engine

An engine is a machine designed to convert energy into mechanical work. Engines draw energy from a fuel, a stored thermal reservoir, or an electrical source and transform it into rotational or linear motion. In engineering practice, engines are distinguished from machines that do not convert energy directly into motion. For example, electric motors convert electrical energy into motion, whereas engines typically derive energy by combustion or by harnessing high-temperature, high-pressure fluids.

Internal combustion engines (ICEs) convert chemical energy from fuels into mechanical work through combustion within cylinders. Common types include spark-ignition engines (gasoline) and compression-ignition engines (diesel). Some designs, such as the Wankel rotary engine, achieve rotation with a different geometry. External combustion engines burn fuel outside the working chamber, heating a separate working fluid; classic examples are steam engines and Stirling engines.

Most engines share basic elements: an energy source, a working medium, conversion mechanism (pistons, turbines, or rotors), and a method for controlling intake, exhaust, cooling, and lubrication. Performance is described by efficiency, power, torque, and specific power. Efficiency depends on cycle, compression ratio, friction, and heat losses; emissions and noise are major modern considerations.

Historical development began with steam engines in the 18th century, transforming industry and transport. The advent of practical internal combustion engines in the late 19th century led to automobiles, aircraft, and ships. Today, engines range from small spark-ignition units in cars to large aero-derivative gas turbines and diesel generators, with advances in lightweight materials, turbocharging, and emission control.

Applications span transportation, power generation, and industrial machinery. Environmental and regulatory concerns drive efforts to increase efficiency and reduce pollutants, including hybridization, electrification, and alternative fuels.