Supervision and control prototyping for an engine exhaust gas heat recovery system based on a steam Rankine cycle

Rankine-cycle waste heat recovery systems for automotive applications have been the focus of intensive research in recent years, as they seem to offer considerable potential for fuel consumption reduction. Because of the highly transient conditions they are subject to, control plays a fundamental role to enable viability and efficiency of those systems. Yet, surprising little research has been devoted to this topic. This paper illustrates the design of a practical supervision and control system for a pilot Rankine steam process for exhaust gas heat recovery from a spark-ignition engine. The proposed control strategy for power production focuses more on ensuring continuity of operation than on the pursuit of optimality. The resulting decentralized control system is implemented via two anti-wind up controllers with feedforward action. Performance has been assessed in simulation on a motorway driving cycle using real engine exhaust data. Despite very transient exhaust gas conditions, we show that the expander can produce power throughout the cycle, avoiding start-stop procedures, which would greatly reduce the global efficiency.