l29 - internal combustion engine models (1)
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Department of Mechanical EngineeringME 322 – Mechanical Engineering
Thermodynamics
Lecture 29
Internal Combustion Engine ModelsThe Otto CycleThe Diesel Cycle
IC Engine TerminologyFinally … here is one of the reasons we spent so much time analyzing piston-cylinder assemblies in the early part of the course!
disp BDC TDC
BDC
TDC
V V V
VCRV
BDCV
TDCV
2
IC Engine Terminology• Fuel-Air ignition
– Spark• Gasoline engines
– Compression• Diesel engines
• 4-Stroke Engine– Four strokes (intake, compression, power stroke, exhaust)
are executed for every two revolutions of the crankshaft, and one thermodynamic cycle
• 2-Stroke Engine– Two strokes (intake, compression, power stroke, and
exhaust) are executed for every one revolution of the crankshaft, and one thermodynamic cycle
3
IC Engine PerformanceThermal Efficiency
netth
in
WQ
Mean Effective Pressure
net work for one cyclemepdisplacement volume
The mep provides a way to compare two engines that have the same displacement volume
4
Modeling the IC Engine• Air Standard Analysis (ASC or hot ASC)
– The working fluid is a fixed mass of air treated as an ideal gas
• No intake or exhaust– The combustion process is replaced with a heat transfer
from a high-temperature source– The exhaust process is replaced with a heat transfer to a
low-temperature sink– All processes are internally reversible
• Cold Air Standard Analysis (cold ASC)– All of the above– Heat capacity of the air is assumed to be constant at the
ambient temperature
5
SI Engine - Otto Cycle
6
• 1-2 Isentropic compression from BDC to TDC
• 2-3 Isochoric heat input (combustion)
1 2 3 4
BDC
TDC
P
1
2
3
4
BDCTDC
T
s1
2
3
4v = co
nst
v = co
nst
v
12 2 1W m u u
23 3 2Q m u u
SI Engine - Otto Cycle
7
• 3-4 Isentropic expansion (power stroke)
• 4-1 Isochoric heat rejection (exhaust)
1 2 3 4
BDC
TDC
P
1
2
3
4
BDCTDC
T
s1
2
3
4v = co
nst
v = co
nst
v
34 3 4W m u u
41 4 1Q m u u
Otto Cycle Performance
34 12 4 1,ASC
23 3 2
1netth
in
W W W u uQ Q u u
Thermal Efficiency
P
1
2
3
4
BDCTDCv
Compression Ratio
1 4
2 3
v vCRv v
11,cold ASC
2
1 1 kth
T CRT
T
s1
2
3
4v = co
nst
v = co
nst
8
Otto Cycle PerformanceMean Effective Pressure
3 4 2 134 12
1 2
mep net
disp disp
u u u uW W WV V v v
P
1
2
3
4
BDCTDCv
T
s1
2
3
4v = co
nst
v = co
nst
3 4 2 1cold ASC
1 2
mep vc T T T Tv v
Btu Btu0.24 0.172lbm-R lbm-R
1.4
p v
p
v
c c
ck
c
Cold ASC values (Table C.13a) ...
9
CI Engine - Diesel Cycle
10
• 1-2 Isentropic compression from BDC to TDC
• 2-3 Isobaric heat input (combustion)
1 2 3 4
BDC
TDC
P
1
2 3
4
BDCTDC
T
s1
2
3
4
P = const
v = co
nst
v
12 2 1W m u u
23 23 3 2Q W m u u
CI Engine - Diesel Cycle
11
• 3-4 Isentropic expansion (power stroke)
• 4-1 Isochoric heat rejection (exhaust)
1 2 3 4
BDC
TDC
34 3 4W m u u
41 4 1Q m u u
P
1
2 3
4
BDCTDC
T
s1
2
3
4
P = const
v = co
nst
v
Diesel Cycle Performance
23 34 12 4 1,ASC
23 3 2
1netth
in
W W W W u uQ Q h h
Thermal Efficiency
Compression Ratio
1
2
vCRv
1
,cold ASC
11
1
k k
th
CR CO
k CO
Cutoff Ratio3
2
vCO
v
P
1
2 3
4
BDCTDC
T
s1
2
3
4
P = const
v = co
nst
v
12
Diesel Cycle PerformanceMean Effective Pressure
3 2 4 123 34 12
1 2
mep net
disp disp
h h u uW W W WV V v v
3 2 4 1cold ASC
1 2
mep p vc T T c T Tv v
P
1
2 3
4
BDCTDC
T
s1
2
3
4
P = const
v = co
nst
v
Btu Btu0.24 0.172lbm-R lbm-R
1.4
p v
p
v
c c
ck
c
Cold ASC values (Table C.13a) ...
13
Cycle Evaluation• Strategy
– Build the property table first, then do the thermodynamic analysis
• Real fluid model– EES (fluid name = ‘air_ha’)
• Air standard model– Ideal gas with variable heat capacities
• Table C.16 (Air Tables)• EES (fluid name = ‘air’)
• Cold air standard model– Ideal gas with constant heat capacities evaluated at the
beginning of compression• Atmospheric conditions
14
IC Engine Performance• Known Parameters
– Number of cylinders in the engine– Enough information to determine
the mass of the air trapped in thecylinder
– Engine ratios (compression and cutoff)– Rotational speed of the engine (rpm)– Engine type
• All cylinders complete a thermodynamic cycle in either two or four strokes
– P and T at the beginning of compression– P or T at the end of combustion
15
IC Engine Performance
16
The power developed by the engine can be determined by
net cyl netr
W N WN
rev
Btu hp-minmincylcyl-cycle Bturev
cycle
netW
From the Otto or Diesel Cycle analysis conversion factor
Crankshaft revolutions per cycle
Crankshaft rotational speedNumber of cylinders
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