EGR Valve Function

"Rodan" wrote:
The EGR valve richens the mixture _____________________________________________
"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? _____________________________________________
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.
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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" wrote:
< Explanation of EGR valve function > _________________________________________________
"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. __________________________________________________
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.
====================================================== 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.
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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
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"Kevin" wrote:
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.
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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
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