Control Oriented Modeling Of The Gas Exchange Process In Variable Cam Timing Engines

2006-01-0660 Control Oriented Modeling of the Gas Exchange Process in Variable Cam Timing Engines ¨ Per Oberg and Lars Eriksson Vehicular Systems, Dept of EE Link¨opings Universitet S-581 83 Link¨oping SWEDEN c 2006 Society of Automotive Engineers, Inc. Copyright ABSTRACT Variable cam timing engines pose new questions for engine control system designers. The cam timing directly influences cylinder air charge and residual mass fraction. Three models that predict residual mass fraction are investigated for a turbocharged dual independent Variable Cam Timing (VCT) engine. The three models (Fox et. al. 1993, Ponti et. al. 2002, and Mladek et. al. 2000) that all have real time capabilities are evaluated and validated against data from a crank angle based reference model. None of these models have previously been validated to cover this engine type. It is shown that all three models can be extended to dual independent VCT engines and that they also give a good description of the residual gas fraction. However, it is shown that the two most advanced models, based on a thermodynamic energy balance, are very sensitive to the model inputs and proper care must therefore be taken when these models are used. 1 INTRODUCTION Air charge is important for engine fuel and torque control while residual mass fraction is a crucial factor that limits stable engine operation since it influences the combustion variability. Therefore it is essential for the control system to know the air charge and residual mass fraction, hence models that are accurate enough are required. When cam timing is used to its full extent existing control oriented models for residual mass fraction and cylinder air charge have the shortcoming of not covering the entire engine operating region. The three models (Fox et al. 1993 [1], Ponti et al 2002 [2] and Mladek et al. 2000 [3]) that are investigated in this paper all predict residual mass fraction. The model in [3] also predicts air charge. When it comes to applicability the model in [1] is stated to be applicable for low to medium engine speeds. In [3] no operating points where the Variable Valve Timing (VVT) system has been active are used in the estimating procedure. In [2] both VVT and External EGR is used in the model but the model has not been validated. None of these three models have previously been validated to for a dual independent variable cam timing engine. There is thus a need to investigate if the models can describe the residual gas fraction for dual independent VCT engines and to validate them over the engines’ full operating range. There are other publications that propose control and or estimation algorithms for different types of VVT systems, but the focus has been on the air charge and not on the residual gas fraction. In [4] and [5] air charge