Q: Marvel Schebler Economizer/Enrichment

[asicer]

Please help me understand the Marvel Schebler Economizer circuit.


If I read things correctly, carburetors normally have the space above the fuel bowl vented to ambient(-ish) air pressure. There's a fuel nozzle in the Venturi and the Venturi has an air pressure inversely proportional to the speed of the air flowing through it. Therefore, the rate at which fuel is sucked out of the fuel bowl through the nozzle is a function of the differential between the venturi's air pressure and the fuel bowl's air pressure.

Marvel Schebler's Economizer adds a passage between the fuel bowl's air space and the throttle body. At full throttle the manifold pressure is near ambient so the Economizer doesn't do much since the air pressure on both sides of the passage are near equal. As you go to a lesser throttle setting, the manifold pressure decreases, which sucks some of the air out of the fuel bowl chamber through the Economizer passage. The decreased air pressure differential between the venturi and the fuel bowl chamber results in less fuel getting sucked through the nozzle, hence a leaner mixture ratio.

Q1: Did I get this right?
Q2: Without the Economizer, would the air/fuel mixture ratio be constant throughout the throttle range?
Q3: If the Economizer were to be disabled, would there be an effect on engine smoothness or even its ability to stay running?
Q4: Does the Economizer really save any fuel? Once settled into cruise with a fixed throttle setting, what's the difference between the Economizer leaning the mixture and the pilot leaning the mixture with the red knob?

Regarding the enrichment circuit, I've heard it said that if you pull back a little bit from full throttle that you disable the enrichment circuit and thus save a bit of fuel.

Q5: Is this really true?
Q6: How does the enrichment circuit work?
Q7: How is disabling the enrichment circuit different from the pilot merely pulling back on the mixture knob?

 

[Capt. Geoffrey Thorpe]

As someone who used to be involved in these things for the day job, I would say that carburetors fall under the category of "Black Arts". One of the problems is that the pressure drop has a square root in the flow/ pressure equation - if you just had a simple venturi / jet you would not get a consistent mixture across a range of airflow. So the Carburetor engineer has to reach into his/her bag of tricks to shape the air fuel curve to the desired shape for performance. This involves air bleeds, jets, mixing tubes, nozzle designs, holes here and there. Air bleeds TEND to make things leaner at higher airflow, jets TEND to make changes across the whole range of air flows, mixing tubes defy logical understanding. In the end, every expert has a different way of approaching the problem that seems to work for them. Many hours are spent on flow benches and dynamometers getting things tuned "just so". One can tweek the jet and bleed sides if one has adequate facilities, but to try and change the basic metering system is not something one takes on lightly.

Q1: Did I get this right?
Don't think so. The Economizer orifice appears to be upstream of the throttle. The manifold pressure would then be irrelevant. It would be more about pressure losses in the intake / venturi. Based on the drawing, I would expect some interaction between the throttle plate and the orifice. I'm sure someone spent hundreds of hours experimenting with the location of that hole.
Q2: Without the Economizer, would the air/fuel mixture ratio be constant throughout the throttle range?
It would be different. I seriously doubt that it would end up being constant. All of the jets / bleeds / mixing tubes interact and it's a balancing act. If you disable one part of the circuit you change everything.
Q3: If the Economizer were to be disabled, would there be an effect on engine smoothness or even its ability to stay running?
Most likely.
Q4: Does the Economizer really save any fuel? Once settled into cruise with a fixed throttle setting, what's the difference between the Economizer leaning the mixture and the pilot leaning the mixture with the red knob?
It's all about shaping the air/fuel ratio curves. Traditionally, one wants to be a bit on the leaner side during mid-range operation to save fuel where you can get away with it. You tend to be richer at idle to compensate for the weakness of the combustion. At WOT you generally go richer for power and/or cooling. You could fiddle fart with the red knob to adjust the mixture, but the carburetor design is intended to give you the best result with the minimum interaction from the pilot.


Q5: Is this really true?
Probably.
Q6: How does the enrichment circuit work?
Depends. Some carburetors have extra jets that pull fuel in during high airflow operation, some just rely on the magic interactions between bleeds, jets, etc. I'm not familiar with this specific carburetor. As I said, every expert has their own approach.
Q7: How is disabling the enrichment circuit different from the pilot merely pulling back on the mixture knob?
It would not allow richer operation for power or cooling when you need/want it.

 

[asicer]

Q7: How is disabling the enrichment circuit different from the pilot merely pulling back on the mixture knob?
It would not allow richer operation for power or cooling when you need/want it.

I was wondering more along the lines of why a pilot might want to pull back on the throttle a little bit to disable the enrichment versus pulling back on the mixture at WOT.

 

[Capt. Geoffrey Thorpe]

I was wondering more along the lines of why a pilot might want to pull back on the throttle a little bit to disable the enrichment versus pulling back on the mixture at WOT.

You could, but then it would be more of a power reduction. For the most efficient operation, one would want to run at WOT to reduce pumping losses (work required to suck the air through the carburetor) and pull the mixture if you don't need the extra fuel. An example would be at altitude where cooling/knock would not be an issue - you can reduce fuel use with less power loss by using the red knob.

Personally, I think carburetors are amazing (in addition to being a pain in the ass - I was not sorry to see them go away where I worked ) - all of the thought and ingenuity that has gone into shaping the air/fuel curves using nothing more than carefully sized holes - there are some real unsung geniuses out there.

 

[GeorgeC]

For the most efficient operation, one would want to run at WOT to reduce pumping losses...

This always made sense to me. What's your take on the advice/OWTs to close the throttle enough to disable the enrichment circuit / make the airflow turbulent (see also, partial carb heat)? Seems to me that carb manufacturers would have added VGs by now if that helped. Maybe that's the weird old banned trick to get a 100mpg carb...

 

[Capt. Geoffrey Thorpe]

This always made sense to me. What's your take on the advice/OWTs to close the throttle enough to disable the enrichment circuit

I wouldn't know just how far to close it without putting it on a flow stand first

/ make the airflow turbulent (see also, partial carb heat)? Seems to me that carb manufacturers would have added VGs by now if that helped.

On the automotive side, I have seen ridges molded into the bottom of intake manifolds to better direct the liquid fuel film and help reduce the imbalance in the cylinder to cylinder fuel distribution. Also, "diving boards" mounted above throttle body injection systems to re-direct flow. (The throttle plate tends to push fuel to one side.) Hot spots under the carburetor (exhaust crossover) were typical. Heated spacers under the carburetor were experimented with. On the aircraft side, it seems to me that the systems were designed when fuel was cheep and more fuel was the solution to any problem. Some carb heat probably doesn't hurt - how much it may help (if any) would depend on the particular engine.

Maybe that's the weird old banned trick to get a 100mpg carb...

That's locked up in the basement of Ford's World Headquarters in Dearborn.

 

[pfarber]

The economizer is wildly misnamed. It enrichens the mixture at part throttle for better cooling of the engine.


http://www.insightavionics.com/pdf files/MA-4 Carb Manual.pdf. Paragraph 1.d

 

[Dan Thomas]

The economizer is wildly misnamed. It enrichens the mixture at part throttle for better cooling of the engine.
http://www.insightavionics.com/pdf files/MA-4 Carb Manual.pdf. Paragraph 1.d

Wildly misnamed, you say? What does Marvel Schebler mean then, in the third word of the first sentence of that paragraph? And in the 9th-to-last word of the last sentence?

 

[Dan Thomas]

Most MS carbs, like the MA3 and MA4s commonly found on the airplanes we fly, have no economizer stuff. They are pretty basic. The HA-6 might have a mechanical system. We find those on some Lyc 360s and 540s. Horizontal-draft carbs.

There are a LOT of different MS carb models. An MA4 carb is not just an MA4 carb. There are part numbers on every carb that tell us what's in it as far as metering jets, accelerator pump settings and a lot of other stuff.

 

[Domenick]

Most MS carbs, like the MA3 and MA4s commonly found on the airplanes we fly, have no economizer stuff. They are pretty basic. The HA-6 might have a mechanical system. We find those on some Lyc 360s and 540s. Horizontal-draft carbs....

Than MS manual seems to think the MA-3 and MA-4-SPA, MA-4-5 and MA-5, and HA-6 have economizers:
https://www.dropbox.com/s/kqknpocrqc87oup/00.Marvel-Schebler.Carb.Manual.pdf?dl=0

My Warrior has an MA-4-SPA

Here are pretty good descriptions of MS carb internals and functioning:
https://www.dropbox.com/s/auguebz7xr9jq6s/00.Marvel-Schebler.Carb.pdf?dl=0
https://www.dropbox.com/s/3a9u0nrr0kvpvp4/00.Marvel-Schebler.Carb.AMT.pdf?dl=0

 

[Dan Thomas]

Than MS manual seems to think the MA-3 and MA-4-SPA, MA-4-5 and MA-5, and HA-6 have economizers:
https://www.dropbox.com/s/kqknpocrqc87oup/00.Marvel-Schebler.Carb.Manual.pdf?dl=0

Same manual as already posted by pfarber.

https://www.dropbox.com/s/auguebz7xr9jq6s/00.Marvel-Schebler.Carb.pdf?dl=0

Tractor and industrial carbs, considerably different from aircraft carbs.

https://www.dropbox.com/s/3a9u0nrr0kvpvp4/00.Marvel-Schebler.Carb.AMT.pdf?dl=0

This one covers the aircraft series of MS carbs. Note that nowhere is the economizer circuit mentioned, nor is is it shown in the cutaways.

 

[Capt. Geoffrey Thorpe]

Wildly misnamed, you say? What does Marvel Schebler mean then, in the third word of the first sentence of that paragraph? And in the 9th-to-last word of the last sentence?

View attachment 117919

That makes more sense given the diagram in the original post. The "Economizer" is a high speed enrichment - pressure losses in the venturi result in a back suction on the bowl.

Every carb designer has their own personal bag of tricks that they reach into to shape the flow curve. And in the end, it's the curve that counts.

 

[Dan Thomas]

I had a Stromberg (Bendix) carb on my A-65. It has a back-suction mixture control. A small cable-operated valve in the carb gradually shuts off the external bowl vent, allowing venturi vaccum, via a tiny passage into the bowl, to start pulling back on the fuel, leaning the mixture. It worked well, but had no effect at low power or idle. It would not act as an idle cutoff as the MS mixture does.

When I got it, that carb had a blank cover plate over the valve cavity. No valve, no mixture control, as was common in most Cubs and Champs and many others. Those valve parts are rare, so I machined my own. Homebuilts are great.

 

[Domenick]

...Tractor and industrial carbs, considerably different from aircraft carbs.

My Warrior has tractor mags, tractor gascolator, fuel pump, starter, and alternator. But it has an MA-4SPA aircraft carb. According to Marvel-Schebler's own technical documentation posted above, it has an economizer circuit. MS aircraft eligibility/applicability document confirms my carb as an MA-4SPA:
https://msacarbs.com/technical-data/engine-eligibility/

This one covers the aircraft series of MS carbs. Note that nowhere is the economizer circuit mentioned, nor is is it shown in the cutaways.

It's not an official MS publication. It's from a maintenance magazine.

I've never disassembled my MA-4SPA so whether or not it has an economizer circuit or not is a matter of faith.

 

[MRC01]

...
Regarding the enrichment circuit, I've heard it said that if you pull back a little bit from full throttle that you disable the enrichment circuit and thus save a bit of fuel.

Q5: Is this really true?
Q6: How does the enrichment circuit work?
Q7: How is disabling the enrichment circuit different from the pilot merely pulling back on the mixture knob?

Q5: Yes - my experience with the O-320 and O-360 engines is a 10-15% reduction in efficiency. That is, if you leave it at WOT and lean the mixture to compensate for the enrichment circuit (to peak RPM), you get 10-15% higher fuel burn rate for the same power/airspeed, compared to pulling back the throttle just enough to disengage the enrichment circuit, and leaning to peak RPM. But same airspeed either way.

Q7: I believe 2 factors at play here. First, when you pull back the throttle just enough to disengage the enrichment circuit, the butterfly is slightly non-parallel to the airstream, which disrupts airflow just enough to improve air/fuel mixing. Second, pulling back slightly improves the mixture distribution across the intake manifold / cylinders, reducing the difference between your richest & leanest cylinder.

Here's a simple way to confirm the existence of the economizer circuit, and optimize cruise speed & efficiency:
1. Above 5000', set WOT and lean until you get an RPM drop.
2. Now slowly pull back on the throttle.
3. If your carb has a WOT enrichment circuit, you'll get a sudden RPM drop as you are pulling back, because the engine was already lean of peak power, and it will suddenly get even leaner as the circuit disengages. This typically happens about 1/4" to 1/2" back from WOT.
4. Leave the throttle there and enrich the mixture back to your normal setting.

 

[Magman]

With MS carbs it’s important to note the significance of the “ 10 number”.

This identifies the “ sub- model” of a MAx-xxxx carb.

Though similar externally; there are differences in internal passages and

adjustment of components. The Mechanical Adjustment of the Economizer is one

of those on some larger engines/carbs. Roughly 180 hp and up.

On those carbs the setting is accomplished with a Depth Gage to allow the

Economizer to activate at a pre- determined Throttle Position.


A bud had a 172 with a 180 upgrade. CHT’s were very high.

Several carbs were furnished to address this with some improvement .

Finally Lycoming took the readings and with an identical engine in a test cell

they “ doctored” the carb for acceptable performance.

It’s been fine for many years. It’s possible those settings may have become the new standard.

 

[Dan Thomas]

This is Aircraft Spruce's list of carburetors just for Lycoming engines.



I see seven different MA-4SPA carbs there. Those carbs are not all the same. They are built according to the engine manufacturer's demands, and if they need enrichment/economizer circuits, they have them. If they don't need them, they won't be there.

Continental carbs:



Another bunch of parts numbers. No MA-4SPAs.

This is the list made by Facet (former owner of the MS carb line, between the original marvel Schebler and Precision Airmotive), for the various engines. Long list of different part numbers for carbs that often look exactly alike, but are not/

http://www.kellyaerospace.com/fuel_charts/Carburetors/Facet_Marvel_Schebler_Carb.pdf

And from an old MS overhaul manual, which might be (probably is) out of date and not include many modern 10- part numbers, the carbs with economizers are these:


From http://republicseabee.com/Files/Marvel-Schebler Service Manual.pdf

I do note that no MA4-SPA 10- number is there, nor have I ever seen that economizer mechanism on any MA4 carb I ever had anything to do with.

That manual shows NO venturi-driven economizer channels in any of their drawings.

An up-to-date manual would tell use where to expect economizer channels or valves, but I find nothing (free ) online.

 

[Dan Thomas]

First, when you pull back the throttle just enough to disengage the enrichment circuit, the butterfly is slightly non-parallel to the airstream, which disrupts airflow just enough to improve air/fuel mixing. Second, pulling back slightly improves the mixture distribution across the intake manifold / cylinders, reducing the difference between your richest & leanest cylinder.

I ran into a fairly obvious case of that in a 172 with a Lyc O-320. In the climb, I found that pulling the throttle back a bit resulted in an RPM increase and a slightly better climb. It was that throttle plate deflecting and breaking up the fuel stream into smaller droplets that atomized better and vaporized quicker.

Now that, in turn, was a result of the 1990s ADs that forced a change from the two-piece venturi to the one piece. The two-piece had a reputation of coming loose and letting the engine swallow the secondary venturi, which pretty much killed the engine, since it's that secondary venturi that generates the majority of the vacuum for the fuel nozzle, which terminates inside that venturi. This venturi change resulted in some engines running rough (due partly to non-streamlined secondary venturi support legs), so they revised the AD to require a crossdrilled fuel nozzle. In some airplanes that made things even worse, so they revised it again to allow reinstallation of the original two-piece venturi and nozzle, with a requirement for inspection of venturi security every 100 hours. The whole affair was appalling. Aircraft quality, indeed.

That crossdrilled nozzle resulted in more fuel going up one side of the bore and giving the rear cylinders too much fuel and the front ones too little. Pulling the throttle back a bit deflected some of it forward, and broke it up, improving the performance.

The MS carb, as far as I, an aircraft mechanic, am concerned, is a poorly-designed thing. The Marvel Scheblers were widely called the "Marvellous Dribblers." The ancient Stromberg on my A-65 ran far smoother and better than any MS carb. It didn't even have an accelerator pump, just an accelerator well. No economizer circuit, either. Just a well-designed mechanism to atomize fuel into the airstream, one that gave consistent mixture across the power spectrum.

 

[JAWS]

I just saw this thread and I mentioned this system in the maintenance thread about going full rich for landing. Small world!

The economizer system leans fuel flow automatically at less than full throttle. Or enrichens fuel flow at full throttle.

Supposedly, from max throttle, if you pull back just until manifold pressure decreases a bit, the enonomizer circuit will start to function and decrease fuel flow. It will slightly close the throttle plate, causing slight turbulence in the airflow, allowing the fuel and air to mix better. Less fuel flow and better mixing will give more overall power.

 

[Domenick]

This is Aircraft Spruce's list of carburetors just for Lycoming engines.

I see seven different MA-4SPA carbs there. Those carbs are not all the same. They are built according to the engine manufacturer's demands, and if they need enrichment/economizer circuits, they have them. If they don't need them, they won't be there.
...

Thanks.
I'll test my MS-24SPA 10-5217 according to MRC01's suggestion.