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

"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

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard" wrote:
It diverts low pressure air from the back of the venturi into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand that the
standard config (not this stromberg, but rather something like in a
basic 172N) has a fuel-bowl vent upstream of the venturi, making the air
pressure in the fuel bowl pretty much equal to the air intake
pressure.... Kind of like a standard auto carb from a few years ago when
they still had them. Let me know if that's not right.

Tman wrote in news:-
:

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard" wrote:
It diverts low pressure air from the back of the venturi into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand that the
standard config (not this stromberg, but rather something like in a
basic 172N) has a fuel-bowl vent upstream of the venturi, making the air
pressure in the fuel bowl pretty much equal to the air intake
pressure.... Kind of like a standard auto carb from a few years ago when
they still had them. Let me know if that's not right.


They're basically a float bowl and a tube. not a lot to complicate them,
really. Probably the simplest carbs in operation today.

Bertie

"Bertie the Bunyip" wrote in message
...
Tman wrote in news:-
:

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard" wrote:
It diverts low pressure air from the back of the venturi into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand that the
standard config (not this stromberg, but rather something like in a
basic 172N) has a fuel-bowl vent upstream of the venturi, making the air
pressure in the fuel bowl pretty much equal to the air intake
pressure.... Kind of like a standard auto carb from a few years ago when
they still had them. Let me know if that's not right.


They're basically a float bowl and a tube. not a lot to complicate them,
really. Probably the simplest carbs in operation today.

Bertie

You're an idiot.

"Maxwell" #$$9#@%%%.^^^ wrote in :


"Bertie the Bunyip" wrote in message
...
Tman wrote in news:-
:

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard"
wrote: It diverts low pressure air from the back of the venturi
into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand that
the standard config (not this stromberg, but rather something like
in a basic 172N) has a fuel-bowl vent upstream of the venturi,
making the air pressure in the fuel bowl pretty much equal to the
air intake pressure.... Kind of like a standard auto carb from a few
years ago when they still had them. Let me know if that's not
right.


They're basically a float bowl and a tube. not a lot to complicate
them, really. Probably the simplest carbs in operation today.

Bertie

You're an idiot.




Nope.

But please don't let that stop you maxie.


Bertie

Bertie the Bunyip wrote:
"Maxwell" #$$9#@%%%.^^^ wrote in :


"Bertie the Bunyip" wrote in message
...
Tman wrote in news:-
:

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard"
wrote: It diverts low pressure air from the back of the venturi
into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand that
the standard config (not this stromberg, but rather something like
in a basic 172N) has a fuel-bowl vent upstream of the venturi,
making the air pressure in the fuel bowl pretty much equal to the
air intake pressure.... Kind of like a standard auto carb from a few
years ago when they still had them. Let me know if that's not
right.


They're basically a float bowl and a tube. not a lot to complicate
them, really. Probably the simplest carbs in operation today.

Bertie

You're an idiot.




Nope.

But please don't let that stop you maxie.

What's new on teh gasket-front, Cap'N?
--
ah

Tman,

I've been pondering your question quite a bit. I believe I have it. In
deference to Dan my Camaro never idled correctly.

Ultimately your engine depends on the mass flow of air divided by the mass
flow of fuel. But the amount of fuel drawn up is a function of the pressure
difference in the carbureture venturi. So here goes:

The carb throat is a double venturi and a manometer between the opening and
the neck would show a theoretical pressure drop of:

p(opening) - p(neck) = .5 * density of air * { velocity(neck)^2 -
velocity(opening)^2 }
(Lets ignore carb ice for a second and say that the air is
incompressible).

Bernoulli got the idea from Newton thats why the 1/2 m v squared. Now
discouragingly this has the density
in front of it, which is why you posed the question. The difference in
pressures is directly proportional to the density.

Now the low pressure in the neck of the venturi is what is drawing the fuel
up (or properly the difference between the neck and ambient). Again we use
Bernoulli to describe the forces acting on a particle moving along a
streamline -- and this time it is properly incompressible.

{ Pressure / density } + .5 * { velocity ^ 2 } + gravity * change_in_height
= a constant

Again, Bernoulli copped it all from Newton and was just telling us that
kinetic energy + potential energy = a constant.

If we apply this to your fuel being drawn up we get:

(#) p (ambient) - p (neck) / density of fuel = .5 * (velocity of fuel) ^2 +
(gravity * vertical distance from bowl to jet)

However mass flow is the density * the size of the pipe * the velocity.

So the mass flow of air = density of air * carb barrel size * velocity
(opening)

But from (#) the mass flow of fuel is being determined by the pressure
difference (which also carries the air density)

so Air over Fuel cancels your density term.

An altitude compensating carburator puts a small vacuum on the fuel to
prevent the rho from dividing out.

Mr Wizard could have explained this better that as the air gets thinner it
sucks on the straw with less force but it takes less force to slurp up the
gas becuase of the reduced pressure. So the gas drawn up stays about the
same, however the mass flow of air drops off with density so the mixture
richens.

Q.E.D. Good question. If I ever become a physics teacher I am going to
put this one on the final!

I think you are making this waaaay to complicated. The volume of air going
through the venturi remains the same as you climb but the amount of oxygen
(the component needed to burn the fuel) decreases. The volume of air
remains the same so the fuel drawn out of the float bowl remains the same.
There is less oxygen so the mixture become rich.

That's my story and I am sticking to it.

--

*H. Allen Smith*
WACO - We are all here, because we are not all there.
"Todd W. Deckard" wrote in message
...
Tman,

I've been pondering your question quite a bit. I believe I have it. In
deference to Dan my Camaro never idled correctly.

Ultimately your engine depends on the mass flow of air divided by the mass
flow of fuel. But the amount of fuel drawn up is a function of the
pressure difference in the carbureture venturi. So here goes:

The carb throat is a double venturi and a manometer between the opening
and the neck would show a theoretical pressure drop of:

p(opening) - p(neck) = .5 * density of air * { velocity(neck)^2 -
velocity(opening)^2 }
(Lets ignore carb ice for a second and say that the air is
incompressible).

Bernoulli got the idea from Newton thats why the 1/2 m v squared. Now
discouragingly this has the density
in front of it, which is why you posed the question. The difference in
pressures is directly proportional to the density.

Now the low pressure in the neck of the venturi is what is drawing the
fuel up (or properly the difference between the neck and ambient). Again
we use Bernoulli to describe the forces acting on a particle moving along
a streamline -- and this time it is properly incompressible.

{ Pressure / density } + .5 * { velocity ^ 2 } + gravity *
change_in_height = a constant

Again, Bernoulli copped it all from Newton and was just telling us that
kinetic energy + potential energy = a constant.

If we apply this to your fuel being drawn up we get:

(#) p (ambient) - p (neck) / density of fuel = .5 * (velocity of fuel) ^2
+ (gravity * vertical distance from bowl to jet)

However mass flow is the density * the size of the pipe * the velocity.

So the mass flow of air = density of air * carb barrel size * velocity
(opening)

But from (#) the mass flow of fuel is being determined by the pressure
difference (which also carries the air density)

so Air over Fuel cancels your density term.

An altitude compensating carburator puts a small vacuum on the fuel to
prevent the rho from dividing out.

Mr Wizard could have explained this better that as the air gets thinner it
sucks on the straw with less force but it takes less force to slurp up
the gas becuase of the reduced pressure. So the gas drawn up stays about
the same, however the mass flow of air drops off with density so the
mixture richens.

Q.E.D. Good question. If I ever become a physics teacher I am going to
put this one on the final!

On Jan 21, 7:19 am, "Allen" wrote:
I think you are making this waaaay to complicated. The volume of air going
through the venturi remains the same as you climb but the amount of oxygen
(the component needed to burn the fuel) decreases. The volume of air
remains the same so the fuel drawn out of the float bowl remains the same.
There is less oxygen so the mixture become rich.


More properly, the weight of the air decreases. Oxygen still
makes up 21% of the air.

Dan