qip ice 08 fuel air cycles

7/24/2019 Qip Ice 08 Fuel Air Cycles

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Air standard cycles had simplifiedapproximations, and therefore, performanceestimate of the engine is greater than the actual

performance.

With a compression ratio of 7:1, the actualindicated thermal efficiency of an SI engine is of

the order of 30 %, while the ideal (or air-standard)efficiency is about 55 %.

Background

This divergence is due to partly due to non-instantaneous burning, incomplete combustion,valve operation etc. However, the main reason lieswith the over-simplification of using values of

properties of the working fluid.


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Ideal Case:

Working fluid is air

Air is a perfect gas Has constant specific heats

Background-Contd.

Working fluid is air + fuel + residual gas Specific heats increases with increase in

temperature Combustion products are subjected todissociation at high temperature

Actual Case:


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Actual composition of the cylinder gas (fuel+ air + water vapor in air + residual gas)

Fuel-Air Cycle Considerations

F/A ratio change during operation, andhence changes in amount of CO2, watervapor etc.

Specific heat changes with temperature(except for mono-atomic gas), and hence,ratio of specific heats (k) also changes.

Changes in no. of molecules in cylinder withthe change in pressure and temperature.


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There is no chemical change in either fuel or airprior to combustion.

There is no heat transfer between the gases andcylinder walls in any process (adiabatic).

Fuel-Air Cycles - Assumptions

Compression and expansion processes are

frictionless.

The velocities are negligibly small.


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#The air-standard analysis allows how the efficiencyis improved by raising the compression ratio of air.

Remark

# It does not give any idea on the effect of F/Aratio on thermal efficiency.

Fuel Air Cycles Allows study of F/A ratio on thermal efficiency.

Allows study of pmax and Tmax as F/A ratio isvaried. This helps in structural design of the engine.

Gives a good estimate of the power expected

from an actual engine.


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#Except mono-atomic gases, all other gases showan increase in specific heats at high temperature.This increase does not obey any law.

Variable Specific Heats:


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#Above 1500 K, specific heats increase more rapidly,

and may be expressed in the form

Cp = a1 + k1T + k2T2

Cv= b1 + k1T + k2T2

#

Over the temperature range in general use forgases in heat engines (300 K 1500 K), the specificheat curve is nearly a straight line, and can beexpressed as

Cp = a1 + k1TCv= b1 + k1T

R = Cp - Cv= a1-b1


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Physical Explanation

Cp = 1.005 kJ/kgK at 300 K

Cp = 1.343 kJ/kgK at 2000 K

Cv= 0.718 kJ/kgK at 300 K

Cv= 1.055 kJ/kgK at 2000 K

Increase of specific heat is that astemperature is raised, larger and larger fractionsof heat input go to produce the motion of atomswithin the molecules.


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As temperature is an indication of motion ofmolecules as a whole, therefore, the energythat goes into the motion of atoms does not

contribute to temperature rise.

This is the reason, why more heat is requiredto raise the temperature of unit mass by onedegree (This heat, by definition, is the specificheat). As Cp-Cv =Constant , and k (=Cp/ Cv)d e c re a se s w ith inc re a se o f tem p e ra ture .

Therefore, variation of specific heats leadsthe FINAL temperature and pressure to lower

values (as c om p a re d to c onsta nt sp e c ific hea ts) .

Explanation


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2 is lower than 2 : due to variable specific heats3 is lower than 3 : temperature rise due to a given heat

release as Cp , and also as 2 is lower than 2.3 to 4 : resulting adiabatic expansion.

3 to 4 : correct expansion (Specific heatas Temperature during expansion).

1-2-3-4 : with constant specific heats1-2 -3 -4 : with constant specific heat from point 31-2 -3 -4 : with variable specific heats


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Dissociation : disintegration of combustion productsat high temperature. During dissociation, heat isabsorbed, whereas during combustion heat is

liberated.At 10000C, CO2 CO + O2 + heatAt 13000C, H2O H2 + O2 + heat

Presence of CO and O2 in the gases tends toprevent the dissociation of CO2 in rich mixture,

which, by producing more CO suppresses thedissociation ofCO2. That means, there there is nodissociation in the burnt gases of a lean mixture,because the temperature produced is too low for

the phenomenon to occur.

Dissociation Loss


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Lean Mixture : No dissociation takes placedue to low temperature.

Maximum dissociation : Chemically correctmixture when the temperature is high.

Rich Mixture : Dissociation is prevented by

the available CO and O2.

Remarks

Further, heat transfer to cooling mediumcauses a reduction in maximum temperatureand pressure. As temperature falls (during theexpansion stroke) the separated constituentsrecombine and heat absorbed (duringdissociation) gets released. But, it becomestoo late to recover.


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The curve shows the reduction in exhaust gas temperature

due to dissociation with respect to air-fuel ratio.


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Effect of dissociation on Power (SI Engine)

For Lean mixture : No dissociation.For Stoichiometric : Maximum dissociation.For Rich mixture : Effect declines due to incomplete combustion

and also due to increased quantity of CO.

Power Output ismaximum atstoichiometric

ratio where thereis no dissociation.

Shaded arearepresents loss ofpower due to

dissociation.


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Effect of Operating Variables

Compression Ratio: For agiven , efficiency (fuel-air cycle) increases withcompression ratio (r) ina similar manner as thatof air standard cycle.

pmax increases withincreasing r andliberation of chemicalenergy at high pressure

gives more scope forexpansion work. Thus,there is higher efficiencybut to a certain value ofcompression ratio (r).


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FA

FA

Equiv alence Ratio

Actual Ratio

Stoicheom etric R atio

=

=

At the same compressionratio, efficiency (fuel-air)decreases with increasing .

< 1 implies a lean mixture.Tmax

becomes lower due toexcess air. This results inlower specific heats andhigher values of k. Hence,

efficiency increases withdecreasing (gases expandto a larger temperature beforeexhaust).

When >1, efficiency (fuel-air) decreases withincreasing , becauseinsufficient air leads toincomplete oxidation of fuel.


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At a given r, maximumtemperature is reachedwhen the mixture is slightlyrich (about 6 - 8 %). This is

because, at =1, there isstill some oxygen presentat point 3 because ofchemical equilibrium

effects, and a rich mixturewill cause more fuel tocombine with oxygen atthat point thereby raising

the temperature T3.However, at rich mixturesincreased formation of COcounteracts this effect.

Effect of Equivalence Ratio on Temperature


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The pressure of a gasin a given spacedepends upon its

temperature and thenumber of molecules.

The curve ofp3,therefore follows T3,but because of theincreased no. ofmolecules, p

3starts

decreasing when themixture is about 18 to20 % rich.

Effect of Equivalence Ratio on Pressure


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2.2. Eastop TD,Eastop TD, andand McConkey A,McConkey A, (1993), Applied Thermodynamics for Engg.Technologists, Addison Wisley.

3.3. Fergusan CR,Fergusan CR, andand Kirkpatrick ATKirkpatrick AT,, (2001), Internal Combustion Engines, JohnWiley & Sons.

4.4. Ganesan VGanesan V,, (2003), Internal Combustion Engines,Tata McGraw Hill.

5.5. Gill PW, Smith JH,Gill PW, Smith JH, andand Ziurys EJZiurys EJ,, (1959), Fundamentals of I. C. Engines, Oxfordand IBH Pub Ltd.

6.6. Heisler H,Heisler H, (1999), Vehicle and Engine Technology,Arnold Publishers.

7.7. Heywood JB,Heywood JB, (1989), Internal Combustion Engine Fundamentals, McGraw Hill.

8.8. Heywood JB,Heywood JB, andand Sher E,Sher E, (1999), The Two-Stroke Cycle Engine,Taylor & Francis.9.9. Joel R,Joel R, (1996),(1996), Basic Engineering Thermodynamics,Addison-Wesley.

10.10. Mathur ML, and Sharma RP,Mathur ML, and Sharma RP, (1994), A Course in Internal Combustion Engines,Dhanpat Rai & Sons, New Delhi.

11.11. Pulkrabek WW,Pulkrabek WW, (1997),Engineering Fundamentals of the I. C. Engine, Prentice Hall.

12.12. Rogers GFC,Rogers GFC, andand Mayhew YRMayhew YR, (1992), Engineering Thermodynamics, AddisonWisley.

13.13. Srinivasan S,Srinivasan S, (2001),Automotive Engines,Tata McGraw Hill.

14.14. Stone R,Stone R, (1992), Internal Combustion Engines,The Macmillan Press Limited, London.

15.15. Taylor CF,Taylor CF, (1985), The Internal-Combustion Engine in Theory and Practice,Vol.1 & 2,The MIT Press, Cambridge, Massachusetts.

References

qip ice 08 fuel air cycles

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