GEK Wiki / How to Make a Woodgas Carburetor
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How to Make a Woodgas Carburetor

Page history last edited by jim mason 10 years, 11 months ago

return to How to Build and Run the GEK Gasifier 


Here's a very simple method to make your first woodgas carburetor.  More elaborate methods are always possible, but this will get you started quick and easy.  The goal is simply to create a tee in the intake to the native engine carburetor, where you can vary the syngas/air mixture on the way to the engine.


All that is needed to do this is one variable valve on the air side of the tee.  You do not need two valves.  As you choke down on the air side, the engine will pull proportionally more syngas from the gasifier.     You have to manually adjust the ball valve to get the correct mixture, and continue to adjust it as the engine load changes.  It can be touchy, but you'll get the feel for it quickly. 


I find the easiest way to do the start is to fill the open side with starter fluid, pull the cord, then fiddle with the valve as things choke to life.






The more fancy way to do the same thing is with a purpose made butterfly and servo to control it.  This can then be controlled electronically, in relation to air/fuel mixture info fedback from the O2 sensor on the engine.  This is how we did this in the Honda Accord.  You could also start do the same with an obtainium throttle body from a fuel injection engine.









And yes, we covered the hole in the middle of the butterfly later.  The butterfly was made from a fender washer, thus the hole needing covering.


More pictures of the Honda installation are in the Flickr set here:



How to size the air-gas mixer


(forum note from december, 2010 on sizing)


we're currently going round and round trying to get custom manufactured a small electronic throttle body for the automated mixing system on the power pallet.  we currently make these ourselves using the method here: [url][/url]  while these work, a proper efi drive-by-wire throttle body would be better.  they are sadly very rare at small sizes, thus we need to get it custom manufactured for us.

while doing this, i've reviewed what we've learned about air-syngas mixer sizing, and how this relates to the existing speed throttle already on the engine.  i've been surprised to find we've ended up with essentially the same size for both.

i took apart the kubota throttle body tonight and found it to be a 26mm bore diameter.  the 1" npt butterfly valve on the shop made servo valve is basically identical.  chosing between, 1/2, 3/4, 1, and 1.25" npt tube/valve options, we found the 1" one to give us the best mixing range and butterfly movement for this engine size.  

at first i thought this odd and that we were trying to oversize the air throttle body.  the air throttle body is only seeing about half the volume of the engine throttle body, given the 1:1 syngas/fuel mix ratio.

but then i realized that the engine throttle and air mix throttles are working against very different vac sources.  the engine throttle is controlling about 0-10"hg.  the air mix is controlling 0-10"h2o.  given that each inch of hg is about 14 inches of water, we have over an order of magnitude difference in force the two butterflies are needing to contend with.

calculating with some handwaving and anecdotal experience, it seems factors balance out so that about the same size of mixer is needed for these two very different tasks.

maybe this is a new rule of thumb: size your air mixer throttle/valve at the same diameter as the carburetor bore standard with your engine.

how does this relate to what others have experienced in sizing the mixer valve?






Comments (4)

Carlo Guerra said

at 6:07 am on Jul 29, 2009

Dear Sir,
in the above explanation, the simplest carburetor, it is mentioned an "open side" for filling up with starting fluid. What is this open side? Do you refer to the air intake of the tee ? Or is it part of the engine itself?
What is the starting fluid? Is it gasoline ?
thank you
Carlo Guerra

Andrew Schofield said

at 2:49 am on Jul 30, 2009

Dear Carlo,

You are correct. The open side is the air-inlet. You may have success using a small amount of gasoline (gasolina), for starting , however not too much.
Be very careful to prevent fire.


Andrew Schofield said

at 4:30 am on Jul 30, 2009

Hello Jim,

Woodgas, and air have nearly the same mass per unit volume, however our woodgas is at a lower pressure at the inlet of the mixing system. This is because woodgas must be drawn through the gas-generator system, which has variable pressure drop across the generator system, as conditions change over time.

Generally,on mass basis, the correct mixture at the engine throttle-plate, is one part woodgas, and one part air. This is why we all use a valve to control air mass to the mixer.
A certain type of valve, is superior to the ball or butterfly types in this service, I have found. Using this type, eliminates the "It can be touchy " problem.

Multi-cylinder engines have smoother mixture flow to begin with, compared to single cylinder engines which have more pulsating flow. Discounting this difference, and assuming a given smoothness of the total mass flow, I note the hose on the single cylinder engine has woodgas entering on the run of the pipe tee.
Conversely, on the Honda automobile engine, the woodgas enters the mixing tee on the side of the tee shown in the picture.
The mixing system you assembled for the Honda, is superior mainly with respect to blending woodgas, with air.

Smoothly blending large amounts of two gases, can be explained by an analogy of two streams of cars merging tangently on the freeway, rather than head-on.

Have you assembled the tee on the single-cylinder engine in any other way than shown?

Andrew Schofield
GL Renewable Fuel Systems

Andrew Schofield said

at 3:19 am on Aug 22, 2009

Hi GEKos,

A friend of mine who lives near London, now has a woodgas carby I made for him, to test woodgas in a single cylinder diesel engine.
Liquid-fuel will only be used to ignite the woodgas mixture, and woodgas from his GEK will supply the lion's share of the energy to generate electric power.

Andrew Schofield

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