Transient Operation and Over-Dilution Mitigation for Low-Pressure EGR Systems in Spark-Ignition Engines

Low-Pressure cooled Exhaust Gas Recirculation (LP-cEGR) is proven to be an effective technology for fuel efficiency improvement in turbocharged spark-ignition (SI) engines. Aiming to fully exploit the EGR benefits, new challenges are introduced that require more complex and robust control systems and strategies. One of the most important restrictions of LP-cEGR is the transient response, since long air-EGR flow paths introduce significant transport delays between the EGR valve and the cylinders. High dilution generally increases efficiency, but can lead to cycle-by-cycle combustion variation. Especially in SI engines, higher-than-requested EGR dilution may lead to combustion instabilities and misfires. Considering the long EGR evacuation period, one of the most challenging transient events is throttle tip-out, where the engine operation shifts from a high-load point with high dilution tolerance to a low-load point where EGR tolerance is significantly reduced. In this study, different strategies are proposed and evaluated based on their performance regarding misfire avoidance during aggressive throttle tip-outs. A simulation-based characterization of combustion instability is established for evaluation purposes. An Artificial Neural Network (ANN) methodology is developed to control the intake and exhaust valve timing in order to limit the total dilution during the EGR evacuation period by reducing the internal residual. A spark-throttle methodology is also investigated that initiates the tip-out through combustion phasing retardation while keeping the throttle open to allow for higher EGR evacuation rates. Finally, a secondary air-path is evaluated which bypasses the main intake path during the initial period of the tip-out. Simulation results show that the ANN-controlled valve timing method provides significant improvements to the transient LP-cEGR response. When this method is coupled with a hardware modification to include the secondary air-path, combustion instabilities during such conditions are completely eliminated.