Real-Time Predictive Modeling of Combustion and NO _x Formation in Diesel Engines Under Transient Conditions 2012-01-0899

The present work has the aim of developing a fast approach for the predictive calculation of in-cylinder combustion temperatures and NO_x formation in diesel engines, under steady state and transient conditions. The model has been tested on a PC, and found to require very little computational time, thus suggesting it could be implemented in the ECU (Engine Control Unit) of engines for model-based control tasks. The method starts with the low-throughput predictive combustion model that was previously developed by the authors, which allows the predictive estimation of the heat-release rate and of the in-cylinder pressure trace to be made on the basis of the injection parameters and of a few quantities measured by the ECU, such as the intake manifold pressure and temperature. A three-zone thermodynamic model is used for the in-cylinder temperature evaluation: the combustion chamber is divided into a vapor fuel zone, an unburned gas zone and a stoichiometric burned gas zone, to which the energy and mass conservation equations are applied. The temperature values are calculated by means of second-order polynomial equations. The temperature evaluation allows the in-cylinder NO_x concentration to be calculated by means of the prompt and Zeldovich thermal mechanisms. The procedure also takes into account the intake charge NO_x concentration, and is therefore suitable for both engines equipped with traditional short-route EGR (Exhaust Gas Recirculation) systems, and engines equipped with SCR (Selective Catalytic Reduction) and long-route EGR systems. The tuning parameters of the combustion model for pressure estimation were calibrated on six NEDC (New European Driving Cycle) key-points, by means of the DoE (Design of Experiment) methodology, while the thermodynamic model did not require any specific calibration. The NO_x model features a single calibration coefficient, which is mainly engine dependent. NO_x prediction can be further improved, for a specific engine, by changing the model tuning coefficient as a function of the engine load. The complete model was applied to analyze steady state operating conditions and the urban and extra-urban phases of the NEDC cycle in a modern EURO V low-CR (Compression Ratio) diesel engine, equipped with piezo-driven injectors. A very good matching was found with experimental results in terms of in-cylinder pressure and NO_x emissions, with very little computational effort.