Spark ignition and spark-assisted controlled auto-ignition in an optical gasoline engine

The results of an experimental study of the low speed and low load operation of an optical research engine are presented for spark-ignition (SI) and spark-assisted, controlled auto-ignition (SA-CAI). A direct injection gasoline engine was modified for optical access into the combustion chamber. At 1000 rpm and 3 bar NIMEP, stable SA-CAI combustion was achieved with predicted EGR rates in excess of 45%. The coefficient of variation (CoV) in NIMEP was 4.8% compared to 6.5% recorded in the SI case, with no EGR. Particle image velocimetry measurements of the airflow showed lower mean and turbulent velocities in the SA-CAI case at the end of the compression stroke. Planar Laser Induced Fluorescence (PLIF) measurements of the fuel vapour signal in the air-fuel-residual distribution were significantly lower than in the SI case. Indicating analysis showed that the main combustion burn duration was considerably greater in the SA-CAI case. This was corroborated by high-speed photography of the flame development. In the SA-CAI case, the flame was additionally interspersed with individual ignition sites. Some evidence of re-compression auto-ignition was observed. The phasing of the main combustion period for SA-CAI showed a dependency upon ignition timing. In all of the low speed SA-CAI cases, stable combustion occurred, despite the high levels of EGR present. This was illustrated by the slower burning in the main combustion phase, an absence of the high rates of heat release synonymous with CAI and visual flame development entirely different to conventional SI operation.