Fuel Effects on the Onset of Knock and the Intensity of Superknock at Stochastic Preignition-Relevant Engine Conditions

Abstract

To have a more complete understanding of the fuel effects on each subsequent stage of a stochastic preignition event in a spark-ignition engine and to build on the previous work of understanding the propensity of fuel to initiate and sustain a preignition flame, this work is focused on examining the role of fuel on the onset of knock and the intensity of superknock once the unburned mixture reaches certain conditions ahead of the preignition flame. Using a “skip advance” spark test method to simulate preignition flames initiated at different cylinder conditions, more than 20 single- and multicomponent fuels were ranked based on the condition required to reach the onset of knock (the start of end-gas autoignition) and the condition that leads to severe superknock intensities. It was found that average knock intensity can be mainly explained by the unburn mixture fraction and the thermodynamic condition of the unburned mixture and, not surprisingly, that the fuel ranking for the onset of knock and superknock based on average knock intensity is correlated to octane index. However, outlier cycles with extremely high knock intensities cannot be fully explained by the average cycle behavior. More interestingly, different fuels exhibit different superknock characteristics. Some fuels, such as toluene, have fewer extreme cycles once the same average knock intensity condition is reached, whereas other fuels, such as ethanol, have more extreme cycles that tend to break engine hardware in a single cycle event. A preliminary study based on the modes of reaction front propagation show that fuels with low-temperature heat release and negative temperature coefficient (NTC) behavior can lead to a higher propensity to produce extreme knock intensities when coupled with the right in-cylinder pressure wave.