Lecture -7 Ice

8/6/2019 Lecture -7 Ice

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Internal CombustionEngines

1

Lectur



Air Standard Cycles

1. Carnot - maximum cycle efficiency

2. Otto - spark-ignition (SI) engine3. Diesel - compression-ignition (CI) engine4. Brayton - gas turbine

2



Air Standard Cycles

• Air standard cycles are idealized cyclesbased on

the following approximations:

– the working fluid is air (ideal gas)– all the processes are internally reversible

– the combustion process is replaced by

heat input from an external source

– heat rejection is used to restore fluid toinitial



Thermodynamic Cycles

• Air-standard analysis is used to perform elementary analysesof IC engine cycles.

• Simplifications to the real cycle include:1) Fixed amount of air (ideal gas) for working

fluid 2) Combustion process not considered3) Intake and exhaust processes not

considered4) Engine friction and heat losses not

considered5) Specific heats independent of temperature



SI Engine Cycle vs Thermodynamic Otto Cycle

Fuel/Air Mixture

CompressionStroke

Q in

TC

Const volumeheat addition Process heat rejection

Process Process

FUEL

IR

IntakeStroke

Air

5

CompressionProcess

Ignition

Power Stroke

Expansion

CombustionProducts

ExhaustStroke

Q out

BC

Const volume

A

ActualCycle

OttoCycle



Air-Standard Otto cycle

Isentropic compressionConstant volume heat additionIsentropic expansionConstant volume heat rejection

Q in

Q out

v 1BC

Process 1 2Process 23

Process 34

Process 41

TC BC

6

Compression ratio:

v1 v4r = =

v2 v3



In Otto cycle, thecombustion is so rapid

that the piston does notmove during the process,and therefore,combustion is assumed totake place at constant

volume.Otto cycle efficiency

T (T / T - 1)T (T / T - 1)

7

q in

wnetη = = 1q in

qout = 1 1 4 1

2 3 2= 1

T - TT - T

4 1

3 2



Otto Cycle (Contd.)For isentropic process:

pv k = constantFor process 1-2:

p1 v1k = p 2 v2

k

RT2

v1 p 2 v2

2

v1 v 2

vk v1

T2=

T1

RT1

v1

with k=c p/cv

T2 v1=

T1 v2

8

k

= =v k p1

T2

= T1

k

2

⎛ ⎞

⎝ ⎠

k - 1v1

⎜v ⎟2

= ⎜ ⎟

kv1- 1

vk - 1

2



Since m = constant:k - 1 k - 1

⎜V ⎟ = rk - 1

TDC

For process 3-4, using the same analysis:VBD C ⎞VT D C

⎟

T3 T3or =

T4 T2

η = 1 -

T2

T

T3

T4

T2=

T1

1r k - 1

V1

T4

T1

9

⎛ ⎜⎝

⎛ ⎞ ⎛

⎝ v ⎠ ⎝ v1

k - 1

= ⎜ ⎟ = ⎜2

⎞ ⎛ ⎞

⎠ ⎝ ⎠VBD C

⎟ = ⎜ ⎟

k - 1

⎟ = r k - 1

⎠

k - 1⎛ ⎞

⎝ ⎠V4

⎜V ⎟3

Then

= ⎜ ⎟ = ⎜



Increasing Compression RatioIncreases the Efficiency

TypicalCompression

Ratios forGasoline Engines

10



Higher Compression Ratios?

• Higher compression ratio leads toauto-ignition (without spark)

• Causes knock • Engine damage• Thus, there is an upper limit of high

compression ratio

11



Engine Cycle and the Thermodynamic Diesel Cycle

A

IR

Air

Intake CompressionStroke Stroke

Q in

Air

Compression Const pressureProcess heat addition

Process 12

Fuel injectedat TC

CombustionProducts

Power ExhaustStroke Stroke

Q out

BC

Expansion Const volumeProcess heat rejection

Process

CI

ActualCycle

DieselCycle



Air-Standard Diesel cycle

Process 12

Isentropic compressionProcess 2 3 Constant pressure heat additionProcess 3 4 Isentropic expansionProcess 4 1 Constant volume heat rejection

Q in

Q out

v 2TC

v 1BC

BC

13

TC

Cut-off ratio:

v3r c =

v2



Due to ignition delay and finite timerequired

for fuel injection, combustionprocesscontinues till the beginning of powerstroke.This keeps the cylinder pressureat peak levels for a longer period.Therefore, thecombustion process can beapproximatedas constant pressure heataddition.Remaining processes are similar tothat of Otto cycle.

qout

q in

14



• Cycle efficiency,

η = = 1 -

wnet

q in



Cutoff Ratio , r =

Compression Ratio , r =

Expansion Ratio , r =

Cutoff Ratio Χ Expansion Ratio = Compression Ratio



15

V 1

V 3V 2c

V 4V 3

e



assuming constant specific heats:

(T / T - 1)

p 3 2 3 2 2 3 2

for isentropic process 1-2:

⎛ ⎞=

T2 ⎜v1

⎟

for constant pressure process 2-3: p 2 = p 3

ideal gas law:T3 v3

= = rcT2 v2

16

η = 1 = 1

v3

RT2 RT3= =>

v2

(T - T )k(T - T

4 1 4 1c (T - T)

v 4 1 = 1

k -

1T1 v2

T k(T / TT1



for isentropic process 3-4:k - 1

v 4

⎜v ⎟3

⎛ v3 ⎞ ⎛ v

⎝ ⎠ ⎝ r - 1

r k(r - 1)k

≥ 1, for given rc

diesel

but diesel cycle has higher r!

T3

T4

=>v3

v2

T2=

T1

T4

T1

T3==

T2

then,

sin ce

η = 1

k - 1

⎜ ⎟ = ⎜⎜v ⎟

⎜v

2

⎛ ⎞

⎝ ⎠

k - 1v2

⎜v ⎟3

⎜ ⎟

k - 1v1 ⎞⎟ =

v3 ⎟ ⎠

⎛ ⎜⎝

k T2

vk -1

v1- 1

T12

= 3 3

⎛ v3 ⎞

⎝ ⎠

k - 1

⎜ ⎟ =⎜v ⎟2

⎛ ⎞

⎝ ⎠= ⎜ ⎟ = ⎜

k

= r ck

17

⎞⎟⎟

⎠

3

2

≤ η

r - 1k(r- 1)η

kc1

k -1c

c



Thermal Efficiency

r k

- 1c

r - 1c

η Otto = 1 -

Note that the term in the square bracket isalways largerthan one so for the same compression ratio( r) , theDiesel cycle has a lower thermal efficiencythan theOtto cycle.

1

r k -1

η D ie sel = 1 -1

r k - 1

1⋅

k

Recall,



Note: CI needs higher r compared to SI toignite fuel

18( )⎤( )⎦

⎥

⎥

⎡⎢

⎢⎣



Remark

When r c (= v 3 /v 2 ) 1 the Diesel cycleefficiency

approaches the efficiency of the Otto cycle

Compression ratio = 10-22 (Diesel)Compression ratio = 6-10 (Otto)Thus, efficiency of Diesel Cycle is greater than Otto Cycle.

Higher efficiency and low cost fuel makesdieselengine suitable for larger power units such

aslarger ships, heavy trucks, power



Diesel Cycle Otto Cycle

The onlydifferenceis inprocess2-3

20



Remark

Both Otto cycle (Constant volume

heat addition) and Diesel cycle (Constantpressure

heat addition) are over-simplisticand

unrealistic . In actual case, combustiontakesplace neither at constant volume

(timerequired for chemical reactions), nor

at constant pressure (rapiduncontrolled

combustion).



Modern CI Engine Cycle and the Thermodynamic Dual Cycle

Fuel injectedat 15 o bTC

I R

Air

Intake Compressio n Power Stroke Stroke Stroke

Q in Q in

Air TC

Const pressureheat addition

Process

A

ActualCycle

DualCycle

Const volumeheat addition

Process

CompressionProcess

CombustionProducts

ExhaustStroke

Q out

BC

Expansion Const volumeProcess heat rejection

Process22



Dual Cycle

Process 1 2 Isentropic compressionProcess 2 2.5 Constant volume heat addition

Process 2.5 3 Constant pressure heat additionProcess 3 4 Isentropic expansionProcess 4 1 Constant volume heat rejection

Q in3

Q in2.5

4

Q out

23

2.5

2

2

1

3

4

1



Thermal Efficiency

u4 - u1

(u2 .5 - u2) + ( h3 - h2.5 )

α r ck - 1(α - 1) + αk (r c - 1)⎥

P 2.5

v2 .5 and α = P 2

Note, the Otto cycle (r c=1) and the Diesel cycle ( α =1) are special cases:

η Diesel = 1 -

const c V

1r k - 1

η D ual =1 -

cycle

η Dual

=1 -

η Otto = 1 - 1r k -1

Qout m

Q in m

=1 -1

r k - 1

v3

=

⎡1

⎢k ⎣

⎤

⎦

⎤

⎦(r ck

⋅

(r c -1)⎥

24

⎡

⎢⎣const c v

where c



The use of the Dual cycle requires information about either:i) the fractions of constant volume and constant pressure heat

addition (common assumption is to equally split the heataddition), or

ii) maximum pressure P 3.

For the same inlet conditions P 1, V1 and the same compression ratio:

η Otto > η Dual > η Diesel

For the same inlet conditions P 1, V1 and the same peak pressure P 3

(actual design limitation in engines):

η Diese l > η Dual > ηotto

25

F h i l di i P V



For the same inlet conditions P 1, V1

and the same compression ratio P 2 /P 1:

“x” →“2.5”

P o

Specific Volume

Entropy

For the same inlet conditions P 1, V1

and the same peak pressure P 3:

P max

P o

Specific Volume

Tmax

Entropy



References11 .. CrouseCrouse WH,WH, andand AnglinAnglin DLDL , (1985), Automotive Engines ,

Tata McGraw Hill.22 .. EastopEastop TD,TD, andand McConkeyMcConkey A,A, (1993), Applied Thermodynamics for Engg.

Technologists , Addison Wisley.33 .. FergusanFergusan CR,CR, andand Kirkpatrick Kirkpatrick AATT ,, (2001), InternalCombustion Engines , John

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

McGraw Hill.55 .. GillGill PW,PW, SmithSmith JH, JH, andand ZiurysZiurys EJEJ ,, (1959), Fundamentals of I. C . Engines , Oxford

and IBH Pub Ltd .66 .. HeislerHeisler H,H, (1999), Vehicle and Engine Technology, ArnoldPublishers.77 .. HeywoodHeywood JB, JB, (1989), Internal Combustion EngineFundamentals , McGraw Hill.88 .. HeywoodHeywood JB, JB, aa nn dd SherSher E,E, (1999), The Two-Stroke CycleEngine , Taylor & Francis.99 .. Joel Joel R,R, (199(199 6)6) ,, Basic Engineering Thermodynamics, Addison-Wesley.11 00 .. MathurMathur ML,ML, andand SharmaSharma RP,RP, (1994), A Course in

I t l C b ti E gi



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

1111 .. Pulkrabek Pulkrabek WW,WW, (1997), Engineering Fundamentals of the I .C . Engine , Prentice Hall.11 22 .. RogRog ee rsrs GFC,GFC, andand MayhewMayhew YR YR, (1992), EngineeringThermodynamics , Addison

Wisley .11 33 .. SriSri nn ivasanivasan S,S, (2001), Automotive Engines , Tata McGrawHill.11 44 .. StoneStone R,R, (1992), Internal Combustion Engines , TheMacmillan Press Limited, London.

11 55 .. TaylorTaylor CF,CF, (1985), The Internal-Combustion Engine inTheory and Practice , Vol.1 & 2, The MIT Press, Cambridge, Massachusetts.

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Web Resources1 . http://www.mne.psu.edu/simp son/courses2 . http://me.queensu.ca/courses3 . http://www.eng.fsu.edu4 . 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/14081/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/~kunrotc/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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