On the Combustion Characteristics of Closely-Coupled LD Diesel Injection Strategies

Abstract: Multiple-pilot injection strategies with both moderate and short time separations (dwells) are investigated in a light-duty diesel engine. Both conventional and optical engine experiments are performed. By implementing triple-pilot injection strategies with short dwells between the individual injections, combustion noise can be significantly reduced. The sound pressure level was reduced by nearly 4 dB compared to a conventional double-pilot injection strategy. This is significant since every 3 dB is a doubling of sound power. Reducing the combustion noise allowed for combustion to be phased to its peak efficiency point, improving gross efficiency by nearly 4%.A new metric, the ratio of reduced heat release (RRHR) was developed in order to analyze the heat release rate (HRR). This metric quantifies the magnitude of the undulations in the HRR and explains their influence on the combustion noise. It can explain the relative strength of frequency content in the combustion noise, indicate how the HRR should be modified to reduce combustion noise, and predict the total noise level at a specific load-speed point for different injection strategies.The combustion processes of conventional and closely-coupled strategies were studied in an optical engine. It was discovered that contact ignition stabilizes the combustion of subsequent fuel injected into prevailing combustion regions. Where a physical interface between the fuel and combustion region does not occur, that region has a significantly delayed ignition. Closely-coupling injections can lead to lower stability since the injections arrive sooner, at less developed stages of combustion. In this interface, hot combustion products are extinguished and replaced with cool ones which can add to the undulation in the HRR. In order to minimize its effects on combustion noise, this extinguishing phenomenon should be limited.A balance of mixture dilution, injection pressure, short injection separations, and injection rate shaping are utilized to create a quadruple-pilot injection strategy creating a near linear HRR progression and very low noise level.