Coordinated Air-Fuel Path Control in a Diesel-E85 RCCI Engine

Reactivity Controlled Compression Ignition (RCCI) combines very high thermal efficiencies with ultra-low engine out NOx and PM emissions. Moreover, it enables the use of a wide range of fuels. As this pre-mixed combustion concept relies on controlled auto-ignition, closed-loop combustion control is essential to guarantee safe and stable operation under varying operating conditions. This work presents a coordinated air-fuel path controller for RCCI operation in a multi-cylinder heavy-duty engine. This is an essential step towards real-world application. Up to now, transient RCCI studies focused on individual cylinder control of the fuel path only. A systematic, model-based approach is followed to design a multivariable RCCI controller. Using the Frequency Response Function (FRF) method, linear models are identified for the air path and for the combustion process in the individual cylinders. From timing and blend ratio (BR) sweeps, it is decided to realize the high-level control objectives by controlling CA50, IMEP, BR and λ. Based on the identified models, a static decoupling is designed for the combined air-fuel system. For the decoupled system, a PI air path controller and three next cycle PI fuel path controllers are designed. The potential of the proposed control strategy is demonstrated on a six cylinder test set-up, which is equipped with the standard direct injection system for diesel and with an added port fuel injection system for E85. For engine speed and load steps, the RCCI controller is shown to have good tracking performance during transients. Compared to the open-loop control case, this controller is found to enhance combustion stability and to reduce THC and CO emissions.