Identification of the dominant combustion characteristics on homogeneous charge compression ignition engine noise

The upper load range of homogeneous charge compression ignition engines is limited by extreme noise levels. Previous studies have investigated the sources of this noise by investigating high-frequency (>4 kHz) acoustic energy typically associated with cylinder cavity resonances excited during high in-cylinder pressure rise rate combustion events. In this work, a broader frequency band (0.5-15 kHz) is investigated experimentally for a single-cylinder homogeneous charge compression ignition engine inside an acoustic sub-enclosure. It has been found that the 0.5-4-kHz frequency band being excited by the rapid in-cylinder pressure rise rate of the homogeneous charge compression ignition combustion event is dominating the acoustic signature of the engine. This indicates that the noise is generated from the structural response of the engine structure to the impulse-like load from the rapid cylinder pressure rise rather than ringing associated with the cavity resonances. A band-level analysis is used to explore the contribution of mechanical and combustion noise on the acoustic signature for various load conditions and combustion phasings. Based on this analysis, a method is proposed where a cylinder pressure rise rate threshold is established for determining the point at which the homogeneous charge compression ignition combustion noise will significantly exceed the mechanical noise level. This threshold, based on the theoretical band-level analysis, agrees with thresholds found from previous literature.