A wall sub-model in KIVA3 for the prediction of transient exhaust HC concentrations

Abstract

Unburned fuel is one of three main pollutants from spark ignition internal combustion engines. The mechanisms of escape of this fuel from the main combustion event have been well studied. However, while up to 10% of the main chamber fuel may escape the initial combustion phase, 80–90% of this is oxidized during the postflame period. Modelling of this oxidation requires a detailed understanding of heat and mass transfer in the boundary layer on the surfaces of the combustion chamber. The resolution of the temperature and concentration gradients requires cell sizes less than 0.1 mm. This means that the processes may not be directly estimated with standard computational fluid dynamics (CFD) representations of combustion. In addition, combustion in the vicinity of the wall is usually quite different from that in the main chamber. A one-dimensional (1D) wall model for the oxidation of HCs was implemented into the wall cells of a KIVA3 computational model. The wall layer was used to follow the postfame oxidation of HCs during the expansion stroke. The CFD model was retained to predict combustion, quench and boundary conditions for the wall model, and to follow the entrainment, mixing and oxidation of HCs throughout the exhaust stroke. This paper describes the model. It presents some of the results of the model in the prediction of various features of interest within the processes of postflame oxidation. Most importantly it allows the prediction of mixing and entrainment of HCs from the exhaust gases in the exhaust stream. A comparison of the predictive capabilities of the model with some experimental HC traces is also presented.