The Competing Effects of Fuel Enrichment During Heavy Knock in an Ethanol-Fuelled Optical SI Engine

Paper #:
  • 2017-01-0665

Published:
  • 2017-03-28
Abstract:
The experimental work was concerned with improving understanding of the effects of latent heat of vaporisation of different ethanol blends during heaving knocking combustion in modern spark ignition engines. The unique single cylinder engine employed included full bore overhead optical access capable of withstanding unusually high in-cylinder pressures. Heavy knock was deliberately induced under moderate loads using inlet air heating and a primary reference fuel blend of reduced octane rating. High-speed chemiluminescence imaging and simultaneous in-cylinder pressure data measurement were used to evaluate the combustion events. Under normal operation the engine was operated under port fuel injection with a stoichiometric air-fuel mixture. Multiple centred auto-ignition events were regularly observed, with knock intensities of up to ~40bar. Additional excess fuel 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% (75 RON fuel) before lower unburned gas temperatures suppressed knock under extremely rich conditions. This trade-off is not usually observed during conventional low intensity knock suppression via over-fuelling and has been associated with the competing effects of reducing auto-ignition delay time and charge cooling/ratio of specific heats. Ethanol has the highest latent heat of vaporization among other fuels in this study and it was more effective to reduce the knock intensity throughout. Overall, the results demonstrate the risks in employing excess fuel to suppress knock deep within a heavy knocking combustion regime (potentially including a Low Speed Pre-ignition regime).
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