抄録
Diesel jet controlled compression ignition (JCCI) with dual-direct injection was proposed to control the ignition timing and combustion phasing of premixed charge compression ignition (PCCI) actively and effectively. In diesel JCCI mode, the direct pre-injection gasoline prepares a flexible premixed charge, which is ignited by a small amount of jet-injection diesel near the compressed top dead center. In this research, the combustion and emission characteristics of diesel JCCI mode were optimized by using three-dimensional computational fluid dynamics (CFD) combined with genetic algorithm based on the test engine at 75% load. The simulation results show that under the current conditions, higher economy and lower emissions can be obtained simultaneously when the energy ratio is 50~59%, the initial in-cylinder temperature is 338~360 K and the pressure is 1.53~1.75 bar, meanwhile, the combustion phasing is in the range of 4~9 °CA ATDC. In addition, the combustion phasing has a positive proportional relationship with the diesel jet-injection timing, indicating that, in diesel JCCI mode with a reentrant type combustion chamber, the jet-injection timing plays a decisive role in the combustion process. Two typical cases are selected for deeper understanding. The Case A with both early pre- and jet-injection presents the single-stage high-temperature heat release process. By contract, the Case B with both late injections presents the two-stage high-temperature heat release process. Compared to Case A, the O2 at the corner of the combustion chamber at 90 °CA ATDC is more the that of Case A, and the THC and CO produced during the main combustion can be further oxidized. Therefore, the THC and CO emissions reduce by 2.17 and 15.09 g/kWh, respectively, the equivalent indicated specific fuel consumption (EISFC) reduces by 6.78 g/kWh compared with case A. Besides, the distribution of the spray particulars is leaner. So, the soot emission of Case B is about 0.0011 g/kWh lower than Case A. At the same time, the local temperature is lower, resulting in the NOx emission reduction by 0.06 g/kWh.
