INTERNAL COMBUSTION ENGINE

(SKMV 3413)

Dr. Mohd Farid bin Muhamad Said

Room : Block P21, Level 1, Automotive

Development Centre (ADC)

Tel : 07-5535449

Email: mfarid@fkm.utm.my

• Actual Cycles:

– Variable Composition (combustion) with

Gas Mixtures (CO2, H2O, N2)

– Open Cycle

• Air-Standard Cycle:

– Simplified and more manageable

– Determines Important Design Parameters

ENGINE CYCLE

• Working Fluid is air for the entire cycle.

• Most of the gas in cylinder is air. (about 7% fuel vapor).

• Closed Cycle the gases being exhausted are fed back

into the intake system.

• The combustion process is replaced with a heat addition

term Qin of equal energy value. No combustion.

• The open exhaust process is replaced with a closed

system heat rejection process Qout of equal energy value.

AIR STANDARD CYCLE

Assumptions

• Ideal Processes

– Constant pressure exhaust at 1 atms.

– Normally aspirated cycles have constant pressure intake at 1 atms

– Turbo/Supercharged cycles have constant pressure > 1 atms.

– Compression and Expansion are isentropic processes (reversible and adiabatic)

– With constant specific heats

AIR STANDARD CYCLE

Assumptions

• Heating is at constant volume (SI) or constant

pressure (CI)

• Cooling Heat Rejection at constant volume

• All processes are reversible

AIR STANDARD CYCLE

Assumptions

• It is considered as an ideal gas

dTcdh

RTP

mRTPV

RTPv

p

AIR STANDARD CYCLE

Applicable Equations

where,

AIR STANDARD CYCLE

Applicable Equations

• Air flow before it enters an

engine is usually closer to

standard temperature.

KkgkJvcpcR

vcpc

k

KkgkJvc

KkgkJpc

./287.0

35.1

./821.0

./108.1

• For analyses within engine

during operating cycle and

exhaust flow.

AIR STANDARD CYCLE

Applicable Equations

• Otto Cycle

• Diesel Cycle

• Real Air-Fuel Cycle

• Spark Ignition Cycle

• Exhaust Process

• Dual Cycle

• Miller Cycle

AIR STANDARD CYCLE

Types of Cycles

• 1-2: Isentropic Compression

• 2-3: Constant Volume Heat Addition

• 3-4: Isentropic Expansion

• 4-5: Constant Volume Heat Rejection

• 5-6: Exhaust at Ambient Pressure

• 6-1: Intake at 1 atms

– Higher than 1 atms if super/turbo-charged

OTTO CYCLE

Real Otto

OTTO CYCLE

P-v T-s

OTTO CYCLE

• Applying Applicable Thermodynamic Equations

at WOT

6-1: Intake

1-2: Compression

2-3: Constant Volume Heat Addition

3-4: Power Stroke (Expansion)

4-5: Constant Volume Heat Rejection

5-6: Exhaust

OTTO CYCLE

OTTO CYCLE

OTTO CYCLE

OTTO CYCLE

OTTO CYCLE

OTTO CYCLE

OTTO CYCLE

• Only cycle temperatures need to be known.

• The equation can be simplified further by applying

ideal gas relationship.

OTTO CYCLE

Thermal Efficiency

Ideal gas

relationship

OTTO CYCLE

Thermal Efficiency

• Only compression ratio is

needed to determine the

thermal efficiency of the Otto

Cycle at WOT.

• As the compression ratio

goes up, the thermal

efficiency will increase.

• This is the Indicated Thermal

Efficiency.

OTTO CYCLE

Thermal Efficiency

Example 1

A 4 cylinders, 2.5 liter, SI engine operates at WOT on a 4-stroke air-

standard Otto cycle at 3000 rpm. The engine has a compression

ratio of 8.6 : 1, a mechanical efficiency of 86%, and a stroke-to-bore

ratio S/B = 1.025. Fuel is isooctane with AF = 15, a heating value of

44,300 kJ/kg, and combustion efficiency = 100%. At the start of the

compression stroke, conditions in the cylinder combustion chamber

are 100 kPa and 600C. It can be assumed that there is a 4% exhaust

residual left over from the previous cycle.

• Do a complete thermodynamic analysis of

this engine.