Numerical study on the effect of multiple injection strategies on ignition processes for low-temperature diesel spray

Abstract

The multiple-injection strategies can significantly improve the diesel engine startability at low-temperature conditions, but the processes are extremely complex due to the effect of various factors simultaneously. To find out how the pilot injection affects the main injection, the spray and ignition with different injection strategies at low temperatures, and the effect of the interaction of the pilot and main injections on mixing and ignition are studied numerically. The results show that the main injection fuel can catch up with the pilot injection which has been pre-heated, making the peripheral fuel of the main injection has a higher temperature and equivalence ratio compared to the single injection, which can accelerate the chemical reaction process and thus shorten the ignition delay. The products of the pre-reacted pilot fuel are mixed with the peripheral fuel of the main injection and affect the reaction process. The pre-injected fuel can promote the combustion and shorten the ignition delay of the main injection fuel because the remaining species of pre-injected fuel can make n-dodecane dehydrogenated with the intermediate products such as OH in preference to O2. However, mixing the injections too late prevents the pre-injected fuel from promoting the combustion of the main injection. The concentration of CH2O increases rapidly during the first heat release, and the species at this moment significantly inhibit the combustion of the new fuel.