In order to reduce emissions and improve fuel economy, most recent SI engines are equipped with several subsystems that deeply influence the combustion process. For example, the quality and the quantity of the fluid within the cylinder at Intake Valve Closing may be controlled by acting on Variable Valve Timing systems, external EGR, variable geometry intake systems, and of course on the throttle. On the other hand, tumble/swirl components of the intake flow may be influenced by acting on specifically designed devices. To achieve maximum efficiency, the Spark Advance (SA) controller should therefore compensate for different combustion speeds, in order to control cylinder pressure peak (or 50% mass fraction burnt) position at a constant value.
On one hand closed-loop SA control is still not feasible in most production systems, and on the other open-loop algorithms should take into account the influence of several subsystems that may be present, such as Variable Valve Timing, external Exhaust Gas Recirculation, variable intake/exhaust geometry, and tumble/swirl control devices.
The paper presents one way of dealing with such complexity, by analyzing separately each subsystem's contribution to combustion duration. Optimal SA control is then achieved by feeding black-box combustion duration models with residual gas fraction estimations performed using a simple and reliable model of the gas exchange process. The overall algorithm has been designed by considering Electronic Control Unit constraints in terms of computational effort.
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 and tumble device position.