Experimental and Numerical Investigations of Close-Coupled Pilot Injections to Reduce Combustion Noise in a Small-Bore Diesel Engine

A pilot-main injection strategy is investigated for a part-load operating point in a single cylinder optical Diesel engine. As the energizing dwell between the pilot and main injections decreases below 200 mu s, combustion noise reaches a minimum and a reduction of 3 dB is possible. This decrease in combustion noise is achieved without increased pollutant emissions. Injection schedules employed in the engine are analyzed with an injection analyzer to provide injection rates for each dwell tested. Two distinct injection events are observed even at the shortest dwell tested; rate shaping of the main injection occurs as the dwell is adjusted. High-speed elastic scattering imaging of liquid fuel is performed in the engine to examine initial liquid penetration rates. The penetration rate data provide evidence that rate shaping of the initial phase of the main injection is occurring in the engine and that this rate shaping is largely consistent with the injection rate data, but the results demonstrate that these changes are not responsible for the observed trend in combustion noise. A zero-dimensional model is created to investigate the causes of the observed combustion noise behavior. The trend in simulated combustion noise values agree well with the experimentally determined trend, which is associated with two main factors: relative changes in combustion phasing of the pilot and main heat release events and suppression of the pilot apparent heat release for dwell times near the minimum-noise dwell. Two possible mechanisms by which the relative phasing between the pilot and the main heat release events impacts combustion noise are proposed.