SYNOPSISThe Diesel engine, as an automotive power plant, is assessed in terms of the Second Law of Thermodynamics. The Second Law analysis provides additional insights concerning the distribution of engine thermodynamic losses. The results of these analyses are contrasted with First Law results. Promising concepts for modification of the diesel cycle to improve fuel economy are identified and analyzed in terms of the Second Law. These concepts include turbocharging, charge air cooling, turbocompounding, the implementation of a bottoming cycle, and the use of insulating techniques. The results provide an indication of which processes within the vehicle power plant must be changed to improve fuel economy.