Butterfly valves

I can’t remember what group I was talking with at Argos this year and someone mentioned having difficulty finding the sweet spot mixing air with butterfly valves. Somebody mentioned that they were constantly fiddling with the valve because butterfly valves are not linear but change the volume of air a lot with small movement during one part of the stroke and little change during the rest of the stroke. It was said that gate valves would be better if they only could have lever action of some sort. Someone else mentioned that an Iris valve like on cameras would be best of all. I got to thinking about this and wondered how hard it would be to make a double butterfly valve where part 1 would open first to the halfway point and then part 2 would open. Is this enough of a problem to consider monkeying with or should we just get used to the butterfly valves? Drawing below is a double butterfly concept.


I like the idea of an Iris Valve. These are designed for Air to Fuel ratios

I’ve most likely got the worst setup possible with one 3" valve to control the air and it’s not that bad to control the mixture with. I think Wayne mentioned using one larger valve you could set and leave alone and a smaller one (like the size of a small engine carburetor) for adjusting with. I’ll be going with two on my next build. That being said the valve you are mentioning seems interesting as long as it isn’t to hard or time consuming to build.


Hi, Don!

This is an everlasting problem, as long as one does not go to the bottom of the issue… flow rules.

It is not a thing depending on the linearity of one dispensing “organ”.

The task is to provide “equal opportunities” for the two gases to be dispensed in a desired proportion, independent of the mixed (resulting) flow volume.

It should be absolutely clear, that the two “arriving” gas streams in a traditional setup have COMPLETELY different “flowing opportunities” at cranking, idle and WOT settings of the common main throttle.

The “arriving” secondary air intake is just depending on the air-flap setting, but the “arriving” gas-line has never a constant gas resistace.

Putting together these two flows at cranking, idle and up to WOT is a constant play of cut and try, more or less.

Above the main throttle, the for the moment prevailing underpressure is always set by the momentary gas-line resistance and motor consumption.

So, the air-flow will increase with increased gas-resistance, and vica versa! The more fluctuancies in the gasline, the more variation in the the resulting gas-mix.

These pressure drops of “arriving” air and “arriving” gas above the main throttle flap are relatively smal in comparison to the vacuum under the main throttle, at idle and part load.

This is the picture of “traditional” T- suspension.

BUT, if we use WHOLE the pressure drop down to the manifold for the gas and the air INDIVIDUALLY, then this (part load) vacuum in the manifold will make the “arriving” and fluctuating gas pressure less significant, compared to the “arriving” air pressure.

And the simple rule for equal (or proportionate) flow:


This, in all simplicity means that we suspense gas and air through identical tubes (area and lenght) with identical flaps (ganged) to a swirrel mixer and then to the manifold.

Cranking a 4-cycle motor with closed flaps, with no “leaking” under the flaps from the crankcase or idle slotting, can generate a vacuum of 2 - 5 m H2O. This is a completely dominant vacuum in comparison to the gas system’s resistance-vacuum.

Already a half meter of H2O is totally dominant at cranking. This means that the ganged gas and air flaps can have only a minute opening to maintain the dominant gas and air resistance for proper suspense control.

This can be controlled automatically by a bellow or a cylinder and piston (~1,5" diam.) sucked inward by manifold vacuum and balanced by a countering spiral. The piston rod will try to pull up the flaps when the vacuum overcomes the spiral setting. The spiral setting rules the achieved vacuum.

If the motor starts, it will cause a runaway if there is no manually set stopper to set the wanted idle.

At WOT the suspense tubes with the ganged flaps will no longer be the DOMINANT resistancies, and the gas system resistance can play it’s game again.

To overcom this, a second airflap is installed some distance ahead of the ganged airflap.

One can use this one manually from the instrument panel (dashboard), or then it can be automatically constantly operated by a resonably big membrane comparing the “arriving” gaspressure and the “arriving” air pressure just before the ganged flaps. Maintain symmetry in the placing of the sampling tubes!


Hi Max

Thanks for the explanation. Do Have any pictures or drawings you could share?


Yes. Excellent explanation MaxG
Key to understanding is we are living at the bottom of an atmosphere well buried in a heavy blanket of air so always dealing in POSITIVE PRESSURES. Measure how you will in with displace weights of water or mercury measured then in pushed up inches, millimeters, feet or meters, it is still positive pressure. Measure in absolutes of bars, psi, cm2 it is still positive pressure. Vacuum measures and concept is near voodoo, tribal thinking.
AIR Weight crushed the can - not vacuum sucking it in. The astronaut/sea diver both out-gas “boil” “bleed” from higher relative internal pressures - NOT from being vacuum “sucked” to deaths.
We are always dealing in the DIFFERENCES of this positive pressure.
Mixers, carburetors, gasses valving these pressure differences matters a lot.

Butterfly valving are never, and will never be linear. So what? Nothing more easy and practical to make.
You can easily compensate with basic sizing selection and variable from the shaft round/cam cable or chain opening and closing.
Lever arm actuating will always be non-linear actuating as a matter of course. Set it up at the proper angles and lengths it will do this better. Simple to see the just a longer arm from shaft distance will give you a slower valve opening. Now change the starting and ending angles. In fact simple levers have TOO much non-linear biasing. Really why often the OEM’s learned to go to a cable/chain guided rounded cams to “program” the movements rates to the desired flow rates better.

Interesting concept DonM. I think it would air/gasses leak too much at the increased edges areas. Sure be a son-of-a-bugger to make.

Your iris valve IanM looks wonderful for precision by electric wire controllabilty of spotlessly clean liquids and gases.
Woodgas ain’t.
In my experiences even if four stage fuel gas filtered at normal flow pressure delivered and “clean” you will still get valve back side sooting, “dirt” deposited building up. And unless the valve is heated the valve induced flow restricted pressure drop WILL cool the gases. Cooled woodgas will aways wring out MORE moisture droplets! These can and will ice crystal freeze. Why you see some of the Euro guys and the India gas people intentionally gases overcooling, to super dehumidify the producer fuel gas, then reheating before valve/mixer introducing.
Engine manufacturer Jenbacher super gas filtered, dehumidified and using a non-flap throttle valve (think of a teardrop ended shuttle cock center plug being variably plunged into a tapered seat) then in service in a few hundred hours got growing deposits build ups of whitish/yellow crystals upsetting flow rates. Explanation to that is at the molecular level. Beyond my pay-grade.
Point is the iris the leaf/pedals will stick and not want to move. If forced, tear up the drive cams/levers/grear teeth.
Why you usually only see iris’s inside cmareas and such with very controlled conditions.
Fluids and gases flows restricted face apply a LOT of pressure against the valve elements.
Imagine those overlapping leaf segments being forcefully pushed together and then being forced to move with coating contaminates dragging in between and being edged scrapped off??

Naw fellows a flapper valve is still the answer. Round form they will edge seal the best. Self clean the best. (Yeah, I tried square form - worked kinda/sorta but impossible to side edges seal and would side edges contaminates bind. My Better idea was NOT)
Set up the true flow sizing range better.
Control it better that first 25-30 degrees of opening.
These can easily edge scrape off surface contaminates. Then loss of performance “signal” when they need the spray foam and tooth brush cleaning.
Woodgas, make it out of SS if you have a lathe or can get the lathe work done if you want to craft optimize it.
Can do jim-cracky with hand-fitted honed brass piping, 1/4" brass rod, and cut edge fitted brass sheeting.

Steve Unruh

I remember Wayne saying he considers these butterfly valves as dampers not valves. That says to me that a lathe cut tight fit is probably not necessary. Now I am confused. Arvid did say that his woodgas throttle on the carb he built was not a real close tolerance and that caused that tractor engine to rev up a “million” rpm. My original post was more about air mixing valves than throttles. Maybe it is just the throttles that need close tolerances.

Engine does have to be precision Don or engine speed will be all over the place, but the air and wg valves can be “so so”, on the caddy I had air and wg the same size at first but made air adj to sensitive, made fresh air one about half of wg and was much easier to get that “just right” setting, now I can " set it and forget it " !!!

not a butterfly valve…

Thank you, Steve!

Still, when not explicit enough, I have caused one more confusion, that I am going to try to rectify, wish me luck!


Hi, Don!

It seems that my description of feeding air and gas into the intake manifold has failed.

  1. In the first chapter (before the first point-line…) there is an oversite of the relation between air- and gasflow as they arrive into a T- or Y-piece, and then together pass a single COMMON throttle into the intake manifold.
    Further more, pinpointing that the pressure-drop for these two gases, when they flow into the T- or Y-piece is relatively low (about 1’ of water), and only occasionally their flow is in balance mutually.

Below the common throttle the vacuum can vary between 1’ and 19’ of water, depending on loading.

  1. In the second chapter (after the first point-line…) is a suggestion to UTILIZE the available generally higher vacuum in the manifold, UNDER the sigle throttle, but now as TWIN-THROTTLES, one for air, the other one for gas! (ganged together)
    This is to make the effect of the arriving gas pressure variations as minimal as possible. (compared to the manifold vacuum)

Hopefully the rest is now a bit more understandable?


Thank you Max! Now it is all clear to me.Doing it that way you DO need tight fitting valves.

When using a single common throttle after a “T-mixer”, the opening of the silo lid has usually a remarkable effect on the idle RPM.
The small pressure drop over the nozzles is eliminated, and that being about half of the gasifier’s total idle resistance is enough to upset the balance against the air mixing flap flow!

At the same time the manifold vacuum under the common throttle can be more than 10’ water.

It is quite obvious, that the twin-throttles with the full manifold idle vacuum over them is affected very little by a gasline pressure variation of one or two INCHES of water!

That is the starting point. A twin throttle dispensing is practically unaffected by “shortcutting” the nozzle resistance when opening the silo lid at “bunkering”…

At full throttles, the “extra” air flap takes care of the mixing balance.


Hi, Arvid!

Used those, bigger format, in Germany 2004. They were specified for 30Bar, and were extremely hard to screw up!


Hi Max,

Last question! Are you putting the dual throttle method on a multi port fuel injected engine that can also run on gasoline? If so - how do you do that?


Hi, Don!

To be the least disturbing for the gasoline system, I would feed the ready suspensed and well-mixed air - woodgas mix in through a “banjo” under the current throttle body. Needed height ~1,5".

The rest of the woodgas items have to be “off side”, where enough room can be found.
To avoid leaning or enriching the used system, both systems have to be “free of” idle slotting. Idle setting through a mechanical system which chooses stepless ratio of the wanted fuel(s).


It seems to me that max’s dissertation is mostly academic in this context. The real question is ease of operation of the air fuel mixer. The way our brains and motor skills work, a more or less linear response is much easier to cope with.
I can think of a few ways to accomplish this. Steve mentions that the lever angle on the butterfly has an effect. Which is why most modern vehicles use a quadrant with the cable pulling around it. This gives a constant shaft movement, but still the flow is not linear. It seems like you could use a funny shaped progressive Nautilus like quadrant to get closer to a linear response.
Another possibility is a cone shaped plug sliding into a circular orifice
On my lawn tractor project. I used a sleeve with pie shaped holes sliding around a drum. It seems to work pretty well, but I don’t know how well it would move with a cable.
While we are thinking mixture control, I know some are experimenting with using a gooseberry pie to control it automatically. I don’t know anything about electronics and have little interest in learning it. If I were going to automate the mixture control I think I would use a vacuum diaphragm. on one side gassifer vacuum, and on the other side air intake vacuum. link diaphragm to air mixer of choice.

Hey AndrewW
No, Max is speaking from lots of hours and systems using/working.
He is trying to help other evolve pressure/mechanical self-banacing flow systems as he has seen and used in real life.
I like this much better for woodgas fueled engine controlling DYI than a much less see-able digital electronic approach.

Of course a human active in the control loop will always be the simplist, most direct approach.
Some joke this human then is the trunk-monkey, woodslave, and such. Good jokes actually. I do laugh too.

Other of us like me and WayneK would call this active human in the control loop an Operator.
Like the difference between a gal who can make and bake a pie versus shop and oven pop one.
The one has love and celebrates the human spirit. The other . . . still, just demeaned to be a mere consumer.
Steve Unruh

I was thinking along the lines of a Mikuni slide bore carb either round or flat along with a butterfly for a pilot or ruff adjustment and the slide for fine tuning. Would this not work well?

Jim L. I don’t have any experience with those, but I’ve seen them. It does seem like a workable idea, but would be hard to manufacture from junk as most of us are doing. Surely if you have one to sacrifice, it could be adapted.
Steve, I am always honored when you deem my thoughts worthy of your response. It was not my purpose to minimize max’s input. My thinking was that the original question was less technical in nature and more about ease of adjustment. Perhaps I didn’t fully comprehend the direction on his comments.