Engine is a machine that can convert other forms of energy into mechanical energy, including internal combustion engines (reciprocating piston engines), external combustion engines (Stirling engines, steam engines, etc.), jet engines, motors, etc. For example, internal combustion engines usually convert chemical energy into mechanical energy.

The diesel engine has better efficiency, so it can reduce carbon dioxide emissions, but the pollution is high (for example, long carbon chain organic compounds are prone to produce suspended particulates (SS) mainly composed of carbon particles after combustion); The use of high quality (ultra-low sulfur) diesel fuel and newer engine and pollution control technologies can reduce a lot of pollution, but until the latest technology can make diesel vehicles meet the environmental standards of the United States.