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The purpose of this study was to investigate the effect of residual gas fraction and compositions on the ignition delay of a diesel spray. The air residual gas mixture was produced by injecting diesel sprays into a constant volume combustion bomb with no scavenging burned gas in it. The air initially contained in the bomb was enough to completely burn the fuel supplied by more than 20 injections. The spray injected in the bomb was ignited by the self-ignition process affected by the residual gas. Repetitions of the fuel injection raised the fraction of residual gas in the bomb. The ignition delay in each injection was measured by a photo-transistor. The ignition delay was a minimum when the ambient mixture contained about 4 % residual gas. The effect of residual gas compositions was investigated by adding small amount of CO, CO2 and THC into the bomb. The CO and CO2 compositions in the burned gas produced an elongation of the ignition delay, while the THC shortened the delay period.