Cycle-to-cycle variability in spark-assisted compression ignition engines near optimal mean combustion phasing

Abstract

Stoichiometric spark-assisted compression ignition (SACI) combustion with exhaust gas recirculation (EGR) dilution has demonstrated higher part-load thermal efficiencies compared to spark-ignited engines, while maintaining ultra-low tailpipe emissions. However, SACI is often characterized by high cyclic variability in heat release or torque output, which poses a challenge to its implementation in light-duty vehicles. This paper presents an experimental investigation of cyclic variability in stoichiometric SACI combustion under EGR-dilute conditions, while maintaining mean combustion phasing (θ₅₀) near optimal timing for thermal efficiency. The present work focuses on SACI conditions where the flame-based heat release fraction is between approximately 10% and 40% of the overall heat release, and therefore contributes significantly to the combustion process. For the SACI conditions examined, the variability in θ₅₀ was driven by variability in autoignition timing, which in turn correlated with the start of measurable heat release (θ₀₂). High variability in θ₅₀ caused unstable work output due to very late combustion with poor or no end-gas autoignition. The use of a high ignition energy dual-coil offset ignition system had negligible impact in reducing θ₅₀ variability. Analysis of experimental data from close to 1000 operating conditions showed that the magnitude of θ₅₀ variability correlates with the flame-based heat release fraction ( $x_{B,{\theta }_{\mathrm{AI}}}$ ), for a large range of intake pressures, spark timings and exhaust gas recirculation levels. For the SACI conditions examined, combustion phasing variability was largely determined by flame-based combustion and particularly the initial flame formation (θ_IGN-02), and minimally by the end-gas autoignition heat release. The analysis also demonstrated that θ₅₀ variability is amplified as the contribution of the flame to the overall heat release increases.