Modern Engineering Thermodynamics

486 CHAPTER 13: Vapor and Gas Power Cycles
Fuel
Air
Heat and
combustion
exhaust
Internal
combustion Work
engine
(a) Open loop IC engine
Air
Fuel
Air
Heat and
combustion
exhaust
Heat exchanger Hot
Combustion
chamber
air
(b) Open loop EC engine
Air
exhaust
External
combustion
engine
Work
Air
Fuel
Air
Heat and
combustion
exhaust
Heat exchanger
Hot
Combustion
chamber
air
External
combustion
engine
Work
Cold air
(c) Closed loop EC engine
Heat to
environment
FIGURE 13.35
Internal and external combustion engine thermodynamic loop classifications.
13.11 PART II. GAS POWER CYCLES
Heat engines whose working fluid is a gas rather than a vapor undergo gas power thermodynamic cycles. Like
steam power, gas prime movers fall into two broad mechanical design categories: reciprocating and turbine. In
addition, unlike steam power prime movers, they fall into two heat source categories: external combustion (EC)
and internal combustion (IC). In external combustion engines, the working fluid does not enter into the combustion
process. Combustion, if it occurs at all, occurs outside the engine, with the resulting heat being transferred
into the working fluid at some point. All steam engines, therefore, are external combustion prime movers.
Since the working fluid of an internal combustion engine always enters into the heat-generating combustion
process, thus depleting the fuel and oxygen supply of the working fluid, the combustion products must be
removed and fresh fuel and oxygen added during each thermodynamic cycle. Consequently, all internal combustion
engines operate on an open loop process, whereas external combustion engines can operate on either an
open loop or closed loop process (see Figure 13.35).
Gas power cycle prime movers (engines) developed slightly later than their steam engine counterparts. They
evolved largely as an alternative to steam power technology, and by 1900, they were already very competitive
with small to medium power steam engines. By the mid 20th century, they had replaced all steam power within
the transportation industry and the small to medium electrical generating industry, leaving only large electrical
power plants as the major commercial users of steam.
13.12 AIR STANDARD POWER CYCLES
Most modern gas power cycles involve the use of open loop internal or external combustion engines. The working
fluid has highly variable physical and chemical properties throughout these engines, and this makes their
thermodynamic cycle very difficult to analyze. Since the most abundant chemical constituent of the working
fluid of air-breathing engines is nitrogen, which is largely chemically inert within the engine, it is possible to
devise an effective closed loop engine model in which air alone is considered to be the working fluid. Such an
approximation to real engine thermodynamics is called the air standard cycle, ASCforshort.TheASCallowsa
simple but highly idealized closed loop thermodynamic analysis to be carried out on an otherwise very complex
open loop system. The assumptions embodied in an ASC analysis of an IC or EC engine are as follows:
1. The engine operates on a closed loop thermodynamic cycle and the working fluid is a fixed mass of
atmospheric air.
2. This air behaves as an ideal gas throughout the cycle.