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Old 13-10-14, 12:43 PM   #11
martinpaff
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That makes sense. Not only do you have a "controlled" input pressure, but the pressure differential across the demand valve is far lower and therefore easier to control.

I have seen larger engines running on this set-up; no reason why not as long as there is adequate gas flow capacity?

I'll see if I can do some diagrams to help with the understanding of how this works...

MP
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Old 13-10-14, 01:23 PM   #12
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If you can find a vapourizer/demand valve from a scrap fork truck, (or LPG car) it would probably be ideal.
It will also operate from full cylinder pressure.
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Old 13-10-14, 01:29 PM   #13
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I intend to use a variable pressure regulator off the bottle for my engine gas supply, then fit an in-line home made demand valve (as discussed here, and similar to Nick's example).

As my engine is only 1hp in size, I hope this set-up will be sufficient to get it up and running nicely. The engine has independent mechanically operated gas inlet and air + gas mixture inlet valves, so the demand valve might not be necessary - we will see.

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Originally Posted by martinpaff View Post

.....I'll see if I can do some diagrams to help with the understanding of how this works...

MP
Some diagrams would be very useful if you have time Martin

To put things into perspective here - I've just bought 2 Calor 766 regulators like the one I pictured at the start of this thread for 13 including postage. If I can convert these to operate as demand valves then I will be very happy with that
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Old 13-10-14, 01:32 PM   #14
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Okay; let's see if this makes sense...

This is a regulator;



The gas bottle pressure enters at the right (red). Its passage through the reg is controlled by the poppet valve, which in turn is controlled by the diaphram.

The difficult bit to get your head round is the the area of the diaphram is so much larger than the area of the valve, that a very small pressure can be used to control a much larger pressure.

As the pressure in the output rises (blue), the diaphram lifts against the spring and closes the poppet valve. As gas is used the blue pressure drops and the spring opens the valve. Simples!!!

The output pressure is set by the spring strength and is uneffected by the input pressure.

Now...



For those who may be unsure - a demand valve only supplies gas when the engines "sucks"; if the ouput is open to fresh air the gas flow stops. If the standard regulator is modified as follows, you can create a demand valve.

The steady, low (37mbar) input pressure is still controlled by the poppet valve, which is kept closed by the two springs. The springs are balanced, so that the valve will just stay closed and shut off the input pressure. When a demand is made on the output, the internal pressure drops below atmospheric, and the diaphram opens the valve. As soon as the demand stops, or the ouput rises to atmospheric, the springs close the valve. Also simples!!!

MP
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Old 13-10-14, 01:49 PM   #15
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Thanks Martin

I was struggling to get my head around the balanced springs, but it makes sense now. When the diaphragm is pulled down by the engine inlet suction, it causes an unbalance in the spring tensions and then allows the poppet valve to open. Very simple now I understand how it works

It obviously works well as I have seen several gas engines running with modified regulators like this.

I will keep you posted on how my regulator to demand valve conversion goes in due course
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Old 13-10-14, 02:03 PM   #16
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Out of interest, Kev, what did the previous owner(s) use?

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Old 13-10-14, 02:11 PM   #17
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Quote:
Originally Posted by martinpaff View Post
(snip) Okay; let's see if this makes sense...
Pretty much covers it except that I ditched the internal spring altogether - all depends what you have in the bit box I guess.

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Old 13-10-14, 03:22 PM   #18
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Quote:
Originally Posted by Dazzla View Post
Out of interest, Kev, what did the previous owner(s) use?

Dazzla
Hi Dazzla,

Since owning the National I have managed to trace its history back to approximately 1979 when it was first rescued for preservation in the Stamford area. I have now concluded that despite the engine having received some restoration work since then, and passed through two collectors hands, it has never been run since being in preservation.

So, when I get it running in the next few weeks it will be the first time it has run since well before 1979.

Consequently setting up the gas supply is going to be a little trial and error
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Old 13-10-14, 05:07 PM   #19
martinpaff
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Quote:
Originally Posted by Kevbuts View Post
...Consequently setting up the gas supply is going to be a little trial and error
This would be an extremely good reason to purchase a commercial demand valve. You'll remove one variable from the equation, and give yourself a fighting chance of success...

MP
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Old 13-10-14, 05:13 PM   #20
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Quote:
Originally Posted by martinpaff View Post
This would be an extremely good reason to purchase a commercial demand valve. You'll remove one variable from the equation, and give yourself a fighting chance of success...

MP
It would certainly also reduce the risk of a fire or explosion.
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