Waste Heat Recovery Potential of Advanced Internal Combustion Engine Technologies

Coupling waste heat recovery with internal combustion engines creates opportunities to improve overall system efficiency and power output. The internal combustion engine has multiple pathways for dissipating thermal energy; the engine's exhaust is one that is conveniently accessible for converting to useful work via waste heat recovery. Coincident with increased waste heat recovery efforts, however, is increased engine efficiency improvement efforts. Anecdotally, an increase in engine efficiency will typically result in a decrease in exhaust exergy, thus decreasing the power capability of a waste heat recovery system. Further, other developments are taking place with internal combustion engines, such as the use of alternative fuels and combustion modes designed to decrease engine emissions, which may affect engine exergy. This article explores the relationships that may exist, both fundamentally and in practical application, between engine parameters and the corresponding effect on the maximum waste heat recovery potential (i.e., exergy) of the engine's exhaust. Specifically, the objectives of this study are to quantify (1) the effects of typical trends in internal combustion engine technology (i.e., increased compression ratio, decreased fuel–air equivalence ratio, and increased exhaust gas recirculation level) on waste heat recovery potential, (2) the role certain alternative fuels, particularly biodiesel, may have on waste recovery, and (3) the influence of and opportunities created by certain advanced modes of combustion, particularly low temperature combustion (LTC), on waste heat recovery potential. The study finds that fundamental engine parameters that typically result in increases in engine efficiency (i.e., increased compression ratio, decreased fuel–air equivalence ratio, and increased exhaust gas recirculation level) result in decreased exhaust exergy that decreases both efficiency and maximum power capability of a waste heat recovery system. Practical application of alternative fuels, such as biodiesel, seems to have small to no effect on the waste heat recovery. Application of novel modes of combustion, such as LTC, may result in decreases in waste heat recovery due to decreased exhaust mass flow rates typical of such combustion modes. Waste heat recovery may, however, create an opportunity to increase efficiency of LTC by exploiting chemical-to-thermal exothermic generation associated with the typically observed high concentrations of unburned fuel in the exhaust.