ii multi-dimensional code has been developed to study the effects of injection pressure and nozzle hole inlet conditions on diesel engine performance and emissions. The code includes a new liquid core and spray breakup model. The models were validated using spray Visualization images obtained from a single-cylinder Version of the Caterpillar 3406 heavy-duty truck engine instrumented with an endoscope system. The computational results were also compared with experimental emissions data. The results show that combustion and emissions predictions are controlled by the details of the spray model. With modifications to the spray model to account for Rayleigh-Taylor and Kelvin-Helmholtz drop breakup mechanisms, the predicted liquid and vapor-fuel penetration agrees well with that measured. The models were applied over a wide range of engine operating conditions and were found to provide good prediction accuracy. The simulations also showed that sharp edged-inlet nozzles give significantly lower particulate emissions than rounded-inlet nozzles with the same rate-of-injection profile as has been seen experimentally.