A heat engine is a device that converts thermal energy from a heat source into mechanical work by utilizing temperature differences between hot and cold reservoirs. Heat engines operate on thermodynamic cycles that absorb heat at high temperature, produce work output, and reject waste heat to a cold reservoir. Air conditioning systems function as reverse heat engines, using mechanical work to move heat from cold spaces to warmer environments.
Operating Principles
Heat engines achieve maximum theoretical efficiency according to the Carnot cycle, calculated as 1 minus (cold temperature divided by hot temperature). A coal-fired power plant with 600 Kelvin hot reservoir and 300 Kelvin cold reservoir has maximum theoretical efficiency of 50 percent. Real engines operate at 30 to 40 percent efficiency due to friction and irreversible processes.
HVAC Applications
Compressor-based air conditioning systems operate as heat pumps, a reverse heat engine configuration. These systems use electrical or fuel-based work input to extract heat from indoor air and reject it outdoors during cooling mode. Heat pump efficiency, measured in coefficient of performance, typically ranges from 2 to 4, meaning one unit of work moves 2 to 4 units of heat.