chokes and mixture controls

[photofly]

I used to drive vehicles with a manual choke (don't laugh: my father used to drive vehicles with a manual ignition timing advance).

What's the practical and physical difference between a choke (on a carb) and a mixture control?

[Rudy]

A choke controls the amount of air going into the engine while a mixture controls the amount of fuel.

[Colonel Sanders]

A choke changes the air/fuel ratio by varying the air flow -
it's merely another throttle (butterfly valve) above the venturi.

The mixture control changes the air/fuel ratio by varying the fuel flow.

A choke is good for starting/idling when cold, and a richer
mixture is beneficial, but it's going to hurt power output.

Decades ago I had a big V8 which I put a four barrel carburetor
on. No choke. Wouldn't idle when cold. Had to run it on
the accelerator pump (blipping the throttle) until it warmed
up and would atomize the fuel better.

[photofly]

A choke controls the amount of air going into the engine

So does the throttle. What's the difference?

[Colonel Sanders]

Choke is above the venturi.

Throttle is below the venturi.

When the choke is closed (either manual,
or via an electrical servo) regardless of how
much you open the throttle, you can't put
much air through the carburetor. This results
in a rich mixture, especially combined with
the action of the accelerator pump (squirter).

This is the famous Rochester Quadrajet - another
four barrel carburetor - of which millions were
made.



Note the tiny primaries and the massive secondaries.
View from the bottom:



Even if you mashed the throttle wide open, the secondaries
would flow little air until the vacuum-controlled kinda-chokes
on top, opened up.

Ever run a hose without a nozzle? What happens if you
put your thumb over the metal open end of the hose,
and cover most of it up?

[Tom H]

As I can best put it...

Choke is above the venturi.

Throttle is below the venturi.

The choke limits the amount of air able to reach the venuturis enriching the mixture for starting and cold temperatures.

The throttle controls the amount of mixture (fuel and air) that is allowed to go to the engine

Mixture allows for the manual adjustment of the air/fuel mixture while running...or as you increase altitude you have a limited ability to reduce the amount of fuel entering the venturis.

As I understand it.

Tom H

[iflyforpie]

I still drive a car with a choke until I replace it sometime later this year.

It has a Zenith side draft carb and doesn't have any choke plates. It has a cam which opens up the butterfly valve (one 'barrel') when the choke is on (at normal idle it is completely closed.... idle air coming through another passage which is adjusted by a restrictor set screw).

I'm not sure of the mechanism it uses to enrichen the mixture, it's probably just an extra nozzle located downwind of the throttle much like idle nozzles in an aircraft carb.

Normal mixture and metering is set by a diaphragm controlled needle valve in the Venturi upwind of the throttle (which also varies in size because of a piston controlled by that same diaphragm.... which keep the vacuum the same at all power settings in these constant depression carbs).

For an aircraft, the choke is setting mixture rich and cracking the throttle. As most of you are undoubtedly aware, as the engine warms up you have to reduce throttle. It's also a good idea to lean the mixture too.

I guess with the choke they wanted to make it a little more idiot proof (plus it's hard to set throttle with your foot).

[photofly]

Choke is above the venturi.

Throttle is below the venturi.

That's a nice simple explanation which I can follow.

When the choke is closed (either manual,
or via an electrical servo) regardless of how
much you open the throttle, you can't put
much air through the carburetor. This results
in a rich mixture, especially combined with
the action of the accelerator pump (squirter).

Wouldn't that prevent me from generating any power with the choke full "on"? It's a long time, but I recall driving for the first few minutes with the choke out, until the engine had warmed up, and I could still accelerate relatively normally.

Of course that was a knob labeled "choke"; but it might have been labelled so in order that ignorant people like me would know what do with it; it might have been achieving the same ends via another method.

To be honest, carburetors strike me as the vehicular equivalent of analogue power rf stages. Designed by wizards from another planet into whose arcane workings mortals shouldn't delve.

[Tom H]

I still drive a car with a choke until I replace it sometime later this year.

It has a Zenith side draft carb and doesn't have any choke plates. It has a cam which opens up the butterfly valve (one 'barrel') when the choke is on (at normal idle it is completely closed.... idle air coming through another passage which is adjusted by a restrictor set screw).

I'm not sure of the mechanism it uses to enrichen the mixture, it's probably just an extra nozzle located downwind of the throttle much like idle nozzles in an aircraft carb.

Normal mixture and metering is set by a diaphragm controlled needle valve in the Venturi upwind of the throttle (which also varies in size because of a piston controlled by that same diaphragm.... which keep the vacuum the same at all power settings in these constant depression carbs).

For an aircraft, the choke is setting mixture rich and cracking the throttle. As most of you are undoubtedly aware, as the engine warms up you have to reduce throttle. It's also a good idea to lean the mixture too.

I guess with the choke they wanted to make it a little more idiot proof (plus it's hard to set throttle with your foot).

Know these ones as well...

The Stromberg/SU/Bing and some others are a different breed of cat again.

The Cam moves the needle of the jet way up to enrichen the mixture to create a "Choke Effect" but it's not truly a "choke" in the normal sense, more like an enrichening device.

Because these carbs use airflow to adjust the fuel metering they are also altitude compensating.

Also they have no venturi they theoretically can't ice up.

TH

[iflyforpie]

Also they have no venturi they theoretically can't ice up.

TH

Well I'm not so sure about that. Connected to the air intake is a hose that leads to a shroud around the exhaust manifold that looks suspiciously similar to the carb heat setup on an aircraft.

Of course you never had to worry about that with the old quadrajets or Holleys... on top of the engine they were bathed in heat... so much so that they are still selling suckers on 'cold air intakes' on their fuel injected cars that have had factory cold air intakes for over 20 years now.

[azimuthaviation]

When the choke is closed (either manual,
or via an electrical servo)

They also had an automatic choke that was controlled by a thermostat. The choke was connected to a bimetallic strip, when it was cold it kept the butterfly colsed, as the engine warmed up, the coil warmed up, expanded, and opened it up. Worked great. In theory.

[CamAero]

For the most part, you guys Tom H and iflyforpie, are not describing 'chokes', though they may be attached to controls labeled as such. They are other methods of mixture enrichment.

A true choke, is as previously pointed out by the Colonel, a plate upstream of the venturi.

The effect here, which I think you all are failing to convey is that the choke plate, when selected "on" is blocking the carburetor intake. In doing so, the exposed ends of the carburetor jets within the venturi are subject to very significant low pressure, (or "vacuum"), during the intake stroke(s).

The other ends of these jets are submersed in fuel in the carburetor bowl. The carburetor bowl is vented to atmosphere.

That pressure-differential causes raw fuel to get drawn up through the jets and into the intake.

This is how a true choke works to enrichen the mixture for start.

I realize that you both understand this principal but I don't think it's been adequately conveyed to the O.P.

As to this, from photofly:

A choke controls the amount of air going into the engine

So does the throttle. What's the difference?

The difference is, the throttle, being downstream of the jets does not cause the jets to be subject to the intake 'vacuum' when it is in the closed position. Quite the opposite in fact.

In that state, (Choke open and throttle closed), the jet(s) feel closer to atmospheric pressure as the 'sucking' effect of the venturi is reduced to near nil, hopefully just enough fuel, (and air), to sustain idle.

CA

[photofly]

Thanks.

Choke is above the venturi.

Throttle is below the venturi.

So, the air encounters in order: a choke plate, the venturi and jets, and then the throttle plate. Have I understood that right?

[Tom H]

Thanks.

Choke is above the venturi.

Throttle is below the venturi.

So, the air encounters in order: a choke plate, the venturi and jets, and then the throttle plate. Have I understood that right?

Correct

[Tom H]

Cam Aero

I believe we are saying the same thing...you've just gone a step farther

My comment:

The choke limits the amount of air able to reach the venuturis enriching the mixture for starting and cold temperatures.

The throttle controls the amount of mixture (fuel and air) that is allowed to go to the engine

Mind you...great job of covering it in simple terms. I couldn't find a way to explain it as simply and didn't want to get into a drawn out explanation.

Tom

[CamAero]

Indeed Tom;

You bet.

CA

[Colonel Sanders]

the choke plate, when selected "on" is blocking the carburetor intake.
... the venturi are subject to very significant low pressure

Was I too developmental when I previously said:

Ever run a hose without a nozzle? What happens if you
put your thumb over the metal open end of the hose,
and cover most of it up?

[CamAero]

I think I see where you were going with it but I really don't see your analogy as being all that suitable here Colonel.

CA

[Colonel Sanders]

Really? Here's another picture of another choke-like
obstruction in an air intake:



You'd think it would get more air without it, no?

[CamAero]

Colonel, which one of us has lost the plot here?

Are you saying that a closed choke plate causes air velocity to increase though the venturi which causes a significant pressure drop above the jet??

That is wholly false.

The velocity increase around the restriction of the choke plate is instantly negated by the pressure drop on the downstream side of the plate, upstream of the fuel metering jet.

A closed choke plate causes manifold vacuum, not venturi vacuum.

If you don't believe me, try completely obstructing the mouth of the carburetor on your lawn mower with the palm of your hand, turn the kill switch off and pull it over a few times. Have a look inside the carburetor throat afterwards.

It's full of raw fuel. Where did that come from with no air flow?

CA

[Driving Rain]

So...a choke.....chokes the air off to an internal combustion piston engine. A mixture ...chokes the fuel delivery to an engine. Thanks guys. The only engine I'm operating with a choke at present is the mighty Tecumseh Snow King engine on my 8 -26.....40 year old Mastercraft snowblower.

[iflyforpie]

Really? Here's another picture of another choke-like
obstruction in an air intake:



You'd think it would get more air without it, no?

It does get more air without it.



It's just that a bellmouth intake will cause too much drag and not add any compression at supersonic speeds.

[photofly]

It's a shock cone, isn't it? Creates the first shock front to slow supersonic air to subsonic speeds for the engine intake.

[switchflicker]

What I get from the conversation above is that you pull the choke out to start an engine (if you have a choke) and you pull a mixture control to stop an engine (if you have a mixture control). Did I get that from the conversation? Not really sure.

[burhead1]

Choke is above the venturi.

Throttle is below the venturi.

When the choke is closed (either manual,
or via an electrical servo) regardless of how
much you open the throttle, you can't put
much air through the carburetor. This results
in a rich mixture, especially combined with
the action of the accelerator pump (squirter).

This is the famous Rochester Quadrajet - another
four barrel carburetor - of which millions were
made.



Note the tiny primaries and the massive secondaries.
View from the bottom:



Even if you mashed the throttle wide open, the secondaries
would flow little air until the vacuum-controlled kinda-chokes
on top, opened up.

Ever run a hose without a nozzle? What happens if you
put your thumb over the metal open end of the hose,
and cover most of it up?

Just a note: Your picture sucks as it is missing the choke plate. Excuse my humor, but seriously it is.