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The competing chemical and physical effects of transient fuel enrichment on heavy knock in an optical spark ignition engine

Vafamehr, Hassan; Cairns, Alasdair; Sampson, Ojon; Mohammadmohsen Moslemin, Koupaie

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Chair in Combustion Engineering

Ojon Sampson

Koupaie Mohammadmohsen Moslemin


The work was concerned with improving understanding of the chemical and physical trade-offs when employing transient over-fuelling to control auto-ignition in gasoline spark ignition engines under knock intensities not usually tolerated in optical engines. The single cylinder engine used included full bore overhead optical access capable of withstanding unusually high in-cylinder pressures. Heavy knock was deliberately induced by adopting 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 ~30bar. 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 65% 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. 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).

Journal Article Type Article
Acceptance Date Jul 11, 2016
Online Publication Date Jul 18, 2016
Publication Date Oct 1, 2016
Deposit Date Feb 7, 2017
Publicly Available Date Feb 7, 2017
Journal Applied Energy
Print ISSN 0306-2619
Electronic ISSN 0306-2619
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 179
Keywords Optical, Auto-ignition, Developing detonation, Downsizing, Knock, Super-knock
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