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

• Otto Cycle

• Diesel Cycle

• Real Air-Fuel Cycle

• Spark Ignition Cycle

• Exhaust Process

• Dual Cycle

• Miller Cycle

AIR STANDARD CYCLE

Types of Cycles

REAL AIR-FUEL CYCLE

• The actual cycle experienced by an IC engine is not, in the true

sense, a thermodynamic cycle.

• An ideal air-standard thermodynamic cycle occurs on a closed

system of constant composition This is not what actually happens

in an IC engine,

• For this reason air-standard analysis gives, at best, only

approximations to actual conditions and outputs.

• Major differences include:

Real engines operate on an open cycle with changing

composition.

Air-standard analysis treats the fluid flow through the entire

engine as air and approximates air as an ideal gas.

There are heat losses during the cycle of a real engine that are

neglected in air-standard analysis.

• Major differences include:

Combustion requires a short but finite time to occur, and heat

addition is not instantaneous at TDC, as approximated in an

Otto cycle.

The blowdown process requires a finite real time and a finite

cycle time, and does not occur at constant volume as in air-

standard analysis.

In an actual engine, the intake valve is not closed until after

BDC at the end of the intake stroke.

Engine valves require a finite time to actuate.

Some error is introduced when the lower heating value of the

fuel (QLHV) is used as the energy input to the cycle during

combustion in air-standard analysis.

REAL AIR-FUEL CYCLE

REAL AIR-FUEL CYCLE

• Due to the differences between real air-fuel cycle and ideal cycle,

results from air-standard analysis will:

have errors

deviate from actual conditions

• Indicated thermal efficiency of a real 4-stroke SI engine is always less

than that predicted by air-standard Otto cycle analysis.

• This is due to the heat loss, friction, ignition timing, finite time of

combustion and blowdown and deviation from ideal gas behavior of

the real engine.

• Indicated thermal efficiency of an actual 4-stroke cycle engine can be

approximated by:

SI ENGINE CYCLE at Part Throttle

• When 4-stroke engine is run at less than WOT conditions, air-fuel

input is reduced by partially closing the throttle in the intake system.

• Less than WOT = Part Throttle or Part Load.

• This condition creates a flow restriction and consequent pressure

drop in the incoming air air & fuel input are then reduced.

Real cycle (WOT) Otto cycle (WOT)

• Lower pressure in intake manifold

during the intake stroke and

resulting lower pressure in the

cylinder at the start of compression

stroke (for NA engine).

• Indicated work for Otto cycle:

Part throttle < WOT

Upper loop compression

& power stroke positive

work output.

Lower loop intake &

exhaust stroke negative

work.

SI ENGINE CYCLE at Part Throttle

4-stroke air-standard otto cycle for

SI engine at part load condition.

(Naturally Aspirated)

• The smaller the throttle angle, the

lower the intake pressure during

intake stroke resulting greater

negative pump work.

• Two factors contribute to the

reduced net work at part load:

Lower pressure at start of

compression (point1) results

in lower pressures

throughout the rest of the

cycle.

Less air is ingested into the

cylinder during intake

stroke, thus fuel input is also

proportionally reduced.

SI ENGINE CYCLE at Part Throttle

4-stroke air-standard otto cycle for

SI engine at part load condition.

(Naturally Aspirated)

• For supercharged and

turbocharged engines, the intake

pressure is higher than atmospheric

pressure.

• More air and fuel in combustion

chamber during the cycle, thus

increase the net indicated work.

• Higher intake pressure increases all

pressure throughout the cycle.

• When air is compressed, the

temperature is also increased due to

compressive heating.

• This can cause self-ignition and

knocking problems during

combustion.

SI ENGINE CYCLE at Part Throttle

4-stroke air-standard otto cycle for

SI engine at part load condition

(Supercharger or Turbocharger).

• Exhaust process consist of two steps:

Blowdown

Exhaust Stroke

• When exhaust valve opens near the end of expansion stroke, high

temperature gases are suddenly subjected to a pressure decrease

due to the blowdown occurs.

• Large percentage of the high temperature gases leaves the

combustion chamber during the blowdown process, driven by the

pressure differential across the open exhaust valve.

• When the pressure across the exhaust valve is equalised, the

cylinder is still filled with exhaust gases at the exhaust manifold

pressure of about 1 bar.

• These gases are then pushed out of the cylinder through opened

exhaust valve by the piston as it travels from BDC to TDC during

exhaust stroke.

EXHAUST PROCESS

• To have the best of both worlds, an engine ideally could be CI but

would operate on the Otto cycle.

• CI would operate on more efficient higher rc, while constant-volume

combustion of the Otto cycle would give higher efficiency for a given

rc.

• The modern high speed CI engine accomplishes this by simple

operating change from early Diesel engine.

• In early Diesel engines fuel is injected at late of compression

stroke near TDC.

• Modern CI engines start to inject the fuel much early in the

compression cycle ~ 200 BTDC.

DUAL CYCLE

• The first fuel then ignites late in compression stroke.

• Some of the combustion occurs almost at constant-volume at TDC,

much like Otto cycle.

DUAL CYCLE

Indicator diagram of a modern 4-stroke

CI engine

• Peak pressure still remains high

into the expansion stroke due to

finite time required to inject the

fuel.

• Last of the fuel is still being

injected at TDC, and combustion

of this fuel keeps the pressure

high into the expansion stroke.

• This diagram is a cross between

an SI engine cycle and the early

CI cycles.

• The air-standard cycle used to analysed this modern CI engine cycle

is called a Dual Cycle or sometimes a Limited Pressure Cycle.

• Dual cycle heat input process of combustion can best be

approximated by a dual process of constant volume and followed by

constant pressure.

DUAL CYCLE

• It also can be considered as

modified Otto cycle with a

limited upper pressure.

• The thermodynamic analysis

of an air-standard Dual cycle

is the same as that of the

Diesel cycle, except for the

heat input process

(combustion) 2-x-3.

DUAL CYCLE

Pressure Ratio is defined as the rise in

pressure during combustion:

DUAL CYCLE

DUAL CYCLE

OTHER CYCLE

Atkinson Cycle ?

Miller Cycle ?

* Find out and understand above cycles.

• The thermal efficiency for each cycle can be determined by:

COMPARISON OF

OTTO, DIESEL & DUAL CYCLES

• These three cycles are not operate on the same rc.

• CI engines that operate on the Dual cycle or Diesel cycle have much

higher rc than SI engines operating on Otto cycle.

• More realistic way to compare these three cycles would be to have

the same peak pressure an actual design limitation in engines.

• It is found that: