Promises and Challenges of the Low-Heat-Rejection Diesel

The low-heat-rejection (LHR) diesel promises decreased engine fuel consumption by eliminating the traditional liquid cooling system and converting energy normally lost to the coolant into useful shaft work instead. However, most of the cooling energy thus conserved is transferred into the exhaust stream rather than augmenting crankshaft output directly, so exhaust-energy recovery is necessary to realize the full potential of the LHR engine. The higher combustion temperature of the LHR diesel favors increased emission of NOx, with published results on hydrocarbon and particulate emissions showing mixed results. The cylinder insulation used to effect low heat rejection influences convective heat loss only, and in a manner still somewhat controversial. The cyclic aspect of convective heat loss, and radiation from incandescent soot particles, also deserve attention. The temperatures resulting from insulating the cylinder of the LHR diesel require advancements in lubrication. The engine designer must learn to deal with the probabilistic nature of failure in brittle ceramics needed for engine construction. Whether ceramic monoliths or coatings are more appropriate for cylinder insulation remains unsettled. These challenges confronting the LHR diesel are reviewed.