Diesel four stroke engines  In Diesel engines they use a compression ignition they do not need a separate ignition system; only a glow plug system to get them started, when cold. This is the only major difference between gasoline and diesel engines (diesels don’t have spark plugs). High compression acts as the spark plugs for a diesel engine and diesel fuel being a colder slower burning fuel allows an operating temperature much lower than a gasoline vehicle and also permits larger cylinder displacement and a working RPM range much lower than gasoline engines. Lower operating temperatures and lower revolutions of the crank shaft per minute generally mean that diesel engines are more efficient because they have less friction than a gasoline engine, but emissions of diesel engines are much higher, this keeps them from being more widely used.