Towards model-based control of RCCI-CDF mode-switching in dual fuel engines

The operation of a dual fuel combustion engine using combustion mode-switching offers the benefit of higher thermal efficiency compared to single-mode operation. For various fuel combinations, the engine research community has shown that running dual fuel engines in Reactivity Controlled Compression Ignition (RCCI) mode, is a feasible way to further improve thermal efficiency compared to Conventional Dual Fuel (CDF) operation of the same engine. In RCCI combustion, also ultra-low engine-out NOx and soot emissions have been reported. Depending on available hardware, however, stable RCCI combustion is limited to a certain load range and operating conditions. Therefore, mode-switching is a promising way to implement RCCI in practice on short term.

In this paper, a model-based development approach for a dual fuel mode-switching controller is presented. Simulation results demonstrate the potential of this controller for a heavy-duty engine running on natural gas and diesel. An existing control-oriented engine model is extended with a new CDF model to simulate both CDF and RCCI operation. This model shows good agreement with experimental data. As a first step towards model-based control development, this extended model is used for system analysis to understand the switching behavior and to design a coordinated air-fuel path controller. This closed-loop controller combines static decoupling with next-cycle CA50-IMEP-Blend Ratio control. For a mode-switching sequence in a low load operating point, the closed-loop controlled engine demonstrates stable behavior and good reference tracking. The paper concludes with an outlook on necessary steps to bring model-based control strategies for dual fuel mode-switching in a multi-cylinder engine on the road.