3 Actual Cycles

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ACTUAL CYCLE

Actual engine cycle

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Introduction

Ideal Gas Cycle (Air Standard Cycle)

Idealized processes

Idealize working Fluid

Fuel-Air Cycle

Idealized Processes

Accurate Working Fluid Model

Actual Engine Cycle

Accurate Models of Processes

Accurate Working Fluid Model

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Introduction

Air-Standard Cycle Analysis gives an estimate of engine

performance which is much greater than the actual

performance,For Example for SI

Air-Standard

Cycle

Actual Engine

Cycle

Compressionratio

7:1 7:1

Thermal

Efficiency

55 % 28%

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Introduction

The actual cycles for IC engines differ from the fuel-air cycles and air- standard

cycles in many respects.

Theactual cycle efficiencyismuch lowerthan theair-standard efficiencydue to

various lossesoccurring in the actual engine operation.

The major losses are due to:

Variation of specific heats with temperature

Dissociation of the combustion products

Progressive combustion

Incomplete combustion of fuel

Heat transfer into the walls of the combustion chamber

Blowdown at the end of the exhaust process

Gas exchange process

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Introduction

Air Cycle

Corrected for the

Characteristics of the Fuel-Air

Composition of Cy. GasesVariable sp.heat, Dissociation etc..

Fuel-Air Cycle

modified to account

for Combustion loss,

Time loss, Heat loss

Blowdown loss, etcActual Cycle

Actual work loses

Less the friction losses

gives

Useful work

Theoretical Cycle

I II

III

IV

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Comparison Of Fuel-Air Cycle And

Actual Cycles

v. The progressive combustion rather than the instantaneous

combustion.

vi. The heat transfer to and from the working medium

vii. The substantial exhaust blowdown loss, i.e., loss of work on the

expansion stroke due to early opening of the exhaust valve.

viii. Gas leakage, fluid fiction etc., in actual engines.

Points (i) to (iv), are similar to fuel-air cycles

Points (v) to (viii) are the difference between fuel-air cycles

and actual cycles.

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The Major Loss of Actual Cycle

Time loss factor

Loss due to time required for mixing of fuel and air and also

for combustion.

Heat loss factor

Loss of heat from gases to cylinder walls.

Exhaust blowdown factor

Loss of work on the expansion stroke due to early opening

of the exhaust valve.

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Time Loss Factor

In air-standard cyclesthe heat addition is an instantaneous

processwhereasin an actual cycle it is over a definite period

of time.

The crankshaft will usually turn about 30 to 400 b/n the

initiation of the spark and the end of combustion (time loss

due to progressive combustion)

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Time Loss Factor

Due to thefinite time of combustion,

peak pressurewill not occur when the

volume is minimum (TDC)but will occur

some time after TDC The pressure, therefore, rises in the

first part of the working strokefrom b

to cas shown in Fig.

This loss of work reduces the

efficiencyand is calledtime lossdue

toprogressive combustion.

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Time Loss Factor

The time taken for combustion depends upon

Theflame velocitywhich in turn depend up on thetype of

fuel andthe fuel-air ratio

Theshapeandsize of the combustion chamber.

Thedistance from the point of ignitiontothe opposite side of

the combustion space

In order that the peak pressure is not reached too late in the

expansion stroke,the time at which thecombustion startsis varied by

varying thespark timing or spark advance.

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Time Loss Factor

Figure below shows the effect ofspark timing on p-v diagramfrom a typical trial.

With spark at TDC (0o spark advance), the peak pressure is low due to the

expansion of gases.

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Time Loss Factor

If thespark is advancedto achieve complete combustion close to

TDCadditional work is required to compress the burning gasses

35o Spark advance

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Time Loss Factor

With or without spark advance

the work area could be less and

the power output and efficiency

are lowered.

Therefore a moderate or

optimum spark advance (15o-

30o) is the best compromiseresulting in minimum losses on

both the compression and

expansion strokes

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Time Loss Factor

Table shows the engine performance for various ignition timings

(rc=6).

The effect of spark advance on the power output by means of

the p-V diagram

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Time Loss Factor

The effect of spark advance on imep and power loss

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Time Loss Factor

Some times a deliberate spark retarded from

optimum may be necessary in order to

avoid knocking

reduce exhaust

reduce emission of hydrocarbons and carbon

monoxide

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Time Loss Factor

Atfull throttle with the fuel-air ratiocorresponding tomaximum

power and with the optimum ignition advance, the time losses

may account for a drop in efficiency of about

5 percent for actual Engine2 percent fuel-air cycle efficiency

These losses are higher when the

mixture is richer or leanerIgnition advance is not optimum and

at part throttle operations the losses are higher.

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Time Loss Factor

It is impossible to obtain aperfect homogeneous mixturewith

fuel-vapor and air, since,residual gases from the previous are

present in the clearance volumeof the cylinder. further, very

limited timeis available between themixture preparation andignition

Under these circumstances, it is possible that apocket excess

oxygenis present in one part of the cylinder anda pocket of

excess fuel in another part.

Therefore, some fuel does not or burns partially toCOand the

unusedO2appears in the exhaust

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Time Loss Factor

Composition exhaust gases for

various fuel-air ratio

...

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Time Loss Factor

Only about95 % of the energyis released with stoichiometricfuel-air ratios.

Energy released in actual engine is about90% of fuel energy input.

It should be noted that it is necessary to use a lean mixture to

eliminate wastage of fuel,while arich mixtureis required toutilize

all the oxygen.

Slightly leaner mixture would give maximum efficiency but too lean

a mixture will burn slowly increasingthe time lossesor will not burnat all causing total wastage of fuel

In a rich mixture a part of thefuelwill notget the necessary oxygen

and will be completely lost.

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Time Loss Factor

The flame speed in mixtures more than 10% richer is low,

thereby, increasing thetime losses and lowering the efficiency.

Imperfect mixing of fuel and air may give different fuel-air

ratios during suction stroke or certain cylinders in a multi cylinder

engine may get continuously leaner mixtures than others.

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Heat Loss factor

During combustion the heat flowsfrom thecylinder gasesthrough

Cooling water

Lubricating oil

Conduction and convection andradiation

Heat loss during combustion willhave the maximum effect on the

cycle efficiency

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Heat Loss factor

The effect of heat loss during combustionreduce the

maximum temperature and therefore the specific

heats are lower.

Out of various losses heat losses contributearound12 %

For further details, read John B. Heywood, chapter 12 (page 668- 711)

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Blowdown At the end of thepower strokewhen theexhaust valve opensthecylinder pressureis much higher than theexhaust manifold pressure

which is typically at 1 atm (P4

> Pe), so the cylinder gas flows out through the

exhaust valve and the pressure drops to Pe.

Displacement Remaining gas is pushed out of the cylinder by the piston fromBDC moving to TDC.

Exhaust Gas Blowdown

State 6 (TC)

The actual exhaust processconsists of two phases:

i) Blowdown

ii) Displacement

Blowdown Displacement

State 5 (BC)

Pi TiPe

Products

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When to open the exhaust valve?

Thecylinder pressureat the end of expansion stroke is high as 7

bar depending on the compression ratio employed.

If the exhaust valve isopened at BDC, thepiston has to do workagainst high cylinder pressure during the early part of theexhaust

stroke

If theexhaust valve is opened too early, a part of theexpansionstroke is lost

The best compromise is to open the exhaust valve400 to 700 before

BDC thereby reducing the cylinder pressure to halfway (say 3.5

bar) before the exhaust stroke begins

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kk

e

kk

e

P

PT

P

PTT

PP

1

4

4

1

4

5

45

6e565 TTT,P

=

=

====

kk

kk

P

P

P

P

T

T1

4

6

1

4

5

4

5since

=

=

TC BC

DisplacementBlowdown

The residual gas temperature T6 is equal to T5

ke

c

k

c P

P

rP

P

r

f

1

4

1

4

5 11

=

=

=

=

=

===

4

6

5

4

4

6

6

4

6

4

44

66

4

6

1

6

11

1

P

P

T

T

rP

P

T

T

r

v

v

rvV

vV

m

m

m

m

f

cc

c

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Loss due toGas Exchange process(pumping loss)

Thework doneforintake and exhaust strokecancelled

each other

Thepumping lossincreasedat part throttle, because

throttling reduce the suction pressure

Pumping lossalso increase withspeed

Pumping loss affect theVolumetric efficiency when Pi

less than Pe

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dei VPPW )(165 =

dV

WWimep 2143

=

Unthrott led (WOT): Pi = Pe = 1 atm

Throttled: Pi < Pe

Supercharged: Pi > Pe

1

EV closes

IV opens

EV closes

IV opens

EV closes

IV opens6

6

EV opens

IV closes (state1)

EV opens

IV closes

Pumping work

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Volumetric efficiency affected by

The density of fresh charge

The exhaust gas in the clearance volume

The design of intake and exhaust manifold

The timing of intake and exhaust valves

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Volumetric Efficiency

The design of intake and exhaust manifold

The exhaust manifold should be designed to enables the

exhaust products to escape readily,

The intake manifold should be designed so as to bring in

maximum possible fresh charge flowing in to the cylinder

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Volumetric Efficiency

The timing of intake and exhaust valves

Valve timing is theregulation of the points in the cycle at

whichthe valves are set to open and close.

Valves requires afinite period of time to open or close for

smooth operation

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Volumetric efficiency

For high speed

Opening @10o before TDC

Closing @60o after TDC

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Loss due to Running Friction

The losses are due tofrictionbetween

the piston and the cylinder walls

In various bearings Energy spent in operating the auxiliary equipment

(cooling pump, ignition system, fan)

Thepiston ring frictionincreases rapidly withenginespeed.

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Loss @ part and Full load r=838

3 Actual Cycles

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