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In a carbureted engine at higher altitudes, less dense air means there is less air entering the engine. Wouldn't we want the mixture to be full rich to allow the maximum air into the engine?

I understand that the engine will become rough if we don't lean the engine. I'm asking what is going on? It seems like a decrease in air means we would need to open the mixture not close it.

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    $\begingroup$ Full rich = maximum fuel? Why would you want to add more fuel to compensate for less air? $\endgroup$
    – user253751
    Jan 3, 2016 at 1:12
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    $\begingroup$ Mixture controls fuel. You may be mixing things up here. $\endgroup$
    – egid
    Jan 3, 2016 at 16:56
  • $\begingroup$ @egid Throttle controls fuel, mixture controls how much of the fuel-air mixture enters the cylinder $\endgroup$
    – jskypilot
    Jan 4, 2016 at 1:24
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    $\begingroup$ Not true. Mixture controls the fuel-air ratio, by controlling how much fuel is added to the air. Throttle controls how much of the air-fuel mixture enters the cylinders. I promise. $\endgroup$
    – egid
    Jan 4, 2016 at 1:26
  • $\begingroup$ Egid is right. Mixture controls fuel-air ratio and throttle controls the amount of the mixture. However the optimal fuel-air ratio, by mass, is simply given by the chemistry and shouldn't (and doesn't!) depend on pressure/altitude. So one would expect not having to touch the mixture control. Yet, one has to and that still needs to be explained (and none of the existing answers explain). $\endgroup$
    – Jan Hudec
    Jan 4, 2016 at 6:14

4 Answers 4

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The decrease in air means less oxygen. If the fuel is burnt at "rich" there would not be enough oxygen in the air to burn some of the fuel, thus reducing efficiency, increasing the risk of carbon monoxide, and adding grime to the exhaust system. That is why "lean" is used for high altitudes, to burn efficiently and safely.

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  • $\begingroup$ This does not explain anything. Mixture is the fuel-air ratio. When you reduce air by closing throttle, fuel is reduced accordingly to keep the fuel/air ratio the same. So what needs to be explained is why at lower pressure the same setting of mixture results in higher fuel/air ratio (richer). And for carburettors and injection the reason may not actually be the same. $\endgroup$
    – Jan Hudec
    Jan 4, 2016 at 6:05
  • $\begingroup$ @JanHudec engine mixture controls aren't based on mass, they're based on air volume. As altitude increases, air pressure (air mass) decreases for a given volume. This causes the fuel mass to gradually increase relative to the air mass. Leaning the mixture is required to bring that back towards the correct ratio. $\endgroup$
    – egid
    Jan 4, 2016 at 6:21
  • $\begingroup$ @egid, I see it isn't based on weight, at least in carburettor. But why it is based on volume would deserve explanation; I don't see that. $\endgroup$
    – Jan Hudec
    Jan 4, 2016 at 6:54
  • $\begingroup$ As an aside: this is also why you may choose to run lean while the engine is warming up on the ground - to avoid unburnt fuel fouling the plugs and exhaust. $\endgroup$
    – Jon Story
    Feb 1, 2016 at 15:34
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Internal combustion gasoline engines need to have a constant air/fuel ratio of 14.7:1 for efficient combustion.

The air/fuel mixture by volume remains constant as we climb but the air/fuel mixture by weight does not. The carburetor only knows the volume of air passing through it, and because the weight of the air becomes less as we climb, the mixture becomes too rich at altitude. THAT is why we have to lean the mixture as we climb

"Leaning" the mixture mixes less fuel with the air in order to keep the air/fuel ratio at a constant 14.7:1 ratio.

Even with a leaned mixture, as the air becomes less dense, less power is produced because less air and fuel enters the engine.

Forcing additional air into the engine with a mechanical driven compressor (supercharger), or exhaust driven compressor(turbocharger), can restore normal sea level power to the engine.

At sea level, forcing additional air into the engine can also boost performance to more than is available in a "normally aspirated" engine.

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    $\begingroup$ So the term 'leaning' isn't really correct. It suggests the air/fuel ratio is changed, while it is in fact kept constant. Sometimes language tends to make technology more confusing than it already is... $\endgroup$
    – Rob Vermeulen
    Jan 3, 2016 at 9:25
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    $\begingroup$ Actually I think "leaning" it is correct: If you don't pull back on the mixture control to "Lean" the air/fuel mixture as you climb, the engine will be running too "Rich". $\endgroup$
    – Mike Sowsun
    Jan 3, 2016 at 9:38
  • $\begingroup$ It depends on what you mean: I see 'leaning' as decreasing the ratio, not as altering the absolute fuel flow. But that last thing is what actually happens. Same confusion lurks when we get to talk about throttling. So let's stay on topic and not let language get in the way too much ;) $\endgroup$
    – Rob Vermeulen
    Jan 3, 2016 at 9:47
  • $\begingroup$ My original comment was meant to help the OP understand your excellent original answer and avoid Language Induced Confusion) $\endgroup$
    – Rob Vermeulen
    Jan 3, 2016 at 10:05
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    $\begingroup$ Try coffee to understand. You only like it with sugar, but it should be just right, not too sweet. Now imagine there's just a little coffee left in the can (high altitude). You can't get a full cup. So there's less coffee (air), you put in less sugar (fuel). You don't 'lean' your coffee, you get it just the way you always drink it. Afterwards you'll feel less awake though (engine performance). $\endgroup$
    – Rob Vermeulen
    Jan 3, 2016 at 15:08
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Not only is a rich fuel mixture inefficient, if it is too rich there can be zero combustion.

A long time ago in auto shop, the shop teacher demonstrated the effect of too much gas and not enough oxygen. He took a old coffee can and sprayed some gas in it, then covered it with the plastic lid, that had a small hole in it. Placed a match at the hole and the resulting small fire was about the size of a small candle flame. After some time (about a minute), the fire had burned enough fuel, the remaining fuel-air mixture was at the correct ratio, and they burned with a rapid little whoosh.

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    $\begingroup$ Welcome! That's indeed a good explanation of mixture ratio, but it doesn't yet answer the question. Consider editing your post to make it a full answer. See How to Answer for additional details. $\endgroup$
    – mins
    Jan 4, 2016 at 8:42
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Airplanes with a Mixture control have internal combustion engines (like automobiles and lawnmowers, but not like jets). An internal combustion engine runs on fuel (avgas, petrol, butane, alcohol, whatever hydrocarbon you pick...) mixed with air.

The ratio that the mixture of fuel and air is set to is important for several reasons: cost of fuel, engine cooling, avoiding carbon buildup in the engine, how smooth the engine runs, how much power the engine produces, etc.

The ratio is based on the number of molecules of fuel and the number of molecules of oxygen. Air is approximately 20% oxygen no matter if at sea level or 30,000 feet.

But at sea level the air is compressed more than at 30,000 feet. As the airplane, or you, go up in altitude the air gets "thinner", or less dense. Less dense means that the air is not as compressed. For some volume of air, 1 cubic foot for example, higher pressure air will have more air molecules than lower pressure air. Since air is 20% oxygen, fewer air molecules also means fewer oxygen molecules.

The engine performs best when the air/fuel mixture ratio stays "in balance", or as close to ideal as possible. As the engine is running is pulls air into the engine, but the amount of air is based on the volume of air, not the number of molecules of air. As the pressure decreases, so do the number of molecules. As the aircraft climbs into thinner and thinner air the mixture must be "leaned out", where the volume of fuel and the fuel molecules is reduced, to compensate for the fewer number of oxygen molecules being pulled into the engine.

As the aircraft descends into "thicker" air, the number of oxygen molecules increase again, so the mixture must again be adjusted by increasing the number of fuel molecules to keep the air molecules and fuel molecules at the correct ratio.

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