An author's previous studies addressed a combustion system which reduces emissions, noise, and fuel consumption by using PCCI with the split injection of fuel. This concept relies on the premixed combustion of the first injected fuel and accelerated oxidation by the second injected fuel. Although this combustion system requires the optimization of the timing of the second injection, the details of how noise and emissions are reduced have not been elucidated.In this paper, the authors explain the mechanism whereby emissions and noise are reduced by the second injection.In-cylinder visualizations and numerical simulations both showed an increase in smoke and CO as the second injection timing was advanced, as induced by the inhibited oxidation of the rich flame. When the second injection timing is excessively retarded, the amount of soot forming around the near-nozzle increased. The second fuel injection at the optimum timing can mix with the air in the inner-region of the cavity, such that no soot is formed in the near-nozzle region.Combustion noise spectra analysis revealed that noise canceling occurs between two peaks in the pressure rise rate at the optimum timing for the second injection, which reduces the overall combustion noise by lowering the maximum frequency component of the noise spectrum. The period of this frequency is twice the interval between the two peaks of the pressure rise rate. We named this noise reduction technique ‘noise-cancelling spike combustion’, as it relies on the interference of a “spike” in the pressure increase with the preceding peak.