What is the difference between how a gasoline engine and a diesel engine ignite their fuel?

A gasoline engine uses a spark plug to ignite the compressed air-fuel mixture at the right moment. A diesel engine has no spark plugs; instead it compresses air to a very high pressure, which makes it hot enough that injected diesel fuel ignites on contact (compression ignition). Diesel engines run at higher compression ratios and are generally more fuel-efficient and produce more low-end torque, while gasoline engines rev higher and run more smoothly at high speed. Knowing this key difference — spark ignition versus compression ignition — is a frequently tested engine-basics concept.