Fig 3a describes the variation of heat release rate under 3% and 6% mass fraction oxygen replaced in the inlet charge. When the intake oxygen is replaced by water or nitrogen, the ignition delay becomes longer. With the intake oxygen replaced, the heat release rate in early diffusion combustion phase is lower than that of the original diesel engine, but the heat release rate in late diffusion combustion phase is larger than that of the original diesel engine. Because of the decrease of intake oxygen concentration, fuel can't get enough oxygen to form the combustible mixture, which leads to the decrease of burning rate in the early diffusion combustion phase. Meanwhile as the decrease of burning rate, the combustion duration increases, which leads to the increase of heat release rate in the late diffusion combustion phase. What's more, with the increase mass fraction of oxygen replaced, the trend of this difference become larger.