Heat Recovery and Bottoming Cycles for SI and CI Engines - A Perspective

The pursuit of fuel economy is forcing technology change across the range of control and engine management technologies. Improved thermal management has been addressed in order to promote fast warm-up, improved exhaust gas after-treatment performance, and lower variance in combustion through a consistent and high cylinder head temperature. Temperature management of exhaust gas is of increasing interest because of the need to maintain efficiency in after-treatment devices. More effective temperature management places requirements on heat exchange systems, and offers the potential for bottoming and heat recovery cycles that use energy transferred from the exhaust stream.

Turbo-compounding is already established in heavy duty engines, where a reduction in exhaust gas temperature is the consequence of an additional stage of expansion through an exhaust turbine. A new project in electric turbo-compounding offers flexibility in the control of energy extracted from the exhaust stream[1]. Small scale fluid power systems can implement continuous bottoming cycles for both heavy and light duty engines. This represents another means of extracting energy from the exhaust gas stream. For light duty applications, the fluid cycle can utilize stored thermal energy after prolonged high load conditions at a time when the traction battery may be depleted. In heavy duty engines, the cycle will simply provide a means of recovering exhaust energy and thereby boost the vehicle's fuel economy. The particular relevance to hybrid vehicles comes through the architectural advantages of generating electricity using the recovered heat energy. Thermo-electric devices represent one, but by no means the only way to build heat recovery into a hybrid propulsion system.

In this paper we will review and compare a range of previously published techniques from the point of view of their fundamental thermodynamic behavior and their ability to sustain typical drive and duty cycles.