Waste heat recovery in various types of combustion engines is a way to improve the overall efficiency of these systems. Waste heat recovery systems range from power plants that have bottoming cycles, to thermoelectric systems that generate electricity. Power plants that have bottoming cycles utilize the excess heat in the low pressure exhaust gases from the primary work generating cycle. Thermoelectric systems utilize similar waste heat sources.
On piston engines, waste heat recovery systems can consist of a closed loop Rankine Cycle. A Rankine Cycle uses the heat from the exhaust to power the cycle. These systems typically have a separate, dedicated, expander that extracts power from the working fluid and is connected to the crankshaft of the engine.