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Various pilot-main injection strategies are investigated in a single-cylinder optical Diesel engine. It is observed that as the dwell between a single pilot and the main injection is decreased toward zero, combustion noise passes through a minimum, and a reduction of 3 dB is possible. The injection schedules employed in the engine are analyzed with a hydraulic injection analyzer to provide rate shapes for each of the dwells tested. Two distinct injection events are observed even at the shortest dwell tested, and various rate shaping effects are observed with the main injection event as the dwell is adjusted. High-speed elastic scattering imaging of liquid fuel is performed in the engine to examine initial spray penetration rates. These compare favorably to the measured rates of injection, thus providing evidence that rate shaping of the initial phase of the main injection occurs in the engine and that this rate shaping is consistent with the injection rate data. However, experimental evidence suggests that these changes are not responsible for the observed trend in combustion noise as dwell changes. The combination of thermodynamic data and rate of injection data supports the theory that the main injection interacts with the pilot mixture field and influences its heat release process, thus playing a role in decreasing combustion noise. The relative phasing of the pilot and main heat release may play a significant role in reducing combustion noise; further studies will focus on combustion phasing effects.

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