For a little open discussion... why do you have to lean the mixture when you increase in altitude?

Most people know the basic operation of a intake system where the air being sucked in by the pistons will create a partial vacuum and suck in a certain amount of fuel. When the density of the air decreases, the suction acting on the fuel will decrease and less fuel will be mixed with the air. But does the decrease in density/pressure cause the fuel coming in to decrease so that the fuel/air ratio remains constant? Is it because we're creating less power now that we don't need the extra fuel for detonation protection?

Others more knowledgeable than me will no doubt give a better answer but my understanding was that it was to keep the mixture stoichiometric i.e. in the correct ratio, as the partial pressure of oxygen falls with increasing altitude.
Presumably the variation in manifold depression effect is not linear with stoichiometry
CW

to keep the mixture stoichiometric i.e. in the correct ratio, as the partial
pressure of oxygen falls with increasing altitude.

First of all, the stoichiometric value cannot be used at high power because the burn is too hot. Neither is it anything to do with oxygen partial pressure.

The power output of a piston engine depends upon the weight of charge in the cylinders. This in turn depends upon air density.

As altitude is gained the air density (mass per unit volume) reduces. If the mixture were not leaned then the amount of fuel relative to the amount of air would increase.

Thus the mixture has to be progressively leaned to maintain the required air/fuel ratio.

When the density of the air decreases, the suction acting on the fuel will
decrease

The depression in the carburettor ("suction" does not exist) venturi depends upon the velocity of the air flow through it (Bernoulli).

:)

The volumetric fluid flow through a restrictor is proportional to the throat area and the square root of the pressure drop across it.

The mass flow rate is the volumetric flow rate multiplied by the density

We need to mainatin the correct fuel:air mass ratio.

We could consider our carburettor to consist of two restrictors. One is the throttle valve/venturi and the other is fuel jet.

Let's suppose that we have no ram effect and the float chamber and air inlet are at the same ambient pressure.

The pressure drop across the throttle valve/venturi is ambient pressure minus venturi throat pressure.

The pressure drop across the fuel jet is also ambient minus venturi throat pressure.

If we ignore the effecst of temperature, Air density varies with altitude but fuel density is constant.

The cross section of the throttle/venturi and that of the fuel jet are such that when we have sea level air density, we get the correct fuel:air mass flow ratio at sea level.

If we now climb to altitude while maintaining the same pressure drop, the air density will decrease while the fuel density remains constant. This will cause the air mass flow rate to decrease while the fuel mass flow rate remains constant. This in turn will enrich the mixture.

So to restore the mass flow balance we need to reduce the size of the fuel passage.

Thanks guys, told you you'd get a better answer elsewhere!
CW

As a student pilot, I used to lean the mixture on cross country trips as I thought I would get more 'miles per gallon'. It was nothing I was taught - just seemed a good idea! I kept an eye on cylinder head temp's but never exceeded 6,000' so probably didn't achieve any significant savings (or damage, I hope).

Keith,

A nicely more complicated explanation as is required, as usual!

we get the correct fuel:air mass flow ratio at sea level.


NO!!

It's AIR/FUEL RATIO.

YOU SHOULD KNOW BETTER.........

A pressure carburetor solves all this. Auto-rich for takeoff and climb, then auto-lean when you level off.

David,

We can express it as an air:fuel ratio, or as a fuel:air ratio.

Air:fuel ratio is simply the reciprocal of the fuel:air ratio, so if one is correct, then the other must also be correct.

So it really doesn't matter which we choose to use.

The FW190 had an integrated single lever power control that combined throttle, mixture and prop control.


All this in WW2.



Irrelevant I know..:bored:

the FW190 people lost!

But, the 'victors' pinched all of their technology, and......

had a 'race' with it.....

Cheers:ok:

Keith... thanks for the reply. Can you expand on what you said or provide some links to related material? It makes sense but I'd like to get a better understanding of it.

The depression in the carburettor ("suction" does not exist) venturi depends upon the velocity of the air flow through it (Bernoulli).

Suction does exist - Suction - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Suction)

It's a bit more than just velocity - Pressure (http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html)

'the FW190 people lost! '


Not when you look at their economy today..

A pressure carburetor solves all this. Auto-rich for takeoff and climb, then auto-lean when you level off.

Will the pressure carburettor make corrections for variations in temperature of intake air like the pilot can do with manual leaning? For example, will the mass airflow to mass fuel flow ratio be the same with using pressure carburettor if flying with 20" MAP at 30°C or -20°C?

I seriously doubt it. The only way to get rid of manual leaning (and still maintaining correct air-to-fuel or fuel-to-air ratio - and not destroying the engine in the process) is via electronic fuel injection, where ECU (Engine Control Unit) calculates the amount of fuel to be injected into cylinders by using many sensors (temperature and pressure of intake air, etc.).

Can you expand on what you said or provide some links to related material?


The link below is not specific to carburettors, but it covers the physics of flow through restrictors and derives the equations.

Orifice plate - Ask Jeeves Encyclopedia (http://wzeu.ask.com/r?t=p&d=eu&s=uk&c=a&app=a16&dqi=&askid=&l=dir&o=951&oo=951&sv=0a652912&ip=d48b6326&id=82DAD28D45E43052E972240998747437&q=relationship+between+fluid+flow+and+pressure+drop+across+a+restrictor&p=1&qs=1&ac=114&g=67f0PbURjYbo4N&cu.wz=0&en=nwsa&io=0&ep=&eo=&b=a003&bc=&br=&tp=d&ec=1&pt=Orifice%20plate%20-%20Wikipedia%20on%20Ask%20Jeeves&ex=tsrc%3DRFE&url=&u=http%3A%2F%2Fuk.ask.com%2Fwiki%2FOrifice_plate)

I think the terms are being used incorrectly here
Leaning means reducing the fuel:air mixture ratio

When in the climb, engines, even old ones with an altitude capsule, will reduce the weight of fuel going to the engine so as to attempt to maintain the the same fuel:air mixture.

At cruise power when you are safely away from the detonation boundry then the fuel:air mixture can be reduced[Leaned] so as to reduce fuel flow and increase range.

Now my experience is on old engines so modern electronic controls might make this wrong

I think the terms are being used incorrectly here



Yes, I must confess that I have cencentrated only on this part of the original post


When the density of the air decreases, the suction acting on the fuel will decrease and less fuel will be mixed with the air. But does the decrease in density/pressure cause the fuel coming in to decrease so that the fuel/air ratio remains constant?



I have ignored this part


Is it because we're creating less power now that we don't need the extra fuel for detonation protection?


To answer both parts we should say that we adjust the mixture lever for two reasons as we climb to altitude. These are:

1. To compensate for the fact that reducing air density tends to enrich the mixture, for the reasons that I have described.

And

2. As altitude increases, the reducing cylinder pressures reduce the risk of detonation. This enables us to stop feeding in additional fuel that is used to cool the engine to prevent detonation when generating high power at lower altitudes.

Why do you have to lean?

To make a turn.

Since the pressure carburetor uses the density of the air to control the fuel flow it does compensate for non-standard temperatures.