Hi, Guest
Like I said, it's a grey area. I think EGR existed before catalytic converters. In that case, it probably was for emissions. As stated here http://en.wikipedia.org/wiki/Exhaust_gas_recirculation it does meaningfully reduce NOx production.
However, ever since the introduction of catalytic converters, NOx is less of a problem. They take NOx and CO (the other bad combustion byproduct) and turn them into N2 and CO2 which are not harmful (well, ok, other than global warming).
That's when EGR started getting phased out. It wasn't about emissions anymore, it was about milage, and it seemed nobody cared about milage in the 90's.
In fact, I might go so far as to say you don't *want* to reduce NOx too much, because the cat actually needs it to reformulate CO into CO2.
That is why on factory cars, if you put a narrowband sensor on, you'll see it dithering just on either side of stoich while running closed loop. It's not because it can't control the process fast enough, it actually NEEDS to work this way. When lean, you get more N0x, when rich you get more CO. It needs waves of these different mixtures coming in to work properly.
At least, that what the Ford EECIV engineering manual said.
To further the point, one of my buddies still at Ford has a Focus. His EGR is stuck open, throwing the CEL. He's driven around like this for years, because he gets like an extra 5mpg with full egr flow. If the milage benefit is so great, why not flow that much all the time? Emissions. It probably makes the cat less effective. But that's a guess.
However, ever since the introduction of catalytic converters, NOx is less of a problem. They take NOx and CO (the other bad combustion byproduct) and turn them into N2 and CO2 which are not harmful (well, ok, other than global warming).
That's when EGR started getting phased out. It wasn't about emissions anymore, it was about milage, and it seemed nobody cared about milage in the 90's.
In fact, I might go so far as to say you don't *want* to reduce NOx too much, because the cat actually needs it to reformulate CO into CO2.
That is why on factory cars, if you put a narrowband sensor on, you'll see it dithering just on either side of stoich while running closed loop. It's not because it can't control the process fast enough, it actually NEEDS to work this way. When lean, you get more N0x, when rich you get more CO. It needs waves of these different mixtures coming in to work properly.
At least, that what the Ford EECIV engineering manual said.
To further the point, one of my buddies still at Ford has a Focus. His EGR is stuck open, throwing the CEL. He's driven around like this for years, because he gets like an extra 5mpg with full egr flow. If the milage benefit is so great, why not flow that much all the time? Emissions. It probably makes the cat less effective. But that's a guess.