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An electronically-controlled, two-stage high pressure diesel unit injector is the focus of a study of injection characteristics. The fourteenth order nonlinear mathematical model of the injector is developed and a computer simulation written. The simulation is verified by good agreement between predicted and measured accumulator pressure histories and fuel deliveries for fuel and water tests over a range of rail pressure settings. Stroboscopic flash photographic and dynamic laser attenuation measurements of spray penetration distance and spray cone angle are obtained for the injector operating at peak injection pressures of 80, 110 and 145 MPa. The experiments are performed with water injection into quiescent air at laboratory conditions. At the highest value of peak injection pressure of 145 MPa, the penetration length history exhibits a slope of approximately 250 m/s with a corresponding spray cone angle of 16°.