Fuel-Flexible Engine Control of Biodiesel Blends During Mixing-Controlled Combustion

The use of biodiesel blends has the potential to result in many benefits including decreased reliance on imported petroleum, increased sustainability, decreased net carbon dioxide emissions, and decreased particulate matter emissions. There are, however, two major combustion-related challenges to the use of biodiesel blends: (1) decreased torque/power capacity and (2) increased emissions of nitrogen oxides. The work presented in this paper demonstrates that both of these challenges can be met through the use of a physically based fuel-flexible combustion control strategy. The approach consists of two parts: estimation, whereby the engine control module (ECM) detects the biodiesel blend fraction being supplied to the engine, and accommodation, whereby the ECM dynamically changes the control setpoints in order to improve the combustion performance. The proposed control method utilizes only stock engine hardware and does not require the creation of new ECM lookup maps. The proposed framework is incorporated into an already complex engine control system, and as such, it must be ensured that the stability of the overall system is not detrimentally affected. A formal stability analysis is outlined which demonstrates that the engine control system will remain stable. Experimental validation of this control strategy on a 2007 6.7 liter Cummins ISB series engine at several very different operating modes shows that this fuel-flexible control strategy greatly reduced or completely eliminated increases in emissions of nitrogen oxides of up to 30% while largely maintaining the torque/power capacity of the engine when operating with biodiesel.