Towards model-based control of a steam Rankine process for engine waste heat recovery

Control plays a critical role in enabling good performance of Rankine processes for waste heat recovery from prime movers. Despite that, literature on control design for those systems is scarce, and there is a real need for investigating approaches that can be effective in an industrial context, where rapid control prototyping must be followed by a viable calibration procedure. The paper describes modeling and control of a pilot Rankine steam process for exhaust gas heat recovery from a spark-ignition engine, focusing in particular on the use of a reduced-order dynamic model of the system to compute a nonlinear feedforward action to better control the system during nominal operation. Model reduction is obtained at the heat-exchanger level. First, via the moving-boundary approach, which avoids the need for complex, finite-volume, models. Then, via a further reduction of the resulting evaporator model which captures its dominant dynamics. The proposed control system is validated on a detailed reference simulator using a demanding driving cycle.