On the Reduction of Combustion Noise by a Close-Coupled Pilot Injection in a Small-Bore Direct-Injection Diesel Engine

For a pilot–main injection strategy in a single-cylinder light-duty diesel engine, the dwell between the pilot- and main-injection events can significantly impact combustion noise. As the solenoid energizing dwell decreases below 200 μs, combustion noise decreases by approximately 3 dB and then increases again at shorter dwells. A zero-dimensional thermodynamic model has been developed to capture the combustion noise reduction mechanism; heat release (HR) profiles are the primary simulation input and approximating them as top-hat shapes preserves the noise reduction effect. A decomposition of the terms of the underlying thermodynamic equation reveals that the direct influence of HR on the temporal variation of cylinder pressure is primarily responsible for the trend in combustion noise. Fourier analyses reveal the mechanism responsible for the reduction in combustion noise as a destructive interference in the frequency range between approximately 1 kHz and 3 kHz. This interference is dependent on the timing of increases in cylinder pressure during pilot HR relative to those during main HR. The mechanism by which combustion noise is attenuated is fundamentally different from the traditional noise reduction that occurs with the use of long-dwell pilot injections, for which noise is reduced primarily by shortening the ignition delay of the main injection. Band-pass filtering of measured cylinder pressure traces provides evidence of this noise reduction mechanism in the real engine. When this close-coupled pilot noise reduction mechanism is active, metrics derived from cylinder pressure such as the location of 50% HR, peak HR rates, and peak rates of pressure rise cannot be used reliably to predict trends in combustion noise. The quantity and peak value of the pilot HR affect the combustion noise reduction mechanism, and maximum noise reduction is achieved when the height and steepness of the pilot HR profile are similar to the initial rise of the main HR event. A variation of the initial rise rate of the main HR event reveals trends in combustion noise that are the opposite of what would happen in the absence of a close-coupled pilot. The noise reduction mechanism shown in this work may be a powerful tool to improve the tradeoffs among fuel efficiency, pollutant emissions, and combustion noise.