Numerical study on critical ambient temperature for auto-ignition of the diesel spray under cold-start conditions

Mitigating the misfire of fuel spray under low temperature conditions can significantly improve the cold start-ability of diesel engines. In the present study, the computational fluid dynamics model was coupled with the kinetic mechanism to investigate the effects of ambient temperature (Ta) on the auto-ignition of the n-dodecane spray. Results show that two essential processes are needed for a successful ignition of the spray: 1) the cool flame propagates from fuel-lean regions to fuel-rich regions and 2) sufficient heat must be released from low temperature reactions (LTR) to raise the local temperature above H₂O₂ decomposition threshold of 1000 K. Both the first- and second-stage ignition delay increase as Ta decreases. The second-stage pre-ignition duration (τ₂) remains almost constant when Ta is above 740 K, but sharply increases when Ta is close to the critical value. As Ta decreases, the LTR region moves to the downstream of the spray. With this downstream movement, the local equivalence ratio and the heat release from LTR decrease. Eventually, without sufficient heat release, the H₂O₂ decomposition reaction cannot be activated, τ₂ is prolonged, and misfire occurs. The critical Ta for 6 ms duration spray is 715 K, while it is 740 K for 1.5 ms duration spray because the decrease of local equivalence ratio not only results from the LTR downstream movement but also from the continuing air entrainment after the end of injection. These double effects lead the critical Ta of 1.5 ms spray to be higher than that of 6 ms spray.