Author Topic: fuel to air ratio  (Read 11386 times)

Offline Stef

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fuel to air ratio
« on: June 08, 2008, 09:35:20 PM »
Hi Fellas! I have a quick and simple question (which no doubt will spawn into a 100 post thread, but nevertheless  ;) )

What is the average fuel to air ratio in a Cessna like Chuck's?  ???

Offline Oddball

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Re: fuel to air ratio
« Reply #1 on: June 08, 2008, 09:44:11 PM »
as in carburation stef?
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Offline Stef

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Re: fuel to air ratio
« Reply #2 on: June 08, 2008, 10:10:39 PM »
as in carburation stef?

Well, obviously it's good to have a lot of air around you and a lot of fuel in the tank!  ;D But indeed, I was rather thinking about the "mixture" that goes into the engine!

Offline Rooster Cruiser

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Re: fuel to air ratio
« Reply #3 on: June 09, 2008, 01:39:08 AM »
Optimum Air/Fuel ratio in an Internal Combustion engine is approximately 7:1.  Air cooled engines are often run at a lower ratio (richer mixture) than this while operating at higher power settings in order to help keep cylinder head temperatures within limits and prevent detonation.  Liquid cooled engines can run at the optimum air/fuel ratio all the time due to the more efficient heat conduction of the cooling system compared to air.  This also allows much higher cylinder compression ratios in liquid cooled engines which leads to still more efficient combustion and better power output for a given fuel consumption.

OK guys, what'd I miss here?

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Offline Mike

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Re: fuel to air ratio
« Reply #4 on: June 09, 2008, 01:46:06 AM »
you missed an internal discussion where my brother and I couldn't agree if it was 4.1 to 1 or 14.1 to 1 . . .
guess we were both wrong.....



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Offline Frank N. O.

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Re: fuel to air ratio
« Reply #5 on: June 09, 2008, 05:47:22 AM »
I thought the optimum fuel/air-ratio, also known as Lambda 1, was 14.7:1 ?? Some cars can easily drive with less, like my old Orion (that had a vacuum-regulated Weber carburator) could do a lot less, not sure how much but maybe 18:1 or more? Some modern cars do this too, but it's been comming in the carmagazines that they don't do it all the time and in the other times they switch it totally off to give power and then the fuel-economy goes to heck. The early Mitsubishi Carisma 1.8 GDI (gasoline direct injection) claimed 20% better economy but that lean-burn function it had was disabled when you used more than just a little throttle, and permanently when the car was driving over 80 kph. I have no idea why the car's speed should have any direct impact on the engine settings but that's what I've read several cars have, perhaps due to maximum speed where the goverment tests official car polution figures used for car taxes in some countries. Just weird the car can't drive at optimum combustion all the time, but I guess that would be too perfect ::)

I must say, it sounds wild if aircraft engines run at 7:1 but I guess I it sounds logical about the engine temperature in this case, but wouldn't that advocate for liquid-cooled aircraft engines?

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Offline Oddball

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Re: fuel to air ratio
« Reply #6 on: June 09, 2008, 08:45:53 AM »
i shall hang my head in shame that was one of the things i had in one of my previous exam papers  was caught at a weak moment last night.     :-[  :-[
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Re: fuel to air ratio
« Reply #7 on: June 09, 2008, 02:29:39 PM »
HMMM---Google fuel/air ratios, there's some interesting stuff there that might change those numbers a little  ::thinking::

Offline leiafee

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Re: fuel to air ratio
« Reply #8 on: June 09, 2008, 07:55:46 PM »
I learned 12:1 Air:Fuel...

Though the engine will run albeit less eficientny either side of that. 

Am I missing something?

Offline Mike

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Re: fuel to air ratio
« Reply #9 on: June 09, 2008, 09:52:13 PM »
woah ! ::unbelieveable::

looks like we have as many different ratios in here as we do members  ::rofl::

should we ask in the AOPA forum?!


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Offline Oddball

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Re: fuel to air ratio
« Reply #10 on: June 09, 2008, 09:56:31 PM »
now now mike dont be so hasty you might get as many answers there as well!!!!  ::unbelieveable:: ::eek:: ::eek:: ::loony::
"You can teach monkeys to fly better than that!"and "spring chicken to sh**e hawk in one easy lesson"

Offline Stef

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Re: fuel to air ratio
« Reply #11 on: June 09, 2008, 11:02:07 PM »
... thanks for all the hard thinking on your part guys! Somehow I get the feeling that no matter what ratio we would use in this potential strip, somebody's definitely going to write us and say we're wrong!  ;D

Offline AirScorp

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Re: fuel to air ratio
« Reply #12 on: June 09, 2008, 11:53:03 PM »
I too have learned about grossly 15:1 which seems to be the one most of us agree on (14.7:1)

However, 14.7:1 is a "Theoretical" ideal.. And that to my ears sounds like: perfect fuel (iso-octane), a test engine and perfect conditions (1 atm pressure, 20 degs Celcius or whatever)
With normal fuel, I'd expect everything to be one or even two points lower, maybe as low as 12.5:1 on low-octane fuel..
Then you have detonation, and everything shifts again to the safe side which is the rich side.. Remember rich is less efficient, lean is more heat and more dangerous to detonate.

Most of what I said is from intuition, I don't have internal combustion engines in my curriculum , but Leia's 12:1 makes the most sense to me..
R/C, maybe the 7:1 was for diesel fuel or a compression ratio or something? It's too low to make sense unless I'm completely wrong about everything..

AOPA forums won't do much good, pilots were never meant to know such things.. ::rofl:: Maybe you should ask *gasp* an engineer??????? (Oddball, you're the man on this)

Ok, for a question now: How much is the air to fuel ratio on a pilot's brain?????
It's all Greek to me!

Offline TheSoccerMom

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Re: fuel to air ratio
« Reply #13 on: June 10, 2008, 03:09:32 AM »
 ::whistle::

I think we know how much AIR is in the pilot's brain.............   ::whistle::

The dusty old FAA Flight Handbook says: 
"Mixtures as rich as 8:1 and as lean as 16:1 will burn in the cylinder of an engine that develops maxiumum power with about a 12:1 ratio."

I think we should ask CHUCK....   ::silly::

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Offline G-man

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Re: fuel to air ratio
« Reply #14 on: June 10, 2008, 03:18:19 AM »


Ya'll just try to overcomplicate stuff--I mean really



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