Optical study on autoignition and knocking characteristics of dual-fuel engine under CI vs SI combustion modes

Uncontrollable autoignition with engine knock has been the main obstacle for high thermal efficiency and low CO2 emissions in downsized engines. Therefore, it is necessary to study the autoignition and knocking characteristics and reveal the similarities/differences between different combustion modes. In this study, synchronization measurement was performed through simultaneous pressure acquisition and high-speed natural flame acquisition, and knocking experiments were comparatively conducted under spark-ignition (SI) and compression-ignition (CI) conditions in a high-strength optical engine. The CI experimental results show that early ignited fuel injection leads to advanced combustion phase thus concentrated heat release. The advanced combustion phase can mitigate cyclic variation at low energy density while induce knocking combustion at high energy density. The stochastic analysis shows that CI knocking intensity mainly depends on the combustion rate whereas SI knock intensity is more random because of the stochastic end-gas autoignition. Visualized combustion images show that compared to SI combustion, the burning rate of CI combustion is much higher due to the multipoint autoignition. At low energy density, there is no pressure oscillation of CI and SI and the main reason is the low value of peak heat release rate (HRR) regardless of the autoignition. Whereas at high energy density, there are obvious two-stage HRRs for the CI knocking combustion and the high second peak HRR results in the engine knock. Further flame comparison shows that the AI flame speeds in CI and SI modes are similar but much higher than traditional SI flame speed.