Burt Rutan's White Knight and SpaceShip One, Photo Courtesy of Scaled Composites
Thermodynamics and Propulsion

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)