| Research Abstract |
The multidimensional engine simulation code, FREC-3D(CI), has been used to elucidate the effects of injection rate and split injection on diesel combustion, NO, and soot emissions. The combustion submodel has been updated, including the ignition submodel previously based on a one-step global mechanism. In-cylinder NO and soot formations were predicted by a Zeldovich mechanism with a partial equilibrium assumption and Morel's soot formation with an oxidation submodel, respectively. In result, computations give good agreement between measured and predicated trends of in-cylinder pressure, and rate of heat release, and a trade-off relationship between NO and soot emissions at pilot injection with high pressure injection. Computations also show that a high turbulence kinetic energy caused by a higher initial combustion is retained at the late combustion stage after fuel injection, and promotes the soot oxidation process.
Experimental data suggests the roll of initial combustion on character
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istics of exhaust emissions. The experiment were carried out to change the initial combustion by using different nozzle orifice diameters and the electromagnetic controlled spool acceleration type injection system. It is found that smoke and NOx concentrations at smaller nozzle orifice diameter, I.e., dィイD2NィエD2 = 0.08mm, are higher than at bigger one, i.e., dィイD2NィエD2 = 0.14mm. The initial rate of heat release at dィイD2NィエD2 = 0.14mm is higher than that at dィイD2NィエD2 = 0.08mm. This means that the smaller orifice decreases the ignition delay at same injection pressure and controls rapid combustion at the initial stage. In generally, soot is formed at the late combustion stage. At late combustion stage, fuel injection is already terminated, so that combustion and spray at the initial stage have an influence on combustion at the late stage. So, the mixing by turbulence at late combustion stage is one key of important parameters for reduction of smoke, i.e., spray penetration, spray interaction by swirl, turbulent mixing of air and fuel, etc. The experimental results suggest that higher initial rate of heat release will enhance the mixing of soot and oxygen at late combustion stage. Less
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