Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

9–7 ■ STIRLING AND ERICSSON CYCLES

The ideal Otto and Diesel cycles discussed in the preceding sections are
composed entirely of internally reversible processes and thus are internally
reversible cycles. These cycles are not totally reversible, however, since they
involve heat transfer through a finite temperature difference during the non-
isothermal heat-addition and heat-rejection processes, which are irreversible.
Therefore, the thermal efficiency of an Otto or Diesel engine will be less
than that of a Carnot engine operating between the same temperature limits.
Consider a heat engine operating between a heat source at THand a heat
sink at TL. For the heat-engine cycle to be totally reversible, the temperature
difference between the working fluid and the heat source (or sink) should
never exceed a differential amount dTduring any heat-transfer process. That
is, both the heat-addition and heat-rejection processes during the cycle must
take place isothermally, one at a temperature of THand the other at a tem-
perature of TL. This is precisely what happens in a Carnot cycle.

Chapter 9 | 503

Process 2-3 is a constant-pressure heat-addition process, for which the

boundary work and uterms can be combined into h.Thus,

Process 4-1 is a constant-volume heat-rejection process (it involves no work

interactions), and the amount of heat rejected is

Thus,

Then the thermal efficiency becomes

The thermal efficiency of this Diesel cycle under the cold-air-standard

assumptions could also be determined from Eq. 9–12.

(c) The mean effective pressure is determined from its definition, Eq. 9–4:

Discussion Note that a constant pressure of 110 psia during the power

stroke would produce the same net work output as the entire Diesel cycle.

110 psia

MEP

Wnet

VmaxVmin

Wnet

V 1 V 2

1.297 Btu

1117 6.5 2 in^3

a

778.17 lbf#ft

1 Btu

ba

12 in.

1 ft

b

hth

Wnet

Qin

1.297 Btu

2.051 Btu

0.632 or 63.2%

Wnet
QinQout
2.0510.754
1.297 Btu

0.754 Btu

1 0.00498 lbm 21 0.171 Btu>lbm#R 2311425  5402 R 4

Qout
m 1 u 4 u 12 
mcv 1 T 4 T 12

2.051 Btu

1 0.00498 lbm 21 0.240 Btu>lbm#R 2313432  17162 R 4

Qin
m 1 h 3 h 22 
mcp 1 T 3 T 22