Seminar Report

ROTARY ENGINE AND ITS APPLICATION IN AUTOMOBILE SEMINAR SUBMITTED IN PARTIAL FULFILLMENT OF BACHELOR OF ENGINEERING DEGREE OF THE JODHPUR NATIONAL UNIVERSITY JODHPUR

GUIDED BY: -

SUBMITTED BY:-

PROF. S.N.GARG

AMRUTIYA DHAVAL

(H.O.D OF MECHANICAL)

(4

TH

TH

YEAR, 8

SEM)

DEPARTMENT OF MECHANICAL ENGINEERING FACULTY OF ENGINEERING & TECHNOLOGY JODHPUR NATIONAL UNIVERSITY JODHPUR (RAJ.) 1

(SESSION 2011-2012)

Acknowledgement 

It is indeed a pleasure for me to express my sincere gratitude to those who have always helped me throughout my seminar s eminar report work. I would like to thank my report guide PROF. S.N.GARG (H.O.D of  mechanical engg.) who helped me selecting the report topic, understanding the subject, stimulating suggestions, encouragement and also for writing of this thesis. I am sincerely thankful for this valuable guidance and help to enhance my presentation skill.

AMRUTIYA DHAVAL

2

(SESSION 2011-2012)

Acknowledgement 

It is indeed a pleasure for me to express my sincere gratitude to those who have always helped me throughout my seminar s eminar report work. I would like to thank my report guide PROF. S.N.GARG (H.O.D of  mechanical engg.) who helped me selecting the report topic, understanding the subject, stimulating suggestions, encouragement and also for writing of this thesis. I am sincerely thankful for this valuable guidance and help to enhance my presentation skill.

AMRUTIYA DHAVAL

2

CERTIFICATE

I here certify that the seminar entitled ROTARY ENGINE AND ITS APPLICATION IN AUTOMOBILE being submitted by AMRUTIYA DHAVAL KUMAR M. in partial fulfillment for the award of the degree of Bachelor of Technology in Mechanical Engineering to the Department of Mechanical Engineering, Faculty of Engineering & Technology, Jodhpur National University, Jodhpur is a record of the candidates own work carried out by him under my supervision and guidance the matter embodied in the seminar has not been submitted by him for the award of any degree.

Date: Place: -JODHPUR

Prof. S.N.Garg H.O.D Mech. Engg. Faculty of E&T JNU,Jodhpur

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INDEX  Introduction of Rotary Engine

6



History

6



Principles of Rotary Engine

8



The Parts of Rotary Engine

9



Rotary Engine Assembly

11



Material used

13



Sealing

13



Working

13



Ignition

17



Cooling system

19



Different and challenges

21



Fuel consumption and hydrogen emission

22



Rotary engine geometry

22



Different types of rotary engine

25



Research paper

30



Advantages

40



Disadvantages

40



Application

41



Utilities

41



Conclusion

41



References

42

4

FIGURE LIST Figure 1 Rotary Engine Figure 2 Rotary Engine Figure 3 Rotar Figure 4 Housing Figure 5 [Output Shaft; Note the eccentric lobes.] Figure 6 One of the two end pieces of a two-rotor Wankel engine Figure 7 The part of the rotor housing that holds the rotors Figure 8 The center piece contains another intake port for each rotor. Figure 9 schematic fig. of rotary engine Figure 10 intake stage Figure 11 compression stage Figure 12 combustion stage Figure 13 Exhaust stage Figure 14 [The Wankel cycle: Intake (blue), Compression (green), Ignition (red), Exhaust (yellow)] Figure C-1 cooling system and lubrication system Figure 15 Rotary Engine Nomenclature Figure 16 Nomenclature For Epitrochoid parametric equation Figure 17 successive phases in the execution of the otto cycle in the rotary engine Fiugre 18 minimum working fluid with flat-flanked rotary engine Figure 19 maximum working-fluid volume for a flat-flanked rotary engine Figure 20 Geometry of circular arc of rotor Figure 21 Exploded view of twin rotary engine Figure 22 Influence of flank rounding onclearance and compression ratio for an eccentricity ratio of 0.16 Figure 23 After top center pilot, dual injector configuration Figure 24 Stratified-Charge Rotary Engine performance Figure 25. Comparison of energy densities between internal combustion engines and primary batteries. Figure 26 Rotary Engine Operation Figure 27 Mini-Rotary Engine Test Bench Figure 28 Dynamometer calibration chart for 50 W Maxon brushless electric motor. Figure 29 MN30 Mini-Rotary Engine Figure 30 Modified MN30 Rotor with Apex Seals Figure 31 CWRU Micro-Rotary Engine Left: Three.wafer Si mold Right: Molded SiC rotor Figure 32 CWRU Micro-Rotary Engine Housing

5

ROTARY ENGINE 

Introduction of rotary engine:

The Rotary engine is a type of internal combustion engine which uses a rotor to convert pressure into a rotating motion instead of using reciprocating pistons. Its four-stroke cycle is generally generated in a space between the inside of an oval-like epitrochoid-shaped housing and a roughly triangular rotor (hypotrochoid). This design delivers smooth high-rpm power from a compact, lightweight engine. The engine was invented by German engineer Felix Wankel. He began its development in the early 1950s at NSU Motorenwerke AG (NSU) before completing a working, running prototype in 1957. NSU then subsequently licensed the concept to other companies across the globe, who added more efforts and improvements in the 1950s and 1960s. Because of its compact, lightweight design, Wankel rotary engines have been installed in a variety of vehicles and devices such as automobiles and racing cars, aircraft, go-karts, personal water crafts and auxiliary power units. In a piston engine, the same volume of space (the cylinder) alternately does four different jobs - intake, compression, combustion and exhaust. A rotary engine does these same four jobs, but each one happens in its own part of the housing. It's kind of like having a dedicated cylinder for each of the four jobs, with the piston moving continually from one to the next. The rotary engine (originally conceived and developed by Dr. Felix Wankel) is sometimes called a Wankel engine.[1]

Fig.1 Rotary Engine



History:

In 1951, Wankel began development of the engine at NSU (NSU Motorenwerke AG), where he first conceived his rotary engine in 1954 (DKM 54, Drehkolbenmotor ) and later the KKM 57 (the Wankel rotary engine, Kreiskolbenmotor ) in 1957. The first working prototype DKM 54 as 6

running on February 1, 1957 at the NSU re search and development department Versuchsabteilung TX . Considerable effort went into designing rotary engines in the 1950s and 1960s. They were of  particular interest because they were smooth and quiet running, and because of the reliability resulting from their simplicity. In the United States, in 1959 under license f rom NSU, Curtiss-Wright pioneered minor improvements in the basic engine design. In Britain, in the 1960s, Rolls Royce Motor Car Division at Crewe, Cheshire, pioneered a two stage diesel version of the Wankel engine. Also in Britain Norton Motorcycles developed a Wankel rotary engine for motorcycles, which was included in their Commander and F1; Suzuki also made a production motorcycle with a Wankel engine, the RE-5. In 1971 and 1972 Arctic Cat produced snowmobiles powered by 303 cc Wankel rotary engines manufactured by Sachs in Germany. John Deere Inc, in the U.S., designed a version that was capable of using a variety of fuels. The design was proposed as the power source for several U.S. Marine combat vehicles in the l ate 1980s. After occasional use in automobiles, for instance by NSU with their Ro 80 model, Citroën with the M35, and GS Birotor using engines produced by Co motor, as well as abortive attempts by General Motors and Mercedes-Benz to design Wankel-engine automobiles, the most extensive automotive use of the Wankel engine has been by the Japanese company Mazda. After years of development, Mazda's first Wankel engined car was the 1967 Mazda Cosmo. The company normally used two-rotor designs, but received considerable attention with their 1991 Eunos Cosmo, which used a twinturbo three-rotor engine. In 2003, Mazda introduced the RENESIS engine with the new RX-8. The RENESIS engine relocated the ports for exhaust and intake from the periphery of the rotary housing to the sides, allowing for larger overall ports, better airflow, and further power gains. The RENESIS is capable of delivering 238 horsepower (177 kW) from its 1.3 L displacement with better fuel economy, reliability, and environmental friendliness than any other Mazda rotary engine in history.[5]

Dr. FelixWankel 7



Principles of a Rotary Engine:

Like a piston engine, the rotary engine u ses the pressure created when a combination of air and fuel is burned. In a piston engine, that pressure is contained in the cylinders and forces pistons to move back and forth. The connecting rods and crankshaft convert the reciprocating motion of the pistons into rotational motion that can be used to power a car. In a rotary engine, the pressure of combustion is contained in a chamber formed by part of the housing and sealed in by one face of the triangular rotor, which is what the engine uses instead of pistons.

Fig. 2 Rotary Engine The rotor follows a path that looks like something you'd create with a Spirograph. This path keeps each of the three peaks of the rotor in contact with the housing, creating three separate volumes of gas. As the rotor moves around the chamber, each of the three volumes of gas alternately expands and contracts. It is this expansion and contraction that draws air and fuel into the engine, compresses it and makes useful power as the gases expand, and then expels the exhaust. We'll be taking a look inside a rotary engine to check out the parts, but first let's take a look at a new model car with an all-new rotary engine.[5]

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Mazda RX-8:

Mazda has been a pioneer in developing production cars that use rotary engines. The RX -7, which went on sale in 1978, was probably the most successful rotary-engine-powered car. But it was preceded by a series of rotary-engine cars, trucks and even buses, starting with the 1967 Cosmo Sport. The last year the RX-7 was sold in the United States was 1995, but the rotary engine is set to make a comeback in the near future. The Mazda RX-8 , a new car from Mazda, has a new, award winning rotary engine called the RENESIS. Named International Engine of the Year 2003, this naturally aspirated two-rotor engine will produce about 250 horsepower.[8]



The Parts of a Rotary Engine:

A rotary engine has an ignition system and a fuel-delivery system that are similar to the ones on piston engines. If you've never seen the inside of a rotary engine, be prepared for a surprise, because you won't recognize much.[10] 1. Rotor : The rotor has three convex faces, each of which acts like a piston. Each face of the rotor has a pocket in it, which increases the displacement of the engine, allowing more space for air/fuel mixture. At the apex of each face is a metal blade that forms a seal to the outside of the combustion chamber. There are also metal rings on each side of the rotor that seal to the sides of the combustion chamber. The rotor has a set of internal gear teeth cut into the center of one side. These teeth mate with a gear that is fixed to the housing. This gear mating determines the path and direction the rotor takes through the housing.[10]

9

Fig.3 Rotar

2. Housing : The housing is roughly oval in shape (it's actually an epitrochoid).The shape of the combustion chamber is designed so that the three tips of the rotor will always stay in contact with the wall of the chamber, forming three sealed volumes of gas. Each part of the housing is dedicated to one part of the combustion process. [ Rotor] The four sections are: · Intake · Compression · Combustion · Exhaust The intake and exhaust ports are located in the housing. There are no valves in these ports. The exhaust port connects directly to the exhaust, and the intake port connects directly to the throttle.[10]

Fig.4 Housing

10

3. Shaft: The output shaft has round lobes mounted eccentrically, meaning that they are offset from the centerline of the shaft. Each rotor fits over one of these lobes. The lobe acts sort of like the crankshaft in a piston engine. As the rotor follows its path around the housing, it pushes on the lobes. Since the lobes are mounted eccentric to the output shaft, the force that the rotor applies to the lobes creates torque in the shaft, causing it to spin.[10]

Fig.5[Output Shaft; Note the eccentric lobes.]



Rotary Engine Assembly:

A rotary engine is assembled in layers. The two-rotor engine we took apart has five main layers that are held together by a ring of long bolts. Coolant flows through passageways surrounding all of the pieces. The two end layers contain the seals and bearings for the output shaft. They also seal in the two sections of housing that contain the rotors. The inside surfaces of these pieces are very smooth, which helps the seals on the rotor do their job. An intake port is located on each of these end pieces.[12]

Fig.6 One of the two end pieces of a two-rotor Wankel engine 11

The next layer in from the outside is the oval-shaped rotor housing, which contains the exhaust ports. This is the part of the housing that contains the rotor

Fig.7 The part of the rotor housing that holds the rotors (Note the exhaust port location.) The center piece contains two intake ports, one for each rotor. It also separates the two rotors, so its outside surfaces are very smooth.

Fig.8 The center piece contains another intake port for each rotor.

12

In the center of each rotor is a large internal gear that rides around a smaller gear that is fixed to the housing of the engine. This is what determines the orbit of the rotor. The rotor also rides on the large circular lobe on the output shaft.[12]



Materials used:

Wankel engine is constructed with an iron rotor within a housing made o f aluminum, which has greater thermal expansion. Unlike a piston engine, where the cylinder is cooled by the incoming charge after being heated by combustion, Wankel rotor housings are constantly heated on one side and cooled on the other, leading to high local temperatures and unequal thermal expansion. While this places high demands on the materials used, the simplicity of the Wankel makes it easier to use alternative materials like exotic alloys and ceramics. With water cooling in a radial or axial flow direction, with the hot water from the hot bow heating the cold bow, the thermal expansion remains tolerable.[1]



Sealing :

Early engine designs had a high incidence of sealing l oss, both between the rotor and the housing and also between the various pieces making up the housing. Also, in earlier model Wankel engines carbon particles could become trapped between the seal and the casing,  jamming the engine and requiring a partial rebuild. Modern Wankel engines have not had these problems for many years. Further sealing problems arise from the uneven thermal di stribution within the housings causing distortion and loss of sealing and compression. This thermal distortion also causes uneven wear between the apex seal and the rotor housing, quite evident on higher mileage engines. Attempts have been made to normalize the temperature of the housings, minimizing the distortion, with different coolant circulation patterns and housing wall thicknesses.[1] 

Working:

Fig.9 schematic fig. of rotary engine

13

The "A" marks one of the three apexes of the rotor. The "B" marks the eccentric shaft and the white portion is the lobe of the eccentric shaft. The shaft turns three times for each rotation of  the rotor around the lobe and once for each orbital revolution around the eccentric shaft. [10] In the Wankel engine, the four strokes of a typical Otto cycle occur in the space between a rotor, which is roughly triangular, and the inside of housing. In the basic single-rotor Wankel engine, the oval-like epitrochoid-shaped housing surrounds a three-sided rotor (similar to a Reuleaux triangle, a three-pointed curve of constant width, but with the middle of each side a bit more flattened). The central drive shaft, also called an eccentric shaft or E-shaft, passes through the center of the rotor and is supported by bearings. The rotor both rotates around an offset lobe (crank) on the E-shaft and makes orbital revolutions around the central shaft. Seals at the corners of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers. Fixed gears mounted on each side of the housing engage with ring gears attached to the rotor to ensure the proper orientation as the rotor moves.[10] The best way to visualize the action of the engine in the animation at left is to look not at the rotor itself, but the cavity created between it and the housing. The Wankel engine is actually a variable-volume progressing-cavity system. Thus there are 3 cavities per housing, all repeating the same cycle.[10]

As the rotor rotates and orbitally revolves, each side of the rotor gets closer and farther from the wall of the housing, compressing and expanding the combustion chamber similarly to the strokes of a piston in a reciprocating engine. The power vector of the combustion stage goes through the center of the offset lobe.[10] While a four-stroke piston engine makes one combustion stroke per cylinder for every two rotations of the crankshaft (that is, one half power stroke per crankshaft rotation per cylinder), each combustion chamber in the Wankel generates one combustion stroke per each driveshaft rotation, i.e. one power stroke per rotor orbital revolution and three power strokes per rotor rotation. Thus, power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement in a similar state of tune and higher than that of a four-stroke piston engine of similar physical dimensions and weight. Wankel engines also generally have a much higher redline than a reciprocating engine of similar size since the strokes are completed with a rotary motion as opposed to a reciprocating engine which must use connecting rods and a crankshaft to convert reciprocating motion into rotary motion. National agencies that tax automobiles according to displacement and regulatory bodies in automobile racing variously consider the Wankel engine to be equivalent to a f our-stroke engine of 1.5 to 2 times the displacement; some racing regulatory agencies ban it altogether.[10]

Rotary Engine Power: Rotary engines use the four-stroke combustion cycle, which is the same cycle that four-stroke piston engines use. But in a rotary engine, this is accomplished in a completely different way. 14

The heart of a rotary engine is the rotor. This is roughly the equivalent of the pistons in a piston engine. The rotor is mounted on a large circular lobe on the output shaft. This lobe i s offset from the centerline of the shaft and acts like the crank handle on a winch, giving the rotor the leverage it needs to turn the output shaft. As the rotor orbits inside the housing, it pushes the lobe around in tight circles, turning three times for every one revolution of the rotor.[11] As the rotor moves through the housing, the three chambers created by the rotor change size. This size change produces a pumping action. Let's go through each of the four strokes of the engine looking at one face of the rotor.[11]

Intake: The intake phase of the cycle starts when the tip of the rotor passes the intake port. At the moment when the intake port is exposed to the chamber, the volume of that chamber is close to its minimum. As the rotor moves past the intake port, the volume of the chamber expands, drawing air/fuel mixture into the chamber. When the peak of the rotor passes the intake port, that chamber is sealed off and compression begins.[11]

Fig.10 intake stage

Compression: As the rotor continues motion around the housing, the volume of the chamber gets smaller and the air/fuel mixture gets compressed. By the time the face of the rotor has made it around to the spark plugs, the volume of the chamber is again close to its minimum. This is when combustion starts.[11]

15

Fig.11 compression stage

Combustion: Most rotary engines have two spark plugs. The combustion chamber is long, so the flame would spread too slowly if there were only one plug. When the spark plugs ignite the air/fuel mixture, pressure quickly builds, forcing the rotor to move. The pressure of combustion forces the rotor to move in the direction that makes the chamber grow in volume. The combustion gases continue to expand, moving the rotor and creating power, until the peak of the rotor passes the exhaust port.[11]

Fig.12 combustion stage

Exhaust: Once the peak of the rotor passes the exhaust port, the high- pressure combustion gases are free to flow out the exhaust. As the rotor continues to move, the chamber starts to contract, forcing the remaining exhaust out of the port. By the time the volume of the chamber is nearing its minimum, the peak of the rotor passes the intake port and the whole cycle starts again. The neat thing about the rotary engine is that each of the three faces of the rotor is always working on one part of the cycle -- in one complete revolution of the rotor; there will be three combustion strokes. But remember, the output shaft spins three times for every complete revolution of the rotor, which means that there is one combustion stroke for each revolution of  the output shaft.[11]

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Fig.13 Exhaust stage

Fig.14 [The Wankel cycle: Intake (blue), Compression (green), Ignition (red), Exhaust (yellow)]



Ignition:

Since the working chambers of each rotor fire in the same geographic location in the engine, only one set of spark plugs are needed per rotor housing. Due to the complexities of  combusting a long chamber, two spark plugs are used in each housing . The lower one is called the "leading" spark plug, while the top one is called the "trailing" spark plug. Additionally, the front rotor housing is denoted as "1" and the rear rotor housing is denoted as "2". So, in short hand, the Trailing (top) plug on the front rotor housing is referred to as "T1", the Leading (lower) plug in the front rotor housing is referred to as "L1", and so forth.[6]

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As the working chamber approaches Top Dead Center (TDC), the leading plug fires first, starting the ignition of the air-fuel mixture and contributing most to the generation of power. The trailing plug typically fires 10 to 15 degrees later and effectively completes the combustion of  the remaining air-fuel mixture above the minor axis of the trochoid housing. Additionally, the leading plug fires a second time late in the power stroke, which is called a "wastespark". The wastespark is done to simplify the ignition system by allowing both leading plugs to be fired "on the same channel" (one coil, one signal) . Basically, both leading plugs always fire at the same time, so there is an extra (wasted) spark during the power stroke. The trailing plugs cannot be fired in wastespark mode due to the location of the plug above the minor axis -- when one trailing plug is firing, the other trailing plug is already in the next working chamber, where it would preignite the incoming air-fuel mixture.[6] As you may have figured out, there are three discrete ignition "channels" -- one for the leading plugs and two separate ones for the trailing plugs. Early, distributor-controlled, rotary engines could get away with only two coils (one each for leading and trailing ignitions) due to the coil engery being redirected through the distributor to the appropriate trailing spark plug. Later, crank angle sensor-controlled, rotary engines (starting with the 2nd generation RX-7) require three separate coils due to the direct-fire setup.[6]

TIMING COMMENTS Timing of the engine, be it ignition or intake/exhaust porting, is based on degrees of eccentric shaft rotation, rather than rotor rotation. The difference is that the rotor rotates at one third the forward rate of the eccentric shaft, so three degrees of eccentric shaft rotation translates to one degree of rotor rotation relative to the housing. It is also helpful to remember that each working chamber of a rotary engine has two Top Dead Centers (minimum chamber volume) and two Bottom Dead Centers (maximum chamber volume). Timing is usually expressed relative to either TDC or BDC, in degrees of eccentric shaft rotation Before or After those critical points. 5° BTDC, for example, means the event in question occurs 5° of eccentric shaft rotation Before the applicable Top Dead Center of a given working chamber. Further, one may refer to Advancing the timing, which simply means to increase degrees BTDC or BBDC, or decrease degrees ATDC or ABDC. Retarding the timing is the converse. Relative measurements such as these are difficult to grasp at first, but they make the most sense in application.[6] 18



COOLING SYSTEM FOR ROTOR:

Table 1 suggests a classification of the Wankel type engines from the point of view of the cooling system employed for rotor and rotor bearing The best known concept to date is the oil cooled rotor (OCR) which is usually associated with a liquid cooling system of engine housings, i.e., rotor housing, front housing and rear housing. This was the original solution developed by NSU/Wankel, and was successfully applied in production type engines by Mazda, John Deere, and others. For these reasons we considered this solution representing the current full potential of the Wankel engine and we credited it with 100% rank (see Table 1). For light duty applications, the charge cooled rotor (CCR) offers significant manufacturing cost reduction and added simplicity by eliminating the oil cooling system. Combined with a liquid cooling system for the engine housing, the CCR system offers only 80% of the maximum obtainable power when compared to the OCR system applied to the same basic engine design, e.g. volumetric displacement, engine rated speed, port arrangement, etc.[9] Poor performance of the CCR type engine is due to the higher temperature of the rotor, rotor bearing and eccentric shaft and to the diminished volumetric efficiency as a result of heat transfer from the above mentioned engine parts to the f resh charge mixture. The engine performance is even lower when the CCR solution is coupled with an air cooling system for the engine housings. Some CCR engines are using the fresh charge mixture to cool the eccentric shaft and rotor bearing. This method is usually employed when gasoline (mixed with oil) or natural gas are used as fuel. In the first case, the fuel evaporation helps the engine's internal cooling[9]. Table 1 is far from being exhaustive. For example,the introduction of an intercooler for the charge mixture in association with the CCR solution opens

19

fig. C-1 cooling system and lubrication system another branch of the classification tree. Also, there are a few Wankel engines which are using the bleed air to cool the rotor and the rotor bearing. This combination again extends the classification related to the rotor and rotor bearing cooling system.[9] 20

Table 2 shows the influence of the rotor cooling design concept on estimated engine performance, total efficiency potential and production cost. The evaluation was limited to the basic design concept discounting the influence of engine accessories such as intercoolers, etc.

By eliminating the oil cooling system with an oil pump, a heat exchanger, an oil sump and especially the oil sealing system, a cost advantage of up to 30% can be achieved for a LCCR engine when compared with the OCR solution. The CCR solution offers an even better cost advantage but its concept is limited to the relatively small engines. To date, only CCR rotary engines up to 650cc have been developed and produced successfully. The total efficiency of the CCR engines can equal that of the OCR engines due to the former's lower friction losses as long as the overheating phenomena can be controlled. Overheating is especially worrisome at part load conditions. in this respect, the LCCR engine demonstrates a decisive advantage by exactly controlling its internal temperatures.[9]



Differences and Challenges:

There are several defining characteristics that differentiate a rotary engine from a typical piston engine.

Fewer moving parts : The rotary engine has far fewer moving parts than a comparable four stroke piston engine. A two-rotor rotary engine has three main moving parts: the two rotors and the output shaft. Even the simplest four-cylinder piston engine has at least 40 moving parts, including pistons, connecting rods, camshaft, valves, valve springs, rockers, timing belt, timing gears and crankshaft. This minimization of moving parts can translate into better reliability from a rotary engine. This is why some aircraft manufacturers (including the maker of Sky car) prefer rotary engines to piston engines.[11]

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Smoother: All the parts in a rotary engine spin continuously in one direction, rather than violently changing directions like the pistons in a conventional engine do. Rotary engines are internally balanced with spinning counterweights that are phased to cancel out any vibrations. The power delivery in a rotary engine is also smoother. Because each combustion event lasts through 90 degrees of  the rotor's rotation, and the output shaft spins three revolutions for each revolution of the rotor, each combustion event lasts through 270 degrees of the output shaft's rotation. This means that a single-rotor engine delivers power for three-quarters of each revolution of the output shaft. Compare this to a single-cylinder piston engine, in which combustion occurs during 180 degrees out of every two revolutions, or only a quarter of each revolution of the crankshaft (the output shaft of a piston engine).[11]

Slower: Since the rotors spin at one-third the speed of the output shaft, the main moving parts of the engine move slower than the parts in a piston engine. This also helps with reliability.[11] 

Fuel consumption and hydrocarbon emissions :

Just as the shape of the Wankel combustion chamber prevents preignition; it also leads to incomplete combustion of the air-fuel charge, with the remaining unburned hydrocarbons released into the exhaust. While manufacturers of piston-engine cars were turning to expensive catalytic converters to completely oxidize the unburned hydrocarbons, Mazda was able to avoid this cost by enriching the air/fuel mixture and increasing the amount of unburned hydrocarbons in the exhaust to actually support complete combustion in a 'thermal reactor'(an enlarged open chamber in the exhaust manifold) without the need for a catalytic converter, thereby producing a clean exhaust at the cost of some extra fuel consumption. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing exhaust port area.[11]



Rotary Engine Geometry

Fig.15 Rotary Engine Nomenclature

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Fig.16 Nomenclature For Epitrochoid parametric equation

Fig.17 successive phases in the execution of the otto cycle in the rotary engine

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The major elements of the rotary engine. the housing and the rotor. are shown in crosssection in Figure 15. The housing inner surface has a mathematical form known as a trochoid or epitrochoid . A single-rotor engine housing may be thought of as two parallel planes separated by a cylinder of epitrochoidal cross-section. Following the notation of Figure 7.5, the parametric form of the epitrochoid is given by  x = e cos 3α + R cosα [ft | m] _ _ _ (1a) y = e sin 3α+ R sinα [ft | m] _ _ (1b) where e is the eccentricity and R is the rotor center-to-tip distance. For given values of e and R, Equations (1) give the x and y coordinates defining the housing shape when is varied from 0 to 360 degrees. The rotor shape may be thought of as an equilateral triangle, as shown in Figures 15 and 17 (flank rounding and other refinements are discussed later in the chapter). Because the rotor moves inside the housing in such a way that its three apexes are in constant contact with the housing periphery, the positions of the tips are also given by equations of the form of Equations (1):  x = e cos 3α + R cos(α+ 2nπ/3) [ ft | m] _ _ (2a) y = e sin 3α + R sin(α+ 2nπ) [ft | m] _ _ (2b) where n = 0, 1, or 2, the three values identifying the positions of the three rotor tips, each separated by 120°. Because R represents the rotor center-to-tip distance, the motion of the center of the rotor can be obtained from Equations (2) by setting R = 0. The equations and Figure 17 indicate that the path of the rotor center is a circle of  radius e. Note that Equations (1) and (2) can be non dimensionalized by dividing through by R. This yields a single geometric parameter governing the equations, e/R, known as the eccentricity ratio. It will be seen that this parameter is critical to successful performance of the rotary engine. The power from the engine is delivered to an external load by a cylindrical shaft. The shaft axis coincides with the axis of the housing, as seen in Figure 15 . A second circular cylinder, the eccentric, is rigidly attached to the shaft and is offset from the shaft axis by a distance, e, the eccentricity. The rotor slides on the eccentric. Note that the axes of the rotor and the eccentric coincide. Gas forces exerted on the rotor are transmitted to the eccentric to provide the driving torque to the engine shaft and to the external load. The motion of the rotor may now be understood in terms of the notation of Figure 16. The line labeled e rotates with the shaft and eccentric through an angle 3α, while the line labeled R is fixed to the rotor and turns with it through an angle α about the moving eccentric center. Thus the entire engine motion is related to the motion of these two lines. Clearly, the rotor (and thus line R) rotates at one-third of the speed of the shaft, and there are three shaft rotations for each rotor revolution.[5]

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

Different types of ROTARY ENGINES:

(1) A Simple Model for a Rotary Engine:

Fig.18 minimum working fluid with flat-flanked rotary engine

Additional important features of the rotary engine can be easily studied by considering an engine with an equilateral triangular rotor. Figure 18 shows the rotor in the position where a rotor flank defines the minimum volume. We will call this position top center, TC, by analogy to the reciprocating engine. The rotor housing clearance parameter , d , is the difference between the housing minor radius, R -e, and the distance from the housing axis to mid- flank, e + R cos 60 = e + R/2: d = (R -e) - (e + R/2) = R/2 -2e

[ft | m]

_ _ _ (3)

Setting the clearance to zero establishes an upper limiting value for the eccentricity ratio: (e/R)crit = 1/4. Study of Equations (1), at the other extreme, shows that, for e/R = 0, the epitrochoid degenerates to a circle. In this case the rotor would spin with no eccentricity and thus produce no compression and no torque. Thus, for the flat-flanked rotor, it is clear that usable values of e/R lie between 0 and 0.25.

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Fig.19 maximum working-fluid volume for a flat-flanked rotary engine

Now let’s examine some other fundamental parameters of the flat-flanked engine model. Consider the maximum mixture volume shown in Figure 19. For a given rotor width w , the maximum volume can be determined by calculating the area between the housing and the flank of the rotor. Using Equations (1), the differential area 2y dx can be written as: dA max = 2y dx  2 2 = 2(e sin3α + R sinα) d (e cos3α + R cosα) [ft | m ] _ _ _ (4) 2

Dividing by R and differentiating on the right-hand side, we obtain an equation for the dimensionless area in terms of the eccentricity ratio and the angle α 2  Amax/R = - 2 [(e/R)sin3α + sinα][3(e/R)sin3α + sinα]d α [dl] _ _ _(5) In order for the differential area to sweep over the maximum trapped volume in Figure 19, the limits on the angle α must vary from 0° to 60°. Thus integration of Equation (5) with these limits and using standard integrals yields 2 2 1/2  Amax/R = π [(e/R) + 1/3]- 3 /4[1 -6(e/R)] [dl] _ _ _ (6) Similarly, using Figure 6 and the differential volume shown there, the nondimensionalized minimum area can be written as: 2 1/2  Amin/R = π[(e/R)2 + 1/3] - 3 /4 [1 + 6(e/R)] [dl] _ _ _ (7)

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These maximum and minimum volumes (area-rotor width products) are analogous to the volumes trapped between the piston and cylinder at BC and TC in the four-stroke reciprocating engine. In that engine the difference between the volumes at BC and TC is the displacement volume, and their ratio is the compression ratio. A little thought should convince the reader that the analogy holds quantitatively for the displacement and compression ratio of the rotary engine. Therefore, subtracting Equation (7) from Equation (6) gives the displacement for a rotor width w for one flank of the flat-flanked engine as 1/2 2 3 3 disp = 3 ×3 wR (e/R) [ft | m ] _ _ _ (8) Thus the displacement increases with increases in rotor width, the square of the rotor radius, and with the eccentricity ratio, whereas the compression ratio is independent of size but increases with increase in eccentricity ratio.[5]

(2) The Circular-Arc-Flank Model:

Fig.20 Geometry of circular arc of rotor

Fig 21 Exploded view of twin rotary engine 27

Fig.22 Influence of flank rounding onclearance and compression ratio for an eccentricity ratio of 0.16

While the triangular rotor model represents a possible engine and is useful as a learning tool, such an engine would perform poorly compared with one having a rotor with rounded flanks. A more realistic model is one in which the triangular rotor is augmented with circular-arc flanks, as shown in Figure 20. The radius of curvature, r , of a flank could vary from infinity, corresponding to a flat flank, to a value for which the arc touches the minor axis of the epitrochoid. Note that the center of curvature of an arc terminated by two f lank apexes depends on the value of r . It can also be seen from Figure 20 that r is related to the angle,θ, subtended by the flank arc by 1/2 r sin(θ/2) = R sin(π/3) = 3 R/2 [ft | m] or 1/2 r/R = 3 /[2sin(θ/2)] [dl] _ _ _ (10) Thus either the included angle, θ, or the radius of curvature, r , may be used to define the degree of flank rounding for a given rotor radius R. [ 5 ] Rotary engines usually have the maximum rounding possible consistent with adequate engineering clearances.[5]

Effect of the Recess Volume The additional capture volume associated with the recess is seen in Figure 21. Its influence on the displacement and compression ratio may be reasoned in the same way as with the segment volume. The recess increases both minimum and maximum mixture volumes by the same amount. It therefore has no effect on displacement and it decreases the compression ratio. Figure 22 shows the influence of flank rounding and recession on clearance and compression ratio. While flank recession reduces the compression ratio for given values of θ and e/R, it improves the shape of the long, narrow combustion pocket forming the minimum capture volume. Rotary engines usually have more than one spark plug, to help overcome the combustion problems associated with this elongated shape.[5] 28

(3) Stratified-Charge Rotary Engine

Fig.23 After top center pilot, dual injector configuration

Fig.24 Stratified-Charge Rotary Engine performance The design and performance of stratified-charge rotary engines developed for commercial aviation propulsion and APU (auxiliary power unit) application as well as for marine, industrial, and military requirements. Figure 23 shows a direct fuel injection configuration that has performed well under a wide range of speed, load, and environmental conditions and with a variety of liquid fuels. The reference reports a lack of octane and cetane sensitivities, so that diesel, gasoline, and jet fuel can all be used with this configuration. As air in the rotor recess passes below, the spark plug ignites a locally rich pilot stream that in turn ignites the fuel from the main injector. The net fuel-air ratio is lean, resulting in improved fuel economy over normal carburetion. Figure 24 presents data for full-load brake horsepower and specific fuel consumption obtained with Jet-A fuel for the twin-rotor 2034R engine. The maximum takeoff power at 5800 rpm was 430 horsepower, with a brake specific fuel consumption (BSFC) of 0.44 lbm/BHP-hr. Throughout a range of loads and altitude conditions the engine operates with a fuel- air ratio between 0.035 and 0.037, well below the stoichiometric value. The r eference reports a best 29

thermal efficiency of 35.8% (BSFC = 0.387 lbm/BHP-hr) at 3500 rpm and 225-horsepower output.[5]



Research Paper:

DESIGN AND EXPERIMENTAL RESULTS OF SMALL-SCALE ROTARY ENGINES ABSTRACT A research project is currently underway to develop small-scale internal combustion engines fueled by liquid hydrocarbons. The ultimate goal of the MEMS Rotary Internal Combustion Engine Project is to develop a liquid hydrocarbon fueled MEMS-size rotary internal combustion micro-engine capable of delivering power on the order of milli-watts. This research is part of a larger effort to develop a portable, autonomous power generation system with an order of  magnitude improvement in energy density over alkaline or lithium-i on batteries. The rotary (Wankel-type) engine is well suited for the fabrication techniques developed in the integrated chip (IC) community and refined by the Micro Electro Mechanical Systems (MEMS) field. Features of the rotary engine that lend itself to MEMS fabrication are its planar construction, high specific power, and self-valving operation. The project aims at developing a "micro-rotary" engine with an epitrochoidal-shaped housing under 1 mm3 in size and with a rotor swept volume of 0.08 mm 3. To investigate engine behavior and design issues, larger-scale "minirotary" engines have been fabricated from steel. Mini-rotary engine chambers are approximately 1000 mm 3 to 1700 mm 3 in size and their displacements range from 78 mm 3 to 348 mm 3.[7]

A test bench for the mini-rotary engine has been developed and experiments have been conducted with gaseous-fueled mini-rotary engines to examine the effects of sealing, ignition, design, and thermal management on efficiency. Preliminary testing has shown net power output of up to 2.7 W at 9300 RPM. Testing has been performed using hydrogen-air mixtures and a range of spark and glow plug designs as the ignition source. Iterative design and testing of  the mini engine has lead to improved sealing designs. These particular designs are such that they can be incorporated into the fabrication of the micro-engine.[7]

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Design and fabrication of a first generation meso-scale rotary engine has been completed using a SiC molding process developed at Case Western Reserve University. The fabrication of the micro-rotary engine is being conducted in U.C. Berkeley's Micro f abrication Laboratory.[7] Testing of the mini-engine has lead to the conclusion that there are no fundamental phenomena that would prevent the operation of the micro-engine. However, heat loss and sealing issues are key for efficient operation of the micro-engine, and they must be taken into account in the design and fabrication of the micro-rotary engine. The mini-rotary engine design, testing, results and applications will be discussed in this paper.[7] NOMENCLATURE Biot Number Bi e Engine Eccentricity Convection Coefficient h k Ratio of Specific Heats ksolid Conduction Coefficient l Length nE Output Shaft Speed rv Compression Ratio rc CutoffRatio R Rotor Generating Radius Tres Residence Time Tehem Chemical Time Tm Temperature (Melting) U Velocity Vs Swept Volume (Displacement) W Engine Width Eth Theoretical Compression Ratio

INTRODUCTION

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Figure 25. Comparison of energy densities between internal combustion engines and primary batteries. MicroElectroMechanical Systems (MEMS) devices are mechanical elements constructed using the manufacturing techniques and materials used in the integrated circuit / microchip industry [1,2]. Typically, MEMS fabrication uses lithography to mask the silicon substrate, followed by etching and deposition to create the high aspect ratio features in the substrates. The combination of small scale and potentially inexpensive mass production is the Intake primary attraction of the technology. [7]

Early products in the MEMS field (accelerometers and gyroscopes) have given way to other, more complex, engineering systems (head-mounted displays, optical communication systems, and micro-fluidic systems). As new manufacturing techniques and materials have been developed, MEMS-scale thermal devices are now practical [3]. These micro scale engines have numerous applications, because the power output can be electrical or mechanical (torque). Applications range from mobile electrical power supplies (potential battery replacements) to mechanical power.[7] The ultimate goal of the MEMS Rotary Internal Combustion Engine Project is to fabricate a micro-rotary 'Wankel' engine, which produces 10-100 mW of mechanical power. The engine 3 chamber will fit in a cubic millimeter, with a swept volume of 0.08 mm . A power generation system using the micro-rotary engine would have 5 times to 14 times the energy density of a primary battery [4] (see Fig. 25). The MEMS scale "micro-rotary" engine will be constructed primarily of Silicon (Si), Silicon Carbide (SIC), and Silicon Dioxide (SiO2). All parts subjected to high temperatures and stresses will be built either using molded SiC or a Si substrate with a thin SiC coating [7] The research program is planned in progressive steps leading to the fabrication of a MEMS3 scale engine. A series of larger scale "mini-rotary" engines, with swept volumes of 78 mm to 32

3

348 mm , have been constructed to investigate design and combustion issues as the engine scale is reduced. In addition, fabrication of the micro rotary engine has begun. This paper describes the challenges of building a small-scale rotary engine from testing, design, and MEMS fabrication viewpoints and suggests solutions to these challenges.[7]

ROTARY ENGINE LAYOUT AND DESIGN

Fig. 26 Rotary Engine Operation A rotary engine was selected for development as the basis of a MEMS-scale power generation system due to several factors: - the planar design of the rotary engine lends itself to MEMS fabrication - the rotor controls the timing of the intake and exhaust, eliminating the need for the complex valve actuating and valve timing systems found in 4-stroke reciprocating engines - the power output is in the form of rotary motion of the shaft, which is necessary for powering either a micro-vehicle or an electric generator[7]

The rotary engine consists of a triangular rotor rotating within an epitrochoidal-shaped housing (see Fig. 26). The rotor centerline is offset from the housing centerline by an eccentric, e. As the rotor rotates about the center of the epitrochoid, all three apexes of the rotor remain in contact with the walls, forming three sealed chambers. In large-scale systems, an elaborate sealing system is used to prevent leakage: 1) over the face of the rotor (face seals) and 2) around the rotor ends (apex seals). The seals incorporated into the present work will be addressed later in this paper.[7] The rotary engine operates on a 4-stroke cycle, as illustrated in Figure 26. As the rotor apex passes over the intake port, the increasing volume in the chamber draws in a fresh fuel / air charge (intake). The rotor continues to rotate, and the next apex closes the intake port and compresses the fuel / air mixture (compression). The fuel / air mixture is ignited using a spark plug, glow plug, or compression ignition. The resulting pressure ri se acts off the rotor axis due to the eccentricity, e, leading to a torque from which the shaft work is extracted (power). After the power stroke, the rotor apex uncovers the exhaust port. As the rotor continues to rotate, the exhaust gases are ejected from the engine by the decreasing volume (exhaust). The rotor apex then uncovers the intake port again, taking in a fresh charge. Since there are three chambers formed by the rotor and epitrochoid, this cycle occurs three times for every 33

revolution of the rotor. The rotor spins at 1/3 the speed of the output shaft, resulting in one power pulse for every revolution of the shaft. References to engine speed in this paper refer to the output shaft rotational speed.[7] The three primary engine parameters are: the rotor generating radius (R), the rotor width (w), and the eccentricity (e). Once these are established, all other engine parameters (displacement, theoretical compression ratio, maximum tip velocity, and tip angle) are also established [7,8]. The compression ratio in a working rotary engine is also never as high as the theoretical compression ratio. All rotary engines have a rotor cutout to allow the combustion reaction to propagate at the onset of the power stroke. The theoretical compression ratio does not take the rotor cutout into account, because the cutouts are different for each engine.[7] 3

3

A series of mini-rotary engines of intermediate size (78 mm to 348 mm ) have been built and used to study combustion, fluid, and design issues as the engine is reduced in size. The design of the mini rotary mimics the proposed design of the micro-rotary. Each engine has different power outputs and builds upon the lessons learned from the previous engine.[7]

COMBUSTION, THERMODYNAMICS, AND SCALING The rotary engine cycle can be described by either the Otto or Diesel cycle, depending on the ignition source (spark or compression ignition, respectively). The efficiency of both the Otto cycle engine and Diesel cycle engine is dependent on the compression ratio. As can the compression ratio is dependent only upon the rotor generating radius (R) and the eccentricity (e) rather than the length scale of the engine. Therefore, decreasing the size of an engine does not affect the theoretical compression ratio or the theoretical thermal efficiency the theoretical thermal efficiency of a spark ignited rotary engine (Otto Cycle) is 69%. For the same compression ratio, and assuming a cutoff ratio of 4:1 (typical for a diesel engine), the theoretical thermal efficiency of a compression ignited rotary engine (Diesel Cycle) is 56%. However, there are other factors that affect the design and efficiency of the engine due to the change from the macro scale to micro scale. Issues include residence time vs. reaction time, increased heat loss, and quenching.[7] Residence time (swept time of rotor through combustion chamber) in a rotary engine is independent of size. Rather, the residence time is rel ated to the operating speed of the engine. The power stroke of the rotary engine occurs over 90 ° of the rotor turn. At 40,000 RPM, the fuel / air mixture has 375 µsec to react before the exhaust port is uncovered. At normal combustion temperatures, hydrocarbons have a characteristic reaction time of 10 µsec Assuming tccs/tchem =10 for complete reaction, the maximum reaction time limited engine speed for a rotary engine is 150,000 RPM. Since the micro-rotary engine is designed to operate at an engine speed of 40,000 RPM, the reaction time is not a factor, if the temperature is kept elevated. As the length scale of the engine decreases, the surface area to volume ratio increases . [7] 34

The reduced temperature gradient between the hot (combustion / exhaust) and cold (intake) sides of the engine will reduce the effect of thermal expansion of the housing. An isothermal housing implies that measures must be taken to control the amount of heat added to the incoming fuel / air mixture. While pre-heating the intake fuel / air mixture will aid in obtaining combustion in the microscale, care must be taken to avoid autoignition in the inlet port. A further concern is that the heat added to the intake mixture will reduce the thermal efficiency due to the reduced TH/T c ratio of the cycle.[7] Flame quenching occurs due to cooling of the reaction, as well as radical destruction at the walls. The minimum distance in the micro-rotary engine occurs at the point of maximum compression, when the fuel/air mixture is compressed between the rotor cutout and the side of the epitrochoid. The smallest linear distance of the micro-rotary engine is well below the quenching distance for any fuel. It has been determined that a flame can be stabilized in tubes well below the quenching diameter with an external heat supply to heat the tube wall. In a supporting experiment, this strategy has been successfully employed using only the exhaust gas to heat the tube wall. Based on these findings, the obvious solution to the quenching problem is to increase the temperature of the engine wall.[7] The use of a SiC housing helps with this solution since there are no material constraints (SiC Tm= 31000(2). In fact, quenching can be totally eradicated if the wall temperature can be maintained at the adiabatic temperature of the fuel. If thermal management is not sufficient to support gas phase combustion, the combustion reactions can be enhanced through catalytic surface reactions. It should be noted that for catalytic reactions, the increasing surface-area-tovolume ratio leads to overall higher conversion efficiency. For this purpose, the housing and rotor walls can be coated with a catalyst (e.g., Platinum). From the point of view of combustion efficiency, the critical factor in MEMS combustion systems is the increased heat loss at small length scales. Heat loss decreases combustion temperatures, which leads to reduced reaction rate and quenching. Several strategies are being reviewed to reduce heat loss:[7] - Exhaust Gas Re circulation (EGR): A strategy that re circulates the combustion gases around the outside of the combustion chamber to decrease heat loss from the combustion chamber and pre-heat the incoming fuel-air mixture. The effectiveness of this strategy has been demonstrated - Stacking the engines to reduce the heat losses from the center engine(s). The center engine(s) will operate at nearly adiabatic conditions due to insulation and the higher me lting temperatures of SiC - An insulating channel filled with low conduction material, such as SIO2, surrounding the engine A significant problem facing large-scale rotary engines is leakage, either past the rotor tips or over the rotor face. Leakage reduces the engine efficiency (by reducing the compression ratio) and increases the effect of incomplete combustion. Sealing mechanisms have been created to reduce this problem in large-scale engines. These sealing systems consist of spring35

loaded tabs at the rotor apexes and across the rotor face. The tabs maintain contact between the rotor and epitrochoid walls and sealing for the chambers. Apex seals have been fabricated in the mini-engine, shown to be effective, and designed for the micro-rotary engine. A face seal system has not yet been developed that is compatible with MEMS fabrication. However, it should be noted that the smallest commercially available rotary engine (the 5000 mm 30. S. Graupner) does not use a face seal.[7]

MINI-ROTARY ENGINE TEST BENCH At the scale of the mini-rotary engine, there are no commercially available diagnostic engine test stands. Therefore, a test bench (Fig. 27). diagnostic engine test stands. Therefore, a test bench (Fig. 3) was designed and fabricated to test the mini-engine operation. The test bench consists of an electric motor / dynamometer, optical tachometer, ignition system (for spark plug use), and flywheel. A magnet on the flywheel is used to trigger the spark. Ignition and spark timing is achieved with a Hall Effect sensor mounted on a rotary dial and a spark ignition system manufactured by CH Electronics, Inc. Engine speed is measured using a Monarch Instruments ACT-3 tachometer with a ROS-5W remote optical sensor. The mini-engine is rigidly coupled to the dynamometer via a steel shaft.

Fig. 27 Mini-Rotary Engine Test Bench To test combustion in the mini-rotary engine, a pre-mixedhydrogen-air mixture is used rather than a liquid hydrocarbon, given hydrogen's ease of ignition. A gaseous fuel mixture is advantageous for this study because it does not require the complexity of a fuel carburetion system. The gaseous fuel and air are mixed upstream of the engine in a T-junction. The fuel is metered using valves and rotameters upstream of the T-junction.[7] To measure engine power, a dynamometer has been developed using a Maxon TM brushless electronically commutated motor and a rectifier circuit. Power generated by the mini-rotary engine spins the dynamometer, which acts as a generator and produces electrical power. The rectifier circuit converts the dynamometer's three-phase output to a DC voltage potential. Rheostats are used to apply a load to the dynamometer. The rheostats can be adjusted to produce the appropriate load, based on the mini-rotary engine being tested.[7]

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To calibrate the voltage across the dynamometer load to mechanical power output, the dynamometer is driven by an electric motor while connected to a torque arm and load cell. Figure 28 shows the calibration curve for the Maxon 50 W brushless DC motor.

Fig. 28 Dynamometer calibration chart for 50 W Maxon brushless electric motor. Voltage drops across the rectifier circuit and across the engine coils were taken into account. During engine operation, the output voltage of the motor is measured and related to the power output through the calibration curve.[7] Temperature measurements were also made upstream of the engine and at the housing near the combustion chamber using thermocouple or infrared imaging. Pressure measurements upstream of the engine were also made to accurately measure fuel and air flow rates.

MN30 MINI-ROTARY ENGINE DESIGN AND TESTING The MN30 generation mini-rotary engine is shown in Figure 29. The basic engine is simple in design, consisting of 7 parts: front plate, epitrochoid housing, back plate, rotor, internal gear, spur gear, and shaft. Two bearings, mounted in the front and back plate, position the shaft. For speed of fabrication, the mini-rotary engine is made from steel. The most accurate means of  manufacture at this scale is through electrodischarge machining (EDM). Note that the initial design of the MN30 engine does not include apex or face seals.[7]

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Fig.29 MN30 Mini-Rotary Engine

Fig.30 Modified MN30 Rotor with Apex Seals The MN30 mini-rotary engine has a displacement of 78 mm 3. Preliminary sealing tests determined that the engine suffered from 20% leakage during operation due to poor tolerances during manufacturing. From the leakage tests, it was apparent that an apex sealing system was necessary. In order to improve sealing, slots were cut in the rotor apexes and apex seals inserted. The springs consist of brass tabs with leaf springs formed from spring steel (see Fig. 30). Even these simple designs significantly improved leakage. [7] Combustion tests were performed on the MN30 mini-engine with apex seals. In these tests, an electric motor was used to rotate the engine, while a stoichio metric H2-air or H2-C3H8-air mixture was supplied. The ignition system used was a spark plug. Ignition timing was changed from 15 degrees BTDC to 15 degrees ATDC. While the engine generated no net power output, there was a 50% reduction in the power required by the electric motor to turn the mini-engine with combustion. Exhaust temperatures also changed with spark timing, with peak exhaust temperatures at 7.5 degrees BTDC.[7]

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MEMS-ROTARY ENGINE FABRICATION Design and fabrication of the MEMS-scale rotary engine (microrotary engine) has begun. A larger scale "meso-rotary" engine was fabricated at the Case Western Reserve University (CWRU) MEMS Research Center using Si and SiC. The CWRU micro-engine has a displacement of  1.2 mm 3 and a rotor diameter of 3 mm, compared to the ultimate goal of 0.08 mm 3 displacement and 1 mm rotor diameter. The larger engine was fabricated to test the limits of  the CWRU SiC fabrication process. The rotor is made of molded SiC from a three-wafer Si mold (see Fig. 31). The Si housing is fabricated from three separate 500 mm wafers deep reactive ion etched (DRIE) to form the features and then fusion bonded together (see Fig. 32).[7]

Fig.31 CWRU Micro-Rotary Engine Left: Three.wafer Si mold Right: Molded SiC rotor

Fig.32 CWRU Micro-Rotary Engine Housing The CWRU micro-engine highlights some of the difficulties in fabricating an engine using MEMS techniques. Even though the tolerance obtained in MEMS is typically on the order of microns, the percentage tolerance when compared to the length scale is high. For a high precision 39

system such as the rotary engine, the tolerance required is on the order of 0.1% (