Modern Engineering Thermodynamics

518 CHAPTER 13: Vapor and Gas Power Cycles
Compressor
Exhaust out
Intake manifold
Air in
Turbine
Compressor
Air in
Power
Power
Intake
manifold
Exhaust
out
Exhaust
manifold
(a) Supercharging
Engine
Exhaust
manifold
(b) Turbocharging
Engine
Air in
Turbine
Intake
manifold
Exhaust out
Engine
Exhaust
manifold
Power
(c) Turbocompounding
FIGURE 13.56
Air and exhaust flow arrangements for supercharging, turbocharging, and turbocompounding an internal combustion engine.
13.23 SECOND LAW ANALYSIS OF VAPOR AND GAS
POWER CYCLES
The difference between the isentropic thermal efficiency and the actual thermal efficiency of a system is due to
the effects of the second law of thermodynamics. The second law can be used to determine viscous irreversibilities
leading to pressure losses in pipes, valves, and fittings; to determine heat transfer irreversibilities due to
incomplete insulation and large temperature gradients; and to determine mechanical and chemical irreversibilities
in pumps, compressors, and prime movers due to friction and chemical reactions. However, piping and
ancillary viscous losses (irreversibilities) are normally determined through the empirical friction factor material
introduced in Chapter 9 (and found in most fluid mechanics textbooks), and mechanical and thermal losses in
machinery are globally lumped into the empirically determined isentropic efficiency, η s . The application of the
second law to complex engines and turbines is so difficult today that it is not normally used in the engineering
design stage of product development. This will, no doubt, change as technology and engineering analysis
advance in the future.
The thermal efficiency η T is essentially a first law energy conversion efficiency, in that it is concerned with the
effectiveness of an energy conversion process as the ratio of a desired output to a required input. The isentropic
efficiency η s , on the other hand, can be viewed as a second law energy conversion efficiency in that it compares
the actual energy conversion performance of a real (irreversible) device with its idealized reversible counterpart.
Consequently, the primary role of the second law of thermodynamics in the analysis of vapor and gas power
cycles today is through the (largely empirical) determination and use of the isentropic efficiency. Many of the
practical aspects of engineering courses in heat transfer, fluid mechanics, electrical circuit theory, and machine
design today are the result of the consequences of the second law of thermodynamics.