3. The First Law Applied to Engineering Cycles
[VW, S & B: Chapter 9, 11.8, 11.9, 11.10, 11.11, 11.12, 11.13,
11.14]
This chapter is devoted to describing the fundamentals of how
various heat engines work. A heat engine is a device that uses heat
to produce work, or uses work to move around heat. Refrigerators,
internal combustion (automobile) engines, and jet engines are all
types of heat engines. We will model these heat engines as
thermodynamic cycles and apply the First Law of Thermodynamics to
estimate thermal efficiency and work output as a function of
pressures and temperatures at various points in the cycle. This is
called ideal cycle analysis. The estimates we obtain from the
analysis represent the best achievable performance that may be
obtained from a heat engine. In reality, the performance of these
systems will be somewhat less than the estimates obtained from ideal
cycle analysis - you will learn how to make more realistic
estimates later.
Our analyses will use the ``air-standard cycle,'' which is an
approximation to actual cycle behavior. Specifically, we make the
following simplifications:
- Air is the working fluid (the presence of fuel and combustion products is
neglected)3.1,
- Combustion is represented by heat transfer from an external heat
source,
- The cycle is `completed' by heat transfer to the surroundings,
- All processes are internally reversible (described more fully
in Chapter 5), and
- Air is a perfect gas with constant specific heats.
Muddy Points
How can we idealize fuel addition as heat addition?
(MP 3.1)
Subsections
UnifiedTP
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