Compression is the amount of air pressure that is generated by the Piston inside the Engine Cylinder as it is raised from its lowest point to its highest point by the rotation of the Crankshaft. Usually the amount of Compression in an Engine is determined by the Compression Ratio. If an Engine has a 9:1 Compression Ratio, this means that the Air and Fuel mixture is reduced in size by a 9 to 1 ratio. High Compression Engines like Diesels can have a Compression Ratio of 22:1 or more.