Search

Knock is one of the main obstacles to improving the thermal efficiency of spark-ignition internal combustion engines. Although knock has been widely studied and accepted as a result of end-gas auto-ignition, the fundamentals regarding auto-ignition behaviors are still not fully revealed. In this study, the ignition behaviors of primary reference fuels were investigated in an optical rapid compression machine equipped with a quartz combustion chamber allowing for visualizing the combustion process from the lateral view. By combining both the lateral view and the top view photography, ignition behaviors under vortex-existing conditions with a creviced piston and vortex-minimized conditions with a flat piston were comprehensively analyzed to reveal the impact of the vortex on the ignition behaviors of PRF fuels. The influence of fuel reactivity was also investigated. The results showed that mild ignition was prevalent under large Da* numbers. The occurrence of mild ignition was closely related to the ignition delay time of the mixture, and the critical ignition delay time was not fixed but decreased with increasing initial temperature. The propensity of mild ignition could be boosted under vortex-existing conditions due to the increasing hotspot formation probability. Vortices were demonstrated to be capable of mitigating knock intensity via 1) the buffer effect of surrounding burned regions on the shock waves generated from surrounded unburnt pockets; 2) a larger burned mass fraction at the instant of the final ignition under vortex-existing conditions. The results also showed that strong ignition was more likely to occur under vortex-minimized conditions. Besides, higher fuel reactivity also could increase the probability of strong ignition occurrence. Compared with the creviced piston, the use of the flat piston could shift the ignition regime towards regions with higher Re t and lower Da t in the Da t - Re t diagram, where the strong ignition is less pronounced. [ABSTRACT FROM AUTHOR]

We report an experimental study of ignition of flammable mixtures by highly unexpanded, supersonic hot jets. The high-pressure, hot-gas reservoir supplying the jet is created by impacting a projectile on a plunger to rapidly compress and ignite a rich n-hexane/air mixture, resulting in a peak reservoir pressure of more than 20 MPa. A locking mechanism was used to prevent the plunger from rebounding and the jet was created by rupturing a diaphragm covering a nozzle with an exit diameter between 0.25 and 1 mm. The jet development and ignition processes in the main chamber filled with hexane-air mixture were visualized using high-speed schlieren and OH* chemiluminescence imaging. The ignition threshold was determined as a function of composition in the jet and main chamber, the nozzle diameter, and the initial pressure in the main chamber. Unlike the case of subsonic jets in which ignition occurs at the shear layer near the nozzle exit, ignition of combustion in the main chamber was found to take place downstream of the Mach disk terminating the supersonic expansion and within the turbulent mixing region created by the startup of the supersonic jet. The results are interpreted using a constant-pressure, well-stirred reactor model simulating the mixing between the hot jet and cold ambient gas. The critical conditions for ignition are determined by the competition between energy release due to chemical reactions initiated by the hot jet gas and cooling due to mixing with the cold chamber atmosphere. The critical value (maximum for which ignition occurs) of the mixing rate was computed using a detailed chemical reaction model and found to be a useful qualitative guide to our observations. [ABSTRACT FROM AUTHOR]