Multivariable diesel low-pressure EGR controller designed by input-output linearization

A multivariable low-pressure (LP) exhaust gas recirculation (EGR) controller that simultaneously regulates the LP-EGR mass flow rate and differential pressure using a LP-EGR valve and a LP-intake throttle was designed by input-output linearization plus proportional-integral (PI) control. The controller includes a PI limitation strategy that prioritizes tracking the flow-rate setpoint over the differential-pressure setpoint if either one of the two actuators saturates. A consequence of the limitation strategy is that setting the differential-pressure setpoint unachievably low, e.g. zero, forces the multivariable controller to operate as a single-input two-output (SITO) controller that tracks the LP-EGR flow-rate setpoint while using the LP-EGR valve and throttle as a single chained actuator. Two cases-one with the multivariable controller and a second with the SITO chained-actuator controller-were experimentally compared in a 6.7-liter diesel medium-duty truck. The multivariable controller achieved a lower 2-norm control error and required less actuator movement than the chained-actuator controller, while the chained-actuator controller minimized the differential pressure necessary to achieve the desired LP-EGR flow rate.