Carburettor at altitude

[JSeward]

Hello everyone,

It has been bugging me recently because I don't understand how the physics of a carburettor work with regards to having to change mixture at altitude. Why doesn't the lower air density mean less of a Bernoulli effect therefore less fuel coming out of the carburettor? Why doesn't it work like this?

[rudderrudderrat]

Hi JSeward,

I think it is because the pressure drop in the throat of the carburettor is proportional to air density * V squared (of the air flow through the carb).
The fuel flow is dependent on the pressure drop.
However the mass flow of the air into the engine is proportional to air density * V.
Therefore on simple carburettors some device to control the fuel flow (like a tapered needle) is used to match the air mass flow.
The tapered needle will only be correct for one air density, and when operating at different density altitudes, then it is necessary to "tweak" the mixture using the mixture control.

[JSeward]

Ok I see so the air travelling through the carburettor up there is still fast enough so that mixture must be adjusted? So the pressure sucking the fuel out does not drop enough for an automatically correct mixture when your up higher?

[Typhoon650]

The pressure differential either side of the carburettor venturi remains constant no matter the outside air pressure, but the density of the air reduces as you go higher. So, less dense air contains less oxygen, even though the engine is drawing the same volume of air in.
Thus, we need to lean the mixture to compensate for the less dense air and reduced oxygen available.

[JSeward]

Even though the pressure differential is the same, wouldn't the lower density therefore lower pressure result in less fuel flowing out? It obviously doesn't I just need a bit further explanation, it annoys me when I don't know something.

I think rudderrats mention of velocity of the flow might be able to explain it?

[JSeward]

I assume that the pressure pushing the fuel into the low pressure region of the venturi is just outside atmospheric pressure?

[Tarq57]

I think it's because the venturi effect results in the same pressure differential at any constant air velocity, regardless of the density of the air.

I don't know how to explain that, or use "laws" (Boyle's law etc) to justify it.

So, with altitude, and an unchanged venturi diameter, the same weight of fuel is being sucked into the venturi, though there is less air to combust this fuel. So the mixture needs to be leaned with altitude.

Please feel free to correct mistakes.

[JSeward]

That makes sense now, because the fuel is being pushed through by outside air pressure? If that is the case then it makes perfect sense!

[Tarq57]

Well, yes. The ambient air pressure outside the carb - and in the float chamber, will always be higher than the pressure of the moving air in the venturi.

[A Squared]

I think it's because the venturi effect results in the same pressure differential at any constant air velocity, regardless of the density of the air.

No this is not true. The pressure differential for a constant velocity will be proportional to the density of the air.

[rudderrudderrat]

Hi A Squared,
The pressure differential for a constant velocity will be proportional to the density of the air.
I agree.

JSeward, try putting some actual numbers into the relationships I posted in #2.

Assume your carburettor is set up for sea level with the jets and needles etc. all set correctly.
If you now fly where the density altitude is only half that at sea level, and develop the same engine power by doubling the speed of the airflow through the carburettor (density * Velocity of air through the carburettor: 0.5 * 2 = 1), then the pressure drop through the carb's venturi is now double what it was at sea level (density * V*V: 0.5 * 2 * 2 = 2). Therefore about twice as much fuel will be sucked through the petrol jets than is required and hence you will have to lean the mixture off.