Compression ignition engine

A compression ignition engine relies on the high temperature of a compressed gas to ignite the fuel, rather than a separate source of energy (such as a spark plug). This effect is also called the Diesel Cycle.

Charles Law states that if the volume of a container is decreased, the temperature of the gas inside increases. When the piston in an internal combustion engine performs its compression stroke, the air inside the cylinder is squeezed to a fraction of its original volume, which multiplies the pressure and temperature correspondingly.

In a compression ignition engine, fuel is injected into the hot gases and ignites immediately, driving the piston's power stroke.

In a spark ignition engine, a spark is required to ignite the fuel after it has been injected into the cylinder.

A common example is the diesel engine. Esoteric examples include coal powder engines and vegetable oil engines.

A gasoline engine can sometimes act as a compression ignition engine under abnormal circumstances, a phenomenon typically described as "pinging" or "pinking" (during normal running) or "dieseling" (when the engine continues to run after the electrical ignition system is shut off).