Why does one need to LEAN OUT a CARB when climbing?

Somebody posed that seemingly simple question to me, but kept coming
back to the point that they stumped me.... And I am stumped. What do
you see wrong with the logic in this dialog?

Q: Why do I need to lean out my carb when I climb?

A: Ahem, seems you forgot your PPL ground school. The air is less
dense. Fewer air molecules per unit volume. Therefore, you need less
gas, so you lean it out!

Q': Um, ok. Well I looked at my ground school text, and it shows how a
carby operates. Apparently, avgas is kept at a constant level in a
float bowl, which is vented upstream of a venturi. Air flows through
the venturi, and creates a lower pressure, the resulting differential
pressure forcing the avgas across an orfice and into the airstream,
where it mixes it all up in a nice and precise ratio.

A': OK, go on.

Q'': Well, as you climb, I understand the air gets less dense. Let's
assume for simplicity that the volumetric efficiency of the engine
remains fixed, therefore the velocity in the venturi remains the same.
Now the air is less dense, and from the previous chapter in ground
school 101, the differential pressure "p" is related to the density "r"
given a certain velocity "v" like this:
p = 1/2 r v^2
So given a constant velocity, and a decreasing density, won't the
differential pressure decrease, effectively metering less avgas across
that orfice?

A'': OK; I'm sure you're simplifying assumptions are too simple, you
missed something there.

Q''': OK, let's get a little more precise. The mass airflow rate, m,
through a carby is m = c v , where c is a constant for a certain
throttle setting, v is the velocity. Substituting that into the eq's
above , we see that p = 1/2 r m^2 / c^2. Now we know that the mass flow
rate for a liquid across an orfice is very close to proportional to the
square root of the pressure drop [ I actually had to check up on this
one, but it appears to be so:
http://www.efunda.com/formulae/fluid..._flowmeter.cfm ] --
and of course the density of the avgas doesn't change appreciably[!].
Therefore, the avgas flow rate is proportional to "r^(1/2) m". From
this point of view the carby at a constant air density can be viewed as
a device that meters a constant mass proportion mixture of avgas and
air, across a range of mass airflows -- ignoring the effects of
accelerator pumps, full-throttle enrichers, idle circuits and all that.
But note that as the density decreases, the fuel proportion to air
decreases -- suggesting that one would need to ENRICH the mixture when
climbing into less dense air. Assuming that the desired mass proportion
of fuel/air is approximately the same across varying densities (which
seems very reasonable to both of us).

A''': OK, I do follow that (after some work)... and I'm stumped.

Granted, some simplifying assumptions here, but no convincing
explanation of why you would need to lean that red know when climbing...
(and I don't question that you in fact do)....

Anyone see what is amiss?

T

Let me try a simpler explanation.

14.7 to one is the normal fuel air ratio in gasoline engines.

As you climb the air (14.7) gets less and mixture goes rich so you
have to lean (reduce) the fuel flow to keep the14.7 to one ratio that
engine likes.

Big John

************************************************** **************************

On Mon, 19 Jan 2009 14:23:57 -0500, Tman
wrote:

Somebody posed that seemingly simple question to me, but kept coming
back to the point that they stumped me.... And I am stumped. What do
you see wrong with the logic in this dialog?

Q: Why do I need to lean out my carb when I climb?

A: Ahem, seems you forgot your PPL ground school. The air is less
dense. Fewer air molecules per unit volume. Therefore, you need less
gas, so you lean it out!

Q': Um, ok. Well I looked at my ground school text, and it shows how a
carby operates. Apparently, avgas is kept at a constant level in a
float bowl, which is vented upstream of a venturi. Air flows through
the venturi, and creates a lower pressure, the resulting differential
pressure forcing the avgas across an orfice and into the airstream,
where it mixes it all up in a nice and precise ratio.

A': OK, go on.

Q'': Well, as you climb, I understand the air gets less dense. Let's
assume for simplicity that the volumetric efficiency of the engine
remains fixed, therefore the velocity in the venturi remains the same.
Now the air is less dense, and from the previous chapter in ground
school 101, the differential pressure "p" is related to the density "r"
given a certain velocity "v" like this:
p = 1/2 r v^2
So given a constant velocity, and a decreasing density, won't the
differential pressure decrease, effectively metering less avgas across
that orfice?

A'': OK; I'm sure you're simplifying assumptions are too simple, you
missed something there.

Q''': OK, let's get a little more precise. The mass airflow rate, m,
through a carby is m = c v , where c is a constant for a certain
throttle setting, v is the velocity. Substituting that into the eq's
above , we see that p = 1/2 r m^2 / c^2. Now we know that the mass flow
rate for a liquid across an orfice is very close to proportional to the
square root of the pressure drop [ I actually had to check up on this
one, but it appears to be so:
http://www.efunda.com/formulae/fluid..._flowmeter.cfm ] --
and of course the density of the avgas doesn't change appreciably[!].
Therefore, the avgas flow rate is proportional to "r^(1/2) m". From
this point of view the carby at a constant air density can be viewed as
a device that meters a constant mass proportion mixture of avgas and
air, across a range of mass airflows -- ignoring the effects of
accelerator pumps, full-throttle enrichers, idle circuits and all that.
But note that as the density decreases, the fuel proportion to air
decreases -- suggesting that one would need to ENRICH the mixture when
climbing into less dense air. Assuming that the desired mass proportion
of fuel/air is approximately the same across varying densities (which
seems very reasonable to both of us).

A''': OK, I do follow that (after some work)... and I'm stumped.

Granted, some simplifying assumptions here, but no convincing
explanation of why you would need to lean that red know when climbing...
(and I don't question that you in fact do)....

Anyone see what is amiss?

T

If he does not believe that the engine is not operating at peak efficiency..
Climb above 5000MSL in a full throttle full rich condition and watch the
RPMs (fixted pitch) decrease.
Then lean the engine and watch the RPMs increase. Leaner air, lower the gas,
same fuel to air ratio is achieved and the RPMs pick up.

BT

"Tman" wrote in message
...
Somebody posed that seemingly simple question to me, but kept coming back
to the point that they stumped me.... And I am stumped. What do you see
wrong with the logic in this dialog?

Q: Why do I need to lean out my carb when I climb?

A: Ahem, seems you forgot your PPL ground school. The air is less dense.
Fewer air molecules per unit volume. Therefore, you need less gas, so you
lean it out!

Q': Um, ok. Well I looked at my ground school text, and it shows how a
carby operates. Apparently, avgas is kept at a constant level in a float
bowl, which is vented upstream of a venturi. Air flows through the
venturi, and creates a lower pressure, the resulting differential pressure
forcing the avgas across an orfice and into the airstream, where it mixes
it all up in a nice and precise ratio.

A': OK, go on.

Q'': Well, as you climb, I understand the air gets less dense. Let's
assume for simplicity that the volumetric efficiency of the engine remains
fixed, therefore the velocity in the venturi remains the same. Now the air
is less dense, and from the previous chapter in ground school 101, the
differential pressure "p" is related to the density "r" given a certain
velocity "v" like this:
p = 1/2 r v^2
So given a constant velocity, and a decreasing density, won't the
differential pressure decrease, effectively metering less avgas across
that orfice?

A'': OK; I'm sure you're simplifying assumptions are too simple, you
missed something there.

Q''': OK, let's get a little more precise. The mass airflow rate, m,
through a carby is m = c v , where c is a constant for a certain throttle
setting, v is the velocity. Substituting that into the eq's above , we
see that p = 1/2 r m^2 / c^2. Now we know that the mass flow rate for a
liquid across an orfice is very close to proportional to the square root
of the pressure drop [ I actually had to check up on this one, but it
appears to be so:
http://www.efunda.com/formulae/fluid..._flowmeter.cfm ] --
and of course the density of the avgas doesn't change appreciably[!].
Therefore, the avgas flow rate is proportional to "r^(1/2) m". From this
point of view the carby at a constant air density can be viewed as a
device that meters a constant mass proportion mixture of avgas and air,
across a range of mass airflows -- ignoring the effects of accelerator
pumps, full-throttle enrichers, idle circuits and all that. But note that
as the density decreases, the fuel proportion to air decreases --
suggesting that one would need to ENRICH the mixture when climbing into
less dense air. Assuming that the desired mass proportion of fuel/air is
approximately the same across varying densities (which seems very
reasonable to both of us).

A''': OK, I do follow that (after some work)... and I'm stumped.

Granted, some simplifying assumptions here, but no convincing explanation
of why you would need to lean that red know when climbing... (and I don't
question that you in fact do)....

Anyone see what is amiss?

T

"Tman" wrote in message
...
Somebody posed that seemingly simple question to me, but kept coming back
to the point that they stumped me.... And I am stumped. What do you see
wrong with the logic in this dialog?

Q: Why do I need to lean out my carb when I climb?

A: Ahem, seems you forgot your PPL ground school. The air is less dense.
Fewer air molecules per unit volume. Therefore, you need less gas, so you
lean it out!

Q': Um, ok. Well I looked at my ground school text, and it shows how a
carby operates. Apparently, avgas is kept at a constant level in a float
bowl, which is vented upstream of a venturi. Air flows through the
venturi, and creates a lower pressure, the resulting differential pressure
forcing the avgas across an orfice and into the airstream, where it mixes
it all up in a nice and precise ratio.

A': OK, go on.

Q'': Well, as you climb, I understand the air gets less dense. Let's
assume for simplicity that the volumetric efficiency of the engine remains
fixed, therefore the velocity in the venturi remains the same. Now the air
is less dense, and from the previous chapter in ground school 101, the
differential pressure "p" is related to the density "r" given a certain
velocity "v" like this:
p = 1/2 r v^2
So given a constant velocity, and a decreasing density, won't the
differential pressure decrease, effectively metering less avgas across
that orfice?

A'': OK; I'm sure you're simplifying assumptions are too simple, you
missed something there.

Q''': OK, let's get a little more precise. The mass airflow rate, m,
through a carby is m = c v , where c is a constant for a certain throttle
setting, v is the velocity. Substituting that into the eq's above , we
see that p = 1/2 r m^2 / c^2. Now we know that the mass flow rate for a
liquid across an orfice is very close to proportional to the square root
of the pressure drop [ I actually had to check up on this one, but it
appears to be so:
http://www.efunda.com/formulae/fluid..._flowmeter.cfm ] --
and of course the density of the avgas doesn't change appreciably[!].
Therefore, the avgas flow rate is proportional to "r^(1/2) m". From this
point of view the carby at a constant air density can be viewed as a
device that meters a constant mass proportion mixture of avgas and air,
across a range of mass airflows -- ignoring the effects of accelerator
pumps, full-throttle enrichers, idle circuits and all that. But note that
as the density decreases, the fuel proportion to air decreases --
suggesting that one would need to ENRICH the mixture when climbing into
less dense air. Assuming that the desired mass proportion of fuel/air is
approximately the same across varying densities (which seems very
reasonable to both of us).

A''': OK, I do follow that (after some work)... and I'm stumped.

Granted, some simplifying assumptions here, but no convincing explanation
of why you would need to lean that red know when climbing... (and I don't
question that you in fact do)....

Anyone see what is amiss?

T


My knowledge is limited to small engines (all two cycle), which the vast
majority I specialize in are diaphram/crankcase pulse type carbs. Are you
sure that the carb is regulated by the venturi flow through the carb? Are
carbureted aircraft engine not run off crank case pulse?

"Darkwing" theducksmail"AT"yahoo.com wrote in message
...

"Tman" wrote in message
...
Somebody posed that seemingly simple question to me, but kept coming back
to the point that they stumped me.... And I am stumped. What do you see
wrong with the logic in this dialog?

Q: Why do I need to lean out my carb when I climb?

A: Ahem, seems you forgot your PPL ground school. The air is less
dense. Fewer air molecules per unit volume. Therefore, you need less
gas, so you lean it out!

Q': Um, ok. Well I looked at my ground school text, and it shows how a
carby operates. Apparently, avgas is kept at a constant level in a float
bowl, which is vented upstream of a venturi. Air flows through the
venturi, and creates a lower pressure, the resulting differential
pressure forcing the avgas across an orfice and into the airstream, where
it mixes it all up in a nice and precise ratio.

A': OK, go on.

Q'': Well, as you climb, I understand the air gets less dense. Let's
assume for simplicity that the volumetric efficiency of the engine
remains fixed, therefore the velocity in the venturi remains the same.
Now the air is less dense, and from the previous chapter in ground school
101, the differential pressure "p" is related to the density "r" given a
certain velocity "v" like this:
p = 1/2 r v^2
So given a constant velocity, and a decreasing density, won't the
differential pressure decrease, effectively metering less avgas across
that orfice?

A'': OK; I'm sure you're simplifying assumptions are too simple, you
missed something there.

Q''': OK, let's get a little more precise. The mass airflow rate, m,
through a carby is m = c v , where c is a constant for a certain throttle
setting, v is the velocity. Substituting that into the eq's above , we
see that p = 1/2 r m^2 / c^2. Now we know that the mass flow rate for a
liquid across an orfice is very close to proportional to the square root
of the pressure drop [ I actually had to check up on this one, but it
appears to be so:
http://www.efunda.com/formulae/fluid..._flowmeter.cfm ] --
and of course the density of the avgas doesn't change appreciably[!].
Therefore, the avgas flow rate is proportional to "r^(1/2) m". From this
point of view the carby at a constant air density can be viewed as a
device that meters a constant mass proportion mixture of avgas and air,
across a range of mass airflows -- ignoring the effects of accelerator
pumps, full-throttle enrichers, idle circuits and all that. But note that
as the density decreases, the fuel proportion to air decreases --
suggesting that one would need to ENRICH the mixture when climbing into
less dense air. Assuming that the desired mass proportion of fuel/air is
approximately the same across varying densities (which seems very
reasonable to both of us).

A''': OK, I do follow that (after some work)... and I'm stumped.

Granted, some simplifying assumptions here, but no convincing explanation
of why you would need to lean that red know when climbing... (and I don't
question that you in fact do)....

Anyone see what is amiss?

T


My knowledge is limited to small engines (all two cycle), which the vast
majority I specialize in are diaphram/crankcase pulse type carbs. Are you
sure that the carb is regulated by the venturi flow through the carb? Are
carbureted aircraft engine not run off crank case pulse?


I guess I need to add this, I realize that airplanes have fuel pumps, but
are they ran off the engine belt or pulse? I would think the smaller LS
engines like the Rotax might be a pulse carb with float bowl.

On Jan 19, 3:23*pm, "Darkwing" theducksmail"AT"yahoo.com wrote:
"Darkwing" theducksmail"AT"yahoo.com wrote in message

...





"Tman" wrote in message
m...
Somebody posed that seemingly simple question to me, but kept coming back
to the point that they stumped me.... And I am stumped. *What do you see
wrong with the logic in this dialog?

Q: Why do I need to lean out my carb when I climb?

A: *Ahem, seems you forgot your PPL ground school. *The air is less
dense. Fewer air molecules per unit volume. *Therefore, you need less
gas, so you lean it out!

Q': *Um, ok. *Well I looked at my ground school text, and it shows how a
carby operates. *Apparently, avgas is kept at a constant level in a float
bowl, which is vented upstream of a venturi. *Air flows through the
venturi, and creates a lower pressure, the resulting differential
pressure forcing the avgas across an orfice and into the airstream, where
it mixes it all up in a nice and precise ratio.

A': *OK, go on.

Q'': *Well, as you climb, I understand the air gets less dense. *Let's
assume for simplicity that the volumetric efficiency of the engine
remains fixed, therefore the velocity in the venturi remains the same.
Now the air is less dense, and from the previous chapter in ground school
101, the differential pressure "p" is related to the density "r" given a
certain velocity "v" like this:
p = 1/2 r v^2
So given a constant velocity, and a decreasing density, won't the
differential pressure decrease, effectively metering less avgas across
that orfice?

A'': OK; I'm sure you're simplifying assumptions are too simple, you
missed something there.

Q''': *OK, let's get a little more precise. *The mass airflow rate, m,
through a carby is m = c v , where c is a constant for a certain throttle
setting, v is the velocity. *Substituting that into the eq's above , we
see that p = 1/2 r m^2 / c^2. *Now we know that the mass flow rate for a
liquid across an orfice is very close to proportional to the square root
of the pressure drop [ I actually had to check up on this one, but it
appears to be so:
http://www.efunda.com/formulae/fluid...flowmeter.cfm] --
and of course the density of the avgas doesn't change appreciably[!].
Therefore, the avgas flow rate is proportional to "r^(1/2) m". *From this
point of view the carby at a constant air density can be viewed as a
device that meters a constant mass proportion mixture of avgas and air,
across a range of mass airflows -- ignoring the effects of accelerator
pumps, full-throttle enrichers, idle circuits and all that. But note that
as the density decreases, the fuel proportion to air decreases -- *
suggesting that one would need to ENRICH the mixture when climbing into
less dense air. *Assuming that the desired mass proportion of fuel/air is
approximately the same across varying densities (which seems very
reasonable to both of us).

A''': *OK, I do follow that (after some work)... and I'm stumped.

Granted, some simplifying assumptions here, but no convincing explanation
of why you would need to lean that red know when climbing... (and I don't
question that you in fact do)....

Anyone see what is amiss?

T

My knowledge is limited to small engines (all two cycle), which the vast
majority I specialize in are diaphram/crankcase pulse type carbs. Are you
sure that the carb is regulated by the venturi flow through the carb? Are
carbureted aircraft engine not run off crank case pulse?

I guess I need to add this, I realize that airplanes have fuel pumps, but
are they ran off the engine belt or pulse? I would think the smaller LS
engines like the Rotax might be a pulse carb with float bowl.

Aircraft engines like the Lyc or Continental often have NO fuel
pumps if the aircraft is a high-wing type with the tanks in the wings.
Gravity does the job. If there's a pump it's driven off a cam in the
accessory section, like an old automobile engine-mounted fuel pump,
and there'll be a backup electrical pump in case the mechanical pump
quits.
A four or six cylinder engine's crankcase doesn't have pulsation like
a two-strokes because the air/fuel mix doesn't travel through the
case.
The float carb is mostly velocity-sensitive. It doesn't care very
much what the air density is, so as the density decreases the fuel
flow won't decrease as much and the mixture will get richer.
I'm no physicist, but I still have to lean my engine as I climb.
That tells me more than any number of formulae.

Dan

Tman opined

Somebody posed that seemingly simple question to me, but kept coming
back to the point that they stumped me.... And I am stumped. What do
you see wrong with the logic in this dialog?

Q: Why do I need to lean out my carb when I climb?

A: Ahem, seems you forgot your PPL ground school. The air is less
dense. Fewer air molecules per unit volume. Therefore, you need less
gas, so you lean it out!

Because the carb measures volume, and adds enough fuel for that volume. So, if
the air is less dense, there is less mass of air entering the cylinders, and too
much fuel.

To correct that, you lean the mixture.


-ash
Cthulhu in 2012!
Vote the greater evil.

"Tman" wrote in message
...
Somebody posed that seemingly simple question to me, but kept coming back
to the point that they stumped me.... And I am stumped. What do you see
wrong with the logic in this dialog?

Q: Why do I need to lean out my carb when I climb?

I'll take a stab at this one. Its a very good question.

A Stromberg carb does not require a mixture adjustment (at least below
8000'). It diverts low pressure air from the back of the venturi into the
fuel float bowl. In this way it is "self regulating" just as you describe.
The amount of
fuel drawn in is proportional to the air pressure.

Older classic airplanes will use this type of carberator system and thus
have no mixture.

I believe more "conventional" systems use a mixture simply becuase the
logistics of balancing all the
jets is difficult and because slight misadjustments in the orifice that
tunes the ratio might result in
a catastrophically lean mixture. A carburetor can have four jet circuits
for idle, midrange, main and accelerator (I am quoting more Rochester
Quadrajet than Lycoming -- so aircraft mechanics please jump in).

Your mixture valve is in front of all of these and so restricts the fuel
thru all of them. If you have a higher performance
engine, or operate at a higher range of altitudes, I suspect you cannot
build a mechanical metering system that covers the range without regions of
overly rich or overly lean so we have the man in the loop.

I am pretty sure of my answer but I'd invite any clarification.

Todd

On Jan 19, 7:05 pm, "Todd W. Deckard" wrote:
"Tman" wrote in message
Q: Why do I need to lean out my carb when I climb?

I'll take a stab at this one. Its a very good question.

A Stromberg carb does not require a mixture adjustment (at least below
8000'). It diverts low pressure air from the back of the venturi into the
fuel float bowl. In this way it is "self regulating" just as you describe.
The amount of
fuel drawn in is proportional to the air pressure.

Older classic airplanes will use this type of carberator system and thus
have no mixture.

The Stromberg carb's bowl is vented to a dead airspace behind
the venturi just like all the other carbs and has the same rich
problem with altitude that the others do. The Stromberg was built with
a mixture control valve cavity in the upper casting and many were left
empty and capped off to run full rich, or had the valve installed and
the lever wired to the full rich position. Most of the population, at
least years ago, lived near the coasts and flew puddlejumpers that
didn't fly very high. Fuel was cheap, too. So the makers didn't see
another control as having much value, expecially the back-suction
mixture type that the Stromberg uses and which will not act as an idle
cutoff for shutdown.
I have one of those old carbs on my airplane. I operate off a
strip that's around 3000' ASL. I machined the necessary parts for the
mixture control, they being very rare now, and installed them. It
works fine. It's a homebuilt and so such doings are permitted.
Air from behind the venturi where the air, being still, is at or
near ambient pressure, and this air passes into the cavity I
mentioned. There's a much smaller port in the cavity that leads to the
venturi itself and has considerably less pressure when the throttle is
open. Air is sucked out here. A third port into the cavity leads to
the bowl itelf. As long as the port from behind the venturi is wide
open, air can come in from behind the venturi and get sucked into the
venturi proper without exerting any negative pressure on the bowl.
When we lean, the mixture control simply starts choking off the air
supply from behind the venturi and allows the lower venturi pressure
to lower the bowl pressure, which reduces fuel flow through the jet
into the nozzle. Because the venturi's pressure drop is about zero
when at idle, it won't suck back on the bowl to act as an idle cutoff
at shutdown.

I believe more "conventional" systems use a mixture simply becuase the
logistics of balancing all the
jets is difficult and because slight misadjustments in the orifice that
tunes the ratio might result in
a catastrophically lean mixture. A carburetor can have four jet circuits
for idle, midrange, main and accelerator (I am quoting more Rochester
Quadrajet than Lycoming -- so aircraft mechanics please jump in).

The typical light-aircraft carb like the Marvel Schebler/
Precision Aeromotive/Tempest carb has one jet. The mixture control is
in the bottom of the float bowl and is a small valve that varies fuel
flow directly, from max to nothing at all.

Your mixture valve is in front of all of these and so restricts the fuel
thru all of them. If you have a higher performance
engine, or operate at a higher range of altitudes, I suspect you cannot
build a mechanical metering system that covers the range without regions of
overly rich or overly lean so we have the man in the loop.

Yup, you can, and it's done, too. It's just not cheap or
simple. There are various aneroid actuators that adjust fuel flow for
altitude, along with power valves that increase fuel flow for takeoff
and climb and other overriding devioces that prevent overly lean
conditions. Some pressure carbs (no float bowl) have these systems and
are similar in some ways to the fuel controllers used on fuel
injection systems. These types measure ambient air pressure, air
velocity through the carb, fuel supply pressure and fuel delivery
pressure and so forth and make the adjustments constantly.
This is diagram of a pressure carb, without the aneroid mixture
control. I can't find one on the 'net with it:
http://www.navioneer.org/riprelay/Th...bFlowChart.jpg


Dan

On Mon, 19 Jan 2009 14:23:57 -0500, Tman
wrote:

Somebody posed that seemingly simple question to me, but kept coming
back to the point that they stumped me.... And I am stumped. What do
you see wrong with the logic in this dialog?

Q: Why do I need to lean out my carb when I climb?


like most arguments in aviation this one arises because a question is
asked with a misleading part in it.

"why do I lean my carb when I climb" is a bull**** question open to
much misinterpretation.

the fact is that most airfields are near sea level, say under 1,000ft
elevation anyway. there is actually no need to lean a carby at these
altitudes.
in fact the mixture is left rich so that as you climb the over rich
mixture aids in engine cooling.
continental's pilot notes will tell you this.

"why do I need to lean my carburettor at higher altitudes" is probably
a better wording of the question.

that is simple. the air is less dense so the amount of fuel it needs
to achieve full combustion is reduced. the density of the fuel doesnt
decrease so you need less of it.

learn to think in more precise terms and a lot of the confusion
vanishes.
Stealth Pilot