department of mechanical engineering me 322 … of mechanical engineering me 322 – mechanical...
TRANSCRIPT
Department of Mechanical Engineering
ME 322 – Mechanical Engineering
Thermodynamics
Lecture 28
Internal Combustion Engine Models
The Otto Cycle
The Diesel Cycle
IC Engine Terminology
Finally … 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
VCR
V
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 Performance
Thermal Efficiency
netth
in
W
Q
Mean Effective Pressure
net work for one cyclemep
displacement 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
BDC TDC
T
s1
2
3
4
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
BDC TDC
T
s1
2
3
4
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 u
Q Q u u
Thermal Efficiency
P
1
2
3
4
BDC TDC
v
Compression Ratio
1 4
2 3
v vCR
v v
11,cold ASC
2
1 1 k
th
TCR
T
T
s1
2
3
4
8
Otto Cycle Performance
Mean Effective Pressure
3 4 2 134 12
1 2
mep net
disp disp
u u u uW W W
V V v v
P
1
2
3
4
BDC TDC
v
T
s1
2
3
4
3 4 2 1
cold ASC
1 2
mepvc T T T T
v v
Btu Btu0.24 0.172
lbm-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
BDC TDC
T
s1
2
3
4
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
BDC TDC
T
s1
2
3
4
v
Diesel Cycle Performance
23 34 12 4 1,ASC
23 3 2
1netth
in
W W W W u u
Q Q h h
Thermal Efficiency
Compression Ratio
1
2
vCR
v
1
,cold ASC
11
1
k k
th
CR CO
k CO
Cutoff Ratio
3
2
vCO
v
P
1
2 3
4
BDC TDC
T
s1
2
3
4
v
12
Diesel Cycle Performance
Mean Effective Pressure
3 2 4 123 34 12
1 2
mep net
disp disp
h h u uW W W W
V V v v
3 2 4 1
cold ASC
1 2
mepp vc T T c T T
v v
P
1
2 3
4
BDC TDC
T
s1
2
3
4
v
Btu Btu0.24 0.172
lbm-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 the cylinder
– 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 net
r
W N WN
rev
Btu hp-minmincyl
cyl-cycle Bturev
cycle
netW
From the Otto or
Diesel Cycle analysis conversion factor
Crankshaft revolutions per cycle
Crankshaft rotational speed
Number of cylinders