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Lean condition = Idle hang, why, the scientific reasoning ?

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Ibrahim M

Ibrahim M

Automotive
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Oct 21, 2021
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Hello,
Why does usually a lean idle causes the rpm to drop very slowly when you shut the throttle ?

Rather than very quickly when it is too rich ?

Is it because it burns hotter and 100%of the fuel is burned ?

Thanks
 
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Lean doesn't burn hotter, it burns later. This reduces the effective expansion ratio so the exhaust is hotter.
 
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Whether that's the case or not, depends on the reason for being lean. Vacuum leaks tend to do that. Vacuum leaks can be variable depending on what else is going on.
 
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I can imagine a few mechanisms that could be going on, but no point speculating on such limited information. If it is really important to figure out, you either need to get a system engineer looking at it, or become one yourself.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz
 
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To a certain point lean flames are hotter, the average cutting torch shows that. If you go super lean then yes it cools, and that is how the TC-18 aircraft engine could obtain such a good BSFC.
 
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Acetylene and CO, yes, but HCs are hottest very close to stoich, which is why NOx formation is a maximum there

The-adiabatic-flame-temperature-as-a-function-of-the-equivalence-ratio-for-various_acgp6b.png


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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That chart shows acetylene and CO hottest when rich - not lean.

je suis charlie
 
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To a certain point lean flames are hotter, the average cutting torch shows that
Click to expand...
Actually, what an average cutting torch shows, is that intense heat plus a strong jet of pure oxygen allows the cutting of metal via oxidization.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz
 
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NOx formation needs excess oxygen molecules, and high temperature, so maximum NOx formation is just lean of Lambda 1.0.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz
 
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