Imaging and Analysis of Auto-Ignition and Heavy Knock in a Full Bore Optical SI Engine

The work involved a fundamental study of auto-ignition under unusually high knock intensities for an optical spark ignition engine. The single-cylinder research engine adopted included full bore overhead optical access capable of withstanding continuous peak in-cylinder pressures of up to 150bar. A heavy knock was deliberately induced under relatively low loads using inlet air heating and a primary reference fuel blend of reduced octane rating. High-speed chemiluminescence natural light imaging was used together with simultaneous heat release analysis to evaluate the combustion events. Multiple centered auto-ignition events were regularly observed to lead into violent knocking events, with knock intensities above 140 bar observed. The ability to directly image the events associated with such a high magnitude of knock is believed to be a world-first in an optical engine. The multiple centered events were in good agreement with the developing detonation theory proposed elsewhere to be the key mechanism leading to heavy knock in modern downsized SI engines. The accompanying thermodynamic analysis indicated a lack of relation between knock intensity and the remaining unburned mass fraction burned at the onset of the auto-ignition. Spatial analysis of the full series of images captured demonstrated the random location of the initial auto-ignition sites, with new flame kernels forming at these sites and initially growing steadily and suppressing further growth of the main flame front before violent detonation at speeds well over the imaging frequency of the camera.