Exhaust Gas Recirculation: Description and Operation
EGR Valve Control Schematic:
Purpose:
The exhaust gas recirculation (Linear EGR) system is used to decrease the emission levels of oxides of nitrogen (NOx). NOx is a group of chemical compounds containing nitrogen and varying amounts of oxygen that can have harmful environmental effects in large quantities.
Operation:
NOx forms during the combustion process in amounts that is dependent on the concentration of oxygen in the combustion chamber and the duration that the combustion process temperatures exceed 822°C (1500°F). Decreased NOx levels are accomplished by reducing the peak combustion temperatures through dilution of the incoming fuel/air charge with exhaust gas. When combusted, exhaust gas (largely nonreactive carbon dioxide and water vapor) acts to absorb a portion of the combustion energy, resulting in lower temperatures throughout the combustion process and yielding lower amounts of NOx.
Desired amounts of EGR depend upon the geometry of the combustion chamber and the operating condition of the engine. Extensive laboratory and vehicle tests are used to determine optimal EGR rates for all operating conditions. Too little EGR can yield high NOx, while too much EGR can disrupt combustion events. The linear EGR valve is computer controlled to precisely regulate the amount of EGR delivered to the engine for all operating conditions.
The linear EGR system consists of the PCM, the linear EGR valve, and EGR valve signal modifier module assembly. The EGR module is required because the PCM driver does not have the current capability to drive the EGR valve on its own.
The linear EGR system is designed to offer more precise EGR flow metering, improved emission control and driveability than a typical back pressure system. The PCM monitors the following sensors to control the linear EGR valve:
^ Engine coolant temperature (ECT)
^ Throttle position (TP) sensor
^ Manifold absolute pressure (MAP)
^ Inlet air temperature (IAT)
^ Revolutions per minute (rpm)
^ Vehicle speed sensor (VSS)
Based on the inputs to the PCM a pintle position (pulse width) is commanded to the EGR module. The EGR module then reads this signal and commands a pintle position to the linear EGR valve directly. The EGR module connection consists of ignition voltage, a ground circuit, an input signal from the PCM and the EGR valve control circuit.
The linear EGR valve also contains a position feedback circuit which provides a variable voltage (0-5 volts) signal back to the PCM. This signal provides for the control of EGR to be a closed loop system because the actual position of the EGR pintle can be communicated back to the PCM so that corrections can be made. Fuel and spark compensation for EGR are determined off actual EGR position.
Refer to DTC 32 for explanation of the diagnostics. Testing and Inspection
Output messages are then sent to the EGR system indicating the proper amount of exhaust gas recirculation necessary to lower combustion temperatures. This electronic metering of exhaust gas is ten times faster than a vacuum operated system and has improved diagnostic capabilities.
Solenoid Resistance:
Positioned at the top of the linear EGR assembly are five terminals:
^ "A" is the pulse width modulated negative signal from the control module.
^ "E" is the positive from the ignition.
^ "B," "C" and "D" are terminals from the control module for the integral pintle position sensor.
- "B" is sensor ground.
- "C" is sensor output.
- "D" is +5 volts supply.
Electrical Components Of Linear EGR Valve:
The solenoid (bobbin and coil) assembly is energized by current which enters the valve through an electrical connector (terminal "E"), then flows through the solenoid assembly to the control module and creates an electromagnetic field. This field causes the armature assembly to be pulled upward, lifting the pintle a variable amount off the base.
The exhaust gas then flows from the cylinder head exhaust port (through the orifice) to the intake manifold. The height of the pintle is read by the pintle position sensor, and the control module closes the loop as desired position versus actual position read, changing the pulse width modulated command to the solenoid accordingly, until the actual pintle position equals the desired pintle position.
This results in improved flow accuracy. In most EGR designs, the flow is "Open Loop," in which the system has no feedback mechanism to monitor actual flow and then to correct it. The linear EGR valve is unique in that the control module continuously monitors pintle height and continuously corrects it in order to obtain accurate flow, making linear EGR a "Closed Loop" system. When the solenoid is de-energized (control module breaks the circuit), the pintle is sealed against the orifice, blocking exhaust flow to the intake manifold. When the solenoid is energized, engine vacuum acts to draw exhaust gases through the cylinder head passages, linear EGR valve, intake manifold and into each cylinder. Although the opening looks small, the EGR valve pintle is the flow limiter in the system.
The results of incorrect operation with too much EGR flow at idle, cruise, or cold operation, any of the following conditions could occur.
^ Engine stops after cold start.
^ Engine stops at idle after deceleration.
^ Vehicle surges during cruise.
^ Rough idle.
If the EGR valve should stay open all of the time,the engine would not idle. The results of incorrect operation with too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause:
^ Spark knock (detonation).
^ Engine overheating.