Physics of Combustion Noise Reduction with Multiple Injections in a DI Diesel Engine - A Computational Study

With the ability of modern high pressure diesel injectors to deliver accurate, closely coupled multiple pulse injections, it is possible to minimize engine combustion noise without negative effect on exhaust emissions. Literature shows that, splitting the cycle heat release into several parts helps to lower peak heat release rate and combustion noise. The charge cooling caused by fuel vaporization can be effectively used to influence ignition delay and achieve lower noise, emissions and fuel consumption. With the traditional pilot-main injection scheme, researchers have shown that, the injection dwell time between the pilot and main is primarily responsible for noise reduction. The current objective is to analytically explore the fundamental physics behind the experimentally observed noise reduction phenomena with multiple injections. This computational study was conducted at a key part-load operation (2000RPM and 5Bar BMEP) with five injection pulses. In agreement with experimental trends, computational results showed that as the dwell between the third injection pulse and the fourth (main) injection pulse is reduced, combustion noise decreased and, if the dwell is decreased beyond an optimum value, combustion noise started to increase. The heat of vaporization needed for the evaporation of fuel spray in the main (fourth) injection pulse, locally cools the environment and profoundly influences the ignition delay of fuel injected in the first three (smaller) injection pulses. This cooling affects the rate of pressure rise in the cylinder, which in turn affects the combustion noise.