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7/23/2019 Chap 9 Lecture
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Chapter 9GAS POWER CYCLES
Mehmet Kanoglu
Universit o! Ga"iantep
Copright # $he M%Gra&'(ill Companies) *n%+ Permission re,uire- !or repro-u%tion or -ispla+
$hermo-nami%s. An Engineering Approa%hSeventh E-ition in S* Units
Yunus A+ Cengel) Mi%hael A+ /oles
M%Gra&'(ill) 0122
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O34e%tives Evaluate the performance of gas power cycles for which the
working fluid remains a gas throughout the entire cycle.
Develop simplifying assumptions applicable to gas power
cycles.
Review the operation of reciprocating engines.
Analyze both closed and open gas power cycles.
olve problems based on the !tto" Diesel" tirling" and
Ericsson cycles.
olve problems based on the #rayton cycle$ the #rayton cycle
with regeneration$ and the #rayton cycle with intercooling"
reheating" and regeneration. Analyze %et&propulsion cycles.
'dentify simplifying assumptions for second&law analysis of
gas power cycles.
(erform second&law analysis of gas power cycles.
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)
/AS*C CO5S*6ERA$*O5S *5 $(E A5ALYS*S
O7 POWER CYCLES
*odeling is a
powerful
engineering toolthat provides great
insight and
simplicity at the
e+pense of some
loss in accuracy.
*ost power&producing devices operate on cycles.
*-eal %%le.A cycle that resembles the actual cycle
closely but is made up totally of internally reversible
processes+
Reversi3le %%les such asCarnot %%lehave the
highest thermal efficiency of all heat engines
operating between the same temperature levels.,nlike ideal cycles" they are totally reversible" and
unsuitable as a realistic model.
-hermal efficiency of
heat engines
-he analysis of many comple+
processes can be reduced to
a manageable level by
utilizing some idealizations.
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-he ideal cycles are internally reversible, but" unlike the 0arnot cycle" they are not
necessarily e+ternally reversible. -herefore" the thermal efficiency of an ideal
cycle" in general" is less than that of a totally reversible cycle operating between
the same temperature limits. 1owever" it is still considerably higher than the
thermal efficiency of an actual cycle because of the idealizations utilized.
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$he i-eali"ations an- simpli!i%ations in the
analsis o! po&er %%les.
3. -he cycle does not involve any friction.
-herefore" the working fluid does not
e+perience any pressure drop as it flows in
pipes or devices such as heat e+changers.
2. All e+pansion and compression processes
take place in a quasi-equilibrium manner.
). -he pipes connecting the various
components of a system are well
insulated" and heat transfer through them
is negligible.
0are should be e+ercised
in the interpretation of the
results from ideal cycles.
!n both P-v and T&s diagrams" the area enclosed
by the process curve represents the net work of the
cycle.
!n a T&s diagram" the ratio of the
area enclosed by the cyclic curve to
the area under the heat&addition
process curve represents the thermal
efficiency of the cycle.Any
modification that increases the ratio
of these two areas will also increase
the thermal efficiency of the cycle.
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$(E CAR5O$ CYCLE A56 *$S
8ALUE *5 E5G*5EER*5G
P-v and T&s diagrams of
a 0arnot cycle.
-he 0arnot cycle is composed of four totally reversible
processes isothermal heat addition" isentropice+pansion" isothermal heat re%ection" andisentropic
compression.
For both ideal and actual cycles:Thermal efficiency
increases with an increase in the average temperature
at which heat is supplied to the system or with a
decrease in the average temperature at which heat isrejected from the system.
A steady&flow 0arnot engine.
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A*R'S$A56AR6 ASSUMP$*O5S
-he combustion process is replaced by
a heat&addition process in ideal cycles.
Air'stan-ar- assumptions
3. -he working fluid is air" whichcontinuously circulates in a closed loop
and always behaves as an ideal gas.
2. All the processes that make up the
cycle are internally reversible.
). -he combustion process is replaced by
a heat&addition process from an
e+ternal source.
/. -he e+haust process is replaced by a
heat&re%ection process that restores the
working fluid to its initial state.
Col-'air'stan-ar- assumptions6hen the working fluid is considered
to be air with constant specific heats at room temperature72809.
Air'stan-ar- %%le.A cycle for which the air&standard assumptions are
applicable.
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A5 O8ER8*EW O7 REC*PROCA$*5G E5G*5ES
;omenclature for reciprocating engines.
Spar'ignition :S*; engines
Compression'ignition :C*; engines
0ompression ratio
*ean effectivepressure
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$&o'stroe %%le
3 cycle > 2 stroke > 3 revolution
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-he thermal efficiency of the
!tto cycle increases with the
specific heat ratio k of the
working fluid.
-hermal efficiency of the ideal
!tto cycle as a function of
compression ratio 7k =3./9.
'n ' engines"
the
compression
ratio is limited
byautoignition
or engine
no%.
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3)
6*ESEL CYCLE. $(E *6EAL CYCLE
7OR COMPRESS*O5'*G5*$*O5 E5G*5ES
'n diesel engines" the spark plug is replaced
by a fuel in%ector" and only air is compressed
during the compression process.
'n diesel engines" only air is compressed during the
compression stroke" eliminating the possibility ofautoignition 7engine knock9. -herefore" diesel engines
can be designed to operate at much higher compression
ratios than ' engines" typically between 32 and 2/.
2'0isentropic
compression0'
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-hermal
efficiency of the
ideal Diesel cycle
as a function of
compression and
cutoff ratios
7k=3./9.
0utoff
ratio
for the same compression ratio
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>UES$*O5S ???
Diesel engines operate at
higher air&fuel ratios than
gasoline engines. 6hy?
Despite higher power to
weight ratios" two&stroke
engines are not used in
automobiles. 6hy?
-he stationary diesel
engines are among the
most efficient power
producing devices 7about
=@9. 6hy?
6hat is a turbocharger?6hy are they mostly used
in diesel engines
compared to gasoline
engines.
P-v diagram of an ideal dual cycle.
6ual %%le.A more realisticideal cycle model for modern"
high&speed compression ignition
engine.
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S$*RL*5G A56 ER*CSSO5 CYCLES
A regenerator is a device that
borrows energy from the working
fluid during one part of the cycle
and pays it back 7without interest9
during another part.
Stirling %%le2'0T =constant e+pansion 7heat addition from the e+ternal source9
0'
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-he e+ecution of the tirling cycle. A steady&flow Ericsson engine.
-he Ericsson cycle is very much like the
tirling cycle" e+cept that the two constant&
volume processes are replaced by two
constant&pressure processes.
#oth the tirling and Ericsson cycles are
totally reversible" as is the 0arnot cycle"
and thus
-he tirling and Ericsson cycles
give a message egeneration
can increase efficiency.
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3:
/RAY$O5 CYCLE. $(E *6EAL CYCLE
7OR GAS'$UR/*5E E5G*5ES
An open&cycle gas&turbine engine. A closed&cycle gas&turbine engine.
-he combustion process is replaced by a constant&pressure heat&additionprocess from an e+ternal source" and the e+haust process is replaced by a
constant&pressure heat&re%ection process to the ambient air.
3&2 'sentropic compression 7in a compressor9
2&) 0onstant&pressure heat addition
)&/ 'sentropic e+pansion 7in a turbine9
/&3 0onstant&pressure heat re%ection
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