The need to reduce fuel consumption and harmful pollutants from engines is an important task for automotive industry. It has led to technological advances in new engine design, such as engine downsizing. Due to the reduction of displacement, engine power output is reduced and thus its overall performance is limited. In order to increase torque and power, engines are typically boosted by turbochargers or superchargers. Meanwhile, the improvement on turbo design makes it possible to operate VGT (variable geometry turbocharger) at harsher exhaust environment for gasoline engines as well (e.g., with much higher exhaust temperature than that of diesel engines). This makes VGT related control problems more challenging and requires attention to protecting corresponding engine hardware during an entire engine life.
In this paper we investigate a new model-based engine control feature, which controls engine exhaust pressure in order to protect engine, turbocharger and aftertreatment catalyst devices. The methodology inverts a physics-based model from VGT valve position to engine exhaust pressure to design a model-based feedforward controller to be combined with conventional PID feedback control. The model-based control system makes the actual engine exhaust pressure track the target in fast transients under varying operating environments, such as changes in exhaust temperature and pressure with exhaust pipe restrictions. Dynamic compensation is also introduced to compensate for both transport delay for air flow and combustion delay for the mixture of air and fuel in engine exhaust flow estimation to improve control responses. The exhaust pressure control is seamlessly integrated with the boost pressure control to maintain smooth transitions between the two control systems, which is illustrated through detailed simulation analysis. Similar algorithm can be extended to a wastegated system.