Page 50 - Advanced Thermodynamics for Engineers, Second Edition

CHAPTER

ENGINE CYCLES AND THEIR

EFFICIENCIES 3

The concept of internally reversible cycles was introduced in Section 2.7: these are sometimes known
as endoreversible cycles. The advantage of considering endoreversible cycles is that the area of the
cycle on a T–s diagram represents the work done by, or on, the fluid when it executes that cycle. If a
cycle is not endoreversible then the area on the T–s diagram of the cycle is not equivalent to the work
done. There are a number of important endoreversible cycles and these are introduced below.

3.1 HEAT ENGINES
3.1.1 CARNOT CYCLE
The Carnot cycle, as originally proposed, is both internally and externally reversible. It consists of two
isentropic and two isothermal processes, as shown in Fig. 3.1, for a cycle based on a perfect gas.
Fig. 3.1(b) has been based on processes for a perfect gas. The reversible, adiabatic processes obey
k
the law pV ¼ c, and the reversible, isothermal processes obey the law pV ¼ c. It can be seen from

Reversible
(a) (b) isothermal
expansion Q in
2
Temperature, T T 2 2 Q in 3 T H Pressure, p 3

Reversible
adiabatic T H
compression
1 Reversible
T 1 T adiabatic
1 4 C expansion
Q out Reversible
isothermal Q out 4
compression T C
Specific entropy, s Volume, V

FIGURE 3.1
Carnot cycle based on a perfect gas as the working fluid. (a) Temperature–entropy (T–s) diagram;
(b) pressure–volume (p–V ) diagram.
Advanced Thermodynamics for Engineers. http://dx.doi.org/10.1016/B978-0-444-63373-6.00003-4 35
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