Bill,
Kelvin got it mostly right but his description of EGR valve and what it does
was not right.
EGR is a method of controlling oxides of Nitrogen. (NO, NO2,NO3 , etc.,
usually referred as NOX.)
First a little chemistry. When air goes through an engine about 79% of what
is sucked through the intake is Nitrogen. If the temp in the cylinder stays
below 2500 degrees F the Nitrogen goes out the same way it went into the
engine as N2. However if the temp in the cylinder goes above 2500 degrees F
then the two Nitrogen split apart and start grabbing oxygen atoms 1, 2, 3, 10
what ever they can find. These NOX molecules are not stable, and are very
reactive.
These strange oxides of Nitrogen then go into the air and become part of smog.
Oxides of Nitrogen are what cause the burning in the lungs and eyes on smoggy
days. (the oxides combine with water and create Nitric acid, no wonder your
eyes burn!). Needles to say this stuff is bad for you.
Now in the days when the air was clean and sex was dirty (early 60's), nobody
talked about NOX because the mixtures were so rich the temp in the cylinders
was always 2500 degrees F. Then 68 rolled around and the Feds. made the auto
makers make their mixtures leaner. All of a sudden there was NOX coming out
of the tailpipes of cars.
The automakers were faced with having to get rid of NOX, in the early 70's
there were three ways to do this:
1. Lower compression Less pressure, less temp (really bad for performance)
2. Retard timing same effect as #1 (also not good for
performance)
3. Put two to three percent by volume when needed into the cylinder to lower
the peak temp. This is what EGR does. It allows the car maker to keep some
performance in what would otherwise be a smog strangled engine. (yeah!)
When does EGR work?
When the potential for high cylinder temps and pressures exists. During idle
the pressure in the cylinder is very low, and EGR is not needed. In fact EGR
at idle will cause a rough or possibly no idle condition.
During full throttle there is so much fuel going into the cylinder that the
temp does not go above 2500 degrees F. EGR is designed to work at part
throttle cruise.
Sorry I went on for so long,
I hope this helps
Rick Ewald
67 B roadster
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