Recovering Energy from the Diesel Engine Exhaust Using Mechanical and Electrical Turbocompounding 2007-01-1563

Considering future emission legislation and the global thermal problem, two are the main issues that are of specific concern for the future of the diesel engine, specific gaseous pollutants and CO₂ emissions. Both parameters are related to engine bsfc consumption directly or indirectly. The last is becoming even more important considering current fuel prices and the projection for the future indicating a trend for increasing fuel prices. The last decade significant improvement have been accomplished in the field of diesel engine efficiency that has resulted to considerable reduction of engine bsfc. It is obvious that despite improvements in diesel engine efficiency still a considerable amount of energy is rejected to the environment through the exhaust gas. Approximately 30-40% of the energy supplied by the fuel is rejected to the ambience. Therefore, it appears a possibility for further considerable increase of diesel engine efficiency with the utilization of exhaust gas energy and its conversion to mechanical energy. In this case, the following technological solutions exist: turbocompounding, bottoming cycles (i.e. Rankine with various working media) and use of thermoelectric generators. An attractive technological solution concerning applicability appears to be turbocompounding. In the present work, a thorough investigation is conducted using modeling to estimate the potential of energy recovery from the exhaust of a heavy-duty diesel engine using turbocompounding and its variation with engine operating conditions and exhaust manifold pressure. Two turbocompounding techniques are examined theoretically, mechanical and electrical turbocompounding. The produced results are evaluated comparing engine performance and especially bsfc with the one corresponding to normal diesel operation on the entire engine operating range i.e. speed and load. Results are derived concerning the effect of both technologies on exhaust manifold pressure and temperature that can be a problem especially for future downsized engines. Using the produced data, the two technological solutions are comparatively evaluated. Finally, indicative results are derived concerning the effect of each technology on specific engine out emissions.