heat engines are devices that generate power from heat that flows from a hot to a cold reservoir. The mechanism of operation of the engine can be described by the laws of quantum mechanics. Quantum refrigerators are devices that consume power with the purpose to pump heat from a cold to a hot reservoir. In a reciprocating quantum heat engine, the working medium is a quantum system such as spin systems or a harmonic oscillator. The Carnot cycle and Otto cycle are the ones most studied. The quantum versions obey the laws of thermodynamics. In addition, these models can justify the assumptions of endoreversible thermodynamics. A theoretical study has shown that it is possible and practical to build a reciprocating engine that is composed of a single oscillating atom. This is an area for future research and could have applications in nanotechnology. Miscellaneous engines There are a large number of unusual varieties of piston engines that have various claimed advantages, many of which see little if any current use: Free-piston engine Opposed-piston engine Swing-piston engine IRIS engine Bourke engine Thermo-magnetic motor See also Heat engine for a view of the thermodynamics involved in these engines. For a contrasting approach using no pistons, see the pistonless rotary engine. For an historical perspective see Timeline of heat engine technology. Steam engine Steam locomotive Stirling engine Internal combustion engine Otto cycle Diesel cycle Engine configuration for a discussion of the layout of the major components of a reciprocating piston internal combustion engine. Diesel engine Gasoline engine Notes External links Combustion video – in-cylinder combustion in an optically accessible, two-stroke engine HowStuffWorks: How Car Engines Work Reciprocating Engines at Infoplease. Piston Engines at the US Centennial of Flight Commission. Engine technology