A cooperative research program has been completed evaluating the relative impact of fuel composition and engine design features on the emissions and fuel economy of a Toyota light-duty diesel truck. The fuel set was blended from commercially available refinery stocks and consisted of eight fuels with independently varying 10% and 90% distillation temperatures and aromatic content. The engine design variables included two compression ratios and three injector types with different fuel flow characteristics, and three injection timings.The main fuel effects observed were increasing hydrocarbon and particulate emissions with increasing aromatic content and, to a much lesser degree, increasing emissions with increasing 10% and 90% point. Changing from the standard fuel injectors to the reference injectors, which had both a higher nozzle opening pressure and a higher initial fuel flow rate, resulted in a substantial reduction in all emissions and improvements in fuel economy. However, these injector changes did not completely eliminate the effects of fuel composition.Overall, decreasing the engine's compression ratio from 21.5:1 CR to 18:1 CR resulted in increased emissions but improved fuel economy. Hence the choice of compression ratio for the engine represents a tradeoff between fuel economy and vehicle emissions.Steadily increasing emission levels were found with mileage accumulation with the very high aromatic content fuel and the standard injectors. Advancing the timing 6° provided some improvement, and the injector changes substantially reduced this mileage effect.