Cycle-to-cycle Fluctuations of Combustion Noise in a Diesel Engine at Low Speed

Cycle-to-cycle and cylinder-to-cylinder fluctuations of a Diesel engine are studied in terms of combustion and radiated noise. Cyclic fluctuations of the combustion noise are found to be the highest for the lowest engine speed, notably for idling condition. Thermodynamic parameters qualifying each cycle such as the maximum pressure derivative or the maximum rate of heat release are determined from the cylinder pressure signal. The trapped air mass and the auto-ignition delay are estimated for each cycle. Correlation coefficients between parameters on the same cycle or with a cyclic lag or from cylinder-to-cylinder are determined. Results show that the combustion noise is correlated to the maximum rate of heat release. At low engine speed and for the case of cylinder-to-cylinder fluctuations, correlations are found compliant with cylinder ignition order. This could be the key parameter for cycle-to-cycle fluctuations of the combustion noise at low BMEP.