Nitric oxide and soot emissions determined from a multi-zone thermodynamic direct-injection diesel engine combustion model

A three-step phenomenological soot model and a nitric oxide emission model have been developed by applying the current understanding of conceptual models for direct-injection diesel engine combustion processes. The three-step soot model incorporates the physical processes of fuel pyrolysis, soot inception and soot oxidation. The nitric oxide model is governed by the Zeldovich (thermal) mechanism and N₂O intermediate mechanism. With the local information provided by the previously developed multi-zone thermodynamic diesel engine combustion model, the emission models can be successfully applied via specific detection concerning where and when each of these reactions mainly occur within the diesel fuel jet evolution process. The simulation was completed for a 4.5-L, inline four-cylinder diesel engine. The results demonstrated that this method, which incorporates emission models into the developed multi-zone diesel engine combustion model, has the potential to qualitatively predict the effects of various engine parameters on the engine-out soot and nitric oxide emissions. The results showed that advanced injection timing and higher injection pressure lead to the increase of nitric oxide concentration because of not only the increased residence time but also the higher entrainment rate of fresh gas into combustion products. Soot formed in-cylinder decreases with increasing injection pressure and advanced start of injection timing mainly due to the extension of the lift-off length and lower local equivalence ratio. In spite of decreased ambient oxygen concentration, the extended lift-off length and the reduced combustion temperature contribute to the reduction of soot formation under heavy exhaust gas recirculation levels.