Compression ignition engine Is a diesal. The fuel is injected under pressure into the cylinder after the air is already compressed. This ignites because of the temperature increase caused by the compression. Effectivly the fuel ignites because the heat energy contained in the air charge when it is compressed.(the molecules bump into each other more) Air conditioners use this effect in reverse.
--While the spark ignition engine uses spark plugs to ignite the fuel air charge.
