Previous studies have shown that air motion affects the combustion process and therefore also the emissions in a DI diesel engine. Experimental studies indicate that higher engine speeds enhance the turbulence and this improves air and fuel mixing. However, there are few studies that address fundamental combustion related factors and possible limitations associated with very high speed engine operation. In this study, operation over a large range of engine speeds was simulated by using a multi-dimensional computer code to study the effect of speed on emissions, engine power, engine and exhaust temperatures. The results indicate that at higher engine speeds fuel is consumed in a much shorter time period by the enhanced air and fuel mixing. The shorter combustion duration provides much less available time for soot and NOx formations. In addition, the enhanced air/fuel mixing decreases soot and NOx by reducing the extent of the fuel rich regions. A higher engine speed, therefore, is predicted to decrease soot and NOx formation substantially. Several fuel injection timings were chosen to study emissions, engine power and wall heat transfer at high engine speeds. The results indicate that engine speed and fuel injection timing can be used to determine the optimum conditions for increased engine power density with lower emissions and acceptable engine and exhaust temperatures in a DI diesel engine. This work also demonstrates the usefulness of multidimensional models to gain insight into the combustion process and to provide direction for exploring new engine concepts.