The objective of this study is to investigate the effect of homogeneous mixture distributions on the improvement of combustion performance and the reduction of exhaust emissions in CI engine, numerically.
In this work, single cylinder engine was applied based on the 1,500cc engine with 4 cylinder, and the fuel injection nozzles were applied in 6-hole, 8-hole and group-hole for the investigation of the effect of the nozzle geometry on the homogeneous mixture formation. The group-hole nozzle geometries were constituted as paralleled GHN-P, converged GHN-C, diverged GHN-D with ± 3deg.
In order to analyze chemical and physical phenomenon of engine, ECFM-3Z model, Wave model and Dukowicz model were applied to combustion model, break-up model and evaporation model, respectively. Exhaust emission models were applied extended Zel’dovich model which is used for formation and oxidation of NOx and Kennedy-Hiroyasu-Magnussen model which is used for formation and oxidation of soot for the analysis of NOx and soot emissions.
Injection timing, injection pressure, engine operating speed, nozzle geometry and injection strategy were varied for the optimization of homogeneous mixture distributions in the engine cylinder. At the same time, the results of this work were compared in terms of cylinder pressure, rate of heat release, IMEP, NOx and soot emissions.
It was observed that cylinder pressure was increased, as the injection timing was advanced. Therefore, the NOx emissions was increased by sufficiently enough time to create NOx. However, soot emissions was decreased by enough oxidative time. Cylinder pressure and rate of heat release were increased as the injection pressure was increased due to the improvement of fuel atomization. When engine operating speed was increased, NOx emissions was decreased because of insufficiently combustion time. However, soot emissions was increased.
Effect of inclined spray angle shows that in case of inclined spray angle 156deg, a lot of fuel mass impinged to the piston rim and entered into the piston bowl simultaneously. As the result, the soot emissions was decreased.
Effect of group-hole nozzle shows that converged GHN-C type had the best combustion characteristics because of the improvement atomization by the collision effect. On the other hand, diverged GHN-D type was the worst due to the coalescence effect.
Effect of injection strategy observes that combustion phase was changed from premixed combustion to diffusion combustion by pilot injection. Because of this, the NOx emission was the lowest. When two pilot injection timing was advanced. the injected fuel entered to crevice volume. Thus, the rate of heat release was decreased. The soot emissions was decreased due to the oxidation by combustion of post injected fuel.