EGR Temperature Estimation Model Including the Effect of Coolant Flow Rate for EGR Control

Recently developed gasoline engines utilize more aggressive EGR rate to meet the emissions and fuel economy regulations. The EGR temperature is often estimated by the ECU and its accuracy affects the estimations of EGR flow rate and intake air flow rate and temperature. Therefore, the accuracy of EGR temperature estimation becomes more important than ever for precise EGR rate control. Typical lookup map based EGR cooler model without the sensitivity to the coolant flow rate is acceptable and widely used if the heat capacity of the coolant side is high enough. However, the coolant flow rate under real vehicle driving conditions often visit low-speed high-load part of the engine map where the lookup map based model suffers from the accuracy issues. This paper presents an investigation of the accuracy of the lookup map based model under different heat capacity conditions. In this study, a simple EGR cooler model based on effectiveness-NTU method was also developed. Coolant flow rate used in this model is calculated with a regressed model, which is a function of engine speed and coolant control valve opening. The flow rate data for the regression was generated using a system simulation for an engine with an integrated thermal management system. The results show that this model improves the accuracy of the temperature estimation especially at lower coolant flow rates.