A study of pre-ignition and knock in an optical spark ignition engine

Abstract

The currently reported work involved fundamental study of auto-ignition under unusually high knock intensities in an optical spark ignition engine. The single cylinder research engine adopted included full bore overhead optical access capable of withstanding continuous peak in-cylinder pressure and knock intensity of up to 150 bar and 60 bar respectively. Heavy knock was deliberately induced under relatively low loads (5 bar IMEP) using inlet air heating up to 66 °C and a primary reference fuel blend of reduced octane rating (75 RON). High speed chemiluminescence natural light imaging was used together with simultaneous heat release analysis to evaluate the combustion events. The key out comes of this study could be listed as follow: • Proof and improved understanding of multi centred auto-ignition events under high KIs • Improved understanding of the potential pitfalls of over-fuelling for heavy knock suppression • Optical validation of ‘natural’ oil droplet release and on-off behaviour of knocking cycles Multiple centred auto-ignition events were regularly observed to lead in to violent knocking events, with knock intensities above 140 bar observed. The ability to directly image the events associated with such high magnitude of knock is believed to be a world first in a full bore optical engine. The multiple centred events were in good agreement with the developing detonation theory to be the key mechanism leading to heavy knock in modern downsized SI engines. The accompanying thermodynamic analysis indicated 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 random location of the first captured auto-ignition sites during developing auto-ignition events. Under such circumstances new flame kernels formed at these sites, with initial steady growth sometimes observed to suppress the growth of the earlier spark initiated main flame front prior to violent end gas auto-ignition. It was found that pre-ignition most commonly initiated in the area surrounding the exhaust valve head and resulted in a deflagration that caused the overall combustion phasing to be over advanced. In the cycles after heavy knock, droplets of what appeared to be lubricant were sometimes observed moving within the main charge and causing pre-ignition. These released lubricant droplets were found to survive within the combustion chamber for multiple cycles and were associated with a corresponding “on-off” knocking combustion pattern that has been so widely associated with super-knock in real downsized spark ignition engines. This research also concerned with improving understanding of the competing effects of latent heat of vaporization and auto-ignition delay times of different ethanol blended fuels during heaving knocking combustion. Under normal operation the engine was operated under port fuel injection with a stoichiometric air-fuel mixture. Additional excess fuel of varied blend was then introduced directly into the end-gas in short transient bursts. As the mass of excess fuel was progressively increased a trade-off was apparent, with knock intensity first increasing by up to 60% before lower unburned gas temperatures suppressed knock under extremely rich conditions (γ=0.66). This trade-off is not usually observed during conventional low intensity knock suppression via over-fuelling and has been associated with the reducing auto-ignition delay times outweighing the influence of charge cooling and ratio of specific heats. Ethanol had the highest latent heat of vaporization amongst the other fuels directly injected and was more effective to reduce knock intensity albeit still aggravating knock under slightly rich conditions. Overall, the results demonstrate the risks in employing excess fuel to suppress knock deep within a heavy knocking combustion regime (potentially including a Super-Knock regime).