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Three dimensional computational model has been developed to predict the macroscopic behavior of the fuel spray in D. I. diesel engines. The model was based on the KIVA-II code with modification of some submodels that it can deal with the observed phenomena such as liquid column near the nozzle tip and spray impingement on a wall. Firstly, this model was verified by comparing the prediction with the experimental results in a constant volume vessel. Secondly with application to a D.I. diesel engine, the detailed behavior of the spray in a combustion chamber was revealed. Moreover, the engine performance under different spray angles were discussed with the prediction of this model.

The cause of unburnt hydrocarbon emission (HC) in small DI Diesel engines at light load was studied both by engine emission tests and combustion process visualizing with a common rail injection system. An optically accessible engine, which was enabled to visualize both combustion chamber and squish area, was used to investigate the behavior of spray, mixture distribution and so on. Factors supposed to be the major cause of forming HC in small DI Diesel engines, such as the direct impingement of liquid-phase fuel spray on the combustion chamber wall, the uneven formation of fuel sprays from hole to hole and the spread of the fuel droplets, mixture and flame to the squish area were investigated. Meanwhile, measures for further reducing HC were discussed.