Comparing Expanders for Direct Recovery of Exhaust Energy From a Low-Heat-Rejection Diesel

Abstract

Part-load performance of a compound low-heat-rejection (LHR) engine is estimated at constant speed. The engine consists of an LHR diesel reciprocator geared to a supercharging compressor and an exhaust expander. Two classes of expander differing substantially in both flow characteristics and energy-extraction principles are ranked: aerodynamic (reaction turbine) and positive-displacement (internal expansion). To focus the comparison on differences in fundamental expander characteristics rather than differences in efficiency levels among specific samples of each type of expander, each is assigned an efficiency of 100 percent at its best-efficiency point. Although differences in fundamental characteristics between the expanders were sufficient to rank them on a performance basis, these differences were largely overshadowed by the magnitude of the indicated work developed in the reciprocator relative to the work developed by the expander.