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Exhaust Gas Recirculation: Description and Operation

DPFE EGR System Diagram, OBD II:




PURPOSE
The Differential Pressure Feedback (DPFE), Exhaust Gas Recirculation (EGR) system controls the Oxides of Nitrogen (NOx) emissions by recirculating small amounts of exhaust gases back into the combustion chamber to mix with the air/fuel charge. The combustion chamber temperature is reduced lowering NOx emissions.

OPERATION
The EGR system is a Differential Pressure Feedback (DPFE) EGR system. It is continuously monitored by the On Board Diagnostic II (OBD II) system for component integrity, system functionality and faults that can cause the emission levels to exceed federal regulations. The system is enabled only during part throttle modes when the engine is warm and stabilized.

System Enabling Conditions
The DPFE EGR system receives signals from the Engine Coolant Temperature (ECT) sensor, Intake Air Temperature (IAT) sensor, Throttle Position (TP) sensor, Mass Airflow (MAF) sensor and Crankshaft Position (CKP) sensor to provide information on engine operating conditions to the Powertrain Control Module (PCM).

- The engine must be warm, stable and running at a moderate load and rpm before the EGR system is activated. The PCM deactivates the EGR during idle, extended wide open throttle or whenever a failure is detected in an EGR component or EGR required input.

PCM Control Of System Function
- The PCM calculates the desired amount of EGR flow for a given engine condition. It then determines the desired pressure drop across the metering orifice required to achieve that flow and outputs the corresponding signal to the EGR Vacuum Regulator (EVR) solenoid.

EVR Solenoid Operation
- The EVR solenoid receives a variable duty cycle signal (0-100 %). The higher the duty cycle the more vacuum the solenoid diverts the EGR valve.

EGR Valve Operation
- The increase in vacuum acting on the EGR valve diaphragm overcomes the valve spring and begins to lift the EGR valve pintle off its eat, causing exhaust gas to flow into the intake manifold.

System Exhaust Flow
Exhaust flowing through the EGR valve must first pass through the EGR metering orifice. With one side of the orifice exposed to exhaust backpressure and the other to the intake manifold, a pressure drop is created across the orifice whenever there is EGR flow.

- When the EGR valve closes, there is no longer flow across the metering orifice and pressure on both sides of the orifice is the same. The PCM constantly targets a desired pressure drop across the metering orifice to achieve the desired EGR flow.

Purge Flow Sensor Operation
- The DPFE sensor measures the actual pressure drop across the metering orifice and relays a proportional voltage signal (O-5 volts) to the PCM. The PCM uses this feedback signal to correct for any errors in achieving the the desired EGR flow.







EGR System Monitoring
- Once the EGR OBD II monitor determines that a failure has occurred, the Malfunction Indicator Lamp illuminates to warn the driver and a Diagnostic Trouble Code is also stored in memory. An on-demand test can also be requested to aid in the diagnosis.
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