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The Exhaust Gas Recirculation (EGR) valve is used to send some of the
exhaust gas back into the cylinders to reduce combustion temperature.
Why would we want to do this?
Nitrous oxides (nasty pollutants) form when the combustion
temperature gets above 2,500 degrees F. This happens, because at such
temperatures, the nitrogen in the air mixes with the oxygen to create
nitrous oxides. Did you ever have two friends that were fine by
themselves but just awful when they got together? Well, our good
friend, the sun, is just like that. When it's sunny, the nitrous
oxides from the exhaust get together with the hydrocarbons in the air
to form our not-so-good friend, smog. That's when the EGR valve comes
in handy.
By recirculating some of the exhaust gas back through the intake
manifold to the cylinders, we can lower the combustion temperature.
Lowering the combustion temperature lowers the amount of nitrous
oxide produced. Consequently, less of it comes out the tail pipe.
There are two types of EGR valves. One operates through the use of a
vacuum, and the other operated through the use of pressure. Both
types allow the exhaust gas in to lower the combustion temperature
when it gets too high.
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