ACTUAL CYCLE

Actual engine cycle

1

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

2

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

Compression ratio

7:1 7:1

Thermal Efficiency

55 % 28%

3

Introduction

The actual cycles for IC engines differ from the fuel-air cycles and air- standardcycles in many respects.

The actual cycle efficiency is much lower than the air-standard efficiency due tovarious losses occurring 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

4

Introduction

Air CycleCorrected for the

Characteristics of the Fuel-AirComposition of Cy. Gases

Variable sp.heat, Dissociation etc..

Fuel-Air Cycle

modified to account for Combustion loss,Time loss, Heat lossBlowdown loss, etc…Actual Cycle

Actual work losesLess the friction losses

gives

Useful work

Theoretical Cycle

I II

III

IV

5

Comparison Of Air-standard And Actual Cycles

The actual cycles for internal combustion engines differ from

air- standard cycles in many respects

i. The working substance being a mixture of air and fuel vapor or

finely atomized liquid fuel in air combined with the products of

combustion left from the previous cycle.

ii. The change in chemical composition of the working substance.

iii. The variation of specific heats with temperature.

iv. The change in the pressure, temperature and actual amount of

fresh charge because of the residual gases

6

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.

7

The Major Loss of Actual Cycle

Time loss factor

Loss due to time required for mixing of fuel and air and alsofor 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 openingof the exhaust valve.

8

Time Loss Factor

In air-standard cycles the heat addition is an instantaneousprocess whereas in an actual cycle it is over a definite periodof time.

The crankshaft will usually turn about 30 to 400 b/n theinitiation of the spark and the end of combustion (time lossdue to progressive combustion)

9

Time Loss Factor

Due to the finite time of combustion,peak pressure will not occur when thevolume is minimum (TDC) but will occursome time after TDC

The pressure, therefore, rises in thefirst part of the working stroke from bto c as shown in Fig.

This loss of work reduces theefficiency and is called time loss dueto progressive combustion.

10

Time Loss Factor

The time taken for combustion depends upon The flame velocity which in turn depend up on the type of

fuel and the fuel-air ratio

The shape and size of the combustion chamber.

The distance from the point of ignition to the opposite side ofthe combustion space

In order that the peak pressure is not reached too late in theexpansion stroke, the time at which the combustion starts is varied byvarying the spark timing or spark advance.

11

Time Loss Factor

Figure below shows the effect of spark timing on p-v diagram from a typical trial.

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

12

Time Loss Factor

If the spark is advanced to achieve complete combustion close toTDC additional work is required to compress the burning gasses

35o Spark advance

13

Time Loss Factor

With or without spark advancethe work area could be less andthe power output and efficiencyare lowered.

Therefore a moderate oroptimum spark advance (15o-30o) is the best compromiseresulting in minimum losses onboth the compression andexpansion strokes

14

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 ofthe p-V diagram

15

Time Loss Factor

The effect of spark advance on imep and power loss16

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

17

Time Loss Factor

At full throttle with the fuel-air ratio corresponding to maximumpower and with the optimum ignition advance, the time lossesmay account for a drop in efficiency of about

5 percent for actual Engine

2 percent fuel-air cycle efficiency

These losses are higher when the

mixture is richer or leaner

Ignition advance is not optimum and

at part throttle operations the losses are higher.

18

Time Loss Factor

It is impossible to obtain a perfect homogeneous mixture withfuel-vapor and air, since, residual gases from the previous arepresent in the clearance volume of the cylinder. further, verylimited time is available between the mixture preparation andignition

Under these circumstances, it is possible that a pocket excessoxygen is present in one part of the cylinder and a pocket ofexcess fuel in another part.

Therefore, some fuel does not or burns partially to CO and theunused O2 appears in the exhaust

19

Time Loss Factor

Composition exhaust gases forvarious fuel-air ratio

...

20

Time Loss Factor

Only about 95 % of the energy is released with stoichiometric fuel-air ratios.

Energy released in actual engine is about 90% of fuel energy input.

It should be noted that it is necessary to use a lean mixture toeliminate wastage of fuel, while a rich mixture is required to utilizeall the oxygen.

Slightly leaner mixture would give maximum efficiency but too leana mixture will burn slowly increasing the time losses or will not burnat all causing total wastage of fuel

In a rich mixture a part of the fuel will not get the necessary oxygenand will be completely lost.

21

Time Loss Factor

The flame speed in mixtures more than 10% richer is low,thereby, increasing the time losses and lowering the efficiency.

Imperfect mixing of fuel and air may give different fuel-airratios during suction stroke or certain cylinders in a multi cylinder

engine may get continuously leaner mixtures than others.

22

Heat Loss factor

During combustion the heat flowsfrom the cylinder gases through Cooling water Lubricating oil Conduction and convection and

radiation Heat loss during combustion will

have the maximum effect on thecycle efficiency

23

Heat Loss factor

The effect of heat loss during combustion reduce themaximum temperature and therefore the specificheats are lower.

Out of various losses heat losses contribute around12 %

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

24

Blowdown – At the end of the power stroke when the exhaust valve opensthe cylinder pressure is much higher than the exhaust manifold pressurewhich is typically at 1 atm (P4 > Pe), so the cylinder gas flows out through theexhaust 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 process consists of two phases:

i) Blowdown

ii) Displacement

Blowdown Displacement

State 5 (BC)

Pi TiPe

Products

25

Exhaust Gas Blowdown

When to open the exhaust valve?

The cylinder pressure at the end of expansion stroke is high as 7bar depending on the compression ratio employed.

If the exhaust valve is opened at BDC, the piston has to do workagainst high cylinder pressure during the early part of the exhauststroke

If the exhaust valve is opened too early, a part of the expansionstroke is lost

The best compromise is to open the exhaust valve 400 to 700 beforeBDC thereby reducing the cylinder pressure to halfway (say 3.5

bar) before the exhaust stroke begins

26

Exhaust Gas Blowdown

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DisplacementBlowdown

The residual gas temperature T6 is equal to T5

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27

Exhaust Gas Blowdown

Loss due to Gas Exchange process (pumping loss) The work done for intake and exhaust stroke cancelled

each other

The pumping loss increased at part throttle, becausethrottling reduce the suction pressure

Pumping loss also increase with speed

Pumping loss affect the Volumetric efficiency when Pi

less than Pe

28

Exhaust Gas Blowdown

dei VPPW )(165 −=−−

dVWWimep 2143 −− −

=

Unthrottled (WOT): Pi = Pe = 1 atm

Throttled: Pi < Pe

Supercharged: Pi > Pe

1

EV closesIV opens

EV closes

IV opens

EV closes

IV opens6’

6’

EV opens

IV closes (state1)

EV opens

IV closesPumping work

29

Exhaust Gas Blowdown

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

30

Volumetric Efficiency

The density of fresh charge

As the fresh charge arrives in the hot cylinder, heat is transferred to itfrom

The hot chamber walls

The hot residual gases

Temperature rise reduces the density , which decrease the mass offresh charge admitted and a reduction in volumetric efficiency

The volumetric efficiency increased by

Low temperature

High pressure of fresh charge

31

Volumetric Efficiency

Exhaust gas in the clearance volume

The residual gas occupy a portion of piston displacement

volume, thus reducing the space available to the incoming

charge.

These exhaust products tend to rise the temperature of the fresh

charge.

32

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

33

Volumetric Efficiency

The timing of intake and exhaust valves Valve timing is the regulation of the points in the cycle at

which the valves are set to open and close.

Valves requires a finite period of time to open or close forsmooth operation

34

Volumetric Efficiency

The effect of intake valve timing on the engine air capacity is

indicated by its effect on the air inducted per cylinder, per cycle.

The intake valve timing for both a low and high speed SI engine

For low speed

Opening @10o before TDC

Closing @10o after BDC

35

Volumetric efficiency

For high speed

Opening @10o before TDC

Closing @60o after TDC

36

Loss due to Running Friction

The losses are due to friction between

the piston and the cylinder walls

In various bearings

Energy spent in operating the auxiliary equipment(cooling pump, ignition system, fan…)

The piston ring friction increases rapidly with enginespeed.

37

Loss @ part and Full load r=838