Coordinated air-fuel path control in a Diesel-E85 RCCI engine 2019-01-1175

Reactivity Controlled Compression Ignition (RCCI) is a promising dual fuel combustion concept. It combines very high thermal efficiencies (up to 57%) with ultra low engine out NOx and PM emissions. Moreover, it promotes fuel flexibility. Where other combustion concepts benefit from strict fuel specifications, RCCI enables the use of different combinations of fuels. Although initial results are promising, further developments are needed to assess its potential for meeting future emission targets. Especially on the control side, there remain challenges. First, safe and stable combustion with maximal efficiency has to be guaranteed over the full load range; at low-load, misfires have to be avoided, whereas peak pressure limits have to be respected at high loads. Differences between cylinders can be significant, so cylinder balancing is needed. Second, type approval and on-road application requires adequate transient control; more precisely, besides highly dynamic fuel path control for robust stability, accurate air path control is requisite. This coordinated air-fuel path control is also beneficial for integrated engine-aftertreatment control to meet future HC and CO emission targets, for mode switching, and for optimal thermal management. To the authors knowledge, no results are reported in the open literature on coordinated RCCI control. In this work, a model-based approach is followed for RCCI control design. The studied 6 cylinder heavy-duty diesel engine is equipped with a port fuel E85 and direct diesel injection system. Based on variations of intake conditions and fueling, high efficient operation conditions are determined for different stationary operating points. With this data, also local combustion models are developed. These models are used to design a next-cycle fueling controller as well as a multivariable air path controller. The controller is implemented on the heavy-duty test platform. Its brake thermal efficiency and emission performance is demonstrated for defined speed and load steps.