engines

An engine is a machine that converts energy into mechanical work. It produces motion by applying forces to moving parts such as pistons, rotors, or turbines. In common usage, the term engine is often used for devices that generate motion from energy, while motors are typically electrical devices that produce motion; however, terminology varies by context and field.

Internal combustion engines burn fuel inside cylinders to create high-pressure gases that push components and generate power. This category includes piston engines, which operate on four-stroke or two-stroke cycles, and, less common today, rotary engines. The fuels used span gasoline, diesel, natural gas, and renewable alternatives such as biofuels.

External combustion engines burn fuel outside the working space. The most well-known example is the steam engine, where steam expands to move pistons or drive a turbine. Gas turbines and Stirling engines are other examples of external or semi-external combustion concepts used in power generation and propulsion.

Electric machines also perform energy-to-work conversion, with electric motors converting electrical energy into mechanical motion. Although often treated separately from engines, motors and engines share the goal of turning energy into usable power. Power sources include batteries, generators, and the electrical grid.

Key design considerations for engines include efficiency, power density, durability, and emissions. Modern engines employ advanced technologies such as turbocharging or supercharging, direct fuel injection, variable valve timing, hybridization, and exhaust aftertreatment to improve efficiency and reduce environmental impact. They power transportation, industry, and utilities, making them central to modern energy use.