3D Numerical Simulation Research Of Combustion Characteristics In A Turbocharged Diesel Engine Under High Pressure And Late Injection Conditions

The new combustion concept of diesel engine can be a good way to deal with theTrade-off relations between the NOx and Soot emissions. It is efficient and clear,which can relieve the problems of energy crisis and environmental protection. Thelate injection system of new concept combustion model has attracted a widespreadattention because of its well-controled combustion phase and wider operating load.The platform for the3D numerical simulation of diesel engine combustionprosess was established by KIVA-3V code, and two key technologies were studied，including the compensation volume for compression ratio and the calculationmethods of initial boundary conditions. The3D numerical simulation model wasvertified by the experiment data in different operating conditions, and the simulationresults agreed well with the test data. The simulation results showed that the cylinderflows with different compensation volume types were obviously different. Thecalculation methods of initial boundary conditions, which included the initial pressure,the initial temperature, the initial gas composition and the profile of the injection,were reasonable.The effects of injection pressure, injection timing, EGR levels andUltramultihole Nozzle to the formation of gas-fuel mixture, combustion process andemission characteristics were studied under high pressure and late injectionconditions in a six-cylinder turbocharged diesel engine.The results showed that:(1) Because of a better atomization of fuel, the equivalence ratio was reducedwith higher injection pressure. Advancing or delaying the injection timing relatived tothe TDC increased the ignition delay under late injection conditions, which resultedin more uniform distribution of fuel in the cylinder and lower Soot emission. Whenthe fuel was injected at10oCA ATDC, the equivalence ratio of gas-fuel mixture wasclose to1.0. The lean mixture could obtain an extra-low Soot emission, however theNOx emission increase significantly as the larger premixed combustion rate.(2) Under high pressure and late injection conditions, the ignition timingdelayed with the increasing of EGR rates, while the NOx emission and combustiontemperature in cylinder reduced markedly. With the increase of EGR rates, the Sootemissions increased sharply at the beginning. It may decrease while the EGR rate was large enough (>50%). The cylinder pressure reduced significantly with high EGRlevels because of the worse combustion. Under high pressure and late injectionconditions, the injection timing had less effect to the NOx emission than the EGR.The simulation results also showed that Soot-Bump was changed with injectiontiming. What’s more, the delay of injection timing could effectively reduce the Sootemissions which were caused by large EGR rate (>30%).(3) Ultramultihole Nozzle could significantly reduce the duration of injection,and could obtain a more homogeneous fuel-air mixture which results a premixedcombustion in the diesel engine. The efficient and clean premixed combustion ofdiesel engine was achieved by the combination of Ultramultihole Nozzle, delayedinjection timing, higher injection pressure and EGR, which could simultaneouslyreduce the NOx and Soot emissions.