A combustion kinetic model for estimating diesel engine NO x emissions

Abstract

A phenomenological combustion model, which considers the space and time evolutions of a reacting diesel fuel jet, has been developed in order to estimate the instantaneous NO x concentration in a diesel engine cylinder from the start of the injection until the exhaust valve opening. The total injected fuel mass has been divided into different fuel packages, through the fuel injection rate file, to take into account the heterogeneous nature of the diesel combustion process. Owing to the importance of the kinetics on the formation and destruction mechanisms of the main pollutant species and radicals, the instantaneous composition of each fuel package has been calculated by using a chemical reaction mechanism which considers 27 species and 59 reactions. The main input data are those resulting from the application of the combustion diagnostic procedure to the instantaneous cylinder pressure signal obtained during the engine tests, such as the heat release law (HRL) and the mean temperature. A single-cylinder diesel engine was tested to validate the model and to analyse the influence of the injection parameters (injection pressure, injection timing and injected fuel mass) on the NO x emissions. A good agreement between the theoretical results and the experimental ones was found when the engine conditions were modified. The model proposed also allows a better knowledge of the local mixing fuel/air processes, which represent one of the most important uncertainties when modelling diesel combustion.