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How do EGR Valves work

The EGR valve delivers a controlled reduction of oxygen content to the combustion process by introducing inert exhaust gas into the air/fuel charge to the cylinder at the intake manifold, causing a slower explosion in the cylinder and lower combustion temperature and pressure. Since harmful NOx (Nitrogen Oxides) are mainly produced at high temperatures, this allows for a reduction in NOx concentrates emitted to the environment.

To recirculate exhaust gas back into the intake manifold a small passageway exists between the intake and exhaust manifolds. This is where the Exhaust Gas Recirculation (EGR) Valve is located, where it adjusts the amount of recirculated exhaust gas back into the intake manifold. Intake vacuum in the intake manifold sucks exhaust back into the engine, but the amount of recirculation has to be closely controlled otherwise it can have the same effect on engine performance, idle quality, and driveability as a huge vacuum leak.


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The amount of exhaust gas returned into the intake manifold is only about 5 to 10% of the total, but it is enough to dilute the air/fuel mixture to have a cooling effect on engine combustion temperatures. The EGR is only activated at defined operation stages that vary with specific vehicles. This keeps combustion temperatures below 1500 degrees Celsius to reduce the reaction between nitrogen and oxygen that forms NOx.

When the engine is idling the EGR Valve is closed and there is no Exhaust Gas Return flow into the intake manifold. The EGR Valve remains closed until the engine is warm and operating under load. As the load and combustion temperature increase the EGR Valve is opened and begins to send exhaust gas back into the intake manifold.

Most older EGR systems use a vacuum regulated EGR valve while newer vehicles tend to have an electronic EGR valve to control exhaust gas recirculation. (See Types of EGR Valves)


Vacuum Operated
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Electronically Operated
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