Modelling Diesel Engine Combustion and NOx Formation for Model Based Control and Simulation of Engine and Exhaust Aftertreatment Systems

Emissions standards are becoming increasingly harder to reach without the use of exhaust aftertreatment systems such as Selective Catalytic Reduction and particulate filters. In order to make efficient use of these systems it is important to have accurate models of engine-out emissions. Such models are also useful for optimizing and controlling next-generation engines without aftertreatment using for example exhaust gas recirculation (EGR). Engines are getting more advanced using systems such as common rail fuel injection, variable geometry turbochargers (VGT) and EGR. With these new technologies and active control of the injection timing, more sophisticated models than simple stationary emission maps must be used to get adequate results.

This paper is focused on the calculation of engine-out NO_x and engine parameters such as cylinder pressure, temperature and gas flows. A quasi steady gas exchange model (including models for EGR and VGT) is combined with a two-zone zero dimensional combustion model. The combustion model uses fuel flow parameters to generate heat release data and calculates the corresponding pressure trace. The temperature and equilibrium concentrations in the zones are calculated by the simplified combustion model and the corresponding NO_x concentration is given by the original Zeldovich mechanism.

The result is a low complexity complete engine model which requires far less computational effort than a CFD model but which yet shows good agreement between measured and calculated emissions data. The model is appropriate to use for simulation of complete engine and SCR systems and in a simplified form for model based control.