In an internal combustion engine the energy supplied by a burning fuel is directly converted into mechanical energy by the controlled burning of the fuel in an enclosed space. The explosive fuel-air mixture may be ignited either by an electric spark or by the resulting compression temperature. In reciprocating engines the explosion causes the rotation of some engine parts by driving the piston in the cylinder. The motion is transmitted to the crankshaft by means of the connecting rod.
We can classify internal combustion reciprocating engines according to the number of strokes of the piston in one complete working cycle. Thus, we can speak of two-stroke engines and four-stroke engines.
The complete cycle of events of the former group, that is suction, compression, explosion and exhaust, is accomplished in a single revolution of the crankshaft or in two strokes of the piston, as the compression and expansion of the charge take place during one stroke, while the admission of a fresh charge occurs during the other stroke simultaneously with the escape of the burnt gases. In four-stroke engines the complete engine cycle is accomplished within two revolutions of the crankshaft or four strokes of piston.
There is still another classification according to the process of combustion: explosion or constant-volume combustion engines and constant-pressure combustion or Diesel engines.
- Spark-ignition engine – Internal combustion engine typically running on gasoline or natural gas, where the combustion process is initiated by a spark plug.