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Fuel injection pressure of diesel engines has been increased to improve engine performance and to reduce emission year by year. The higher injection pressure causes a cavitation and leads to erosion in the nozzle hole. Deformation of nozzle hole may result deterioration in engine performance, especially when using heavy fuel oil. It is noted that rounding-offof nozzle hole inlet proves to be effective to maintain the nozzle hole shape and the injection characteristics. In the present paper, from a fundamental viewpoint, measurement of ignition delays of diesel sprays from single hole injection nozzles with and without rounding-off hole inlet were conducted in a high-pressure combustion chamber. Further, fuel flow patterns in the nozzle hole were numerically simulated by three dimensional highly precision nozzle flow model. When ambient pressure and fuel injection quantity are relatively low, sprays from nozzle without rounding-off hole inlet do not ignite at low ambient temperature. On the other hand, there is no difference in ignition delay under high ambient temperature. In cases where an ambient pressure is high enough and a large amount of fuel is injected, ignition delays of nozzle with rounding-off and without rounding-off show almost the same values even at low ambient temperature. The computed results of numerical simulations show that cavitation level decreases significantly for all cases of rounding-off nozzles.

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