Thread: how does the auto-mixture work?

i can find many references to the auto-mixture but no explanations of how it works.

Hi Ed,

Give me a call and I'll explain. Just too long an email.

thanks rog, its fasteddie that needs this info, i'll tell him to call you

I saw your post on Sport Pilot. I don't respond much there, but I look a lot.

In a nut shell...

Everyone knows of the venturi effect and that the air pressure in the throat of the venturi is always lower then the air entering the venturi.

Under the caps of those dome shaped carbs is a large diaphram. This diaphram is conntected to the throttle slide and the fuel mixture needle. On the top side of the diaphram is low pressure air that gets ita vaccum from the two holes drilled in the center of the slide.remember the throttle slide is positioned in the throat of the venturi.

The pressure on the lower side of the diaphram is actually ram air pressure coming into the venturi via the small port on the top of the entrance to the carb. The throttle slide is spring loaded to push the throttle slide down to force the needle to the full lean position. Imagin that the lower pressure in the center of the venturi and the top of the diaphram stay constant. Now if you think about as you go up in altitude there is less ram air pressure coming in. Because there is less pressure pushing up on the diaphram the diaphram and the slide will sit lower which will inturn lean the mixture by also dropping the needle father down. The principle works in reverse as you desend lower air pressure increases which pushes the needle up out of it's jet and allows more fuel to enter into the stream.

Jeremy,

Thanks.

Your explanation is similar to mine in the Sport Pilot forum:

The throttle is hooked up, not to the slide, but to a butterfly valve, only controlling the amount of air flowing into the carb. This air, via Bernoulli, causes a drop in pressure when constricted. This lower pressure air is channeled to the top of a rubber diaghram, which in turns lifts the actual slide. The flow of fuel into the venturi is metered by a tapered needle which rises in the main jet with the slide - the higher the slide, the more fuel.

Now, to my way of thinking, the volume of fuel remains constant for any given slide setting, but since the density of the incoming air decreases with altitude, the mixture gradually gets richer and richer.

So, I don't see how the Bing carb can be said to have "auto-mixture".


I'm still a bit unclear.

It seems as though at full throttle, even at altitude, in order to be efficient the slide would have to be all the way open. If not, you'd never really have full throttle. And with the slide all the way up, the tapered needle would allow in the same volume of fuel as at sea level, too much for 10,000' let's say.

I understand that the level in the float bowl is important, and I'm trying to see how the decreased atmospheric pressure pushing "down" on the fuel in the bowl might decrease the fuel flow.

In my BMW motorcycles with Bings, they sure seemed to get too rich when in the Rockies, and never did seem to "auto-lean". They seem on the surface to be nearly identical to the carbs on my Sky Arrow, but I accept that they may be different somehow. On the motorcycle Bings the solution was to drop the needles a bit by moving the circlip on the needles up a notch, IIRC.

I don't KNOW that my Sky Arrow carbs are not "auto-leaning", or otherwise different from the motorcycle Bings. I just know they seem subtly too rich when above 5,000' or so. The plugs looked OK at the last annual. I plan a climb to 10,000' or above soon to see how the plane feels - I think the highest I've been to date is 8,500'.

BTW, for those not on the Sport Pilot site, here's what my performance charts look like:



Note the continuing reduction in range and the increase in fuel consumption with altitude - the exact opposite of a plane that's being leaned with altitude.

I was wondering if someone could post the performance charts for a CT?

I plan on calling Roger Lee, who volunteered to straighten me out on this.

Originally Posted by Fast Eddie B.

It seems as though at full throttle, even at altitude, in order to be efficient the slide would have to be all the way open. If not, you'd never really have full throttle. And with the slide all the way up, the tapered needle would allow in the same volume of fuel as at sea level, too much for 10,000' let's say.

someone please correct me if i'm wrong, i'm trying to get this as well.

the same volume of fuel might be "allowed" but as the pressure differential changes so does the motivation.

a garden hose without a nozzle allows the same volume but the volume adjusts as you adjust the motivation (valve)

the same volume of fuel might be "allowed" but as the pressure differential changes so does the motivation.

From the mental image I'm getting from some of the answers, it would seem like nearly all carburetors would have "auto-mixture", or that at least all CV carbs would. And they sure don't seem to.

I still feel like I'm missing something somewhere.

Originally Posted by Fast Eddie B.

From the mental image I'm getting from some of the answers, it would seem like nearly all carburetors would have "auto-mixture", or that at least all CV carbs would. And they sure don't seem to.

I still feel like I'm missing something somewhere.

nearly all carburetors have normally vented float bowls. the difference is when the bing has the High Altitude Compensator added creating dynamic venting.

http://www.greenskyadventures.com/bing/hacArticle.htm

"If fuel air mixture can be thrown off, accidentally by improperly venting the the float bowls, why not control the venting to effectively control mixture? That's exactly what your Bing 54 can do, automatically, when a High Altitude Compensator is added. If pressure in the float bowl is reduced, relative to the venturi, less fuel makes its way up the jets, and consequently, mixture is leaner. "

My manuals are all in my hangar.

I may go by tomorrow and see what the Sky Arrow has installed.

Those performance charts are still problematical - why so much more fuel at altitude if the mixture is automatically leaning?