EGR Valve Function
Rodan
The EGR valve richens the mixture
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"MasterBlaster" wrote:
Thank God you don't work on cars for a living.
Perhaps you could explain how Recirculating a portion of the
now-inert Exhaust Gases could possibly richen the mixture?
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The only reason for the existence of the EGR valve is to richen the mixture.
Modern engines run as lean as possible to reduce hydrocarbon emissions and
to improve fuel mileage. During operation if the fuel/air mixture becomes
too lean, combustion temperatures rise, threatening preignition, burned
valves and unwanted NOx generation. The best way to reduce combustion
temperature is to richen the mixture. The engine control system actuates
the EGR valve to richen the mixture, thereby reducing combustion temperature.
So how does the introduction of hot exhaust gas richen the mixture?
Glad you asked. Before the EGR opens there is a mechanical (not chemical)
mixture of 20 per cent reactive oxygen and 80 per cent inert nitrogen in the
intake manifold, flowing toward intake valves for further mixing with metered
fuel being injected into combustion chambers. If additional inert gas is added
(from the EGR valve) it mixes with the nitrogen and with the oxygen and the
resultant mixture has a higher percentage of inert gas than before (example:
now 18 per cent reactive oxygen and 82 per cent inert gases).
If fuel is being supplied to maintain a lean air/fuel ratio of 15.0 with a 20/80
mix of oxygen/inerts, then changing the oxygen/inerts mixture to 18/82 yields
a new air/fuel ratio of 15.0 x (18/20) = 13.5, a richer mixture. No fuel has
been added. The available oxygen has been reduced. Result: Richer mixture.
Intake manifold gases do warm up slightly when the hotter EGR gas is mixed in,
but combustion temperature can drop hundreds of degrees with a richer mixture.
The only reason for the existence of the EGR valve is to richen the mixture.
Rodan.
Note: EGR valve enrichment is for combustion temperature management,
not for power demand. For more power, apply more throttle.
Kevin
your full of it. don`t give up the paying job. The egr "is" for cyl
temp reduction but your explaniation is not even close. KB
(here is a hint, already burned gas can`t burn again, soooo)
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Rodan
< Explanation of EGR valve function >
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"Kevin" wrote:
You're full of it. Don`t give up your paying job. The EGR
is for cyl temp reduction but your explanation is all wrong.
Here's a hint: Already burned gas can't burn again.
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Good observation. Exhaust gas is inert. And inert exhaust
gas is recirculated through the EGR valve to dilute the air
mixture in the intake manifold. Once a person understands
the need for cyl temp reduction, it only remains to understand
how the introduction of additional inert gas into the intake
manifold dilutes the existing mixture of nitrogen and oxygen,
thereby reducing the oxygen percentage of the total mixture.
The original complete and detailed explanation of the EGR
function, with supporting calculations, is repeated below.
Any technical analysis is subject to being disproved, so an
alternate explanation of the EGR system's functions would
be welcome. Hopefully the new explanation would include
a courteous, articulate and intelligent analysis: something
more than "You're full of it. Your explanation is all wrong!"
Best regards to all.
Rodan.
=======================================================
Kevin
ok explaination is, inert gas can not burn so adding it to the cyl
mixture dilutes the burnable portion of the cyl mixture, less burnable
less heat produced. the computer will compensate for any richening
effect and lean it out again, as egr is pretty much continous no
richening effect will occur. so the cooling effect is strictly from less
burnable mixture. also as a side effect it slows the burn rate so
timing can be increased some to gain back some lost power. KB
Rodan
ok explanation is, inert gas can not burn so adding it to the cyl
mixture dilutes the burnable portion of the cyl mixture, less burnable
less heat produced. the computer will compensate for any richening
effect and lean it out again, as egr is pretty much continous no
richening effect will occur. so the cooling effect is strictly from less
burnable mixture. also as a side effect it slows the burn rate so
timing can be increased some to gain back some lost power. KB
================================================
Thanks for providing a reasonable alternate explanation for EGR
function, particularly for pointing out that the EGR pretty much
continuously operates in its feedback mode to compensate for
transient variations in combustion temperature.
My explanation for EGR function assumed a port-injected fuel
system (no fuel in the intake manifold, only air and recirculated
inert EGR gases). For throttle-body injectors or carburetors,
the manifold mixture includes fuel so calculation of the resultant
enrichment is more complex.
I have only two small disagreements with your approach:
1.) My understanding is that excessive combustion temperature
is much more strongly related to the leanness of the burn mixture
than to the amount of the burn mixture, so reducing the quantity
of a lean mixture to be burned, without enrichment, is of little help.
2.) Increasing injector fuel flow for enrichment would make an
EGR valve unnecessary, but adding fuel would cause an unwanted
power surge along with the desired combustion cooling. This is
avoided by using EGR feedback to enrich the burn mixture by
decreasing the oxygen content without increasing the fuel flow,
thereby maintaining smooth power.
In any case, I appreciate your point of view.
Best regards,
Rodan.
Ted Mittelstaedt
"Rodan" <Ro...@verizon.NOT> wrote in message
news:UYPPj.8039$NK1.5551@trndny05...
Both of you are incorrect about how EGR actually works.
Yes, the inert gas lowers combustion temps
However, it ALSO increases engine efficiency and extracted energy-per
gallon of gas.
The idea that a richer mixture would cool the cylinders is false. A
leaner mix runs slightly hotter but a richer mixture produces even
more NoX than a leaner mix. EGR cools much more than a richer
mixture.
For an explanation of the chemistry, see the following:
http://en.wikipedia.org/wiki/Egr
Ted