Optical Experiments on Strong Knocking Combustion in Rapid Compression Machines with Different Fuels

Nowadays the strong knocking combustion involving destructive pressure wave or shock wave has become the main bottleneck for highly boosted engines when pursuing high thermal efficiency. However, its fundamental mechanism is still not fully understood. In this study, synchronization measurements through simultaneous pressure acquisition and high-speed direct photography were performed to comparatively investigate the strong knocking combustion of iso-octane and propane in a rapid compression machine with flat piston design. The pressure characteristics and visualized images of autoignition and reaction wave propagation were compared, and the correlations between thermodynamic trajectories and mixture reactivity progress were analyzed. The results show that iso-octane behaves a greater propensity to strong knocking combustion than propane at similar target pressures. Visualized images show that strong knocking combustion results from secondary autoignition and subsequent supersonic reaction wave propagation. Further analysis on thermodynamic trajectories shows that the secondary autoignition may closely correlate with the negative temperature coefficient behavior controlled by low-temperature chemistry.