TDIMeister
Phd of TDIClub Enthusiast, Moderator at Large
Consider the combustion in the cylinders of an engine as a dance in a big hall. The oxygen (O2) molecule is a girl. I'll consider myself a fuel molecule and another group of guys as nitrogen (N2) molecules. In a room of, say, about 21% girls, 78% nitrogen and 1% fuel -- which reminds me of my University days, but we didn't even have it this good with 21% girls -- the nitrogen and the fuel are all in competition to pair up with the girls, but each at different rates of success: oxygen and nitrogen don't find each to be other attractive mates at all -- kinda like dating a sibling -- and have rather slow chemical reaction kinetics -- unless it gets REALLY HOT in places, like over 2000°C -- so they pair much less easily and frequently than fuel, where there's definitely lots of chemistry 
. Also, the incestuous offspring of nitrogen and oxygen are quite nasty, called NOx. If you pack the dance hall with more or fewer number of people (increase/decrease the pressure) but keep the percentages of the girls, nitrogen and fuel the same, your chances as a fuel molecule pairing with a girl remains about the same.
On the other hand, suppose now a whole bunch of other people come to the dance - goths, stoners, LGBT, eunuchs, etc., but also a bunch more nitrogen joining them. They have no interest or ability to pair with the girls; we'll call this group of people EGR or "inert species." Nevertheless, they pack the room and now the mix of people is about 10.5% girls, 39% nitrogen and 0.5% fuel from the original group and the rest (50%) are EGR. The party atmosphere in the dance hall has taken a decided turn for the worse with the presence of EGR; the reaction kinetics of the whole dance has been disturbed. Now girls almost don't pair at all with nitrogen, and even I am finding it more difficult to find a girl on the dance floor and when I do, it takes a lot more time and effort to pair with a girl successfully and the result is rather less passionate.
The important part is, the pairing of nitrogen and oxygen are greatly reduced or almost eliminated, so that we get a lot less of their nasty offspring NOx.
This is how EGR works in a dance hall - I mean, internal combustion engine.
. Also, the incestuous offspring of nitrogen and oxygen are quite nasty, called NOx. If you pack the dance hall with more or fewer number of people (increase/decrease the pressure) but keep the percentages of the girls, nitrogen and fuel the same, your chances as a fuel molecule pairing with a girl remains about the same.On the other hand, suppose now a whole bunch of other people come to the dance - goths, stoners, LGBT, eunuchs, etc., but also a bunch more nitrogen joining them. They have no interest or ability to pair with the girls; we'll call this group of people EGR or "inert species." Nevertheless, they pack the room and now the mix of people is about 10.5% girls, 39% nitrogen and 0.5% fuel from the original group and the rest (50%) are EGR. The party atmosphere in the dance hall has taken a decided turn for the worse with the presence of EGR; the reaction kinetics of the whole dance has been disturbed. Now girls almost don't pair at all with nitrogen, and even I am finding it more difficult to find a girl on the dance floor and when I do, it takes a lot more time and effort to pair with a girl successfully and the result is rather less passionate.
The important part is, the pairing of nitrogen and oxygen are greatly reduced or almost eliminated, so that we get a lot less of their nasty offspring NOx.
This is how EGR works in a dance hall - I mean, internal combustion engine.
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