Working fluid selection for a subcritical bottoming cycle applied to a high exhaust gas recirculation engine

The selection of a suitable working fluid for a BC (Bottoming Cycle) system is one of the most important steps in maximising system performance and minimising the system size and cost. The presented work details a systematic approach in the selection of working fluids applied to a subcritical cycle with minimum superheat. Over 60 different synthetic, organic, and inorganic fluids were studied. The fluid selection study was decomposed into numerous fluid screening and fluid ranking criteria with common boundary conditions and assumptions. After the cycle was optimised for maximum overall conversion efficiency, the fluid ranking criteria allowed the objective assessment of the working fluids. Acetone, dichloromethane and trans-1,2-dichloroethylene were found as the best candidates for optimal performance and system related trade-offs, contrary to commonly used R245fa, ethanol and water. A BC integrated into an EGR (Exhaust Gas Recirculation) only engine platform to meet Euro 6 oxides of nitrogen emission is examined for improved fuel economy and reduced load on the engine cooling module. The BC simulation results for EGR and partial high temperature after-cooler heat recovery using the proposed new fluids show a specific fuel saving potential between 9.8 and 13.7% for a typical cruise and high load conditions.