New insights into the phenomenological modelling of diesel engine NOx and soot emissions

This thesis describes the development of a thermodynamic phenomenological diesel engine heat-release model and its use for the prediction of emissions of NOᵪ and soot via the use of detailed sub-models. An improved heat-release model that includes many of the recent developments in the conceptual model of diesel engine combustion is also presented. Validation of model predictions is done by comparison with experimental results. The approach taken here is to model individual phenomena such as fuel injection, fuel evaporation, air entrainment, ignition delay, fuel combustion and product formation. The spray is divided into packets in the radial and axial directions, and each packet experiences its own time history of thermodynamic properties and chemical composition. The formation of combustion products is calculated using a chemical equilibrium model. Heat release from the combustion of fuel is used to calculate cylinder pressure, temperatures and heat losses. The target variables for validation of the heat-release model were cylinder pressure and heat release, because these quantities are readily obtained from experimental results. Calibration of model constants is an important aspect of the model and is discussed in detail. Model predictions of cylinder pressure and heat-release are validated by experimental results. Experiments were conducted on a direct-injection Hino diesel engine that was fully instrumented to record engine performance parameters and in-cylinder measurements. In general, cylinder pressure is predicted with acceptable accuracy. Ignition delay is generally overpredicted and the premixed burn peak in the heat release curve is delayed. Emissions of NOᵪ are predicted using a chemical kinetic NOᵪ model that includes the important N₂O mechanism; predictions from this scheme are compared to the widely used extended Zeldovich NOᵪ model and to experimental results. The NOᵪ scheme used for this investigation was found to perform significantly better than the extended Zeldovich model over a range of engine conditions when calibration of the heat-release was done independently of the NOᵪ prediction. Predictions from the extended Zeldovich model can be improved via recalibration of the heat-release model, but overall performance is still inferior to the NOᵪ scheme presented here. A nine-step phenomenological soot model was also implemented within the framework of the existing diesel engine heat-release model to provide predictions of soot emissions over a range of engine conditions. This model is significantly more detailed than the two-step empirical soot models that are currently widely used with phenomenological diesel engine models, and includes gas and solid phase reactions as well as detailed particle dynamics. Predictions of soot mass, acetylene, precursor species, particle number and particle sizes are presented over a range of engine conditions. Good agreement is observed for the final soot mass value, and the range of particle numbers and sizes, although not measured here, corresponds to values given in the literature. A parametric study of typical diesel engine operating parameters is conducted using the model. Good agreement is found between predicted trends and those given in the literature