Thermodynamic Cycles
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Comparison of Power Cycles: Gas vs. Vapor, Closed vs. Open, Internal vs. External Engines, Schemes and Mind Maps of Law
An overview of various thermodynamic cycles, including power cycles, gas vs. vapor, closed vs. open, and internal vs. external combustion engines. It covers the Otto Cycle, Diesel Cycle, Carnot Cycle, and Air-Standard Assumptions. The document also discusses the differences between the Otto and Carnot cycles and the advantages and disadvantages of using different compression ratios in Otto and Diesel engines.
What you will learn
- What is the role of heat in thermodynamic cycles?
- How does the Otto Cycle differ from the Carnot Cycle?
- What are the advantages and disadvantages of using different compression ratios in Otto and Diesel engines?
- What are the different types of thermodynamic cycles?
- How does the Air-Standard Assumption impact the analysis of thermodynamic cycles?
Typology: Schemes and Mind Maps
2021/2022
Uploaded on 09/12/2022
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Thermodynamic Cycles
- Look at different cycles that approximate real
processes
- You can categorize these processes several
different ways• Power Cycles vs. Refrigeration• Gas vs. Vapor• Closed vs. open• Internal Combustion vs. External Combustion
Ideal Cycles
- We’ll be using ideal cycles to analyze
real systems, so lets start with the onlyideal cycle we’ve studied so far
Carnot Cycle
W
Q
Q-W=
Æ
Q=W
In addition, we know that the efficiency for a
Carnot Cycle is:
th Carnot
L H
T T
,^
A
ir-Standard Assumptions• Air continuously circulates in a closed
loop and behaves as an ideal gas
- All the processes are internally
reversible
- Combustion is replaced by a heat-
addition process from the outside
- Heat rejection replaces the exhaust
process
- Also assume a constant value for C
,p
evaluated at room temperature
T
erminology for Reciprocating Devices
r^
V V
V V
BDC TDC
=
=
maxmin
Compression Ratio
v v
1-2 Isentropic Compression2-3 Constant Volume Heat Addition3-4 Isentropic Expansion4-1 Constant Volume Heat Rejection
Apply First Law Closed System to Process 4-1,V = Constant
(^14)
(^41) ,
(^41) ,
(^41) ,
41
(^41) ,
(^41) ,
∫
PdV
W
W
W
U
W
Q
b
other
net
net
net Q
U
Q
Q
mC
T
T
Q
mC
T
T
mC
T
T
net net
out
v
out
v^
v
, ,^
(
)
(^
)
(^
)
41
41
41
1
4
1
4
4
1
= = −
=
−
= −
−
=
−
∆
η
th Otto
out in
v v
Q Q mC
T
T
mC
T
T
,
(^
)
(^
)
=
−
−
− −
1 1
4
1
3
2
η
th Otto
T
T
T
T
T T
T
T
T
T
,
(^
)
(^
)
(^
/^
)
(^
/^
)
=
−^
− −
=^
−^
− −
1 1
1 1
4
1
3
2
1
4
1
2
3
2
Æ
Recall processes 1-2 and 3-4 are isentropic, so
1
4 3
3 4
1
1 2
(^21)
T
and
−
−
⎞ ⎟⎟ ⎠
⎛ ⎜⎜ ⎝
⎞ ⎟⎟ ⎠ ⎛ ⎜⎜ ⎝ =
k
k
v v
T
v v
T T^ v^3
= v
2
and v
4
= v
1
T T
T T
or T T
T T
2 1
3 4
4 1
3 2 = =
Æ
Efficiency of the Otto Cycle vs. Carnot Cycle
- There are only two temperatures in the
Carnot cycle• Heat is added at T
H
- Heat is rejected at T
L
- There are four temperatures in the Otto
cycle!!• Heat is added over a range of temperatures• Heat is rejected over a range of temperatures
1
1
2 1
1 2
−
k
k
r
V V
Since process 1-2 is isentropic, T T
η
th Otto
k r
,^
=
−
−
1
1
1
th Otto
T T
,^
(^12)
Æ
Increasing Compression RatioIncreases the Efficiency
Typical CompressionRatios for GasolineEngines
Diesel Engines
- No spark plug• Fuel is sprayed into hot compressed air
State Diagrams for the Diesel Cycle