l-06 ice lecture

8/6/2019 L-06 Ice Lecture

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Internal CombustionEngines

1

Lectur


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Engine Operating Cycle

Spark plug for SI engineFuel injector for CI engine

Clearance

volume

Cylinderwall

rev

Piston

270o

2

s= CA

1 rev

360 CA

TDC

0o

180o

BDC

Valves

Top

Center

(TC)

Stroke

Bottom

Center

(BC)

Crank shaft

90o

crank angles

crank speedtime =


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SI and CI Engines

Spark ignition (SI):

combustion is initiated by spark

air and fuel can be added together

Compression ignition (CI):

combustion is initiated by auto ignition

requires fuel injection to control ignition

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Fuel-Air Mixing

In spark ignition engines, air and fuel areusually

mixed prior to entry into the cylinder.

The ratio of mass flow of air to the massflow of

fuel must be held roughly constant at about15 for

proper combustion.

Conventionally, a mechanical device knownas

a carburetor is used to mix fuel and air.Most

modern cars use electronic fuel-injection


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8-5

The Net Work Output of a Cycle

The net workoutput of a cycleis equivalent tothe product of the

mean effectpressure and thedisplacementvolume

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Indicated Power

Power can be increased by increasing:

the engine size, Vd compression ratio, rc engine speed, N

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Power Regulation

An IC engine is basically an air engine, the more air you get

into the cylinder, the more fuel you can burn, and more thepower you get.

Vary throttle position - Maximum intake pressure (and power)

achieved at wide-open-throttle (WOT), and minimum at idle.

Fuel

Pint < Patm

Intake

manifold

Idle

WOT

7

Patm


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Internal Combustion Engine

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Background on the Otto Cycle

The Otto Cycle has four basicsteps or strokes:

An intake stroke that drawsa combustible mixture offuel and air into the cylinder.

A compression stroke withthe valves closed whichraises the temperature ofthe mixture. A spark ignitesthe mixture towards the endof this stroke.

An expansion or power

stroke, resulting fromcombustion.

An Exhaust stroke thepushes the burned contentsout of the cylinder.

An idealized representation of

the Otto cycle on a PVdiagram.


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8-6

Actual and Ideal Cycles in Spark-Ignition Engines

and TheirP-v Diagram

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Typical Theoretical P-V and Valve TimingDiagrams of a Four-Stroke Spark Ignition Engine

Observations:

P-V diagram shows sharp edges

i.e., valves open/close instantaneously at dead centres

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Actual Case:

IV and EV open/close before and after dead centres

Mechanical Factor Dynamic Factor of Gas Flow

Valves are opened and closed by cam mechanism

Valves will bounce on its seat if closed abruptly

Opening/closing of valves spread over a certain crank angle

Every Corner in

the P-V diagram isROUNDED

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IV opens 200 before TDC.

IV closes 350

after BDC to takethe advantage of momentum of

rapidly moving gases (Ram Effect).

Ignition occurs 350 before TDC;

this is to allow the time delay

between the spark andcommencement of combustion.

EV opens at 350 before BDC; else

pressure will rise enormously, and

the work required to expel the gaswill increase.

EV closes at 100 after TDC; this is

to increase the volumetric efficiency.

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Actual Valve Timing Diagram of a

Four-Stroke Spark Ignition


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Remark

Valve Overlap: The time during which boththe valves (inlet and exhaust) remain open

at the same instant.

The values quoted in the actual valve timingdiagrams are the typical values, and may

vary from engine to engine.

For HIGH SPEED engines, higher valuesof angles are desirable to take intoaccount the short time interval.

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Explanation

The IV opens before TDC and closes after

BDC.This is arranged so as to get maximum

chargeinside the cylinder. When the IV opens,

thecharge outside the valve has to

beaccelerated upto the inlet velocity, and

thistakes time. In order that maximum

inletvelocity occurs at the earliest

possible


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Explanation Contd.

The EV opens before BDC so as to exhaustthe

combustion products efficiently. Thus, byvirtue

of its excess pressure above

atmosphere,some exhaust gas leaves the cylinder.

Thismakes the exhaust gas to flow freely from

thecylinder by the time piston commencesthe

exhaust stroke. Again, by closing the EVafter


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Valve Timing for Low and High SpeedFour-Stroke Spark Ignition Engines

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Sequence of Events in a Four-Stroke Spark

Ignition Engine Pressure vs. Crank Angle

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p-V diagram of a Four Stroke SI Engine at WOT

The pressure at the intake port is just below atmospheric pressure

Po Pintake

Pintake

The upper loop consists of compression and powerstrokes and the area represents positive indicated

work. The lower loop indicates negative work of the

intake and exhaust stroke. This is called indicated

pumping work.19


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p-V diagram of a Four Stroke SI Engine at Part-Throttle

The pressure at the intake port is significantly

lower than atmospheric pressure

Pintake

The upper loop consists of compression and power

strokes and the area represents positive indicated work.

The lower loop indicates negative work of the intake and

exhaust strokes. This is called indicated pumping work.

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p-V diagram of a Four Stroke SI Engine

with Supercharger

Compressor

Pintake

The engines with superchargers or turbochargers have

intake pressures greater than the exhaust pressure,

yielding a positive pump work.

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Typical Theoretical and Actual p-V Diagrams ofa Four-Stroke Compression Ignition Engine

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Actual Valve Timing Diagramof a Four-Stroke Compression

Ignition Engine

2. BDC

3. IV opens at 250 before TDC

4. IV closes at 300 after BDC

5. Fuel injection starts at 50before TDC

6. Fuel injection closes at 250 after TDC

7. EV opens at 450before BDC

8. EV closes at 150 after TDC

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1.


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Summary Ideal and Actual p-VDiagram of a Four Stroke SI Engine

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Real vs. Ideal 4 Stroke SI Engine

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Summary Ideal and Actual Valve TimingDiagram of a Four Stroke SI Engine

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Valve Timing 4 Stroke SI Engine

Conventional engines operate at low rpms, with idle and part load importantHigh performance engines operate at high rpms at WOT, with power and

volumetric efficiency important

At high engine speeds, less time available for fresh gas intake so need more

crank angles to get high volumetric efficiency large valve overlap

At low engine speed and part throttle valve overlap is minimized by reducing

the angle duration for valves staying open.

Variable Valve Timing used to obtain optimum performance over wide range

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Valve timing angles:


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References

11.. CrouseCrouse WH,WH, andand AnglinAnglin DLDL, (1985),Automotive Engines,Tata McGraw Hill.

22.. EastopEastop TD,TD, andand McConkeyMcConkey A,A, (1993), AppliedThermodynamics for Engg.

Technologists, Addison Wisley.

33.. FergusanFergusan CR,CR, andand KirkpatrickKirkpatrick AATT,, (2001), InternalCombustion Engines, John

Wiley & Sons.44.. GanesanGanesan VV,,(2003), Internal Combustion Engines,Tata

McGraw Hill.55.. GillGill PW,PW, SmithSmithJH,JH, andand ZiurysZiurys EJEJ,,(1959), Fundamentals ofI. C. Engines, Oxford

and IBH Pub Ltd.

66.. HeislerHeisler H,H, (1999), Vehicle and Engine Technology, ArnoldPublishers.

77.. HeywoodHeywoodJB,JB,(1989), Internal Combustion EngineFundamentals, McGraw Hill.88.. HeywoodHeywoodJB,JB, aanndd SherSher E,E, (1999), The Two-Stroke CycleEngine,Taylor & Francis.

99.. JoelJoel R,R, (199(1996)6),, Basic Engineering Thermodynamics, Addison-Wesley.

1100.. MathurMathur ML,ML, andand SharmaSharma RP,RP,(1994), A Course in

I t l C b ti E i


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Internal Combustion Engines,Dhanpat Rai & Sons, New Delhi.

1111.. PulkrabekPulkrabekWW,WW,(1997), Engineering Fundamentals of the I.C. Engine, Prentice Hall.

1122.. RogRogeersrs GFC,GFC, andand MayhewMayhew YRYR, (1992), EngineeringThermodynamics, Addison

Wisley.

1133.. SriSrinnivasanivasan S,S,(2001),Automotive Engines,Tata McGrawHill.

1144.. StoneStone R,R,(1992), Internal Combustion Engines, TheMacmillan Press Limited, London.

1155.. TaylorTaylor CF,CF,(1985), The Internal-Combustion Engine inTheory and Practice, Vol.1 & 2,The MIT Press, Cambridge, Massachusetts.

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Web Resources

1. http://www.mne.psu.edu/simp son/courses2. http://me.queensu.ca/courses3. http://www.eng.fsu.edu

4. http://www.per sonal.utulsa.edu5. http://www.glenro seffa.org/6. http://www.howstuffworks.com7. http://www.me.psu.edu8. http://www.uic.edu/classes/me/ me429/lecture-air-cyc-web

%5B1%5D.ppt9. http://www.osti.gov/ fcvt/HETE2004/Stable.pdf

10. http://www.rmi.org/s itepages/pid457.php11. http://www.tpub.co m/content/engine/1 4081/css12. http://webpages.csus.edu13. http://www.nebo.edu/misc/learning_resources/ ppt/6-1214. http://netlogo.modelingcomplexity.org/Small_engin es.ppt15. http://www.ku.edu/~kunrot c/academics/180/Lesson

%2008%20Diesel.ppt

16. http://navsci.berkeley.edu/NS10/PPT/17. http://www.career-center.org/ secondary/powerpoint/sge-parts.ppt

18. http://mcdetflw.tecom.usmc.mil19. http://ferl.becta.org.uk/display.cfm20.

http://www.eng.fsu.edu/ ME_senior_design/2002/ folder14/ccd/Combustion

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