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Apparent effects of in-cylinder pressure oscillations and cycle-to-cycle variability on heat release rate and soot concentration under long ignition delay conditions in diesel engines

In diesel engines, long ignition delay due to cold in-cylinder conditions has been shown to lead to high cycle-to-cycle variability, as well as result in pressure oscillations due to rapid localised pressure rise rates from the resulting premixed combustion. These pressure oscillations appear as superimposed pressure waves on the engine indication graph, with an oscillation frequency corresponding to the first radial vibration mode. In the current study, the influences of pressure oscillations on heat release rate and the progress of in-cylinder soot concentration are investigated. Results showed that cycles where pressure oscillations occur reach a higher peak pressure than average or low pressure oscillation cycles, as a result of increased diffusion combustion rate and apparent mixing rate. Additionally, using in-cylinder soot pyrometry, cycles with high pressure oscillations were shown to exhibit increased soot oxidation rates. The combination of the two above-mentioned observed effects leads to the conclusion that pressure oscillations in direct injection diesel engines result in more rapid mixing due to increased turbulent intensity.