FREE ASE PREP – Exhaust GAS Recirculation
This is another segment from my Smog Repair License course, which is run in January.
The Exhaust Gas Recirculation valve is also known as the EGR valve, which helps control Nox, “Oxides of Nitrogen.”
The EGR valve will recirculate hot exhaust gases back into the combustion chamber, and for some strange reason, this will cool down the combustion chamber, by lowering the temperature within the combustion chamber below 2500 degrees f.
This is an overview what actually happens within the combustion chamber.
The burnt exhausts cannot burn again, so when the recycle gases enter the combustion chamber it actually takes up space and prevents any more oxygen from entering the combustion chamber, less oxygen less heat.
The Exhaust Gas Recirculation valve has to be controlled at the right time, we don’t need EGR operation at idle, on a cold engine, deceleration or wide open throttle.
The EGR or Exhaust Gas Recirculation Valve only works at light to medium cruise speeds; where Nox emissions are created the most, under load.
The earlier EGR valves were controlled by ported vacuum and a thermal vacuum switch. The thermal vacuum switch is closed on a cold car and opens up when the engine reaches operating temperature.
At this point vacuum is then allow to flow through the thermal vacuum switch to operate the EGR valve, the valve diaphragm then over comes a spring and opens a pintle were the exhaust fumes recirculates back to the combustion chamber. This is the simplest or conventional form of and an EGR system.
How Does a Back Pressure EGR Work ?:
There are also other versions of an EGR system, another one is the positive back pressure EGR, this valve has an internal vent that bleeds off vacuum from the EGR Valve.
At light cruise speeds back pressure builds up in the exhaust system, this backpressure will close the internal vent within the EGR valve. When vacuum is applied to the diaphragm, and the internal vent is close the vacuum will not bleed off. The diaphragm overcomes a spring within the EGR Valve and now the diaphragm lifts up a pintle, which lets the exhaust fumes recirculate to the combustion chamber.
When back pressure decreases the vent opens and the vacuum is bleed off from the EGR Valve chamber and the valve closes. As the back pressure increases more vacuum is allow to the valve chamber and then the valve opens more.
Some manufacturers have a transducer on the outside of the valve but uses the same principal.
The tube in the plate is exposed to the exhaust gases and pressure flows through the tube to the valve. A vacuum line from the tube to the exhaust back-pressure sensor provides the sensor with an exhaust pressure signal.
The sensor as you can see from the picture is mounted between the temperature thermo valve and Exhaust Gas Recirculation the EGR valve.
A spacer plate is mounted between the EGR valve and the intake manifold. A tube in the plate is exposed to the exhaust gases and pressure. A vacuum line from the tube to the exhaust backpressure sensor provides the sensor with an exhaust pressure signal.
This signal is necessary to close the vent in the Back Pressure Sensor to allow the vacuum to travel to the EGR Valve or Exhaust Gas Recirculation Valve.
The sensor mounted between the temperature thermo valve and the EGR valve, contains a spring-loaded diaphragm and an air bleed.
When exhaust back-pressure is low, the air bleed is open. Air is bled into the EGR vacuum line, reducing the vacuum signal and preventing EGR operation. So any obstruction in this line will cause the EGR Valve to be inoperative and cause high Nox.
As Exhaust Gas Recirculation; back-pressure builds, the diaphragm in the back-pressure sensor is forced upward, blocking off the air bleed and allowing EGR operation.
Remember there is no operation at wide open throttle, due to the low vacuum from the engine. The high exhaust pressure still blocks the air bleed, but the valve is non-operational.
Negative Back Pressure EGR Valve:
The vent on the negative back pressure EGR is normally close and when vacuum is applied to the diaphragm the pintles open.
As the back pressure changes in the exhaust system, the internal bleed is controlled which helps control the operation of the EGR.
Then there are also computer control circuits, which are more accurate, the computer monitors all the input sensors and can more accurately control the EGR valve.
Computer Controlled EGR Valves:
The computer switches a solenoid on and off to control the vacuum to the EGR and in some cases, it may pulse a solenoid or apply a duty cycle signal to the solenoid which controls a variable vacuum to the EGR.
Do not use a Test Light to test the control side of a computer control solenoid!
Unless you have the right test light, if you are not sure do not use one!
Another type of EGR is the Digital Valve that uses three different solenoids controlling three different sized holes. The computer can switch on one or more solenoids to mix different openings for the correct amount of EGR flow that it may need for a certain driving conditions.
A linear EGR that uses only one solenoid, but pulses it at different duty cycles to get different amounts of variable EGR flow.
The linear EGR valve has a potentiometer “variable resistor” mounted on top of the EGR housing and connected to the EGR valve by a metering rod. The position sensor for the EGR Valve works just like a Throttle Position Sensor, which indicates the position of a component.
Now not all cars have an EGR, the computer can control the ignition timing, and spark control circuit which will not let the timing advance no more than necessary.
Some vehicles increase valve over lap, holding the intake and exhaust open longer, which in some cases will keep the inert gases in the combustion chamber longer.
This is just another process in reducing Nox, no matter which procedure the manufacturer uses, the goal of the Exhaust Gas Recirculation Valve (EGR) is to lower Nox, by circulating exhaust gases from the exhaust, through the intake to the combustion chamber to lower the temperature and reduce Nox.