Physics based diesel turbocharger model for control purposes 2009-24-0123

Model-based tuning is a way followed by car manufacturers to reduce development costs. In this context, a new methodology has been developed in order to adapt a tur-bocharged diesel engine in the case of non-standard external conditions. Indeed, variable geometry turbine and fuel injection command laws are developed for standard conditions (20°C, altitude=0m). Turbocharger and fuel injection actuators pre-positioning maps should be adjusted regarding the inducted air mass density (influenced by the external temperature and pressure), in order to meet thermal, mechanical and pollutant emissions constraints. In order to reduce the use of climatic tests bench and extreme conditions tests in foreign countries, a model of a turbocharged diesel engine coupled to an optimization loop has been used to take into account the effect of non-standard external conditions on pre-positioning maps. This tool is based on ACHILLE, a PC-based simulation system for the internal combustion engines, developed internally at Renault.

The paper is structured in three parts:

• The mean-value model of a 2.0l turbocharged direct injection diesel engine is described, with an emphasis on the data map-based turbocharger model. The air path is modelled using mean-value restriction models (air filter, intercooler, throttle) and volumes. In-cylinders physics are processed by a black box approach, in order to determine the combustion parameters and the temperature at the exit of the engine. Data relative to the compressor and turbine are read from data maps: pressure ratio and efficiency are measured as functions of mass flow rate and rotary speed on 2 distinct data maps. However, these data are usually incomplete and poorly discretized, leading to the use of extrapolation and interpolation methods to cover the entire turbocharger operating range. As standard mathematical methods give poor results, an extrapolation treatment based on mathematical methods and turbomachinery physical laws has been used in order to ensure the accuracy of the model.
• The optimization process which determines adjusted turbocharger command laws is described. Test cases for inlet air density from 0.70 to 1.20 kg/m³ are calculated and the results given by the optimisation tool analyzed. Two types of constraints are taken into account by the optimization process: thermo-mechanical limitations (turbocharger speed limit, pressure and temperature upstream turbine and after the compressor) and pollution (soot emissions). For a given altitude and outside temperature input, the tool determines the engine new full-load curve. The goal is to get the turbocharger actuator pre-positioning which guarantees the good performance of the engine under these modified external conditions.
• Finally, simulation-based turbocharger and fuel injection actuator pre-positioning maps are compared to those determined experimentally. Limitations of both model and optimization process are also studied.

It is concluded that the optimization tool based on a mean-value engine model gives very satisfying results: it requires low CPU charge, while turbocharger and fuel injection actuators pre-positioning maps given by the simulation tool are predictive. Additionally, the treatment of the turbocharger data maps allows good agreement between calculation and experimental results, while providing a good robustness level to the model.