Control of charge dilution in turbocharged diesel engines via exhaust valve timing

Stringent constraints in oxides of nitrogen (NOx) and particulate emission require high levels of exhaust gas recirculation. In this paper we employ a Variable Valve Timing methodology that in steady-state achieves large levels of internal Exhaust Gas Recirculation (iEGR) or charge dilution in Diesel engines. We develop a crankangle based dynamic nonlinear model of a six-cylinder 12 liter turbocharged (TC) Diesel engine. This model captures the transient interactions between VVT actuation, the turbocharger dynamics, and the cylinder-to-cylinder breathing characteristics. Low order linear multi-input multi-output (MIMO) models are then identified using cycle-sampled or -averaged data from the higher order non-linear model. A model-based controller is designed that varies Exhaust Valve Closing (EVC) to maximize the internal exhaust gas recirculation under air-to-fuel ratio (AFR) constraints during transient fueling demands. The closed-loop controller is based on tracking optimal and achievable set-points of burned gas fraction. Simulation results are shown on the full order model.