Mery Explosive Engine

[Chipmaster]

Hi MJM,
 
here are pictures of my regulator followed by a picture of another regulator that I intend to put in the line to the engine.

IMG_2980 by Andy, on Flickr

IMG_2981 by Andy, on Flickr

IMG_2982 by Andy, on Flickr

Andy

[MJM460]

Hi Andy, I like that first regulator with its inlet and outlet pressure gauges.  I assume that is the one you said seems to work well down to 0.5 psi.

The second one, if I am reading the label correctly, is intended to operate with 0.5 psi inlet and 3 to 6 inches of water outlet pressure, which seems in the ball park for the engine inlet, especially with a needle valve to allow fine tuning of the flow rate at the engine inlet.  Others will have a better idea than me of the pressure which has been demonstrated to work well.  The low inlet pressure means the valve will be further open, and the flow controlled by the pressure difference rather than sonic flow, so  can be expected to be more reliable with the low outlet pressure.

Remember that regulator will supply gas whether the engine is running or not, so you will need a quarter turn shut off valve close to the engine to shut off gas positively when it is not required.  This will require your attention as you don’t want to have the engine surrounded by a gas cloud when you try and start it.  Similarly, shut it off as soon as the engine stops and have good ventilation to clear the air before another start.   (A bit like your gas bar-b-Que)

There are regulators which operate as a demand valve, which only supply gas when the engine inlet pressure is below atmospheric.  This would be preferable if it is compatible with your engine.  But if you have that regulator already, it is worth a try with the use of a shut off valve while you find the fuel pressure that suits your engine.

I want to write more about the principles that dictate the two stage arrangement, but it’s been one of those days.  If it’s of interest, I will try and explain over the weekend.

MJM460

[Dave Otto]

I agree with MJM460, trying to flow the proper amount of fuel to the engine using regulators seems to me to be an exercise in frustration. Not saying that it won't work, just would not me my choice. What I have used quite successfully on my engines and is what Max used on his Mery is a demand valve built from the plans attached below. A simple BBQ regulator that screws onto a disposable propane canister is all that is needed to supply the fuel to the demand valve. It really ends up being a set it and forget operation, the only thing that one needs to do is adjust the needle valve on the engine, and you don't need to worry about flooding the engine when it is not running. It is easy to prime if desired by just depressing the diaphragm through the small vent hole in the cover. I drew these plans up in Auto CAD years ago from some sketches, this design has been around the model engineering community for many years and can be adapted to what materials you have on hand.

I do also have Alibre solids is anyone wants them. I think the Tecumseh parts could be sourced in the UK, they are pretty common.

Dave

[Chipmaster]

Thank you very much MJM460 for taking the time to reply, your input is helpful and I look forward to your contributions. I haven't found any articles that deal with the subject of supplying gas to model i/c engines.
I take your point about the risk of having a constant gas supply whether the engine is running or not so I am careful to turn the gas off at the cylinder especially as I'm working in my garage. If the weather was better I'd be doing this outside.

My first attempts at starting the Mery using my home made 'Jerry Howell' demand valve were unsuccessful. After removing the demand valve the engine ran three times over a couple of days then no more, just bangs and chuffs.

An identical demand valve fitted to my Alyn Foundry Retlas fed by a Cadac 35 mbar regulator on a Colman C500 cartridge has been perfectly reliable for several years. No needle jet just a mechanically operated inlet valve and the Retlas is always easy to start.

Building Retlas 1026 by Andy, on Flickr

Another variation is my Alyn Foundry Allman engine fed by a Cadac 35 mbar regulator on a Colman C500 cartridge which is a good starter. The Allman admits gas through a tiny hole in its atmospheric inlet valve seat and it has an in-line commercially made needle valve.

 Inlet valve block viewed from the top. The tiny hole is the gas port covered by the inlet valve seat. by Andy, on Flickr
IMG_2633 by Andy, on Flickr

Also thanks Dave, you posted while I was drafting my reply. Its reassuring to know that Max is using the same demand valve as me. The regulator he uses on the Colman canister appears to be adjustable. They aren't sold over here as far as I'm aware.

It's so frustrating after the initial jubilation when it ran which it seems was a fluke.

Note to self - stay patient.

Andy
 

[Dave Otto]

Yes kind of like the one in the photo here.
https://www.longtimeregulator.com/gas-regulator/disposable-propane-tank-regulator.html
Just a low medium high setting, and you can find them with a NPT outlet so you can connect it to your tubing.
I was going to post yesterday but thought that it might be better if I just stay out of it. I thought the drawing of the demand valve might be of interest.

Dave
 

[MJM460]

Hi Dave, glad you joined in, your contribution is most welcome and much appreciated.  And thank you for the demand valve drawing.  I was planning mostly only to address the regulator issues in the gas supply, which of course is only a small part of what is required to get a recalcitrant engine running.  It’s good to confirm that the design intent was originally to use a demand valve.

Hi Andy, my intent was to comment on why the two stage regulation is necessary, but Dave’s comment has confirmed that your original plan to use a demand valve instead of that low pressure regulator is the preferred way to go.  So the question is why did the engine only start after you removed it?

That the engine ran, even if only for a short time suggests everything was properly made and adjusted at the start of that run.  That it would not run again suggests that something changed during that run, some component failed, or moved or was damaged during that first run. 

The fact that it did run after the demand valve was removed, suggests a problem with the demand valve, perhaps corrosion or dirt on the valve seat or diaphragm sealing issues.  None of the above is very specific, but might provide a focus for your continuing investigation.

But that regulator - as the outlet set pressure is adjusted downwards, the gas velocity between the valve and its seat increases.  When the absolute pressure setting is approximately half the bottle pressure, the velocity reaches sonic velocity.  When sonic velocity occurs, the downstream pressure no longer affects the flow, so the valve operates with that nearly closed position at any set pressure below approximately half the bottle pressure.   With such high velocity, the regulator valve is very nearly shut at the flow required by a small engine.  The force to control the valve comes from the outlet pressure acting on the diaphragm to close the valve, while the bottle pressure acting over the seat area is tending to open the valve.  When the outlet pressure is set very low, it is not able to reliably control the valve.

The propane gas pressure in your gas bottle varies from about 15 bar (absolute) at 40 deg C down to about 2 bar (absolute) at -25 deg C.  At a more reasonable 15 deg C, the bottle pressure is around 7 bar (absolute) and sonic velocity occurs around 3.5 bar (absolute), still way too high for the engine inlet.  Hence the need for a second regulator.

The second regulator works reasonably well for reasonably steady flow, but for an engine, the flow is not steady, particularly during start and stopping.

Starting is particularly critical, as the flammable range for propane is only 2.1 - 9.5 % by volume.  It is very easy to make the mixture too rich before the engine starts.  It is very important to have the fuel shut off until the engine is turning, which is where the demand valve comes in.  And of course you still have to adjust the needle valve if you have one on the inlet.

So you have the last word, the requirement is patience as you go through your investigation ( and please let me know if you found a source where it is plentiful).  The engine ran once, it will run again.

I hope a little theory is not too heavy or boring and makes a little contribution to getting your engine running.

MJM460