This report expands EPA's data base on regulated and unregulated emissions from Diesel powered cars and trucks. To the extent possible, comparisons were made to similar vehicles powered by gasoline fueled engines. Emissions, fuel economy and general performance of a pair of gasoline- and Diesel-powered Volkswagen Rabbit and Oldsmobile Cutlass cars are discussed. Characterization of heavy-duty engines included a Mack ETAY(B)673A and a Caterpillar 3208, both Diesels, and a Chevrolet 366 gasoline fueled engine. A pair of Daimler-Benz Diesels, one turbocharged and the other not, were used to evaluate the effect of turbocharging. A high pressure injection system was tried with the Mack engine and compared to the standard system. A Caterpillar 3406 Diesel was used to investigate the effect of injection timing, combustion system and exhaust gas recirculation on exhaust particulate and other emissions.