Engine manufacturers seek to make their engines as efficient as possible. For combustion type engines, the amount of work done is related to the pressure inside the cylinders in the engine and the volume of the enclosed region inside the cylinders. The cycle for a typical engine is shown in the figure below:
The amount of work done by the engine is the area enclosed by the region and is usually measured in Joules (1 Joule of energy is a Newton-meter, denoted N · m). The optimal amount of work the engine can do is determined by the area between the top curve and the Volume axis. The ratio of these two areas is the efficiency of the engine.