Exhaust Gas Recirculation (EGR) Closed Loop Control Limits System Monitor - All Others

The control system is closed loop around EGR rate during normal operation and the limits of the control system are monitored by observing the control deviation calculated as the difference between the desired EGR rate setpoint (after dynamic correction) and the actual EGR rate determined from an air system model. The air system model EGR estimates the percent of EGR in the cylinder by modeling the induction system based on the mass airflow (MAF) sensor reading and an estimate of the airflow over the throttle restriction. The EGR mass flow is then calculated as the difference between the cylinder charge from the speed density equation and this model of the mass airflow into the intake manifold. This model value of the actual EGR flow based on measured values is compared with the desired EGR rate setpoint. If the deviation between the controller setpoint EGR rate and the modeled EGR rate exceed the calibrated thresholds, a system control limit concern is indicated.

If no air system concerns are present, the control limit monitor functions continuously during closed loop operation once the engine coolant temperature is greater than 70°C (158°F). Separate threshold maps indicate the EGR system operation is at its control limits for both the high flow and low flow extremes. Control limit threshold maps are a function of engine torque and engine speed and are based upon the expected distribution of deviations from a statistical sampling. An up/down counter is used that counts upward when the control deviation exceeds the calibrated threshold and counts downward when the control deviation falls below the threshold for the case where the system operation is limited to EGR rates higher than the controller setpoint. Similarly, the up/down counter counts upward when the rate deviation falls below a calibrated threshold and counts downward when it rises above the threshold for the case where the system operation is limited to EGR rates lower than the controller setpoint. Once the counter exceeds a calibrated threshold for either case a concern is detected.