Join Date: Mar 2006
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Your question may be better placed in another thread.
EGR "cools" the peak combustion temperature. Here is how it works.
When the fuel burns, it has a fixed amount of heat to release. If there is only air and fuel, all the heat warms the products of combustion, say up to temperture X
Now, adding exhaust gas as a diluent puts an inert material in the combustion chamber with the air and fuel. When the air and fuel burn, the heat released has to not only heat up the products of combustion, but also the inert exhaust gas.
So, the exhaust gas that has been recycled to the cylinder takes a "free ride" and sucks up some of the heat of combustion. This limits the peak temperature in the cylinder to a value that is below X.
It takes very high temperatures to "fix" (react) nitrogen. By limiting the peak combustion temperature, you reduce the tendancy to form nitric-oxides.
Now, the EGR is cooled before it gets to the cylinder by the cooler. This helps the EGR to flow and also makes the EGR denser so you can either get more in the cylinder or "compact" the EGR so that you can get more air/fuel in the cylinder.