Download - THERMODYNAMICS:Otto vs Diesel cycle
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles1
Carnot Cycle Otto Cycle Diesela CycleStirling & Ericsson Cycle Brayton Cycle
Contents:
Carnot cycleOtto cycleDiesel cycleStirling cycleEricsson cycle Brayton cycle
Jet Gas Turbine
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles2
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Gas power cycles Heat engines in which working fluid is gas
Heat sink
Heat source
QH
QL
WnetHeat engineSample
applications
Internal Combustion Engines
Gas Turbines
Introduction
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles3
Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton CycleCarnot Cycle
Represents most efficient cycle that operates
between two fixed temperatures TH and TL
Efficiency of Carnot heat engine:
Not practical for real-life applications
H
LCarnotth T
T−=1,η
Acts as reference against which actual cycles can be compared.
Carnot Cyle
Carnot
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles4
Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Carnot Cyle
Carnot Cycle
Processes in a Carnot cycle:
1 - 2 Isothermal heat addition
2 - 3 Isentropic expansion
3 - 4 Isothermal heat rejection
4 - 1 Isentropic compression
Enclosed area in T-s & P-v diagrams
=> net work done by the cycle
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles
5Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Reciprocating engines
Carnot Cycle Otto Cycle
Examples of gas power cycle applications that involve piston-cylinder units
Types of reciprocating engine
Combustion initiated by a spark
Ideal process described by Otto cycle
Spark-ignition engine
Compression-ignition engine
Combustion initiated by compression
Ideal process described by Diesel cycle
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles6
Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Reciprocating engines
Carnot Cycle Otto Cycle
TDC : Top dead centre
BDC : Bottom dead centre
Stroke : Distance between TDC and BDC
Bore : Diameter of the piston
Clearance volume : Minimumvolume when piston at TDC
TDC
BDC
VV
VV
r ==min
max
r : Compression ratio
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles7
Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Reciprocating engines
Carnot Cycle Otto Cycle
MEP : Mean Effective Pressure : Fictitious pressure that if it
acted on piston during entirepower stroke would producesame amount of net workproduced during actual cycle
minmax VVw
MEP net
−=
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles8
Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Otto cycle
Carnot Cycle Otto Cycle
Represents ideal cycle for spark-ignition (SI) engines
Processes in 4-stroke engine cycle:
Air-fuel mixture is
compressed
Spark plug ignite and
combustion starts
High pressure gas
drives piston down
Exhaust gas driven out by piston
Fresh air-fuel mixture
drawn in
Otto: stroke by stroke
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles9
Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Otto cycle
Carnot Cycle Otto Cycle
Differences between Otto and actual 4-stroke engines
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles10
Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
4-stroke engine
Carnot CycleOtto Cycle
1 - 2 1. Piston moves upward from BDC to TDC 2. Air-fuel mixture is compressed isentropically.
Isentropic compression (Compression stroke)
2 - 3 1. Spark plug fires and combustion takes place 2. Piston moves downward from TDC to BDC,
converting heat energy to work
Constant-volume heat addition (Power or expansion stroke)
3 - 4 1. Piston moves upward from BDC to TDC 2. Exhaust valve open and exhaust gas is removed
Isentropic expansion (Exhaust stroke)
4 - 1 1. Piston moves downward from TDC to BDC 2. Intake valve open and air-fuel mixture drawn in
Constant-volume heat rejection (Intake stroke)
Actual Cycle
Otto Cycle
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles11
Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
2-stroke cycle
Carnot Cycle Otto Cycle
All four processes in take place in 2 strokes
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles12
Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
2-stroke cycle
Carnot Cycle Otto Cycle
Compression stroke: Air-fuel mixture drawn in,squeezed in combustion chamber
Power stroke: Combustion takes place, burned gas removed
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles13
Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
2-stroke cycle
Carnot Cycle Otto Cycle
2-stroke engines generally less efficient than 4-stroke due to:
incomplete expulsion of exhaust gases
partial expulsion of fresh air-fuel mixture
Advantages of 2-stroke engines:
simple and inexpensive
high power-to-weight and power-to-volume ratios
=> suitable for small size and light applications
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles14
Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
1st. Law Analysis:
Carnot Cycle Otto Cycle
v
P
1
2
3
4
Qin
Qout
s
T
1
2
3
4
Qin
Qout
uwq ∆=−For closed system:
outinnet
out
in
qqwuuquuq
−=−=−=
14
23
in
netOttoth q
w=,η
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles15
Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
1st. Law Analysis:
Carnot Cycle Otto Cycle
If specific heat is considered constant (i.e. approximate method):
)()(
14,14
23,23TTcuuqTTcuuq
avvout
avvin−=−=−=−=
1,11−
−= kOttoth rη Attention:
*Use suitable method(exact or approximate)
consistently*
v
p
CC
k
r
=
= ration compressio
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Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Some notes:
Carnot Cycle Otto Cycle
Efficiency of Otto cycle increases with compression ratio and specific heat ratio 1,
11−
−= kOttoth rη
At high compression ratio (above 8):
further increase in efficiency is
insignificant
premature ignition occurs =>
engine knock. Reduced by anti-
knock agent, e.g. tetraethyl lead
Typical efficiency of SI engines: 25 - 30%
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles17
Stirling & Ericsson Cycle Brayton CycleCarnot Cycle Otto Cycle Diesel Cycle
Represents ideal compression-ignition (CI) engine
Diesel Cycle:
Consists of 4 processes
=> Almost similar to Otto cycle
Air compressed to pressure & temperature above self-ignition temperature of fuel
Combustion starts on contact as fuel is injected to hot air
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles18
Stirling & Ericsson Cycle Brayton CycleCarnot Cycle Otto Cycle Diesel Cycle
1st. Law analysis:
Exact method: variable specific heat
Approximate method: constant specific heat
)(,)( 1423 TTCqTTCq voutpin −=−=
ratio cutoff2
3
2
31, ,
)1(111 ===
−−
−= − vv
VVr
rkr
r cc
kc
kDieselthη
), 1423 UUqhhq outin −=−=
in
netDieselth q
w=,η
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles19
Stirling & Ericsson Cycle Brayton CycleCarnot Cycle Otto Cycle Diesel Cycle
Some notes:
At same compression ratio, Otto has greater efficiency than Diesel engines
Advantages of Diesel engines:☺ able to operate at much higher
compression ratio (12 to 24)i.e higher efficiency (35 - 40%)
☺ able to use cheaper fuel, becauseless constraint on premature ignition problem
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles
20Brayton CycleCarnot Cycle Otto Cycle Diesel Cycle
Stirling & Ericsson cycle:
Stirling & Ericsson Cycle
Stirling: Two constant-volume regeneration
Ericsson: Two constant-pressure regeneration
Robert Stirling
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles21
Brayton CycleCarnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle
Stirling Engine
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Brayton CycleCarnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle
Advantages:
☺ Ideal Stirling and Ericsson cycles
have Carnot cycle efficiency
☺ Combustion can be done externally
=> more choices of fuel types
Disadvantages:
Difficult to achieve in practice:
- involve heat transfer through
small temperature difference.
- require very large heat transfer
area and very long time.
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles23
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Brayton cycle:
Actual gas turbine operate on open cycle
Assumptions:Combustion process => const-pressure heat additionExhaust process => const-pressure heat rejection
Represents ideal gas-turbine engine cycle
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles24
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Made up of 4 processes:
1 - 2 Isentropic compression (compressor)
2 - 3 Const Pressure heat addition (heat exchanger)
3 - 4 Isentropic expansion (turbine)
4 - 1 Const Pressure heat removal (heat exchanger)
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles25
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
1st law analysis:
1423 , hhqhhq outin −=−=
1
2/)1(, ,11
PPr
r pkkp
Braytonth =−=−
η
in
netoutBraytonth q
w ,, =η
If specific heats are assumed constant (approximate method)
4312 , hhwhhw outin −=−=
inoutnetout www −=,
outinnetout qqw −=,
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles26
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Deviation from ideal Brayton cycle, due to:
pressure drops during heat addition and rejection
irreversibilities in compressor and turbine
a
s
a
scompressor hh
hhww
21
21
−−
≅=η
s
a
s
aturbine hh
hhww
43
43
−−
≅=η
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles27
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Efficiency of gas-turbine power plant can be increased significantly by combining with steam power cycle=> combined cycle gas turbine (CCGT)
Main applications of Brayton cycle:electricity generation => gas-turbine power plantsaircrafts => jet propulsion enginesmarine => propeller prime mover
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles28
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Jet propulsion cycle:1 - 2 Air pressure increased slightly in diffuser2 - 3 Air is compressed in compressor3 - 4 Heat addition (combustion) process in burner at constant pressure4 - 5 Partial expansion of exhaust gas in turbine, producing just
enough power to drive compressor and other auxiliaries5 - 6 Gas expansion in the nozzle to ambient pressure at high velocity6 - 1 Heat rejection to surrounding at constant pressure
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles29
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles30
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles31
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
Turbofan engine
Turboprop engine
Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles32
Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle
SummaryGas power cycles => Heat engineswith gas as working fluidOtto cycle => spark ignitioninternal combustion engine
v
P
1
2
3
4
Qin
Qout
s
T
12
3
4
Qin
QoutDiesel cycle => compression ignition internal combustion engine
Brayton cycle => open cycle gas turbine