Analysis of multicomponent surrogate gasoline blends under pre-ignition conditions

Abstract

Pre-ignition is an abnormal combustion phenomenon, in which the fuel–air charge ignites before the spark timing. Pre-ignition is a challenge for boosted gasoline engines, as it may lead to damaging super-knock during the combustion process. The phenomenon may be caused by different initiation mechanisms and the development of robust boosted engines requires understanding of how these mechanisms are formed. This study targeted pre-ignition occurring at hot cylinder surfaces or through bulk gas autoignition by sweeping the intake temperature (30–95 °C) in a single-cylinder spark-ignition engine at high intake pressures (2350 mbar) and a late combustion phasing. Seven different gasoline surrogate blends (composed of the species n-heptane, iso-octane, toluene, cyclopentane, 1-hexene, ethanol and methanol) were evaluated through their limiting intake temperatures for causing pre-ignition. The blend with the highest fraction of cyclopentane (Surrogate A) was found to cause pre-ignition at the lowest intake temperature (30 °C), showing that a fuel with high research octane number and a high tendency for surface ignition can be pre-ignition sensitive. Methanol splash blends and ethanol match blends were shown to be relatively resistant to pre-ignition. Pre-spark heat release of normal cycles was found to be a good indicator for conditions where pre-ignition occurs for each fuel. Chemical kinetic simulations with a tuned mechanism revealed the underlying chemistry behind the first-stage heat release before pre-ignition and established bulk gas autoignition as a realistic pre-ignition mechanism for most of the multicomponent blends studied in this work.