Mixture Control

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pilotrs

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PilotRS
I learned that a venturi is used that determines the pressure difference in the incoming air to calculate how much air is going into the cylinder and that calculates how much fuel needs to go in.

So if closing the throttle reduces air into the cylinder, which also reduces fuel into the cylinder, then why do we need a fuel air mixture control. As the airplane climbs, less air goes into the cylinder, and shouldn't that also cause less fuel into the cylinder?
 
"calculates" - remember that these are fairly crude analog devices

The reason is mass verse volume.. the mixture is proportional to the volume, not the mass. The air will weigh less but the fuel weighs the same. So you end up too rich. Keep in mind as well that the relationship is not linear.. they give us rules of thumb for the colesman window but the change in air pressure with altitude is not a linear relationship
 
pilotrs said:
I learned that a venturi is used that determines the pressure difference in the incoming air to calculate how much air is going into the cylinder and that calculates how much fuel needs to go in.

So if closing the throttle reduces air into the cylinder, which also reduces fuel into the cylinder, then why do we need a fuel air mixture control. As the airplane climbs, less air goes into the cylinder, and shouldn't that also cause less fuel into the cylinder?
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Are you talking about a carburetor or fuel injection? When you use the word "calculate" it sounds like you're taking about FI, but a venturi is part of a carburetor.
 
We'll assume carburetor. I'm just confused as to what's the difference between a slightly closed throttle or flying at a higher altitude. Both would cause less air to go into the cylinder. So why a mixture control for altitude only. We drive regular cars to Denver, yet no mixture control in the car. Just trying to understand all this.
 
pilotrs said:
We'll assume carburetor. I'm just confused as to what's the difference between a slightly closed throttle or flying at a higher altitude. Both would cause less air to go into the cylinder. So why a mixture control for altitude only. We drive regular cars to Denver, yet no mixture control in the car. Just trying to understand all this.
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Your car has a computer in it to adjust these things.

The throttle controls the airflow and the fuel flow will be proportional to airflow. The mixture changes the proportion.
 
pilotrs said:
We'll assume carburetor. I'm just confused as to what's the difference between a slightly closed throttle or flying at a higher altitude. Both would cause less air to go into the cylinder. So why a mixture control for altitude only. We drive regular cars to Denver, yet no mixture control in the car. Just trying to understand all this.
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the air is ‘thinner’ at altitude, but the same volume of air gets in. Because it’s thinner, less oxygen gets in. It’s the ratio of oxygen to fuel that matters.
 
pilotrs said:
We drive regular cars to Denver, yet no mixture control in the car. Just trying to understand all this.
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The credit goes to the technological advancements made with microchips, which all modern day vehicles are equipped with. They automatically meter the fuel and air ratio needed for adequate combustion. Piston aircraft (most of them) don’t have this technology, so it’s still done by hand, just like you would do back in the early 1900’s.
 
pilotrs said:
We'll assume carburetor. I'm just confused as to what's the difference between a slightly closed throttle or flying at a higher altitude. Both would cause less air to go into the cylinder. So why a mixture control for altitude only. We drive regular cars to Denver, yet no mixture control in the car. Just trying to understand all this.
Click to expand...
Your car has a MAP sensor--Manifold Absolute Pressure, a MAF sensor--Mass Air Flow, and an EGO sensor--Exhaust Gas Oxygen--that send signals to the computer to adjust the fuel flow at the injectors. Old cars with carbs did not have this sort of thing and when they got well above sea level their performance fell right off as the mixture got so rich. Some later carbs had an aneroid to adjust the mixture for altitude, but temperature, and therefore mass, didn't figure into it.

The carb's venturi accelerates the air, thus lowering its pressure to draw fuel in. It's a velocity-sensing device, not a mass-sensing device. A partially-closed the throttle at either sea level or at 10,000 feet doesn't change the weight of the volume going through it, but the mass sure has changed between sea level and 10K.

The owners of cars at high-altitude cities sometimes had smaller fuel metering jets installed in their carbs. This was also standard practice in the boating world until EFI was incorporated.
 
asicer said:
Some fuel injection systems have a venturi.
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But it doesn't suck fuel. It's only there to generate vacuum and dynamic pressures for the fuel metering servo. The RSA system, used on Lycomings. It also has a fuel metering valve, connected to the throttle lever, and a manual mixture control.

2-31-min.png
 
pilotrs said:
calculate how much air is going into the cylinder and that calculates how much fuel needs to go in.
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It measures the volume of flow rate of air and provides a volume flow rate of fuel. As you gain altitude, the density of the air changes but the density of the fuel does not. So, on a mass basis, for a volume of air, you get less mass of air, same mass of fuel. Combustion depends on the mass ratio - so lower air mass, same fuel mass = too rich.
 
Pressure drop in a venturi (per Bernoulli) is proportional to the air density. But liquid flow through an orifice (i.e a carburetor jet) is proportional to the square root of the differential pressure, not linearly. So if you halve the air density at attitude (and thus the differential pressure), the mass airflow will be halved, but the fuel flow is 71%, not half, and thus will be rich.
 
The BING carburetors on many ROTAX engines and BMW airhead motorcycles do seem to be able to self-adjust the mixture to some extent with gains in altitude. Always seemed like magic to me, but it seems to work, at least to some extent*.

Here’s an attempt to explain how they do it: http://www.omnilex.com/public/bmw78/cvcarb.pdf

Cirrus SR20’s use a different method to self-adjust the mixture at full throttle, but they still provide a mixture control for fine-tuning at cruise.


*I took my Sky Arrow to 13,500’ once and it gave no indication of running too rich at that altitude.
 
FastEddieB said:
The BING carburetors on many ROTAX engines and BMW airhead motorcycles do seem to be able to self-adjust the mixture to some extent with gains in altitude. Always seemed like magic to me, but it seems to work, at least to some extent*.

Here’s an attempt to explain how they do it: http://www.omnilex.com/public/bmw78/cvcarb.pdf

Cirrus SR20’s use a different method to self-adjust the mixture at full throttle, but they still provide a mixture control for fine-tuning at cruise.


*I took my Sky Arrow to 13,500’ once and it gave no indication of running too rich at that altitude.
Click to expand...


As does the stromberg carb on small old continentals below 5,000 or so.
 
pilotrs said:
I learned that a venturi is used that determines the pressure difference in the incoming air to calculate how much air is going into the cylinder and that calculates how much fuel needs to go in.

So if closing the throttle reduces air into the cylinder, which also reduces fuel into the cylinder, then why do we need a fuel air mixture control. As the airplane climbs, less air goes into the cylinder, and shouldn't that also cause less fuel into the cylinder?
Click to expand...
Others have explained why your mixture gets richer as you climb. This chart shows how mixture affects the relative relationship among cylinder-head temperature (CHT, which is just a proxy for internal cylinder pressure here), power, and fuel consumption, using the difference from peak exhaust gas temperature (EGT) on the lean or peak side.

Managing mixture manually is an art, but (until we all have computer-controlled mixture) an essential part of a pilot's flying skills. There are lots of competing things to consider.
  • If your internal cylinder pressure gets too high (using CHT as a proxy), the fuel/air mixture in the cylinder will preignite like in a diesel engine, before the spark fires; because that happens at the wrong part of the stoke, it will damage or ruin your engine. Peak internal pressure and peak CHT happen when the EGT is about 50°F below its peak on the rich side.

  • You get the most power when the EGT is about 150°F below its peak on the rich side. With a fixed-pitch prop, that's where you'll see the highest RPM while you're leaning.

  • You get the best fuel efficiency when the EGT is about 50°F below its peak on the lean side of the curve.

  • Note that the power curve drops much more steeply on the lean side than on the rich side — that's why engines with uneven fuel/air distribution will run rough when they're leaned beyond peak EGT, because some cylinders are still richer than others, and producing much more power.

  • When you fly too rich, you'll produce a lot more carbon monoxide (which is dangerous if there's a leak into the cabin), leave lead deposits on your plugs (which will foul them), and burn more gas than the POH says for that power setting.

  • Below 65% power, you won't really damage the engine no matter how you lean it.
A good, simple rule of thumb is to lean above (rich of) peak power/RPM, or below (lean of) peak EGT, but stay out of the area in-between, because that's the danger zone (especially at higher power settings).
leaning.png
 
FastEddieB said:
The BING carburetors on many ROTAX engines and BMW airhead motorcycles do seem to be able to self-adjust the mixture to some extent with gains in altitude. Always seemed like magic to me, but it seems to work, at least to some extent*.
Click to expand...
Ain't saying you are wrong, and this is often reported, but I can't get the math to work to convince me that this is true.

Constant velocity carburetors are also called constant depression carburetors because they move the slide in response to the vacuum (suction) under the slide valve. And, as they move up, they increase the fuel by withdrawing a tapered needle from the main metering jet. But, it seems to me that you still have the problem that the vacuum, and thus the slide position, and thus the mass of fuel delivered is still a function of the volumemtric flow rate under the slide. At least as far as I can figure. Also, from the reference you cited, " Dealers in high-altitude areas can also adjust the Bing CV carb for optimal performance in thinner atmosphere."

But, perhaps I just have not sorted it out in my head yet.
 
Capt. Geoffrey Thorpe said:
Ain't saying you are wrong, and this is often reported, but I can't get the math to work to convince me that this is true.

Constant velocity carburetors are also called constant depression carburetors because they move the slide in response to the vacuum (suction) under the slide valve. And, as they move up, they increase the fuel by withdrawing a tapered needle from the main metering jet. But, it seems to me that you still have the problem that the vacuum, and thus the slide position, and thus the mass of fuel delivered is still a function of the volumemtric flow rate under the slide. At least as far as I can figure. Also, from the reference you cited, " Dealers in high-altitude areas can also adjust the Bing CV carb for optimal performance in thinner atmosphere."

But, perhaps I just have not sorted it out in my head yet.
Click to expand...

The Tecnam P92 I used to fly has a constant depression carburetor. Never really knew how it worked. But the flight manual shows the same fuel consumption (4 gph) for 55% power at 10,000 feet as it does at sea level.
 
This was cool. Love this YouTube channel

fast forward to 8:25 if curious

 
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