Optimal control of marine propulsion diesel engine during transient loading operation: application for smoke opacity reduction through manifold air injection avoiding compressor surge

Abstract

The reduction of smoke emissions and improvement of load acceptance in a turbocharged marine diesel engine, during transient operation involving rapid load increases, were considered in the present work. Model predictive control (MPC) was used in a system for external compressed air injection directly in the air manifold. Concurrently, the aim was to avoid surge in the turbocharger compressor upstream of the air manifold. Previous work in the Laboratory of Marine Engineering/NTUA had proved experimentally the applicability of the air injection method for smoke reduction and had also investigated the compressor surge behavior via en- gine performance simulations. Compressor system surge is a self-excited limit cycle oscillation, charac- terized by large amplitude pressure rise and mass ow uctuations. It starts to occur when the pressure rise and mass ow characteristics for constant speed exceed certain values determined by characteristics of the compressor and load. A surge avoidance method was used , where stable operation was achieved by operating the compressor at a safe distance, dened as surge margin, from the unstable region. In MPC, a dynamical model of the process is used to construct and solve an optimization problem aiming to achieve prescribed performance, under constraints on input/output variables. MPC is widely accepted in the process industry and recently has been considered for the control in combustion engines, despite the high computational overhead. Due to the diculty of deriving a process model from rst principles for the smoke density (opacity), system identication was used in order to derive control models relating air injection and fuel to opacity and intake manifold pressure. Air injection was considered as manipulated variable, and opacity as controlled variable. The fuel was considered as measured disturbance. Intake manifold pressure was related to compressor instability. In the MPC, the ob jective function was the minimization of smoke density (opacity), with constraint not to exceed a limit in intake manifold pressure. Experiments at the Laboratory were performed on a full scale marine diesel engine, with two dierent types of predictive controllers. Results comparing the opacity under air injection model predictive control with the standard engine operation, i.e. without air injection, during the same transient were presented. It can be seen that with air injection, opacity was reduced considerably. The peak value remained the same in both cases, about 80%. However, in the case with air injection, this peak dropped considerably after about 0.5 sec to a steady value of about 40%, until the end of disturbance. The signicant reduction obtained in the full scale testbed experiments demonstrated the eectiveness of the proposed system of controlled air injec- tion for smoke abatement during engine transients, as well as the suitability of the control method used.

Η μείωση των εκπομπών καπνού και η βελτίωση απόκρισης φορτίου σε ένα ναυτικό υπερτροφοδοτούμενο κινητήρα Diesel, σε μεταβατική λειτουργία που περιλαμβάνει απότομες αυξήσεις στο φορτίο, αποτέλεσαν το αντικείμενο αυτής της εργασίας. Χρησιμοποιήθηκε σύστημα ελέγχου με μοντέλο πρόβλεψης (Model Predictive Control-MPC), σε ένα σύστημα εξωτερικής παροχής παροχής πεπιεσμένου αέρα απευθείας στο δοχείο αέρα. Ταυτόχρονα, στόχος ήταν η αποφυγή πάλμωσης του συμπιεστή του υπερπληρωτή, ανάντι του δοχείου αέρα.