Exhaust Gas Recirculation (EGR) Rate System Monitor - Transit

The percentage of EGR is calculated using two different methods and the difference between these two calculations is used to determine if the system is operating correctly. The expected amount of exhaust gas in the cylinder is calculated using a model that is based on the commanded EGR rate and the intake throttle position. The actual amount of exhaust gas in the cylinder is measured by subtracting the mass airflow (MAF) sensor reading from a calibrated value of the intake air charge in the cylinder. The expected EGR model accounts for both fresh air and the exhaust gas, and is based on the volumetric efficiency of the engine. An excessive EGR flow is detected when the measured amount of EGR is greater than the expected amount of EGR. An insufficient EGR flow is detected when the measured amount of EGR is less than the expected amount of EGR.

The monitor compares the two calculations, when a set of entry conditions are met, and determines if the system is operating correctly. The concern must be detected for a minimum amount of time before being reported. A timer counts up when the entry conditions are met and the concern is present. The timer counts down when the entry conditions are met, the concern is not present, and the current count is greater than 0. When this timer exceeds the time required to detect a concern, a DTC is set.