The influence of residual gas NO content on knock onset of Iso-Octane, PRF, TRF and ULG mixtures in SI engines

Reported in the current paper is a study of the effects of Nitric Oxide (NO) within a simulated Exhaust Gas Residual (sEGR) on Spark Ignition (SI) engine end gas autoignition. A modified version of the single cylinder Leeds University Ported Optical Engine Version 2 (LUPOE-2) engine was designed to completely eliminate retained residual gas and so allow unambiguous definition of the composition of the in-cylinder charge. The engine was alternately operated on stoichiometric mixtures of iso-octane, two Primary Reference Fuels (PRF), a Toluene Reference Fuel (TRF), and a commercially available Unleaded Gasoline (ULG) and air. These mixtures were diluted with sEGR (products of the complete stoichiometric combustion of the given fuel/air mixture) in mass fractions ranging from 0-15%; with and without 5000ppm NO (0.52% by mass) within that sEGR. For those fuels exhibiting little or no Negative Temperature Coefficient (NTC) behaviour, NO addition to the sEGR resulted in increased rate of reaction in the end gas and advanced autoignition and thus knock onset. Conversely, autoignition (and thus knock onset) was suppressed with the addition of NO for fuels exhibiting strong NTC characteristics if the cycle unburned pressure/temperature history spent a significant period of time to autoignition in the NTC regime. Ideas relating to the effects of residual gas NO concentration on autoignition, and thus knock onset are subsequently discussed.