Investigation on Effects of Ignition Configurations on Knocking Combustion Using an Optical Rapid Compression Machine under Lean to Stoichiometric Conditions

Abstract

Multiple ignition is beneficial to the improvement of engine thermal efficiency since it helps to shorten combustion duration and enhance lean-combustion stability. However, more researches are still needed to further understand knocking combustion under multiple ignition conditions, especially under lean-burn conditions. This study presents an investigation into the knocking characteristics under lean to stoichiometric conditions using multiple ignition on an optical rapid compression machine (RCM). Up to six ignition configurations were tested under three equivalence ratios (0.7/0.85/1). High-speed photography and high-frequency pressure acquisition were used to record the combustion processes. The results showed that the burning velocity increased nonlinearly with the increase of spark plug numbers while the knocking intensity (KI) first decreased and then increased, exhibiting a "U-shaped curve" trend. The burned mass fraction (BMF) at the instant of end-gas auto-ignition was also affected by ignition configuration. Generally, the smaller the BMF is at the instant of auto-ignition, the stronger the KI becomes. Furthermore, the key product during iso-octane oxidation, ·OOQOOH, was selected as an indicator to conduct chemical kinetics analysis on end-gas oxidation process. Results showed that the ignition configuration had strong impact on the heat release during flame propagation, which further influenced the reaction process of end-gas by influencing its low-temperature reactions. Under moderate spark plug number (around 3), the heat release during the flame propagation was most intensive, which further suppressed the low-temperature reactions of the end-gas and thus weakened auto-ignition intensity.

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