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#4768338 - 05/25/18 10:11 PM air fuel ratio
red7404 Offline


Registered: 01/24/18
Posts: 228
Loc: KS
What characteristic of a liquid fuel sets the A / F ratio?

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#4768402 - 05/26/18 12:15 AM Re: air fuel ratio [Re: red7404]
CR94 Offline


Registered: 03/20/16
Posts: 1265
Loc: Western S.C. since 1996
Air/fuel ratio is not a characteristic of the fuel. It's the ratio air mass to fuel mass in the fuel-air mixture. Stochiometric ratio for air:gasoline is about 15:1.
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#4768561 - 05/26/18 08:58 AM Re: air fuel ratio [Re: red7404]
Hammehead Offline


Registered: 03/31/18
Posts: 118
Stochiometric ratio is a result of the calorific power of the fuel. The higher the cp, less fuel is used in the stoichiometry of its mix.


Edited by Hammehead (05/26/18 09:00 AM)
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#4768573 - 05/26/18 09:22 AM Re: air fuel ratio [Re: red7404]
supton Offline


Registered: 11/09/08
Posts: 12319
Loc: NH
Not a chemist but what the heck, I'll play one for now.

Most fuels are composed of hydrogen and carbon. In order to fully release all the energy stored in the fuel, there is an ideal amount of oxygen required for the chemical reaction. Too little air and it doesn't fully react. Too much, seems hard to occur but it certainly does inside of an internal combustion engine (due to the small window of time for it to do "useful" work and the need for the flame to go from combustible molecule to another one). Ultimately you get carbon dioxide and water--and heat which is what is usually we are after--and to do that there is a proper amount of oxygen (from the ambient air) required.

Now I realize fuels can be other than hydrogen and carbon based, and often there is stuff in the fuel which also requires combusting (which may or may not help with releasing heat) but most stuff that we like to burn is largely hydrogen&carbon (oil, gas, wood, coal).
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#4768580 - 05/26/18 09:26 AM Re: air fuel ratio [Re: red7404]
Leo99 Online   content


Registered: 03/30/14
Posts: 3150
Loc: NJ
When we talk about the stoichiometric ratio, that the is the ratio of air to fuel for a "perfect" burn. For gasoline it's 14.7:1. If other fuels can burn with less oxygen, the A/F goes down. If they need more oxygen, the A/F goes us.
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#4768665 - 05/26/18 11:20 AM Re: air fuel ratio [Re: red7404]
SHOZ Offline


Registered: 06/28/03
Posts: 5865
Loc: Illinois
My A/F gauge can be programmed for whatever stoichiometric ratio you want. I use E10 all the time so I set it at 14.2. Lambda is much better to use.
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#4768727 - 05/26/18 12:30 PM Re: air fuel ratio [Re: Hammehead]
eljefino Offline


Registered: 06/15/03
Posts: 32882
Loc: ME
Originally Posted By: Hammehead
Stochiometric ratio is a result of the calorific power of the fuel. The higher the cp, less fuel is used in the stoichiometry of its mix.


This is a good answer.

Another is that if you examine the exhaust gases, neither oxygen nor fuel particles remain as everything was used up and converted.

If one introduces nitrous oxide, they have to run much richer fuel. But they make more power as the oxygen content of the "air" inside is higher than 21%. Some NOS systems are "wet" and include the needed fuel; others require ECM mods and bigger injectors.

The Space shuttle had to carry liquid oxygen so it could have a stoichometric burn in outer space where there was otherwise no other oxidizer available.

Ethanol for all its issues splits apart and leaves a little extra oxygen in the combustion chamber, so if you're tuned for it, even 10%, you can make more power than with pure gasoline alone.

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#4768732 - 05/26/18 12:39 PM Re: air fuel ratio [Re: red7404]
talest Offline


Registered: 04/29/18
Posts: 41
Loc: Texas
Originally Posted By: red7404
What characteristic of a liquid fuel sets the A / F ratio?


As mentioned, for pump gas it is 14.7 to 1, or 14.7:1

This would be the ideal mix for the best, and most complete, burn in combustion per stroke.

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#4768757 - 05/26/18 01:14 PM Re: air fuel ratio [Re: talest]
SHOZ Offline


Registered: 06/28/03
Posts: 5865
Loc: Illinois
Originally Posted By: talest
Originally Posted By: red7404
What characteristic of a liquid fuel sets the A / F ratio?


As mentioned, for pump gas it is 14.7 to 1, or 14.7:1

This would be the ideal mix for the best, and most complete, burn in combustion per stroke.
And the highest exhaust temps.
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#4769057 - 05/26/18 09:10 PM Re: air fuel ratio [Re: red7404]
Hammehead Offline


Registered: 03/31/18
Posts: 118
Yes with ethanol, the 14.7:1 doesn´t apply for ages. Only for the scarce E0. E15 may use 13.9:1 or even a tad less, hence the higher fuel consumption of these oxigenated gasolines, since you´re adding a lesser cp to the final fuel.

And thank you Eljefino


Edited by Hammehead (05/26/18 09:13 PM)
Edit Reason: cp
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#4773551 - 05/31/18 03:57 PM Re: air fuel ratio [Re: red7404]
DGXR Offline


Registered: 01/21/11
Posts: 163
Loc: California
Shell fuels provide the best air-fuel ratio :P
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#4776956 - 06/04/18 08:19 AM Re: air fuel ratio [Re: DGXR]
racin4ds Offline


Registered: 05/07/10
Posts: 1045
Loc: Ocala, Florida
Originally Posted By: DGXR
Shell fuels provide the best air-fuel ratio :P


Let me fix that for you-

The vehicle's ECM or tune provides the best air-fuel ratio wink


Edited by racin4ds (06/04/18 08:19 AM)
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#4779274 - 06/06/18 03:21 PM Re: air fuel ratio [Re: racin4ds]
DGXR Offline


Registered: 01/21/11
Posts: 163
Loc: California
Originally Posted By: racin4ds
Originally Posted By: DGXR
Shell fuels provide the best air-fuel ratio :P


Let me fix that for you-

The vehicle's ECM or tune provides the best air-fuel ratio wink


Clearly. We really need to develop a sarcasm font. I thought my tongue out thingy would indicate a comment in jest.
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