A comparison of two NOx prediction schemes for use in diesel engine thermodynamic modelling

Abstract

This paper describes the use of two NOx models coupled to a multizone quasi-dimensional phenomenological diesel engine model to provide NOx predictions for diesel fuel in a direct-injection naturally aspirated diesel engine. Phenomena modelled are spray evolution, fuel evaporation, air entrainment, ignition delay, ignition, combustion and product formation. Mellor and extended Zeldovich NOx sub-models are applied.Predictions of cylinder pressure, heat release and NOx emissions are validated against experimental results using a calibration process based on the error in cylinder pressure predictions. The model is shown to be able to predict cylinder pressure and heat release with good accuracy over a range of engine loads and speeds. When using the same calibration, the Mellor NOx model performed significantly better than the extended Zeldovich model over the range of engine conditions considered. However, when empirical constants were modified to improve NOx predictions at one engine condition, the extended Zeldovich model provided good predictions. The Mellor model is shown to provide superior performance over a range of engine conditions.