The results of an experimental study in a DI Diesel engine are presented which shows that partial premixing, using direct diesel fumigation of the inlet air, achieved a reduction in the ignition delay, the magnitude of high frequency rapid pressure fluctuations, the maximum rate of pressure rise and the amplitude of the rate of the high frequency pressure oscillations. Two methods of diesel fumigation were investigated. The difference between these two methods was the degree of premixing of diesel fuel with the inlet air. The first technique used a fine (5 micron) diesel spray onto a glow plug and the second technique used prevaporised diesel. A Perkins 4-236 engine was run both with and without fumigation at two different steady state speeds roughly covering both city and highway running conditions. The maximum fumigation rate used was 22% and this caused an increase in the peak rate of pressure rise prior to the end of the ignition delay (peak compression rate) by 90% with a reduction in the amplitude of the high frequency pressure oscillations by 90%. a reduction in the of maximum rate of change of pressure due to main fuel combustion by 90%, in increase in the rate of pressure rise at the end of the ignition delay by 130% and a reduction in the ignition delay by 50%. It was also shown that the magnitude of the pressure rise and maximum rate of pressure rise were strongly dependent upon the amount of liquid fuel injected directly into the engine cylinder, as expected for normal diesel diffusion controlled combustion.