Purpose of the EGR?
Re: Purpose of the EGR? The purpose of EGR is to kill the NOx (oxides
of nitrogen) which are formed particularly under
high pressure, high temperature, lean conditions.
Basically if you don't give the oxygen enough to
burn, you burn some of the nitrogen. NO2 and NO3
are especially the noxious (heh) components of smog.
EGR's main contribution is to add a substantial
amount of inert gasses (CO2 & H2O) to the intake
charge. This lowers burn temp and suppresses NOx
formation.
However, the amount of axhaust gas that is
recirculated is only a small fraction of the total
exhaust stream, certainly not enough to make more
than a dent in HC / CO by "reburning". It's the
lowered burn temperature and pressure from the
degraded cylinder charge that is the key effect.
The degraded cylinder charge makes for a "soggy"
mid-throttle performance but the idea is that you
can live with that and just give it the pedal it
needs; EGR is taken away at WOT (and its orifice-
limited effect would be small, there, anyway).
A secondary contribution to fuel economy comes
from the reduction in pumping losses across the
throttle. EGR operates only at light and mid
throttle, high vacuum means high pumping effort.
EGR reduces the vacuum as it provides the gasses.
For some reason (I've heard, cam, but maybe also
head design?) the late-years cars can meet the EPA
NOX requirements without EGR. This simplifies the
whole thing considerably and GM loves to save the
dime. For once, it's a bonus.
of nitrogen) which are formed particularly under
high pressure, high temperature, lean conditions.
Basically if you don't give the oxygen enough to
burn, you burn some of the nitrogen. NO2 and NO3
are especially the noxious (heh) components of smog.
EGR's main contribution is to add a substantial
amount of inert gasses (CO2 & H2O) to the intake
charge. This lowers burn temp and suppresses NOx
formation.
However, the amount of axhaust gas that is
recirculated is only a small fraction of the total
exhaust stream, certainly not enough to make more
than a dent in HC / CO by "reburning". It's the
lowered burn temperature and pressure from the
degraded cylinder charge that is the key effect.
The degraded cylinder charge makes for a "soggy"
mid-throttle performance but the idea is that you
can live with that and just give it the pedal it
needs; EGR is taken away at WOT (and its orifice-
limited effect would be small, there, anyway).
A secondary contribution to fuel economy comes
from the reduction in pumping losses across the
throttle. EGR operates only at light and mid
throttle, high vacuum means high pumping effort.
EGR reduces the vacuum as it provides the gasses.
For some reason (I've heard, cam, but maybe also
head design?) the late-years cars can meet the EPA
NOX requirements without EGR. This simplifies the
whole thing considerably and GM loves to save the
dime. For once, it's a bonus.