air standard cycles
DESCRIPTION
air std cycles ppt by iit bombayTRANSCRIPT
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Internal Combustion Engines
Lecture-7
Ujjwal K Saha, Ph.D.Department of Mechanical Engineering
Indian Institute of Technology Guwahati
Prepared underQIP-CD Cell Project
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Air Standard Cycles
1. Carnot - maximum cycle efficiency2. Otto - spark-ignition (SI) engine 3. Diesel - compression-ignition (CI) engine4. Brayton - gas turbine
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3
Air standard cycles are idealized cycles based 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 to initial state
Air Standard Cycles
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Thermodynamic Cycles
Air-standard analysis is used to perform elementary analyses of IC engine cycles.
Simplifications to the real cycle include:1) Fixed amount of air (ideal gas) for working fluid2) Combustion process not considered3) Intake and exhaust processes not considered4) Engine friction and heat losses not considered5) Specific heats independent of temperature
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SI Engine Cycle vs Thermodynamic Otto Cycle
AI
R
CombustionProducts
Ignition
IntakeStroke
FUEL
Fuel/AirMixture
AirTC
BC
CompressionStroke
PowerStroke
ExhaustStroke
Qin Qout
CompressionProcess
Const volume heat addition
Process
ExpansionProcess
Const volume heat rejection
Process
ActualCycle
OttoCycle
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Process 1 2 Isentropic compressionProcess 2 3 Constant volume heat additionProcess 3 4 Isentropic expansionProcess 4 1 Constant volume heat rejection
v2TC TC
v1BC BC
Qout
Qin
Air-Standard Otto cycle
3
4
2
1
vv
vvr ==
Compression ratio:
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Otto cycle efficiency
)1T/T(T)1T/T(T
1TTTT
1qq
1qw
232
141
23
14
in
out
in
net
=
===
In Otto cycle, the combustion is so rapid that the piston does not move during the process, and therefore, combustion is assumed to take place at constant volume.
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Otto Cycle (Contd.)For isentropic process:pvk = constant with k=cp/cvFor process 1-2: p1 v1k = p2 v2k
2
1
1
2
1
1
2
2
1
2k2
k1
vv
TT
vRTv
RT
pp
vv
===
1k
2
11k
2
1k1
1
2
1
2
1
2k2
k1
vv
vv
TT
TT
vv
vv
==
=
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Since m = constant:
For process 3-4, using the same analysis:
Then
1k
1k
TDC
BDC
1k
2
1
1k
2
1
1
2 rVV
VV
vv
TT
=
=
=
=
1k
1k
TDC
BDC
1k
3
4
4
3 rVV
VV
TT
=
=
=
1k
1
4
2
3
4
3
1
2
r1
1
TT
TT
orTT
TT
=
==
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Increasing Compression RatioIncreases the Efficiency
Typical Compression Ratios for Gasoline Engines
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Higher Compression Ratios?
Higher compression ratio leads to auto-ignition (without spark)
Causes knock Engine damage Thus, there is an upper limit of high
compression ratio
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CI Engine Cycle and the Thermodynamic Diesel Cycle
AI
R
CombustionProducts
Fuel injectedat TC
IntakeStroke
Air
Air
BC
CompressionStroke
PowerStroke
ExhaustStroke
Qin Qout
CompressionProcess
Const pressure heat addition
Process
ExpansionProcess
Const volume heat rejection
Process
ActualCycle
DieselCycle
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Process 1 2 Isentropic compressionProcess 2 3 Constant pressure heat additionProcess 3 4 Isentropic expansionProcess 4 1 Constant volume heat rejection
Air-Standard Diesel cycle
Qin
Qout
2
3vvrc =
Cut-off ratio:
v2TC
v1BC TC BC
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Cycle efficiency,
in
out
in
net
qq
1qw
==
Due to ignition delay and finite time required for fuel injection, combustion process continues till the beginning of power stroke. This keeps the cylinder pressure at peak levels for a longer period. Therefore, the combustion process can be approximated as constant pressure heat addition. Remaining processes are similar to that of Otto cycle.
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3
2
1
2
4
3
,
,
,
c
e
VCutoff Ratio rV
VCompression Ratio rV
VExpansion Ratio rV
=
=
=
Cutoff Ratio Expansion Ratio Compression Ratio =
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assuming constant specific heats:
for isentropic process 1-2:
for constant pressure process 2-3: p2 = p3ideal gas law:
)1T/T(k)1T/T(
TT
1)TT(k)TT(
1)TT(c)TT(c
123
14
2
1
23
14
23p
14v
=
=
=
=
k 1
1 2
2 1
T vT v
c
2
3
2
3
3
3
2
2 rvv
TT
vRT
vRT ===>=
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for isentropic process 3-4:
but diesel cycle has higher r!
kc
k
2
3
1k
2
3
2
3
1k
2
3
2
3
1
4
1k
3
2
1
21k
3
1k2
1
2
1k3
1k1
1k
3
1
1k
3
4
4
3
rvv
vv
vv
vv
TT
TT
vv
TT
v
vTT
vv
vv
vv
TT
=
=
=
===>
===
=
=
Ottodiesel
c
kc
c
kc
1k
rgivenfor,1)1r(k
1rcesin
)1r(k1r
r1
1,then
=
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( )( )1
11 111
kc
Diesel kc
rr k r
=
Thermal Efficiency
111 = kOtto r
Recall,
Note that the term in the square bracket is always larger than one so for the same compression ratio (r), the Diesel cycle has a lower thermal efficiency than the Otto cycle.
Note: CI needs higher r compared to SI to ignite fuel
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When rc (= v3/v2) 1 the Diesel cycle efficiency approaches the efficiency of the Otto cycle
Remark
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 makes diesel engine suitable for larger power units such as larger ships, heavy trucks, power generating units, locomotives etc.
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Diesel Cycle Otto Cycle
The only difference is in process 2-3
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Both Otto cycle (Constant volume heat addition) and Diesel cycle (Constant pressure heat addition) are over-simplistic and unrealistic. In actual case, combustion takes place neither at constant volume (time required for chemical reactions), nor at constant pressure (rapid uncontrolled combustion).
Dual cycle is used to model the combustion process. It is a compromise between Otto and Diesel cycles, where heat addition takes place partly at constant volume and partly at constant pressure. This cycle is also known as mixed cycle. In fact, Otto and Diesel cycles are special cases of Dual cycle.
Remark
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Modern CI Engine Cycle and the Thermodynamic Dual Cycle
AI
R
CombustionProducts
Fuel injectedat 15o bTC
IntakeStroke
Air
AirTC
BC
CompressionStroke
PowerStroke
ExhaustStroke
Qin Qout
CompressionProcess
Const pressure heat addition
Process
ExpansionProcess
Const volume heat rejection
Process
ActualCycle
DualCycle
Qin
Const volume heat addition
Process
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Process 1 2 Isentropic compressionProcess 2 2.5 Constant volume heat additionProcess 2.5 3 Constant pressure heat additionProcess 3 4 Isentropic expansionProcess 4 1 Constant volume heat rejection
Dual Cycle
Qin
Qin
Qout1
1
2
2
2.5
2.5
33
44
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Thermal Efficiency
)()(11
5.2325.2
14
hhuuuu
mQmQ
in
out
cycleDual +
==
( )
+
= 1)1(
111 1 c
kc
kcconst
Dual rkr
rv
111 = kOtto r
( )( )
= 11111 1
c
kc
kconst cDiesel r
rkrV
Note, the Otto cycle (rc=1) and the Diesel cycle (=1) are special cases:
3 2.5
2.5 2where and c
v Pr v P= =
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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 heat addition), or
ii) maximum pressure P3.
For the same inlet conditions P1, V1 and the same compression ratio:
DieselDualOtto >>
For the same inlet conditions P1, V1 and the same peak pressure P3(actual design limitation in engines):
ottoDualDiesel >>
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For the same inlet conditions P1, V1and the same compression ratio P2/P1:
For the same inlet conditions P1, V1and the same peak pressure P3:
Diesel
Dual
Otto
Diesel
DualOt
to
x 2.5
Pmax
Tmax
Po
Po
Pres
sure
, P
Pres
sure
, P
Tem
pera
ture
, T
Tem
pera
ture
, T
Specific VolumeSpecific Volume
Entropy Entropy
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1.1. Crouse WH, Crouse WH, andand Anglin DLAnglin DL, (1985), Automotive Engines, Tata McGraw Hill.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, John
Wiley & 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, Oxford
and 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, Addison
Wisley. 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
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1. http://www.mne.psu.edu/simpson/courses2. http://me.queensu.ca/courses 3. http://www.eng.fsu.edu4. http://www.personal.utulsa.edu5. http://www.glenroseffa.org/6. http://www.howstuffworks.com7. http://www.me.psu.edu 8. http://www.uic.edu/classes/me/ me429/lecture-air-cyc-web%5B1%5D.ppt9. http://www.osti.gov/fcvt/HETE2004/Stable.pdf10. http://www.rmi.org/sitepages/pid457.php11. http://www.tpub.com/content/engine/14081/css12. http://webpages.csus.edu13. http://www.nebo.edu/misc/learning_resources/ ppt/6-1214. http://netlogo.modelingcomplexity.org/Small_engines.ppt15. http://www.ku.edu/~kunrotc/academics/180/Lesson%2008%20Diesel.ppt16. http://navsci.berkeley.edu/NS10/PPT/ 17. http://www.career-center.org/ secondary/powerpoint/sge-parts.ppt18. http://mcdetflw.tecom.usmc.mil19. http://ferl.becta.org.uk/display.cfm20. http://www.eng.fsu.edu/ME_senior_design/2002/folder14/ccd/Combustion21. http://www.me.udel.edu22. http://online.physics.uiuc.edu/courses/phys14023. http://widget.ecn.purdue.edu/~yanchen/ME200/ME200-8.ppt -
Web Resources
Air Standard CyclesAir Standard CyclesOtto Cycle (Contd.)Increasing Compression RatioIncreases the EfficiencyHigher Compression Ratios?