Browse Publications Technical Papers 2019-01-1142
2019-04-02

Optical experiments on strong knocking combustion in rapid compression machines with different fuels 2019-01-1142

Knocking combustion has become the greatest bottleneck of internal combustion engines with advanced combustion technologies for pursuing thermal efficiency limits. Because of the complexities of combustion conditions, the mechanism for strong knocking combustion in engines under different combustion modes is still not fully understood. In this study, synchronization measurement through simultaneous pressure acquisition and high-speed direct photography was performed, and strong knocking combustion for premixed iso-octane/air mixture was studied in a high-strength optical rapid compression machine with flat piston design. First, strong knocking phenomena under both spark-ignition and compression-ignition conditions are identified through varying initial thermodynamic conditions. Our results show that knocking severity is positively correlated with the effective energy density that can universally quantify the weighting from different combustion boundary conditions (i.e. initial temperature, pressure and equivalence ratio). Then, high-speed combustion images with synchronous pressure traces for strong knocking cycles are discussed, which indicate that knocking severity essentially depends on autoignition development modes. The strong knocking scenarios manifesting engine super-knock involve deflagration-to-detonation transition and even direct detonation, with distinct characteristic in reaction front propagation. Meanwhile, this becomes more prevailing under high effective energy density conditions. Finally, to clarify the ignition modes for different knocking scenarios, a prior ignition regime quantified by turbulent Damköhler and Reynolds numbers is introduced. It shows that the current experimental data show favourable scaling agreement with the Sankaran and Shock Wave Amplification by Coherent Energy Release criteria that are used for classifying mild and strong ignition.

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