Surge index and compressor efficiency estimation for Diesel engines with variable geometry compressor system

Variable geometry turbochargers are a well established technology for Diesel engine systems to optimize the energy recovery over a large operating range. More recently, Variable Geometry Compressors (VGC) systems are being considered as a further improvement of automotive turbochargers for their potential of providing a more direct control over the stability margin and the efficiency of the compressor. In particular, a direct control on the stability margin allows to size the compressor more aggressively to obtain better peak performances. When such an aggressive design is adopted the control of the VGC to prevent surging becomes essential and a reliable Surge Index (SI) estimator needs to be included in the control algorithm. This paper presents two estimator designs for an automotive Diesel engine equipped with a variable geometry compressor. The estimators aim at tracking the value of two important performance variables, namely the Surge Index and the compressor efficiency. The first estimator design relies on an open-loop scheme that is based on the analytical inversion of a grey-box model for the prediction of the compressor flow and efficiency maps. The second estimator is a Kalman filter observer that allows to improve estimation performances when sensor measurements are corrupted by noise. Simulation results are provided in steady-state and transient conditions to verify the accuracy and robustness of the estimators and compare their performances.