As it is well known, the combustion process in Spark Ignition (SI) engines is strongly affected by the quality and quantity of the fluid within the cylinder at Intake Valve Closing (IVC). Residual gas affects the engine combustion processes (and therefore emissions and performance) through its influence on charge mass, temperature and dilution. Moreover, in Gasoline Direct Injection (GDI) engines, the amount of oxygen in the residual gas may be significant if the engine is operated in stratified charge mode (low loads and speeds), while almost no oxygen may be found in the residual gas during homogeneous-charge operation. In this paper, different approaches to residual gas fraction estimation are analyzed and compared. The main objective is to obtain a simple and reliable model also in presence of Variable Valve Timing (VVT, both on intake and exhaust valves) and External Gas Recirculation (EGR) systems, that could be used to control combustion duration and position. In fact, the two main contributions to residual gas fraction (backflow of the burned gas during the valve overlap period, and amount of gas trapped within the cylinder) are strongly affected by intake and exhaust valves timing, and EGR flow should be taken into account in order to determine the total exhaust gas mass within the cylinder at IVC. Therefore, estimation of residual gas mass and composition is crucial for designing VVT and EGR management strategies, integrated with optimal control of Spark Advance (and therefore of the combustion process). Experimental data have been acquired on a 3.2 liter V6 GDI engine, equipped with intake and exhaust VVT systems. Tests were performed throughout the engine operating range for different combinations of intake and exhaust valve timings, while varying EGR flow.

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