The diesel engine is a type of internal combustion engine that was invented by Rudolf Diesel. He received a patent for the diesel engine in 1892 and the primary goal was to create an efficient alternative to the gasoline engine.
Both gasoline engines and diesel engines work by creating a controlled explosion in a sealed piston chamber. The small explosion rapidly moves the piston which in turn rotates the output shaft. In a gasoline engine, a mixture of fuel and air are injected into the chamber and then ignited with a spark created by a sparkplug. A diesel engine, on the other hand, does not rely on a sparkplug to ignite the mixture. Fuel is forced into the chamber and the high pressure generates enough heat to ignite the fuel/air mixture.
Some diesel engines rely on a glow plug to heat the chamber to minimize the amount of pressure required for the engine to turn. Without such an added heat source, the pressure required to achieve ignition (especially when the engine is cold) would be prohibitively high.
Diesel engines require diesel fuel for the combustion process to function properly. Diesel fuel is cheaper than regular gasoline because less refining is required. In addition, diesel engines are more efficient and therefore diesel cars get better mileage than their gasoline counterparts.