Modern Engineering Thermodynamics

13.12 Air Standard Power Cycles 487
3. The combustion process within the engine is replaced by a simple heat addition process from an external
heat source.
4. The intake and exhaust processes of the engine are replaced by an external heat rejection process to the
environment.
5. All processes within the thermodynamic cycle are assumed to be reversible.
Note that item 5 implies that all processes within a cycle that have no associated heat transfer are also isentropic
processes (i.e., they are reversible and adiabatic). Since the numerical results of an analysis using the ASC
depends on how the ideal gas specific heat issue is handled, an ASC analysis is further characterized as either a
cold air standard cycle, if the specific heats of air are assumed to be constant and evaluated at room temperature,
or a hot air standard cycle, ifthespecificheatsofairareassumedto be temperature dependent. When a more
complex analysis is done, in which the actual fuel-air mixture and exhaust gases are used, it is usually called a
real mixture standard cycle.
An IC or EC engine operating on an ASC can be represented schematically as shown in Figure 13.36. Since the
actual operating thermal efficiency of an IC engine is often compared to its ideal ASC thermal efficiency to evaluate
the impact of real world irreversibilities on the engine’s performance, the ASC analysis serves the same type
of idealized benchmark comparison function as the isentropic Rankine cycle analysis of vapor power cycles.
If the working fluid in the Carnot cycle shown in Figure 13.37 is air (functioning as an ideal gas), then we
would have a Carnot ASC. Since (by definition) a Carnot cycle is thermodynamically reversible, using the notation
of Figure 13.37, s 1 = s 2 and s 3 = s 4 .
Isentropic ideal gas compression and expansion processes with constant specific heats are discussed in Chapter 7,
and the p-v-T relation for these processes is given by Eq. (7.38), which shows that it can be written directly in
terms of either the temperature ratio, pressure ratio, or compression (volume) ratio.
For an isentropic expansion process, s 1 = s 2 = s 2s ,
T 2s
= p ðk−1Þ/k
2s
= v 1−k
2s
T 1 p 1 v 1
For an isentropic compression process, s 3 = s 4 = s 4s ,
Q H (from external heat source)
where
T 3
T 4s
= p 3
p 4s
ðk−1Þ/k
= v 3
v 4s
1−k
Air standard cycle
internal combustion engine
Work
out
T 1 = T 4s = T H
and
T 2s = T 3 = T L
We further define
v 2s /v 1 = v 3 /v 4s = isentropic compression ratio, CR
and
p 1 /p 2s = p 4s /p 3 = isentropic pressure ratio, PR
Inlet
air
Environment
Q L
Exhaust
air
FIGURE 13.36
IC or EC engine operating on a closed loop air standard cycle.
T H
T
T L
FIGURE 13.37
The Carnot air standard cycle.
Q H
4s
1
3 2s
Q L
s
1
T = Constant s= Constant
p
2s
4s
T = Constant
s= Constant
3
V