Prelecture 31: Slide 2

You are probably already familiar with the concept of an engine: it consumes energy and in return makes something useful happen. An internal combustion engine uses gasoline or diesel fuel to turn a drive shaft to make cars or trucks move; the steam engine of an old locomotive uses coal as a fuel to turn the locomotive's wheels; and a nuclear power plant uses uranium as fuel to turn generators that generate electricity.

These engines are clearly quite different, yet they do share some common features. In this course we will focus on heat engines, namely those that convert heat into mechanical work.

All heat engines are driven by a temperature difference. The idea is to extract some work as heat flows from a hot region to a cold region. In our car engine, gasoline is mixed with air and burned, causing the gas on one side of a piston to be hotter than the gas of the other side. This results in a pressure difference between the hot and cold side, causing the piston to move and the engine to do useful work.

Another defining feature of a heat engine is that its operation must be cyclical. In other words, it keeps doing the same thing over and over again, each time with the same result. In the car engine, as the piston moves back to its original position, the hot gas that just caused the piston to move must be replaced by a new batch of colder air mixed with gasoline. In this way, the whole process can repeat again.