnitude of force with the distance between the center of rotate to the force applying point. In another aspect of engine performance, how much work is possible within certain time period is also important. This work amount is the power represented by horse power. 3. Representing Method for Power Generally, the most important element of the engine performance is the output (power). When a new engine is installed, some one says “What horse power has it?” This horse power is the work efficiency, that is, the unit indicating the work amount during specific time period, so called the dynamic output (power).
This concept was suggested by the James Watts who invented the steam engine in England. To compare the dynamic performances of the some kinds of the steam engines, as the horse power used for water pumping works in the coal mine, the one horse power is 550 ft·lbf/s. Converting into the metric system, it will be 75 kg·m/s. This is the power for pulling up the 75kg weight per 1m in one second. For the unit of the Horsepower, it will be represented HP in abbreviation, or the PS Pferdestarke from Germany. The PS is more usually used. In SI unit system, representing with W (watt), 1 PS is about 735.4W. So, 100PS is 73.5kW, 100kW is 136PS. In the vehicle catalogue, we can see additional word such as (Net) or (Gross) in front of the unit of PS/rpm. As the engine output is generally measured with being set the engine to the measuring equipment, the measured output is changed according to the measuring condition, and there is dispersion in measured values. Therefore, to indicate the output, Net value and Gross value are used. The Gross value is measured value with engine only, and the Net value is measured value with being set the engine to the vehicle. For the gasoline engine, the Net value is 15% less than the Gross value. If there is no notice, the larger value is the Gross one. 92
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Engine Principles The power is the function of the time. The engine power will be increased proportional to the rpm because the work amount per time is increased when the rpm is higher. However, when the engine rpm is increasing, the dynamic parts can not run over the certain value, or the engine can not intake or exhaust faster than limit, or the engine power is excessively wasted for driving engine itself if the rpm is over than certain rpm. That is the engine power has certain limit value. This is the maximum power output. In catalogue, it is indicated with the rpm thereat. 4. What is Torque?
The torque or the twisting force which is applied to a rotational matter such as bolt, axis rod and wheel. It depends not only on the applied force but also on the length of the lever arm upon which the force acts. By definition, torque is equal to force multiplied by the leverage; the length from the center of the rotor to the point which a force is applied.
engine is decided by the force of which the piston presses the connecting rod, that is, the combustion force. The performance graph of torque is representing that which piston is pressing the crankshaft with how much force when the engine is rotating at what rpm. As this force will be transmitted to the wheel finally, the impulsive force of the vehicle is small if the engine torque is small, the impulsive force of the vehicle is high if the engine torque is high. The expansion (exploded) force is determined by many elements, especially, by the amount of the inhaled air into the cylinder. With plentiful of air, it is possible to get high power. Considering the relationship between the inhaled air amount and the rpm of engine, when the engine has low speed of rotation, the movement of piston is also slow and inhaled air amount is low. When the engine has high speed of rotation, the movement of the piston is fast and the inhaled air amount is high. However, if the engine has too high speed, then the intake valve may close before the air is not fully inhaled into the cylinder yet. In this case, the inhaled air amount per stroke (volumetric efficiency) is decreased. Therefore, the engine torque curve has the peak shape. For example, compare the engine torque between the 2500rpm pick engine and the 5000rpm pick engine. The former engine has the best performance at the 2500rpm but not so good performance at the 5000rpm.
In engine, the torque is equal to force of which the piston going down force multiplied by the distance from the center of crank pin to the center of crankshaft. So, the magnitude of the torque of certain 93
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Engine Principles structure and the high cost.
On the contrary, the 5000rpm pick engine has a good performance at high speed but it has low performance at low speed. So, the engine characteristics are different between above engines even though the maximum torques are same. 5. To enhance Power The engine power is a working amount during a specific time. The engine power can be increased as high as the cylinder volume high, the combustion force high and the rpm high. The engine size is represented by the displacement volume. The cylinder displacement volume is the exhausted gas amount from certain cylinder during the piston moves from the BDC to TDC. The total displacement volume is the sum of volume of all cylinders. The cylinder displacement volume is calculated from multiple the cross sectional area by the stroke of the cylinder. It is represented by unit of cc or ℓ.
The pressing force at the piston is calculated by dividing a work amount per cycle by the displacement volume. By this calculation, the pressing force at the piston is the pressure. But the pressure at the piston is continuously changed by the position of piston and the stroke. So, the mean pressure per cycle is used for a calculation. This is called as the mean effective pressure, the average pressure in the cylinder. To enhance the engine power, the main three elements including the total displacement volume, the mean effective pressure, and the rpm should be enhanced. The work per certain time shall be increased by enhancing the rpm. Deciding the total displacement volume of the engine, how the mean effective pressure is increased and how the limitation of the rpm is overcomed are the major focusing point for a developing of an engine to enhance the engine power.
When the total displacement volume is bigger, the engine output will be also higher. To compare the engine performance, the PS/ℓ is used. The PS/ℓ means that how much horsepower is generated per 1ℓ of displacement volume. For the passenger car, generally, the more cylinder has higher value of PS/ℓ because volumetric efficiency is higher with multiple cylinder. But the demerit of multiple cylinder is the much complicated 94
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Engine Principles
Methods for increasing of maximum power Method
Item Increasing compression ratio Increasing combustion pressure High combustion speed
Increasing mean piston effectivepressure
Good shape of combustion chamber High heat efficiency Small Surface/Volumn ratio of combustion chamber Proper ignition timing Proper location of spark plug Place the duct for taking cold air
Low intake air temperature
Heatproof air duct and manifold Duct layout for heat proof Utilize intercooler
Increasing intake pressure
Utilize turbocharger or supercharger Multi valve
Increasing charging efficiency
Increase inner diameter for intake and exhaust Reduce resistance for intake and exhaust
Increase radius of curvature Smooth inner surface Proper capacity of surge tank Reduce back pressure Wide cam nose
Valve system
Increase valve overlap Proper valve timing
Using inertia energy & pulsation
Variable intake system Proper length of manifold Multi valve Multi cylinder
Reduce intake air speed
Short stroke Increase valve diameter Wide cam nose
Increasing rpm limit
Increase valve lift Reduce inertia mass of moving parts Reduce piston mean speed
Light weight of valve system Light weight of main moving parts Double over head camshaft, twine cam Short stroke
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Engine Principles 6. S/B Ratio & Output
The displacement volume of cylinder is calculated by the cross sectional area of cylinder and the stroke. Also the cross sectional area is calculated by the diameter of cylinder (Bore). So, the main factors of displacement are the Bore and the Stroke. The bore and the stroke may differ from each engine even though the same number of cylinder and the same cylinder displacement. That is, some engine have the thin and long cylinder, other engines have the fat and short cylinder. The ratio between the length of stroke and diameter of bore is called Stroke/Bore ratio.
ratio is about 0.7∼1.3. The S/B ratio is less than 1, the stroke is smaller than the bore, it is called Short Stroke. The S/B ratio is higher than 1, the stroke is larger than bore, it is called Long Stroke. The S/B ratio is just 1, the stroke is equal to the bore diameter, it is called Square. With the same displacement volume, the Short Stroke engine has more potential to have higher power because larger bore engine can make a larger valve diameter and can make a high engine rpm without increasing the piston speed. First of all, consider about the bore. The gas amount will be larger as the valve diameter or the Valve Lift is larger. With larger gas amount, it is possible to get higher output because more gasoline will be combusted. Additionally, if the valve diameter will be larger, for the same gas amount, the valve lift can be made smaller. So the valve movement will be small at the high speed. However, the larger valve diameter has larger diameter of intake port, the large intake port engine can not make the fast flow of intake gas at low speed so combustion may be degrade. Next, considering about the piston speed. With the same rpm of engine, the piston in long stroke should be move as fast as the stroke length. The piston speed has a limitation. When the piston moves in high speed, the lubricant oil may not be properly worked, or the piston inertia force will be too high. The rpm of short stroke engine can be more increased than the long stroke engine, if the engines has same limitation of piston speed. Recently, the limit of the average piston speed is about 15∼22m per second.
For the passenger car, the Stroke/Bore (S/B) 96
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Engine Principles compression chamber volume. The cylinder volume is the total volume of compression chamber volume and the displacement of cylinder.
In general, the engine for high speed and output such as sports car shall accept Short Stroke or Square type S/B ratio, the engine for commercial car accepts the Long Stroke to increase the torque instead of speed. 7. Compression Ratio & Output
In the previous section, the power can be enhanced by increasing the intake air and increasing the engine rpm. Also there is one more method to increase the engine power. That is the compression ratio. When the piston is at the TDC, the space which is surrounded by the piston and the cylinder head including the intake-exhaust valves is the combustion chamber. The compression ratio is that the cylinder volume is divided by
The compression ratio represents how much the intake mixture is compressed. The compression ratio in the vehicle catalogue is a theoretical value by a calculation. In general, it is about 9 ∼ 10 for normal gasoline engine and about 12 ∼ 13 and for racing engine. As the compression ratio is high, the mixture is compresses strongly. So the mixture temperature will be high and the combustion will be performed in short time. Then the combustion pressure will be high and the torque and power will also be high. Furthermore, in the combustion stroke, the expansion ratio is also high, so the exhaust gas has not too high temperature. So the fuel efficiency will be good. However, if the compression ratio is too high, the engine can easily have abnormal combustion such as knocking. So it has limitation. Knocking is related with mixture temperature, flow, chamber wall temperature as well as the compression ratio. So, to increase the compression ratio should be followed by the good cooling system for the cylinder head. Furthermore, the engine should have higher strength for high compression ratio. The high performance engine should be carefully designed. There are theoretical compression ratio and the actual compression ratio. The actual compression ratio indicates how much the intake air is compressed actually. For example, in the intake stroke, if the air is not inhaled sufficiently, then the actual compression ratio is less than theoretical ratio. In the turbo engine, if the boost pressure is 1atm, then the 97
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Engine Principles actual compression ratio will be twice. So, the actual compression ratio is the important factors for the enhancing the power. In above, the knocking is affected by the actual compression ratio.
the opening time: The opening time of intake valve is about 240° in terms of crankshaft rotation angle. In the racing engine, it is about 280∼320°.
8. Increasing power by High rpm (rpm limit) To make a high power engine, the fuel amount of combustion is increased. Even the fuel amount is increased, if the air amount is not increased, then it is no meaning. Therefore, to make a high power engine, the intake air amount be more. The air flow speed at the intake port is divided the intake air quantity by the cross sectional area. The intake air amount will be increased by the increasing of the engine rpm. So, the engine output is proportion to the rpm.
⑷ Design the SHORT STROKE: with the same displacement volume, the SHORT STROKE is to make the valve be enlarged. So, the opening area is also large and then the intake air speed will be slow.
The flow resistance of the air will be increased as the air flow is fast. By enlarging the duct or volume of air cleaner, the flow resistance can be reduced. However, the resistance around the valve is not controlled. So, over the certain rpm, the power can not be increased any more. Therefore, to get high output at the high speed, the intake air speed should be decreased at the high speed. To do so, there are some methods to be considered.
If the intake air speed is sufficiently slow, the engine rpm can be increased more to make more power.
⑴ Increase the number of cylinder: with the same total displacement volume, if the cylinder number is increased, then the cylinder diameter will be reduced and then valve diameter will do so. Therefore, the intake flow velocity will be reduced.
If the engine rpm is increased, the engine should endure against the high rpm. That is, the engine should be strengthening to ensure the high rpm. Generally, the engine should be lightened to reduce the inertia force, and enhanced the strength of the body and parts of engine. 9. Transient Characteristic & Response
⑵ Increase the number of intake valve: with the same reason mentioned in ⑴, the air flow velocity will be reduced around the intake valve. ⑶ Enlarge the lift of intake valve and elongate 98
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Engine Principles the volume of the collector (surge tank) is increased, then the engine output shall be increased.
The engine makes large torque but acceleration response is slow, the engine is not a high performance engine. The acceleration performance or the response of the engine can be affected by the weight of the car or gear ratio. When the driving condition is changed, the intermediate state between before and after is called transient or partial state. The engine characteristic at partial state is called the transient characteristic of the engine. The transient characteristic is basically related with the changing of the rpm and the inertia force. The important things for the response are the weight of the dynamic parts of the engine and the changing ability of the air/fuel while acceleration.
In some fuel system, the gasoline may not be flown smoothly because the gasoline sprayed from the injector. In this case, when the throttle valve is open quickly, the mixture is leaned and then the initial torque of the engine may be delayed. To solve this problem, there is a method in which the injection amount is selectively increased at that moment. 10. Cylinder Array & Performance There are three method for arraying the cylinders, in-line type, V-type, and opposed type. What relationship is there between the cylinder array and the engine performance?
To reduce the inertia force of the dynamic parts of the engine, the dynamic parts should be made as light as possible. In the fuel injection type engine, an intake collector (surge tank) has a similar volume with the total displacement volume. When the accelerator is pressed to open the throttle valve, the air can not be inhaled into the manifold immediately because of the inertia force of the air. Therefore, the first movement of the engine torque will be delay. To solve this problem, if 99
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Engine Principles The in-line type engine has the cylinder in sequentially arrayed. There are from 2-cylinder type to 6-cylinder type. In the in-line type, the structure of the cylinder block is very simple and the cylinder head is one body, so the engine shall be light and compact. It is used widely from commercial car to racing car. In the in-line type engine, generally, the cylinder number is 4 when the displacement volume is up to 2ℓ. The cylinder number is 6 more than 2ℓ less than 3.5ℓ. The 4-cylinder engine having the displacement volume of from 1ℓto 1.5ℓ is used for commercial vehicle, and one having more and up to 2ℓ is used for high performance purposed car. Engine for the displacement volume of 2ℓ is generally made into 4-cylinder or 6-cylinder. The 6-cylinder engine has smaller combustion chamber and is easy to be made with SHORT STROKE. So it can be get large maximum output.
If the angle of V array is set to 60°, the feature is closed to the serial 6-cylinder engine. It can be installed at the FF type vehicle. So it is possible for the FF car to be developed into high performance car. The V-type engine generally consists of 6cylinder. By combining the serial 4-cylinder and the serial 6-cylinder, the V8 and the V12 engines can be manufactured respectively. They are generally installed at the large vehicle or sports car. The V6 engine is not easy to be utilized because the width of the engine is wider and weight is heavier. The opposed engine is the same with the Vtype engine having the 180° of the angle. The center of engine will be lower than others. 11. Fuel Consumption Ratio
The in-line 6-cylinder engine has the long length so it needs somewhat high cost. But, the size is compact in compare with the performance, and the turbocharger can be easily attached. So it can be utilized in high performance engine. Additionally, the inertia force of the piston-crank is well balanced so it shows good features at the anti-vibration. However, it is hard to install widely in the engine room of FF type vehicle; so generally, it is installed at the FR type vehicle in longitudinal direction. The 3-cylinder or 5-cylinder is rarely used in the in-line type.
The fuel efficiency of the engine is represented by the fuel consumption rate. The fuel consumption amount for the driving is changed by the driving condition. When the dynamic performance is measured by attaching to the dynamometer to compare with other engine, the consumption amount of the fuel should be considered.
By dividing 6-cylinder into two set of serial 3cylinder and facing them to array in parallel, the length is reduced almost half of the in-line 6cylinder engine is the V-Type 6-cylinder engine. As this engine has the high intake-exhaust efficiency because the bore diameter can be enlarged easily, it is easy to get high power.
Therefore, the fuel consumption rate is represented by fuel consumption amount per work, and the unit is g/PS·h. Assume that when a engine is rotating with 3000rpm at the dynamometer, the engine outputs 55PS, and 11kg gasoline is used for 1 hour working in this condition, then the fuel consumption rate is 100
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Engine Principles 220g/PS·h. When referring to the graph of fuel consumption rate in the engine performance curve, that the fuel consumption rate is minimized with certain rpm of engine is more concerned than the fuel amount. The actual fuel consumption rate shall be measured in actual driving condition at the vehicle. Generally to say, the catalogue indicates the fuel consumption rate with the 10-15 mode rate and the 60km/h steady rate. On here, the rate is just concerned to the engine itself. To reduce the fuel consumption rate, the fuel is used less as possible and the heat should be thoroughly converted into the dynamic force. So, it is related with the heat efficiency. For example, the mixture should be combusted with high temperature and high pressure, completely and fast as possible. And the heat loss to the exhaust gas and to the cylinder wall should be reduced as possible. As well as the mechanical friction should be reduced also. To calculate how the engine uses the heat from the gasoline in classifying according to the element is the heat balance.
gas and the heat for loss through the cylinder wall are 30% separately, and 10% is for others. Until now, the most heat efficiency of the engine is about 35%, that is, in the term of fuel consumption rate, about 170g/PS·h. 12. Output & Fuel Efficiency As the air amount is increased to enhance the engine output, the fuel amount will be increased, so the fuel efficiency is degraded. However, if the mixture can be completely combusted to increase the heat efficiency and to get higher output, then the high fuel efficiency as well as the high output will be acquired. Additionally, the exhaust gas has less harmful elements. The engine heat efficiency is the ratio of the heat capacity used for working. To increase the heat efficiency, the expansion of the gas should be as large as possible, at the same time; the loss energy should be as small as possible. The loss energies in the engine are the cooling loss by cooling system, the exhaust loss by being taken out with the hot exhaust gas, and the intake-exhaust loss (pumping loss) used for intake-exhaust operation. To increase the heat efficiency by increasing the expansion force of the combusted gas is related with to increase the mixture amount and to increase the compression ratio.
To indicate this heat balance with the graph is the heat balance graph. Generally to say about the heat balance of the gasoline engine. The heat for output, the heat for loss in exhaust
To reduce the cooling loss, the temperature of the combustion chamber should be increased. By changing the shape of combustion chamber to enhance the compression ratio, the chamber temperature will be increased at the compression stroke as well as the knocking is prevented. In other hand, the method using the higher cooling water is also concerned. To reduce the intake-exhaust loss (pumping 101
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Engine Principles loss), The intake-exhaust tube should be short and have less bent portion as possible. To enhance the air flow at the valve, the diameter of the valve may be enlarged or the number of valve may be increased. However, in that case, if the mixture flow is too slow, or if the structure of the chamber is too complicated, then the heat efficiency will be degraded. Therefore, it should be carefully designed. One more, to enhance the heat efficiency, it should be considered that the friction loss generated when the piston moves. The mechanical energy loss in driving the auxiliary equipments should be reduced.
indicates the engine fuel efficiency, this value is not the exact fuel efficiency of vehicle. With the same engine, the fuel efficiency may differ according that it is installed at heavy and large vehicle or it is installed at light and small vehicle. Comparing one case that a vehicle includes a small engine having good fuel efficiency and another case that a vehicle includes a large engine having bad fuel efficiency but high power, the actual fuel efficiency will be changed by the driving condition. For example, if a car is usually used in the low rpm condition, then the small displacement volume is more effective. If the car is usually used in high speed or high power condition, then the large displacement volume engine will be more effective. For the comparing of the fuel efficiency between engines, the specific test mode is required. The specific test mode means that the test method and test conditions are specified. There are many test mode which should be suggested in catalogue, the 10-15 mode running fuel consumption, the 60km/h steady fuel consumption, FTP 75 mode and so on. In representing the fuel consumption rate, for engine only, the unit of g/PS-h is used, the unit of km/ℓ indicating for vehicle what km the car can run with 1ℓ of fuel is used.
13. Fuel Efficiency of Vehicle
Even
though
the
fuel
consumption
rate
The 10-15 mode fuel efficiency is acquired from dividing the running distance by the amount of the used fuel, the vehicle is tested at the dynamometer according to the predetermined pattern cycle of idling → start acceleration → running with constant speed → deceleration. In the past, the driving pattern had the maximum speed of 40km/h, however, it was not proper to the modern traffic condition. Recently, the maximum speed sets 102
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Engine Principles to 70km/h, the 15 mode is added to this test.
engine having high compression ratio and high performance makes more noise. The vibration of the turbo engine makes 20~50% more noise than NA engine. In this case, some devices for preventing from noise are used and the auxiliary devices are attached at the portions less affected by vibration. Additionally, by changing the engine mounting position or adopting the vibration absorber with the mounting portions, the vibration can not be transmitted to the body directly.
The 60km/h steady fuel efficiency is acquired from the used fuel amount when a car is driving with constant speed of 60km/h with the gross weight having full passengers and baggage at the paved plain road without wind. Generally, this value is measured in the ideal state by the manufacturer to suggest to government. The actual value is less than this value. 14. Vibration of Engine
There are many sources to make vibration in engine. There are major three vibrations, one is from the combustion of the engine, one another is from the inertia force of the reciprocal and rotational movement at the dynamic system such as piston, connecting rod, crankshaft, and others. The vibration from the engine is as much as high pressure of the combustion. And, the
The inertia force is one major source of the vibration. As the piston moves from the highest point to the lowest point with various accelerations. The crankshaft makes a vibration waves and an inertia force from the rotation of the crank pin. The Connecting rod makes an inertia force from the combination of the reciprocal and rotational movement. In the multi cylinder engine, the pistons are connected to the crankshaft, so each inertia will be canceled each other. It is very complicated with the number of cylinders, array of them and each timing of combustion. Therefore, using the counter weight, the inertia force is balanced with the total weight. To match the balance of the inertia force completely is very difficult. The inertia force is parts such as piston lighter weight. With volume, the engine 103
less when the dynamic and Connecting rod have the same displacement having more number of
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Engine Principles cylinders has less inertia force because the parts are small and light. When the inertia force is small, the possibility to make a vibration will be few and it will rotate at high speed with the same strength.
louder. Someone may feel that the turbo engine make less noise than the NA engine. The reasons are the turbine absorbs the exhaust energy and the variation of the combustion pressure is smaller.
By lightening the weight of dynamic part, the inertia force at each part will be small. With the same rpm, the strength of these parts will not be maintained highly. Generally, the device having lower strength is lighter than the device having higher strength. To be lightening is most important point to increase performance as well as to prevent vibration.
The mechanical sounds comes from the friction and bump the dynamic parts such as gear, chain, and valves. For example, the cam hits the valve lifter, rocker arm and camshaft hits the valves, the valve bumps with the valve seat and so on.
15. Noise of Engine
The noises from the engine are the combustion noise and mechanical sounds. The mechanical sounds is caused by the friction between the parts. When the engine rotates with high speed, the noise will be changed and be louder. When a driver changes the shift to up or down, generally, the driver may selects the proper gear by engine noise. So, the sound of engine helps the driver for the driving. Therefore, the engine sound should be noticed but it shall not the noise but the sound. The mechanical noise is made from the vibration of the cylinder and cylinder head by the combustion force. When the mixture amount is increased or the combustion pressure is higher, then the noise will be
The resonance noise from the vibration is louder than the direct mechanical noise. So, the causes of the noise from the engine can not be exactly found. Any way, that there is a noise is not good situation because some parts of the engine shall be hit with others and this is bad for the endurance of parts. If you detect abnormal noise, please check the system as soon as possible. Comparing the combustion noise with the mechanical noise, at the low speed, the combustion noise is larger. When the rpm is over 3000rpm, the inertial force is larger and the mechanical noise will be larger. The noise from the engine room will be protected by attaching absorbing materials under the hood and in front of the dashboard, the boundary of the engine room and cabin. The noise absorbing materials are glass wool, 104
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Engine Principles felt and so on.
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