Air-to-fuel ratio control with adaptive estimation of biofuel content for diesel engine LNT regeneration

This paper presents a method of controlling the air-to-fuel ratio (AFR) based upon an adaptively estimated biodiesel fuel content for a diesel engine equipped with the lean NOx trap (LNT) aftertreatment system. The fuel content (or percentage of biodiesel fuel) is estimated by an adaptive estimation scheme based upon the exhaust oxygen sensor signal during the normal engine operations and during the LNT regeneration when the AFR is controlled in a closed loop. The engine system was modeled by a third order linear system in this study. A Linear Quadratic optimal tracking controller was used to regulate the engine AFR to the desired level during the LNT regeneration period. The closed loop system robustness with respect to the air flow measurement error and the fuel content estimation error is also analyzed. Three adaptive control schemes were studied through simulations, and the best performance was obtained for the dual-gain scheme, where the low adaptive estimation gain is used during normal engine operations and high gain is used during the LNT regeneration.