The paper deals with the steady-state optimal tuning of control variables for an automotive turbocharged Diesel engine. The optimization analysis is based on an engine simulation model, composed of a control oriented model of turbocharger integrated with a predictive multi-zone combustion model, which allows accounting for the impact of control variables on engine performance, NOx and soot emissions and turbine outlet temperature. This latter strongly affects conversion efficiency of after treatment devices therefore its estimation is of great interest for both control and simulation of tailpipe emissions.
The proposed modeling structure is aimed to support the engine control design for common-rail turbocharged Diesel engines with multiple injections, where the large number of control parameters requires a large experimental tuning effort. Nevertheless, the complex interaction of injection pattern on combustion process makes black box engine modeling not enough accurate and a more detailed physical model has to be included in the loop. An hybrid modeling approach, composed of black and grey box models is implemented to simulate compressor flow and efficiency maps. The grey box model is used at low engine speeds while the black box model, based on a moving least squares method, provides compressor data at medium- high speed. Both models appear to perform best in their respective area. On the other hand a classical grey box approach is implemented for the turbine, along its overall working range. Compressor and turbine models are implemented in a computational scheme for integration with a predictive multi-zone combustion model that simulates the fuel jet and its interaction with surrounding gases by dividing the jet core into many parcels in order to describe the thermal gradient and the chemical composition within the combustion chamber. The whole engine model allows simulating in-cylinder pressure and temperature, NO and soot emissions as well as turbine outlet temperature, depending on engine control variables (i.e. injection pattern, Exhaust Gas Recirculation - EGR, Variable Geometry Turbine - VGT). Model validation is carried out by comparing simulated in-cylinder pressure trace and exhaust temperature with a wide set of experimental data, measured at the test bench in steady-state conditions on a small automotive Diesel engine. In the paper the overall modeling approach is presented with a detailed description of in-cylinder, compressor and turbine models and the results of the experimental validation vs. measured data are shown. Furthermore, the optimization results over a set of operating points selected among those of interest for the ECE-EUDC test driving cycle are presented and discussed.