13 - Engines

Summary

The general idea

Engines convert heat into work. Thermodynamics owes both its name, “heat-motion”, and much of its early development to the study of engines. The working system for most engines interacts both thermally and mechanically with other systems, so its properties depend on two independent variables. Most engines are cyclical, so that the working system goes through the following stages:

• it is heated;

• it expands and does work, pushing a piston or turbine blades;

• it is cooled further;

• it is compressed back into its original state, ready to begin the cycle again.

The expansion occurs when the working system is hot and is under high pressure or has a larger volume, and the compression occurs when it is cooler and is under lower pressure or has a smaller volume. Therefore, the work done by the engine while expanding is greater than the work done on the engine while being compressed. So there is a net output of work by the engine during each cycle. This is what makes engines useful. If you can understand this paragraph, you understand nearly all engines.

The details vary from one engine to the next. The working system could be any of a large variety of gases or volatile liquids. The source of heating could be such things as a flame, a chemical explosion, heating coils, steam pipes, sunlight, or a nuclear reactor. The cooling could be provided by such things as air, water, ice, evaporation, or radiative coils.