Comparison and analysis of engine exhaust gas energy recovery potential through various bottom cycles

In this paper, aimed to recover engine exhaust gas energy and improve engine thermal efficiency, various means of bottom cycles for engine exhaust gas energy recovery are proposed, and those include direct recovery means through exhaust gas expansion, such as secondary expansion, and indirect recovery means through heat transfer, such as Rankine steam cycle, Brayton air cycle, etc. The performances and characteristics of each bottom cycle are studied by cycle processes calculation and then the energy recovery potentials are compared. The results show that direct recovery bottom cycle through secondary expansion demonstrates little, if any, positive potential for a gasoline engine, and it only suits to diesel engine at full load with high boost pressure. The improvement range also differs with engine speeds and energy recovery potential is low. However, indirect recovery bottom cycles have larger applied range and higher exhaust gas energy recovery potential compared to direct recovery means. In all indirect recovery bottom cycles discussed, the maximum energy recovery potentials reduce in the sequence of over-heated Rankine steam cycle, standard Rankine steam cycle, Brayton air cycle with regeneration and standard Brayton air cycle.