- device converting energy into mechanical work
- types: internal combustion (gasoline, diesel), electric motor, steam, jet, rocket
- internal combustion: fuel-air mixture ignites in cylinders, drives pistons and crankshaft
- electric motor: electromagnetic interaction between stator and rotor, high efficiency
- steam engine: heat boils water, expanding steam drives piston or turbine
- jet engine: continuous combustion, thrust from exhaust gas acceleration
- efficiency measured as ratio of useful work to input energy
- powers transport, pump systems, generators, industrial machinery
- electrification of engines is the core transition of the 21st century energy system
- battery and semiconductor controls enable modern electric drivetrains