Noise-canceling spike between pilot and main-pressure-rise peaks of multiple-injection diesel combustion

A previous study addressed combustion-noise reduction by optimizing the interval between two peaks in the pressure-rise rate (dP/dθ) of premixed charge combustion ignition with the split injection of fuel in diesel engines. Noise canceling occurs between two dP/dθ peaks, 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 dP/dθ peaks. We named this noise-reduction technique “noise-canceling spike” because it relies on the interference between a spike in the pressure rise and the preceding peak in the pressure rise. The time interval between the dP/dθ peaks must be controlled precisely to enable the utilization of the noise-canceling spike. In this article, the theory of the noise-canceling spike between the dP/dθ peak of the pilot injection and a single dP/dθ peak of the main injection is explained relatively simply, using experimental data analysis and zero-dimensional cycle simulations to prove that the noise-canceling spike is a universal phenomenon which occurs between the two dP/dθ peaks independently of the combustion (injection) strategy. Then, the theory of the noise-canceling spike is extended to those cases with three or more dP/dθ peaks, in which the noise-canceling spike can occur between every pair of dP/dθ peaks. Finally, we prove that the noise-canceling spike can occur between the dP/dθ peak of the pilot heat release and the multiple dP/dθ peaks of the main heat release in diesel combustion. The noise-canceling spike between the dP/dθ peak of the pilot combustion and the first dP/dθ peak of the main combustion becomes the dominating factor which reduces the overall combustion noise. In addition, the reasons why there have previously been no reports of the canceling and amplifying between the two dP/dθ peaks are discussed.