A diesel engine is a type of internal-combustion engine that is similar to the gasoline (or car) engine, but requires no electrical ignition system or carburetor. It was invented by Rudolf Diesel, a German engineer, who obtained a patent for the design in 1892. Diesels employ high compression ratios to elevate the compressed air temperature sufficiently to ignite a low-grade fuel that is injected into the cylinder. Components of diesels are usually heavier than those of gasoline engines because of the additional structural strength needed to obtain higher compression ratio and power output.
Diesel engines employ a system of fuel injection to spray the fuel into the cylinder after the air has been compressed by the piston. When this mixture is burning the expanding gases push the piston down and thus supply power. The timing of this fuel injection is just as critical as is the spark that ignites the fuel in the gasoline engine. Therefore, the injection mechanisms are mechanically liked to the crankshaft. Since each cylinder takes in and compresses a fixed amount of air, the power of the engine is varied by the amount of fuel injected. The timing, as well as the duration and pressure of the fuel injection, is designed so that the maximum useful energy is obtained from the fuel for a particular range of speed, power, acceleration, or other working conditions.
Diesel engines, like other internal combustion engines, require an exhaust system, a cooling system, and a starting system. Because of the unusually high compression ratios, diesel engines need a powerful starting system. Some diesel engines use an electric motor or an auxiliary gasoline engine, whereas others used compressed air to spark ignition to start the engine. Diesel engines have always been popular for large vehicles such as buses, trucks, locomotives, and ships. Small diesels have been used in automobiles, although the noise, soot, and pollutants they project have discouraged that used in the United States.