Characterization of knock propensity via observations of end-gas autoignition from laser-ignited, premixed flames in a rapid compression machine

Engine knock in spark-ignited (SI) engines is initiated by autoignition and detonation in the unburned gases (i.e., end-gas) upstream of the spark-ignited, propagating, turbulent premixed flame. Knock propensity of fuel/air mixtures is quantified by research octane number (RON), motor octane number (MON), or methane number (MN; for gaseous fuels), which are typically measured using single-cylinder, variable compression ratio research engines. In this study, we demonstrate the ability to evaluate knock propensity of SI fuels via observations of end-gas autoignition (EGAI) in unburned gases upstream of laser-ignited, premixed flames at engine-relevant pressures and temperatures in a rapid compression machine. Stoichiometric primary reference fuel (PRF; n-heptane/isooctane) blends of varying reactivity (50 ≤ PRF ≤ 100) were ignited, at nominal pressures and temperatures of 23 bar and 630 K, using an Nd:YAG laser. Laser ignition produced outwardly-propagating, laminar premixed flames. High-speed pressure measurements, paired with Schlieren imaging, clearly indicated the presence of EGAI for PRF blends. The magnitude of the EGAI event, as quantified by the fraction of the total heat release occurring during EGAI, varied inversely with octane number. Experiments were accompanied by three-dimensional computational modeling with detailed chemical kinetics performed using CONVERGE™. The model results for PRF50 and PRF75 reveal low-temperature heat release and hydrogen peroxide formation in the end-gas upstream of the propagating laminar flame. Low-temperature heat release increases the temperature and degree of chain branching in the end-gas and ultimately leads to EGAI. Conversely, for PRF100, the model results exhibited neither low-temperature heat release nor hydrogen peroxide formation in the end-gas and EGAI was not observed.