Exhaust Gas Recirculation: Description and Operation
Exhaust Gas Recirculation (EGR) Systems
Overview
The EGR system controls the oxides of nitrogen Ox) emissions. Small amounts of exhaust gases are recirculated back into the combustion chamber to mix with the air/fuel charge. The combustion chamber temperature is reduced lowering NOx emissions.
Differential Pressure Feedback EGR (DPFE) System
The differential pressure feedback EGR system consists of a differential pressure feedback EGR sensor EGR vacuum regulator (EVR) solenoid EGR valve~ orifice tube assembly powertrain control module (PCM) and connecting wires and vacuum hoses.
Operation of the system is as follows:
1. The DPFE system receives signals from the engine coolant temperature (ECT) sensor or cylinder head temperature (CHT) sensor intake air temperature (IAT) sensor throttle position (TP) sensor mass air flow (MAF) sensor and crankshafi position (CKP) sensor to provide information on engine operating conditions to the PCM. The engine must be warm stable and running at a moderate load and RPM before the EGR system is activated. The PCM deactivates EGR during idle extended wide open throttle or whenever a failure is detected in an EGR component or EGR required input.
2. 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 �low and outputs the corresponding signal to the FYR solenoid.
3. The EVR solenoid receives a variable duty cycle signal (0 to 100%). The higher the duty cycle the more vacuum the solenoid diverts to the EGR valve.
4. The increase in vacuum acting on the EGR valve diaphragm overcomes the valve spring and begins to lift the EGR valve pintle off its seat causing exhaust gas to flow into the intake manifold.
5. Exhaust gas 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.
6. The DPFE sensor measures the actual pressure drop across the metering orifice and relays a proportional voltage signal (0 to 5 volts) to the PCM. The PCM uses this feedback signal to correct for any errors in achieving the desired EGR flow.
Hardware
Differential Pressure Feedback EGR (DPFE) Sensor
The DPFE sensor is a ceramic capacitive-type pressure transducer that monitors the differential pressure across a metering orifice located in the orifice tube assembly. The differential pressure feedback sensor receives this signal through 2 hoses referred to as the downstream pressure hose (REF SIGNAL) and upstream pressure hose (HI SIGNAL). The HI and REF hose connections are marked on the differential pressure feedback EGR sensor housing for identification (note that the HI signal uses a larger diameter hose). The differential pressure feedback EGR sensor outputs a voltage proportional to the pressure drop across the metering orifice and supplies it to the PCM as EGR flow rate feedback.
Tube Mounted Differential Pressure Feedback EGR (DPFE) Sensor
The tube mounted DPFE sensor is identical in operation as the larger plastic DPFE sensors and uses a 1.0 volt offset. The HI and REF hose connections are marked on the side of the sensor.
EGR Vacuum Regulator (EVR) Solenoid
The EVR solenoid is an electromagnetic device which is used to regulate the vacuum supply to the EGR valve. The solenoid contains a coil which magnetically controls the position of a disc to regulate the vacuum. As the duty cycle to the coil increases~ the vacuum signal passed through the solenoid to the EGR valve also increases. Vacuum not directed to the EGR valve is vented through the solenoid vent to atmosphere. Note that at 0% duty cycle (no electrical signal applied) the EVR solenoid allows some vacuum to pass but not enough to open the EGR valve.
Exhaust Gas Recirculation (EGR) Valve
The EGR valve in the DPFE system is a conventional vacuum-actuated. The valve increases or decreases the flow of exhaust gas recirculation. As vacuum applied to the EGR valve diaphragm overcomes the spring force~ the valve begins to open. As the vacuum signal weakens at 5.4 kl)a (1.6 in-Hg) or less the spring force closes the valve. The EGR valve is fully open at about 15 kpa (4.5 in-Hg).
Since EGR flow requirement varies greatly providing repair specifications on flow rate is impractical. The on-board diagnostic (OBD) system monitors the EGR valve function and triggers a diagnostic trouble code (DTC) if the test criteria is not met. The EGR valve flow rate is not measured directly as part of the diagnostic procedures.
Orifice Tube Assembly
The orifice tube assembly is a section of tubing connecting the exhaust system to the intake manifold. The assembly provides the flow path for the EGR to the intake manifold and also contains the metering orifice and 2 pressure pick-up tubes. The internal metering orifice creates a measurable pressure drop across it as the EGR valve opens and closes. This pressure differential across the orifice is picked up by the DPFE sensor which provides feedback to the PCM.
Electric EGR (EEGR) System
Highlights of the EEGR System
^ The EEGR valve is activated by an electric stepper motor and does not use vacuum to control the physical movement of the valve.
^ No vacuum diaphragm is used.
^ No DPFE sensor is used.
^ No orifice tube/assembly is used.
^ No EGR EVR solenoid is used.
^ Engine coolant is routed through the assembly.
Overview
The EEGR system uses exhaust gas recirculation to control the oxides of nitrogen (NOx) emissions just like vacuum operated systems. The only difference is the way in which the exhaust gas is controlled.
The EEGR system consists of an electric motor/EGR valve integrated assembly a PCM and connecting wiring. Additionally a MAP sensor is also required. Operation of the system is as follows:
1. The EEGR system receives signals from the engine coolant temperature (ECT) or cylinder head temperature (CHT) sensor throttle position (TP) sensor mass air flow (MAF) sensor crankshaft position (CKP) sensor and the manifold absolute pressure (MAP) sensor to provide information on engine operating conditions to the PCM. The engine must be warm stable and running at a moderate load and RPM before the EEGR system is activated. The PCM will deactivate the EEGR during idle extended wide open throttle or whenever a failure is detected in an EEGR component or EGR required input.
2. The PCM calculates the desired amount of EGR for a given set of engine operating conditions.
3. The PCM in turn will output signals to the EEGR motor to move (advance or retract) a calibrated number of discrete steps. The electric stepper motor will directly actuate the EEGR valve independent of engine vacuum. The EEGR valve is commanded from 0 to 52 discrete steps to get the EGR valve from a fully closed to fully open position. The position of the EGR valve determines the EGR flow.
4. A MAP sensor is used to measure variations in manifold pressure as exhaust gas recirculation is introduced into the intake manifold. Variations in EGR being used will correlate to the MAP signal (increasing EGR will increase manifold pressure values).
Hardware
The EEGR valve is a water cooled motor/valve assembly. The motor is commanded to move in 52 discrete steps as it acts directly on the EEGR valve. The position of the valve determines the rate of EGR. The built in spring works to close the valve (against the motor opening force).
EGR System Module (ESM)
Overview
The ESM is an updated DPFE EGR system. It functions in the same manner as the conventional DPFE system however the various system components have been integrated into a single component called the ESM. The flange of the valve portion of the ESM bolts directly to the intake manifold with a metal gasket that forms the measuring orifice. This arrangement increases system reliability response time and system precision. By relocating the EGR orifice from the exhaust to the intake side of the EGR valve the downstream pressure signal measures manifold absolute pressure (MAP). The system provides the PCM with a differential DPFE signal identical to a traditional DPFE system.
The delta pressure feedback EGR monitor is comprised of a series of electrical tests and functional tests that monitor various aspects of the EGR system operation.
First the DPFE sensor input circuit is checked for out of range values (P1400/P0405 P1401/P0406). The EVR output circuit is checked for opens and shorts (P1409/P0403)
Note: EGR normally has large amounts of water vapor that are the result of the engine combustion process. During cold ambient temperatures under some circumstances water vapor can freeze in the DPFE sensor, hoses, as well as other components in the EGR system. In order to prevent MIL illumination for temporary freezing, the following logic is used.
If an EGR system malfunction is detected below 0°C (32°F), only the EGR system is disabled for the current driving cycle. A DTC is not stored and the uM readiness status for the EGR monitor will not change. The EGR monitor will, however, continue to operate. If the EGR monitor determines that the malfunction is no longer present, the EGR system will be enabled and normal system operation will be restored.
If an EGR system malfunction is detected above 0°C (32°F), the EGR system and the EGR monitor is disabled for the current driving cycle. A DTC is stored and the MW is illuminated if the malfunction has been detected on 2 consecutive driving cycles.
After the vehicle has warmed up and normal EGR rates are being commanded by the PCM, the low flow check is carried out. Since the EGR system is a closed loop system, the EGR system will deliver the requested EGR flow as long as it has the capability to do so. If the EVR duty cycle is very high (greater than 80% duty cycle), the differential pressure indicated by the DPFE sensor is evaluated to determine the amount of EGR system restriction. If the differential pressure is below a calibrated threshold, a low flow malfunction is indicated (P0401/P0406).
Finally, the differential pressure indicated by the DPFE sensor is also checked at idle with zero requested EGR flow to carry out the high flow check. If the differential pressure exceeds a calibrated limit, it indicates a stuck open EGR valve or debris temporarily lodged under the EGR valve seat (P0402).
If the inferred ambient temperature is less than 0°C (32°F), or greater than 60°C (140°F), or the altitude is greater than 8,000 feet (BARO less than 22.5 in-Hg), the EGR monitor cannot be run reliably. In these conditions, a timer starts to accumulate the time in these conditions. If the vehicle leaves these extreme conditions, the timer starts to decrement, and, if conditions permit, will attempt to complete the EGR flow monitor. If the timer reaches 800 seconds, the EGR monitor is disabled for the remainder of the current driving cycle and the EGR Monitor I/M Readiness bit will be set to a ready condition after one such driving cycle. Vehicles will require 2 such driving cycles for the EGR monitor to be set to a ready condition.
