An engine emissions and performance study was conducted in conjunction with a series of experiments using a constant volume cold spray chamber. The purpose of the study was to explore the effects of number of holes and hole size on the emissions and performance of a direct injection heavy duty diesel engine. The spray experiments provide insight into the spray parameters and their role in the engine's combustion processes.The fuel system used for both the engine and spray chamber experiments was an electronically controlled, common rail injector. The injector nozzle hole size and number combinations used in the experiments included 225X8 (225 gm diameter holes with 8 holes in the nozzle), 260X6, 260X8, and 30OX6.The engine tests were conducted on an instrumented single cylinder version of the Caterpillar 3400 series heavy duty diesel engine. Data were taken with the engine running at 1600 RPM, 75% load. Engine emissions and performance results include oxide of nitrogen emissions (NOx), particulate emissions, and brake specific fuel consumption (BSFC). NOx versus particulate trade-off curves were generated over a range of injection timings for each nozzle.The pressurized spray chamber was used in the room temperature spray visualization experiments. Results of the image analysis that give spray tip penetration length, spray cone angle, and droplet size are presented.To better understand the nozzle geometry effects, computer modeling of the engine was done. The KIVA-II code was used to model the combustion with the different injector nozzle geometries. The results of the modeling effort emphasize the importance of the details of the spray on diesel engine emissions.