The most influential force in aircraft performance is the force that the engine generates, that is, thrust. A powerful engine compensates any deficiency that an aircraft may have in terms of high drag or heavy weight. Three important unconventional engines are man-powered engines, sun-powered engines, and electrically powered engines. A piston engine, also known as a reciprocating engine, is a heat engine that employs one or more pistons to release fuel energy through a combustion process and to convert it to rotating mechanical energy. The additional mass of air that is forced into the engine improves its efficiency, which allows it to burn more fuel, which results in the production of more power. A group of aero-engines such as turbojet, turbofan, and turboprop engines are classified as gas turbines or simply turbine engines in which a turbine is used instead of the piston-cylinder to produce shaft power.