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A diesel engine is a type of internal-combustion engine developed by German engineer Rudolf Diesel (1858–1913) in the late nineteenth century. His original design called for the use of coal dust as fuel, but most modern diesel engines burn low-cost fuel oil. Whereas gasoline engines (found in the majority of present-day automobiles) use an electric spark to ignite the premixed fuel-air blend, diesel engines use compressed air to ignite the fuel.

In both gasoline and diesel engines, fuel is ignited in a cylinder, or chamber. Inside the sealed, hollow cylinder is a piston (a solid cylinder) that is attached at the bottom to a crankshaft. The movement of the piston up and down turns the crankshaft, which transfers that movement through various gears to the drive wheels in an automobile.

In a diesel engine cylinder, the piston completes one up-and-down cycle in four strokes: intake, compression, power, and exhaust. During the...