Experimental and Kinetic Study on the Critical Ambient Density for Auto-ignition of the Diesel Spray under Cold-start Conditions

In the present study, the effect of ambient density ((Formula presented.)) on spray two-stage auto-ignition characteristics under cold-start condition was investigated using both optical and numerical methods. By the high-speed Shadowgraph, the LTI (low-temperature ignition delay) and HTI (high-temperature ignition delay) were obtained and followed an increasing trend with the decreasing (Formula presented.). Both experimental and simulated results show that HTI is more sensitive to the (Formula presented.) than LTI, because the high-temperature chemistry is more suppressed at higher (Formula presented.) due to the increased scalar dissipation rate. Moreover, the simulation showed that the high-temperature ignition emerges at the same mixture fraction of 0.065, regardless of changes of (Formula presented.), which indicates that (Formula presented.) has little effect on the ignition site in the mixture fraction space. Further kinetic analysis found that as the (Formula presented.) decreases, the O₂ concentration reduces and the reaction rates of both low- and high-temperature reactions reduce, leading to the increased LTI and HTI. When the HTI increases to such a long time that no ignitable mixture exists in the space due to the continuing diffusion after the end of injection, the spray misfire happens.