Modeling of Combustion Process of Multiple Injection in HSDI Diesel Engines using Modified Two-Dimensional Flamelet

Ignition delay of the second injection of HSDI diesel engines is generally much shorter than that of the first injection because of the interaction between the radicals generated during the combustion process and the mixed gas of the second injection. Although previous Diesel combustion models could not explain this reaction, Hasse and Peters described the mass and heat transfer of the second injection and estimated the ignition delay of the second injection using two-dimensional flamelet equations. But a simulation of the two-dimensional flamelet equations requires enormous computational time. Thus, to analyze the combustion phenomena of the multiple injection mode in HSDI diesel engines effectively, the two-dimensional flamelet combustion model was modified in this study. To reduce the calculation time, two-dimensional flamelet equations were only applied near the stoichiometric region. If this region was ignited, species and temperatures of other regions were changed to the steady-state solutions of one dimensional flamelet equations. By this method the calculation time for solving flamelet equations was reduced by 80 percent without any loss of calculation accuracy.

A new modified 2-D flamelet combustion model was coupled to commercial CFD code by using user subroutines. At first, to verify this combustion model, autoignition and combustion processes were investigated in a pre-combusted vessel. The simulation results of ignition delay of the second injection were well matched with the experimental results. Secondly, simulation of combustion process and emissions for multiple injections was performed, and verified by the experiment. The pressure simulated by the CFD code well matched with the experimental data. If the mixture of the second injection came in contact with the radicals originated by the first injection immediately, oxygen was consumed immediately due to short ignition delay and this oxygen consumption will generate a lot of soot. Thus the injection timing and amount of the first and second injection should be considered carefully. As a result, the combustion model used in this study can be applied in the design stage of an engine using multiple injections.