Vacuum Automixer project

Projects
#41

Hi Tom,

The 4th opening is only used for gasoline running, and has the Volvo sensors etc on it. It is closed off for woodgas.

You’ll notice it over time. Woodgas is very tolerant of different fuel air ratios and you’re right once its set there’s no reason to mess with it - except when it gives you trouble. A short list of problems I run into that make me touch the valve:

  • Cold startup on woodgas, all over the place, no reference point
  • Hot restart on woodgas, usually too rich then very lean
  • Heavy acceleration, too lean
  • stopping quickly after heavy acceleration, VERY rich almost to stall (sometimes)
  • accelerating off a stop, too lean (sometimes)

‘Sometimes’ means that when everything is *just right…*I don’t have to worry about it. But when the gasifier gets a little off, char bed tight, loose etc. these problems get worse.

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#42

I’ll say it again, I know it eluded me for awhile… this material should NOT be stretchy / elastic. Instead it must move to any position within its travel and stay without any centering force. This is important because the balance point may be found in some place other than dead center on the diaphragm travel… and once it is found, there shouldn’t be any force moving it away from there.

The plastic sheet I used is not stretchy, just flexible. I intentionally put it on loose to give it some travel, it moves about 1.25".

Denim would have worked but it has inherent weakness in the fabric. Plus by the time it was siliconed it was about a quarter inch thick, too stiff to respond quickly to small inputs.

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#43

Denim wasn’t going to work, because of the inherent weakness in the fabric. polyester would work a -lot- better.

It also shouldn’t have been 1/4". You want to press the silicone into the fabric like with a roller or just a pipe on a flat surface.

And you still want the right shape. the curves are the slack, but the force is uniform.
They are usually molded to the proper shape. You can do that with silicon fairly easily. But for 15 bucks, it is hardly worth your time.

Diaphragm actuators use a spring, or you can adjust the linkage. With a flat piece, you have uneven pressure.

What you are trying to make is a pneumatic double acting actuator. Most of those use pistons.

Quite honestly to -me-, it makes more sense to use a MAP sensors and then just program around it. But I have a computer background, not a fab background.

#44

Good luck Chris, I have no understanding of what you are doing, but if anybody can figure it out, I believe you can.

#45

Very interesting Chris, just a few things popped in my head when reading this; power brake booster, or an older vacuum operated cruise control pot. Don’t know if these would work just throwing it out there.Al

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#46

[quote=“gasman, post:38, topic:1778, full:true”]
Hi! I think setting the gasmixing functions in 3 stages will bring a bit more clarity in what is happening where, and what the aim is at each stage… [/quote]

Air from the air filter and gas from the “gasplant” approach the motor in their own equal tubes.

In the airtube there is a “perfectly” symmetrical and tight airflap, immune to "flowpower!

A membrane with ~80% of its diameter reinforced with (aluminium plates)
making a light and stable “piston” to control the airflap.

The membrane material shall exercise NO “spring” action on the movement.

The airflap has to do full movement between open and closed within the full movement of the membrane “piston”. No favorite position given.

The ONLY aim with the membrane is to keep the air (under)pressure on the same level as the arriving gaspressure, when there is some gasflow.

The membrane has nothing to do with DISPENSING air/gas in the right proportion! Only delivering air and gas at the same pressure for LATER DISPENSING by other means.

“By other means” is i.e. a twin throat carburettor body with the acceleration nozzles removed = straight throats.

If throat 1 and throat 2 have different closing angles, get troat 1 flaps from two identical carburettors.

Then gang them with a “paralellogram-beam”, so their movements stay identical!

That takes care of IDENTICAL DISPENSING of gas and air.

The same can be applied to a 4-throat carburettor with identical throats.

Now, at last follows “gasmixing”! It has to be carried out with FULL SYMMETRY IN ALL DETAILS, or otherwise you will end up with the same misery as DJ in his pickup!

When the spiral mixing is done, then there is no more problem affecting the dispensing result…

Max

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#47

I’ve been thinking about this a fair bit, and I’ve come to the conclusion that DJ’s two separate diaphragms are a better idea. He would have full atmospheric pressure on one side balancing the few inches of WC. With a single diaphragm, you would have few inches on one side and ever so slightly different few inches on the other. such a tiny pressure difference would give very tiny force to the valve.
Am I thinking right on this?

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#48

Or would the two atmospheric forces on each diaphragm cancel each other out?

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#49

I think an arduino with pressure sensors and a servo could do this but I’ve decided to try this version first too. I don’t have the computer background.

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#50

I appreciate that you are using what’s available. It’s keeping Wayne’s building practices and more conducive for the majority on the site.
Thanks Chris and Marvin for pioneering this on DOW.

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#51

Ok, I’ve been reading and thinking some more and these are the figures I come up with. Using an 8" diaphragm, I get roughly 50 square inches of surface. Since the difference in pressures is what gives the force, If a have full atmospheric pressure on one side and 15 WC on the other,I have a force of about 27 pounds. Same setup with only 13" WC gives me 23 pounds of force Now if I use 15" WC on one side and 13" WC on the other side I get 4 pounds of force. Either way I get a net gain of 4 pounds of force. It’s just a matter of where the reduction of force takes place. With one diaphragm, it occurs right there at the diaphragm, so the friction of the pushrod and all linkages is working against 4 pounds force. with the two separate diaphragms I have 23 pounds pulling on one end of the lever and 27 pounds on the other end. the friction matters little in those force ranges.

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#52

Yes it does matter. The low differential in pressure is the only thing that will move the flap, and friction reduces the available force.

If you have two equally strong men pulling tug of war on a pole, it will be easy to push it one way or the other by helping one of them. And someone in the middle who grabs the pole (friction) can stop your feeble efforts just the same as if the strong men weren’t there at all.

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#53

I don’t know how to do the math, but if you have 23 pounds on one side pulling against 27 pounds on the other side do you still wind up with 4 pounds of force?

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#54

Andy, I’m not following your math. I admit math is not one of my strong points but the way I look at it at first glance, atmospheric pressure at sea level is 14.7 pounds per square inch pushing one side of the diaphragm, and 15 pounds on the other side, it looks to me that you get point 3 pounds of force moving the diaphragm. Am I thinking straight or not? Is point 3 lbs times fifty square inches 4 lbs?

#55

Here’s the math as I see it. Max says this thing needs to operate smoothly with around 0.5" difference in water column.

There’s 27.6" WC in a psi. 0.5" WC is 0.018 psi. There are 63.62 square inches in a 9" circle.

63.62 x 0.018 = 1.15 lbs total pressure (18.3 oz). One pound of force has to get the job done.

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#56

Where did that come from? He said 15 inches water, vs 13 on the other side… total difference of 2".

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#57

Yup, My bad. You are right again.

#58

No, Don, not 15 pounds on the other, but 15 inches water column, which is 54/100 psi So 14.7 psi positive pressure on one side and 14.7 pos minus point.54 neg gives 14.16 psi on the other side. So net gain of .54 difference x 50 = 27 Now the 4 pounds comes from 15inches WC @ .54,psi on one side of diaphragm minus 13 Inches WC @ .46 psi on the other side, = .08 x50 =4

#59

Ok, so I was just guessing at the vacuum at that point in the system and the difference also, but the equation is the same. the net gain of force at the air damper is the same either way. Using chris’ analogy. if you had two equally weak little girls pulling on the pole it would still be balanced. But if instead of pulling directly on the pole, they were using a rope which was wrapped around a fence post, the little girls couldn’t overcome the friction the rope to even pull the pole, but the strong men could.
At any rate, it’s a great idea and I’ll plan to use it when I get that far.

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#60

All Max was trying to do was to keep the pressure of the gas going to the throttle, and air at the same pressure, so they are drawn into the intake manifold at the same rate to get a consistent mixture.

The gas side seems like it would always have more drag then the air side. So does it just run a butterfly on intake before the carb or does it run one on the gas side too?