Abstract

Knock is a major challenge for high load operation of spark ignited gasoline engines with higher compression ratios, since the end-gas undergoes higher temperature and pressure trajectories during combustion. Pre-chamber combustion creates long-reach ignition jets that have the potential to mitigate knock due to their rapid consumption of end-gas. However, conventional pressure oscillation-based knock metrics may not accurately capture the end-gas autoignition severity in pre-chamber systems due to differences in ignition and combustion behavior. This work investigates the knock behavior of both traditional spark ignition and pre-chamber combustion (including different nozzle designs) in a high compression ratio engine fueled with regular octane certification gasoline. The data was analyzed using statistical methods to show the random nature of knock events. Detailed analysis was used to explain the pressure oscillations of both knocking and non-knocking cycles of pre-chamber jet combustion and show that conventional pressure oscillation-based knock metrics may not adequately quantify end-gas autoignition severity. A novel knock metric is introduced to avoid consideration of the non-knock related pressure oscillation and better quantify the end-gas autoignition severity. The new metric was used to explain the knock mitigation mechanism for pre-chamber jet combustion and demonstrate an additional pre-chamber jet ignition benefit of reduced combustion variability during engine operation with cooled exhaust gas circulation within its dilution limit.

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