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fuel mixture issues. stuart 400 replica build.

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miss_emma_jade

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hi guys and girls. this might be a bit long, but it's my first post, so bear with me. usually, I'm just a lurker here.
a while back (two years ago) I found a copy of Henry Greenly's "model engineering" text on eBay. my copy was printed in 1931, and it was first published in 1915. inside was about 10 pages on building the Stuart models "400" engine kit, from long out of production commercial castings. A friend in the UK at that point published some videos on repairing one of these little horizontal engines, so after thinking about it for 5 mins, I made a start. made patterns at 3/4 scale, and got another friend in the USA to cast a set in iron.
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miss_emma_jade

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anyway, I went ahead and started machining the bits, flywheel from martin model, a base casting I cast here in aluminium (boxed, start calls it) and a couple of
years later it's starting to look like an engine.

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miss_emma_jade

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anyway, I went ahead and pretty much finished it to the drawings.

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miss_emma_jade

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However, like nearly all 1920s era gas engines, there is zero information on the fuel system. I have good compression, valve timing, (best guess) and spark timing. the fuel mixer system is probably on paper, but I've never seen one running on Gas on the internet and only a couple of survivors running on petrol.

The head is fairly basic, with big valves and not much lift. Compression is about 6:1 ratio (calculated) and pressure on a tester gauge is about 50 psi or a little more.

spark is by timer and buzz coil. the first picture here is the x section of the cylinder head, (from the original drawings in full size)
the second picture is the manifold under the head and the steel ball to stop gas leaking out of the chamber. fuel (I'm trying with propane is just fed into the side via a tap. not surprisingly it doesn't work. the second tap below, in the third pic is just water jacket drain.



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miss_emma_jade

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anyway, I have a propane regulator and needle valve, which has little control and seems to deliver down to about 6" or water pressure. I feel like this is way too much.
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miss_emma_jade

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so, it fires and hits next compression and bounces at this point. I think maybe it's just fuel /air mixture, but I have no control over what goes in the cylinder.
the big question, is, what would you all do? should I give up on gas and fit a bubble or surface carb? or should i keep looking for better gas control and fit a carb with a needle for propane? someone here will know. unfortunately, there's not much written or published about this engine anywhere. the few pictures I've found on the net are mostly carburettor and spark, so I'm very glad I didn't make it the original hot tube ignition. would appreciate some feedback.
 

Jasonb

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I've made a couple of hot tube engines which is what this engine originally use and they need a VERY small amount of propane. Is that pure propane or a Butane mix?

A good way to check that you have the right mixture is to leave the ignition turned off and then turn the engine over slowly by hand while holding a lighter or candle about 4" from the exhaust which will ignite the air/fuel mix as it is expelled. You are looking for a quick hard blue flame, if you get a soft orange one then you are running to rich

Both engines also close off the gas supply when the inlet valve is closed and uncover a gas port as the valve gets sucked open on the intake stroke and have fine needle valves to adjust gas flow. You can see the needle valve just after the burner tees off on this one


And I have a pair of valves on this one for engine and burner, finer needle on the engine one.


 

miss_emma_jade

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I've made a couple of hot tube engines which is what this engine originally use and they need a VERY small amount of propane. Is that pure propane or a Butane mix?

A good way to check that you have the right mixture is to leave the ignition turned off and then turn the engine over slowly by hand while holding a lighter or candle about 4" from the exhaust which will ignite the air/fuel mix as it is expelled. You are looking for a quick hard blue flame, if you get a soft orange one then you are running to rich

Both engines also close off the gas supply when the inlet valve is closed and uncover a gas port as the valve gets sucked open on the intake stroke and have fine needle valves to adjust gas flow. You can see the needle valve just after the burner tees off on this one


And I have a pair of valves on this one for engine and burner, finer needle on the engine one.


that's pretty helpful. off to try that now. looks like I might need a needle valve. id have loved hot tube, but I'm sort of glad at the moment I didn't go there this time. they're beautiful engines. thankyou
 

dsage

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miss_emma_jade:
>> anyway, I have a propane regulator and needle valve, which has little control and seems to deliver down to about 6" or water pressure. I feel like this is way too much.

First off - can someone clue me in on how to use the quote feature on a previous message instead of doing what I did above - thanks??


It isn't clear to me where you had the needle valve. Maybe you can elaborate. Barbecue regulators are designed for very high flow at low pressure. Consider the volume of propane required to supply three burners on a barbecue. Yet maintaining low pressure. My Parcell and Weed engine has a 2-1/4" bore and longer stroke and it requires the volume from the regulator to be choked down considerably. Initially just as an experiment I fashioned a block with a hole through it and a couple of hose barbs on each end and a pointed 1/4-20 screw across the path so I could control the flow from zero to full. It took a lot of playing around but I finally found the setting that allowed the engine to run. I just kept the block in there rather than fussing with something more elegant. The screw was barely cracked open.
Don't forget propane has a VERY narrow mixture ratio that will ignite. It's pretty fussy.
The Parcell and Weed is a hit and miss so it runs at a constant speed. I'm not sure if it could be throttled with such a rudimentary fuel system.
As mentioned by another person a carbutetor will probably work but will be fussy and probably require modification to the needle valve. The Parcell and Weed does not have a carburetor. Instead it has a pretty elaborate intake valve that mixes air and propane right at the intake valve seat. When the valve is seated the propane is cut off.
All this to say you'll need to fiddle quite a bit to use propane. But you should be able to get there.
BTW I like the suggestion of the candle on the exhaust port. I'll have to try that.
Nice engines Jasonb.


Full scale Parcell and Weed. Has two 14" flywheels. Engine weighs 90 pounds.
Picture here:


See it running here:
[/QUOTE]
 
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miss_emma_jade

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miss_emma_jade:
>> anyway, I have a propane regulator and needle valve, which has little control and seems to deliver down to about 6" or water pressure. I feel like this is way too much.

First off - can someone clue me in on how to use the quote feature on a previous message instead of doing what I did above - thanks??


It isn't clear to me where you had the needle valve. Ma
[/QUOTE]
Im a beginner here with the quote thing too.

the needle valve is on the bottle though, here's a better picture. I may make a limiting valve as you suggest. I had hoped the tapered plug cock on the line would do the job.
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Bentwings

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so, it fires and hits next compression and bounces at this point. I think maybe it's just fuel /air mixture, but I have no control over what goes in the cylinder.
the big question, is, what would you all do? should I give up on gas and fit a bubble or surface carb? or should i keep looking for better gas control and fit a carb with a needle for propane? someone here will know. unfortunately, there's not much written or published about this engine anywhere. the few pictures I've found on the net are mostly carburettor and spark, so I'm very glad I didn't make it the original hot tube ignition. would appreciate some feedback.
I thing you would be best going to a small engine carb. Tilotson or walboro. Most carbs operate on 4-5 psi max. Small engines even less. These are float carbs. To me it sounds like you have too much spark advance.even our big v8 have pretty short advance that is automatically advanced as speed picks up. Many small engines run about 28 deg total at speed but 5-10 at startup.
 

Tim Wescott

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Forgive me if this is already obvious to you, but a key thing that I think is missing from your scheme is that you need to regulate the amount of propane vs. the amount of air going in. Since the air going in is variable, you need some means of not dispensing propane when the the engine is not drawing in air.

Hence, the Parcell & Weed* engine has a nifty arrangement of holes in the intake valve seat, to dispense gas only when there's intake air. And someone mentioned turning off the gas when the intake is closed. And someone else mentioned the fact that propane carburetors have a demand valve in them.

I think that anything that involves a needle valve and any significant gas pressure, that's not backed up by an intake-actuated valve, is going to fall somewhere between doomed to failure and persnickety to operate. Anything that delivers a constant flow of gas regardless of the amount of air going into the engine is going to be too rich when the engine is lightly loaded, and too lean when it is heavily loaded.

Taking the Parcell & Weed engine as an example, "significant pressure" means something like more than one half or one PSI -- if you get that book you'll see that their carburetor and their gas generator are both operating at atmospheric pressure, depending on the suction of the engine to draw fuel into the thing.

I think if I were you, now, I'd whip up a simple venturi with a needle-valve to regulate, similar to a model-airplane setup. I think I'd fuel the engine with methanol or methanol/ethenol (because I'm pretty sure that those fuels tolerate a wide range of mixtures), and I'd see if the engine can be made to work. Or I'd just get an eyedropper of fuel (methanol or even gasoline) and squirt just the right amount in after every power stroke (fuel injection!). Then once I had confidence in the engine, I'd go back to getting it to work with propane.

* I want to build one of these. I have the book, but not the time!
 

Jasonb

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As has been said the propane needs to be shut off except for during the intake stroke as unlike petrol being sucked in by a carbs venturi it will continue to flow all the time.

This is the intake block and valve for the Robinson in my video above, the ring of larger holes are for the air and the tiny hole to the right is the one for the propane, all are uncovered as the atmospheric valve is opened



The Allman on the other hand draws it's air up from the bottom of the bottom of the valve chamber but the 45deg seat for the valve is drilled for the propane so is again only uncovered as the valve lifts





Any needle valve should be placed as close to the engine as possible which will help restrict the amount of propane to what is in teh short distance of pipework between valve and cylinder.
 

Tim Wescott

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@Jasonb, I find it interesting just how small the propane outlet is on your engines compared to the Parcell & Weed engine, which uses a 5/8" bore in the inlet valve body and nine holes drilled with a #45 bit (so, around .082"). That's a much higher ratio of gas feed area to inlet air than your propane engines are using.

But! The Parcell & Weed engine is designed for town gas -- and while I have no idea of how that burns compared to propane, I suspect that you just need more of it than propane in an equivalent engine.
 

Steamchick

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Remember, Town gas typically was at 4 inches water gauge. I. E. 12 1/2 millibars. Propane regulators for domestic appliances are at a much higher pressure (maybe 30mbar?), so check yours and convert your design accordingly. You need the same MASS of fuel - putting it simply. But because of the carbon-hydrogen ratio of molecules and mixtures, even that isn't perfect for a crude comparison. Find some tables of calorific values of fuel and required air volumes. It is complex, but not difficult for an engineer, even though it is "chemistry". You can't effectively play with this and get it right except by chance, like medieval scientists.... We have a wealth of knowledge - so look it up on the web and learn, and you'll get a working solution much quicker.
E,g. BUTANE - with a bigger molecule than Propane - has 20% more energy per molecule.. so for the same jet at 15 psi for butane, you need 18 psi for propane. ( I think!). Town gas has a mixture of Hydrogen and Carbon monoxide - basically what used to be called Water Gas. The Hydrogen will "explode" or flash very easily, but the CO needs to be over 350deg C to burn. In a combustion engine cylinder it would ignite (hydrogen) very easily and that would raise the temperature so the CO would burn. Propane is like a heavier fuel without the hydrogen to light it. You could try acetylene? But that is horribly exlosive! I tried acetylene from a gas generator (using Carbide and water), and set fire to everything (on the lawn) when it flashed-back..
Take care
K
 

LAS

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anyway, I have a propane regulator and needle valve, which has little control and seems to deliver down to about 6" or water pressure. I feel like this is way too much.
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I have a "Brunner" hit and miss model I built using Brunner air cooled air compressor as the starting point. The bore is approx 2". It runs nicely on propane. Here are the dimensions for the mixer I finally settled with; air intake ID is 0.562" with a venture drilled using a #23 drill. I also have a needle valve (shop machined) on the mixer to control the amount of propane. The mixer is fed from a demand valve (Tecumseh carb parts, plans available on internet. The demand valve is then connected to the regulator. The regulator has a 3 position valve (low, medium, high) from a camp stove which attaches to a 16 oz propane tank. The regulator settings translate to 5" water, 10" water, & 15" water approx. I start at the high setting when cold starting and after a little heat is in the engine reduce to medium setting and then the low setting. I hope this is of some help.
 
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