auto

Basic of Engine Operating Characteristics

2103471 Internal Combustion Engine

Background on IC Engines • “An internal combustion is defined as a heat engine in which the chemical energy of the fuel is released inside the engine and converted directly into mechanical work on a rotating output shaft, as opposed to an external combustion engine in which a separate combustor is used to burn the fuel.”

Background on IC Engines Internal combustion engines are so called because the heat required to drive them is released by oxidizing a fuel inside the engine itself. This approach has advantages and disadvantages, but is still the most popular for transport and small power generation plant. We will be looking at some common types of engine, examining some ways of analysing their performance parameters, and some of the problems encountered in improving efficiency and output.

Background on IC Engines Internal combustion engines include systems which function like "closed" systems (e.g. petrol engines) or as "open" systems (e.g. gas turbines).

• • • •

All the engines we will examine contain the same basic activities: invest some work to compress a working fluid, inject heat into the fluid, recover a greater amount of work, return to initial conditions by removal of some heat.

Typical Processes for an Internal Combustion Engine

Background on the Otto Cycle •

The Otto Cycle has four basic steps or strokes: – F-A : An intake stroke that draws a combustible mixture of fuel and air into the cylinder – A-B : A compression stroke with the valves closed which raises the temperature of the mixture. A spark ignites the mixture towards the end of this stroke. – C-D : An expansion or power stroke. Resulting from combustion. – E-F : An Exhaust stroke the pushes the burned contents out of the cylinder.

Figure idealized representation of the Otto cycle on a PV diagram.

Otto (SI Engine) Operating Cycle Spark plug for SI engine Fuel injector for CI engine Valves

Top Center (TC) Stroke Bottom Center (BC)

TC 0o

Crank shaft

θ 270o

90o

180o BC

Clearance volume Cylinder wall

Piston

Pressure-Volume digram of a 4-stroke SI engine One power stroke for every two crank shaft revolutions

Pressure

Spark Exhaust valve opens

Exhaust valve closes

Intake valve closes

1 atm

TC

BC Cylinder volume

Engine Geometric Parameters

VC

TC B

For an engine with bore B; crank offset a, stroke length L, turning at an engine speed of N:

s = 2a

L BC

An average piston speed is:

U p = 2 LN l

s

θ a

Average piston speed for all engines will normally be in the range of 5 to 15 m/sec with large diesel engines on the low end and highperformance automobile engines on the high Compression ratio: end.

Engine Geometric Parameters

VC

TC B

The distance s between crank axis and wrist pin axis is given by:

(

s = a cosθ + l 2 − a 2 sin 2 θ

L BC

)

1/ 2

Cylinder volume when piston at TC (s=l+a) defined as the clearance volume Vc The cylinder volume at any crank angle is:

l

s

V = Vc +

πB 2 4

(l + a − s )

Maximum displacement, or swept, volume:

Vd =

θ a

πB 2 4

L

Compression ratio:

rc =

VBC Vc + Vd = VTC Vc

Engine Geometric Parameters Cylinder volume at any crank angle can also be written in a non-dimensional form as: VC

TC B L

The cross-sectional area of a cylinder and the surface area of a flat-topped piston are given by: BC

l

s

2 ⎤ ⎡l V 1 l⎞ ⎛ = 1 + (rc − 1)⎢ + 1 − cos θ − ⎜ ⎟ − sin 2 θ ⎥ Vc 2 ⎥ ⎢a ⎝a⎠ ⎦ ⎣

Ap =

πB 2 4

The combustion chamber surface area is:

A = Ach + Ap + πB(l + a − s ) θ a

The combustion chamber surface area at any crank angle is: 2 ⎤ ⎛ πBS ⎞ ⎡⎢ l ⎛l⎞ 2 ⎥ A = Ach + Ap + ⎜ 1 cos θ sin θ + − − − ⎟ ⎜ ⎟ 2 a a ⎥ ⎝ ⎠ ⎢⎣ ⎝ ⎠ ⎦

For most engines B ~ L (square engine)

Geometric Properties

VC

TC

(

s = a cosθ + l 2 − a 2 sin 2 θ

)

1/ 2

B

Average and instantaneous piston velocity are: L

U p = 2 LN BC

l

s

Up =

ds dt

Where N is the rotational speed of the crank shaft in units revolutions per second

⎡ cosθ = sin θ ⎢1 + Up 2 ⎢⎣ (l / a )2 − sin 2 θ Up

θ a

π

(

⎤ ⎥ 1/ 2 ⎥⎦

)

Average piston speed for standard high performance auto engine is about 15 m/s. Ultimately limited by material strength. Therefore engines with large strokes run at lower speeds those with small strokes run at higher speeds.

Piston Velocity vs Crank Angle R is the ratio of connecting rod length to crank offset and usually has values of 3 to 4 for small engines, increasing to 5 to 10 for the largest engine. The effect of R on piston speed are shown below.

R = l/a

Engine Torque and Power Output Torque is measured off the output shaft using a dynamometer. b Force F

Stator Rotor N

Load cell

The torque exerted by the engine is T: T = F ⋅b

units : Nm = J

Engine Torque and Power Output Torque is measured off the output shaft using a dynamometer. b Force F

Stator Rotor N

Load cell

The torque exerted by the engine is T: T = F ⋅b

units : J

The power W& delivered by the engine turning at a speed N and absorbed by the dynamometer is: W& = ω ⋅ T = (2π ⋅ N ) ⋅ T

⎛ rad ⎞⎛ rev ⎞ units : ⎜ ⎟( J ) = Watt ⎟⎜ ⎝ rev ⎠⎝ s ⎠

Note: ω is the shaft angular velocity in units rad/s

Engine Torque and Power Output

Torque is a measure of an engine’s ability to do work and power is the rate at which work is done The term brake power, W&b , is used to specify that the power is measured at the output shaft, this is the usable power delivered by the engine to the load. The brake power is less than the power generated by the gas in the cylinders due to mechanical friction and parasitic loads (oil pump, air conditioner compressor, etc… The power produced in the cylinder is termed the indicated power,W&i .

Indicated Work per Cycle Given the cylinder pressure data over the operating cycle of the engine one can calculate the work done by the gas on the piston. This data is typically given as P vs V diagram. The indicated work per cycle is given by Wi = ∫ PdV

WA > 0

WB < 0

Compression W0

Exhaust W0

Work per Cycle Gross indicated work per cycle – net work delivered to the piston over the compression and expansion strokes only: Wi,g =area A + area C (>0)

Pump work – net work delivered to the gas over the intake and exhaust strokes: Wp =area B + area C (