Because diesel has a higher compression resistance — because it is denser than light distillate fuels — engineers can design and develop diesel engines with higher compression ratios than gasoline engines. The higher the compression ratio of an engine, the greater the thermal efficiency.

In a diesel engine, there is no spark plug. Instead, diesel fuel is injected into the cylinder, and the heat and pressure of the compression stroke cause the fuel to ignite. Diesel fuel has a higher energy density than gasoline, so a diesel engine gets better mileage.

Gasoline engines and diesel engines both work by internal combustion, but in slightly different ways. In a gasoline engine, fuel and air is injected into small metal cylinders. A piston compresses (squeezes) the mixture, making it explosive, and a small electric spark from a sparking plug sets fire to it. That makes the mixture explode, generating