Exhaust Gas Recirculation: Description and Operation
Exhaust Gas Recirculation (EGR) System Description
The Exhaust Gas Recirculation (EGR) System is used to reduce the amount of nitrogen oxide (NOx) emission levels caused by high combustion temperatures. At temperatures above 1,371°C (2,500°F), oxygen and nitrogen combine to form NOx. Introducing small amounts of exhaust gas back into the combustion chamber displaces the amount of oxygen entering the engine. With less oxygen in the air/fuel mixture, the combustion pressures are reduced, and as a result, combustion temperatures are decreased, restricting the formation of NOx.
The EGR system on the LGH engine of a single or dual EGR coolers, an EGR valve and a Mass Air Flow (MAF) sensor.
The EGR system on the LML engine applications utilizes dual EGR coolers, an EGR valve, MAF sensor and an EGR cooler bypass valve, controlled by the ECM, to prevent coking of the EGR coolers during light load and idling.
The EGR and EGR cooler bypass valve motors are Direct Current (DC) motors utilizing a multi-stage gear drive connected to the valve stem. The motors are controlled by the ECM and actively force the valves open and closed. Each valve contains an integral valve position sensor which reflects the true position of the valve. The MAF sensor signal is used by the Engine Control Module (ECM) to detect the proper amount of EGR flow. The ECM continually does a consistency check on the indicated EGR flow by comparing the desired MAF to the actual MAF. If the actual MAF is less than or greater than a calibrated threshold level, the amount of EGR flow is determined to be out of range and a DTC will set.
Exhaust Gas Recirculation (EGR) System Operation
EGR Valve
1 - Outlets to Intake Manifold
2 - Intermediate Gear
3 - DC Motor
4 - Output Gear w/Cam Slot
5 - Stem and Poppet
6 - Inlet Port
The EGR valve is controlled by the ECM through the EGR motor high control and EGR motor low control circuits. The ECM supplies voltage that is near ignition voltage to the high and low control circuits at all times. This voltage is used by the ECM as a reference voltage during non EGR operation in order to detect circuit failures. The ECM Pulse Width Modulates (PWM) the low control circuit to ground when the EGR valve is commanded open. A lower pulse width increases the open position of the valve. In order to close the EGR valve, the ECM will PWM the high control circuit to ground.
When the ignition is turned OFF, the EGR valve executes a series of clean and learn operations. The valve goes through a number of full sweeps, (cleaning operation), followed by a number of learn events that pushes the poppet just off the seat and then pulls it into the seat to determine the correct sensor position voltage. Certain environmental conditions, such as low temperature, may inhibit the clean and learn operations, so they may not occur during every ignition cycle.
The EGR valve has an integral position sensor that feeds back to the ECM. The sensor is located on the output gear of the actuator and is a non-contacting type sensor. The ECM uses the position sensor to determine the position of the valve. The ECM sends a reference voltage through the 5-volt reference circuit to the EGR position sensor. The ECM provides a voltage return path for the sensor through the low reference circuit. A variable voltage signal, based on the EGR valve position, is sent from the sensor to the ECM through the EGR position sensor signal circuit.
The ECM commands the required valve position based on engine requirements. The outlet of the EGR valve flows directly into the intake manifold.
EGR Cooler Bypass Valve
1 - Outlet to EGR Cooler
2 - Outlet to Bypass Pipe
3 - DC Motor
4 - Intermediate Gear
5 - Output Gear w/Cam Slot
6 - Stem and Poppet
7 - Inlet Port
The EGR Cooler Bypass Valve (if equipped) is fed off of the passenger side rear exhaust bank. The ECM commands the Bypass Valve to one of two positions - HOT EGR or COLD EGR, depending on engine operating conditions. Under normal to high engine load and temperature conditions the intake system requests COLD EGR and the bypass valve is commanded to the COLD EGR position. The exhaust flow is directed into the two EGR coolers in series and then to the EGR Valve. Under light load/idle conditions, the intake system requests HOT EGR and the bypass valve is commanded to the HOT EGR position. The exhaust flow is directed through the bypass pipe directly into the EGR valve to prevent fouling of the EGR coolers.
When the ignition is turned OFF, the EGR Cooler Bypass Valve executes its own clean and learn operation. The bypass valve switches from seat to seat several times, effectively cleaning the stem while learning the seat position. Certain environmental conditions, such as low temperature, may inhibit the clean and learn operation, so it may not occur during every ignition cycle.
The EGR Cooler Bypass Valve has an integral position sensor that feeds back to the ECM. The sensor is located on the output gear of the actuator and is a non-contacting type sensor. The ECM uses the position sensor to determine the position of the valve. The ECM sends a reference voltage through the 5-volt reference circuit to the position sensor. The ECM provides a voltage return path for the sensor through the low reference circuit. A variable voltage signal, based on the valve position, is sent from the sensor to the ECM through the position sensor signal circuit.
EGR Valve Control Enabling Conditions
Exhaust gas recirculation (EGR) valve control will only be enabled during idle and cruising conditions when the following conditions are met:
* The Intake Air Temperature (IAT) is greater than -5°C (23°F). EGR valve control will remain enabled until the IAT is less than -8°C (17.6°F). Once disabled the IAT must increase to greater than -5°C (23°F) before EGR valve control is re-enabled.
* The IAT is less than 80°C (176°F). Once disabled the IAT must decrease to less than 70°C (158°F) before EGR valve control is re-enabled.
* The ECT is greater than 45°C (113°F) (LGH applications only). EGR valve control will be disabled when ECT decreases to less than 41°C (106°F)
* The ECT is less than 108°C (226°F) (all applications). Once disabled ECT must decrease to less than 105°C (221°F) before EGR valve control is re-enabled.
* The Barometric Pressure (BARO) is greater than 74 kPa (10.7 psi). EGR valve control will remain enabled until the BARO is less than 72 kPa (10.4 psi). Once disabled the BARO must increase to greater than 74 kPa (10.7 psi) before EGR valve control is re-enabled.
* The battery voltage is greater than 8.0 V.
* No active air system faults are present.