Four-Stroke Engines
In the 4-stroke system, changes in volume of the combustion chamber are alternately used to produce work and to exchange gases. Gas exchange is achieved using valves. An entire 4-stroke cycle consists of the four phases: intake, compression, expansion, and exhaust strokes. Figure shows the gas pressure curve of a 4-stroke reciprocating engine which depends on the working volume and the crankshaft angle, as presented in the pi (V) or pi (α) diagram.