The Heat Engine
A heat engine is a device that uses a source of thermal energy to do work by converting heat into work. Internal combustion engines, steam engines and power station turbines are all heat engines. The ideal heat engine is shown below. It consists of a infinitely large hot reservoir at a temperatureand an infinitely large cold reservoir at a temperatureHeat flows from the hot to the cold reservoir. Being infinitely large, the temperature of neither reservoir is changed as a result of heat flow. Some of the heat transferred can be made to do work.
An ideal gas can be used as a heat engine. The diagram below illustrates a gas undergoing a cycle of expansion (1 – 2), release of heat energy (2 – 3), compression (3 – 4) and absorption of heat energy (4 – 1).
The gas returns to it's initial pressure, volume and temperature. During this process, it has absorbed energy and given out energy. The gas has had work equal to the area under the curve 3 - 4 done on it, and has done an amount of work equal to the area under 1 - 2. The net work done by the gas is the difference between these two, the area of the rectangle.
The thermal efficiency of a heat engine is given as
The cycle that results in the maximum possible efficiency is called the Carnot cycle.