Heat Release Rate Shaping for Reducing Combustion Noise of Partially Premixed Combustion in a Diesel Engine

In order to reduce NOx and soot emissions simultaneously under low- and middle-load conditions in diesel engines, partially premixed combustion (PPC) has been widely studied. However, the excessive combustion noise caused by the rapid combustion and the high unburned species emission by the lean mixture are problems. In this study, numerical and experimental investigations were performed to achieve low noise, high thermal efficiency, and low exhaust emissions under a low load condition in a diesel engine. First, the shape of heat release rate that suppresses combustion noise without lowering the degree of constant volume (DCV) was investigated using 0-dimensional calculation. It was found that suppressing the maximum heat release rate with a gradual rising slope reduces combustion noise while maintaining the DCV. Next, a series of experiments were performed using a single-cylinder diesel engine (displacement: 0.55 L) to investigate the effects of injection parameters on heat release rate shape and engine performance. Two injection systems were installed into the engine to realize highly flexible fuel injection, and three-stage injection and four-stage injection were applied to realize the heat release rate shape obtained by the 0D calculation. The operating conditions including engine speed and gross indicated mean effective pressure were 1,500 rpm and 390 kPa. When the four-stage injection was used, the heat-release rate increased gradually when the overall injection timing was delayed. These injection conditions lead to lower combustion noise, high DCV, high thermal efficiency, and low exhaust emissions. Acctually, the thermal efficiency could be increased from 44.6% of single-injection PPC to 46.9% while the combustion noise was reduced from 83.0 dBA of single-injection PPC to 80.1 dBA without a significant increase in emissions.