Selecting Components for a Compound Low-Heat-Rejection Diesel for Light-Duty Use

To realize the fuel-economy potential of the low-heat-rejection diesel, turbocompounding is usually advocated for the heavy-duty application. The delayed response of the turbocharger in this system is more objectionable in the passenger car and light-duty truck than in the heavy-duty application. As an alternative to turbocompounding, that delay may be avoided by gearing a supercharging compressor and exhaust expander directly to the diesel reciprocator. The choice between centrifugal and helical-screw compressors is analyzed for this arrangement, with the latter promising a superior torque characteristic. Part-load trends in engine efficiency are then compared at constant engine speed for idealized turbine and positive-displacement expanders incorporated in various ways. The most beneficial of the options considered uses a turbine joined to the crankshaft through a variable-speed drive.