Evaluation of Positive-Displacement Supercharging and Waste Heat Recovery for an LHR Diesel

One demonstrated method for improving the shaft thermal efficiency of turbocharged heavy-duty diesel engines is turbo-compounding--the incorporation of a power turbine, geared to the engine output shaft, which can supply additional power extracted from the exhaust. An alternative approach is to use a screw-type positive displacement compressor/expander geared to the engine output shaft, as a substitute for both the turbocharger and power turbine. One concept for the latter approach was described in recent literature, with an accompanying analysis indicating a significant advantage over turbo-machinery at full load. The present paper reports the results of a more detailed comparative analysis between this screw-machine and turbomachinery when applied to supercharging and waste heat recovery in heavy duty diesels. These comparisons are developed through the use of an engine system simulation code and are examined for both full load and part load cases, including curb idle. In order to extend this analysis to advanced diesel engine designs, comparisons also are developed for a highly insulated configuration. The results show that at high loads the screw machine has an advantage over turbomachinery but that at part loads and curb idle the reverse situation exists. Insulation is found to have negligible effect on the relative merits of the two types of systems.