Animation of a four stroke engine's thermodynamic power cycle: intake, compression, combustion, and exhaust. At intake (blue arrow), a fuel air mixture is drawn in by the descending piston. At compression, the piston rises to compress the mixture. Combustion starts when the spark plug ignites the explosive compressed mixture and the piston descends again. Finally, the piston rises pushing the exhaust gases out of the cylinder (red arrow) ready for the cycle to begin again. This type of engine is found in most vehicles.