Not All Slide Valves Are Equal

[Troutsqueezer]

Or so my theory goes...

This is kind of a long boring post, my apologies.

Having just finished my Elmer's #45 single compound, I was confident it would run first time out. Too confident, it seems. It's not running at all and after almost 8 hours poring over the design, checking the valve travel, tweaking the eccentric and pressure and making a few other adjustments, I had to call it a day and think it over in the air conditioned house, with a coconut Rum and Coke in hand.

<Edit:> This cylinder is a single compound variety and as such, the High Pressure is fed below the piston and then cycled above the piston, then out to the exhaust hole.

It's generally assumed that with a slide valve, since it is always over the exhaust port, steam pressure will press it against the cylinder forming a sort of seal as long as the surfaces are finished properly. That is what I've been assuming but now I think there is another scenario and it may be causing my problem here. On my engine, I noted that when the valve was at the end of its travel (far left), the piston in midway position will shoot back to its lowest position, it is under pressure but it isn't supposed to be because its in the exhaust cycle. Observe the pic below. High pressure feeds the rear of the piston and moves it towards the top of the cylinder. For the area above the piston, this is the exhaust cycle. The valve is moving to the right during this power stroke, slowly covering up the high pressure inlet port. As this is happening, the valve begins to open the port to the other side (top) of the cylinder and ports the high pressure to that side. This forces the piston back down. The cycle repeats. What I am seeing is when the piston is midway and the valve is as far as it can go to the left, connecting the volume above the cylinder to the exhaust hole, the piston is shooting back to its bottom position as if there is pressure above the piston.

The way I see it, there is only one way this can happen or rather, only one way I can visualize why this might be happening. The valve is not being held against the cylinder, it is lifting and high pressure is finding its way to the exhaust ports. When the valve is all the way to left, there is existing pressure from the area above the piston pushing up against the slide valve and if that valve has play in the Z direction, it will lift and then here comes the high pressure from the steam chest cavity. I can hear and feel pressure escaping from the exhaust valve at this time.

At the very last I checked the slide valve. I pulled up on it and what do you know, it pops up about 20 thou. Tomorrow I will make a new nut for the valve and make sure it seats the valve against the cylinder with no play. I know I can't make it too tight so there's a balancing point there.

So there's my theory. Does it hold water? I'm pretty new to this steam engine stuff. :idea:

-Trout

 

[don-tucker]

From what I can see the Exhaust port should be in the middle,unless I'm missing something.
Don

 

[Blogwitch]

You are quite right Don.

The attached assy drawing shows how it should be, exhaust in the centre, top and bottom feed ports either side.

This is shown in the neutral position.


Bogs

 

[arnoldb]

Bogs & Don, for a normal double-acting cylinder you would both be correct, but the Elmer engine Trout has built here is a bit different in that it's supposed to be an expansion engine with the high pressure side on the "bottom" of the cylinder and the low-pressure side on the "top" - like in the picture posted by Trout.

Trout, a bit of a tricky problem... The valve shouldn't be lifting. If it is on its far left, are the port holes on the right completely uncovered, as at that point the high pressure side of the cylinder should be receiving full "steam" ? - remember, when the LP side on this engine is in exhaust mode, the HP side gets full steam, and when the HP side exhausts to the LP side, the actual exhaust port is actually closed.

Did you put on the valve the right way around - with the offset "hollow" on the HP side of the cylinder?

Edited to add:
What happens if you manually turn the engine more than mid-point so that the valve moves to the right - do you get air leaking from the exhaust?
It could be that the right-hand edge of your slide valve (between the pocket and the outside of the valve) is slightly thinner than the port holes in the valve plate, causing a moment of full air pressure going to the LP port from the steam chest "through" the HP ports. With the true exhaust port closed by the valve face, and slight leakage from the crank-end bush, it could also push the cylinder back down...

Regards, Arnold

 

[deverett]

Trout

You reckon that the valve is lifting off its seating face? I would have thought that the HP inlet pressure would be higher than the HP (and LP) exhaust pressure, thus keeping the valve seated.

As a 'bodge' fix, to keep the valve seated, could you make a Hairpin shaped flat spring seated on the back of the valve, pressing and sliding on the inside of the valve chest cover?

Normally, there is a small clearance under the valve nut to allow the valve to lift to prevent hydraulic lock when the cylinder is cold.

Dave
The Emerald Isle

 

[Cedge]

Trout
I own a Dual Cylinder Tangye engine that nearly drove me to drink for the same reason. The valve shoe was lifting off the surface and letting air flow freely to the exhaust. After many times taking it apart and putting it back together, I noticed that the slide valve rod was being forced in a lateral movement by a small guide mounted outside on the frame. It was creating a "lever" situation with the packing gland acting as a fulcrum. You might want to check the rod for proper alignment going through the gland.

I had to shim the small guides out to get the alignment right before the engine would run. Once things were traveling true, the engine ran like a top.

Steve

 

[gbritnell]

I can't say I've ever seen anything like this setup before.
Lets say we go with what Arnold has stated, it's an expansion engine. After carefully studying the sketch I finally figured out what the valve events were 'supposed to do'. The problem is it won't work, at least not efficiently.
As has been discussed in the past, expansion engines don't work on compressed air. There is no expansion to air.
On an expansion engine, double, triple etc. the high pressure cylinder is always driving the crankshaft and therefore the exhaust from that cylinder is being forced into the expansion cylinder. On this engine the only thing driving the exhaust flow is the inertia of the crankshaft. It might run on steam but I doubt it will run on air except at much higher pressures.
As far as the 'D' valve goes, it always has a greater pressure applied above it because of the surface area that is exposed to pressure. As long as the valve has a little float it will seal to the port face of the cylinder. That is why they came up with ways of 'balancing' the slide valve because there was so much more down pressure on the slide valve that it would lead to excessive wear and friction.
The adjusting block for the slide valve, the threaded part that rides in the cross slot in the valve, should have a little clearance up and down but none laterally or front to rear.
The bottom line is don't expect much from this engine while trying to operate it on air.
George

 

[Dan Rowe]

This is kind of a long boring post, my apologies.

What I am seeing is when the piston is midway and the valve is as far as it can go to the left, connecting the volume above the cylinder to the exhaust hole, the piston is shooting back to its bottom position as if there is pressure above the cylinder.

Trout,
Not boring at all, I had to read it several times to get what is going on but if the valve is lifting and the piston returning back to the HP end because the LP end acts on a larger piston area then one more thing should be happening and that is the exhaust line should have almost the same pressure as supply pressure.

I had never seen this arangement either thanks for posting the drawings.
Dan

 

[bearcar1]

I'm not so sure that engine will ever work on compressed air. In the example shown, the drawing on the left shows the two ports as being 'connected'. That is all well and good for the expansion of steam from one side of the cylinder to the other, but how is it compressed air is going to 'expand' in order to drive the second half of the power cycle? I don't know for certain. but it just seems wrong to me.

BC1
Jim

 

[Troutsqueezer]

Thanks guys. I know it took quite a bit of concentration on your part to analyze the situation so I appreciate all the replies very much.

Bogs and Don - I fully intended to highlight the fact that this cylinder was of a different construction from most but got so engrossed in the explanation it slipped right by me. Sorry.

Arnold - For the valve positioning over the port holes, I checked those darn things time and again all day. In the end I knew it was correct so had to move on to other theories. I did check the hollow on the valve for orientation and it was correct. I did see that it was .009 too shallow so I cut it deeper but that wasn't the problem. There is no exhaust leakage with the valve fully right but there is when it is fully left and I don't think there should be.

Dave - I discovered I made the nut too shallow, the height should be .125" and I have it at .112. I'm thinking I'll remake that today and see what happens. I'll keep the hairpin idea in mind too.

Steve - A good suggestion, I didn't check for lateral movement of the valve rod. I'll do that today.

George - I sure hope you are wrong on this one although knowing your expertise you've got me worried there. I would have to think that if this configuration doesn't work, old Elmer would not have included it in his book. His article states the one he built ran nicely on 10 lbs of air. I Googled this engine and there isn't much out there. There is one fella who built a 2x version and put it on YouTube. It ran nicely and I think he was running on air but can't be sure. It has occurred to me that maybe it's best to remake the cylinder with standard porting and move on. That's a lot of work though. I should mention that running the cycle of the piston over and over in my mind, oftentimes I too wonder how it works at all. I'm thinking the flywheel should have been made of a heavier metal than aluminum to carry the cycles over but Elmer said any metal was fine for that.

Dan - Thanks for taking the time to re-read it. From your statement I would gather the weight of the flywheel might play an important role if the pressures are equal, right?

Jim - I guess if this works, we'll all be surprised, huh? One thing, I'm getting good at visualizing port action vs. air flow...sorta.

I'll start with the new valve nut today, see if that makes a difference.

-Trout

 

[Dan Rowe]

Trout,
What I was trying to say is if the valve is lifting off the seat equal pressure will be on ALL of the passages which includes the HP, LP and exhaust.

You should be able to hear the pressure blowing from the exhaust as the piston moves from the middle in the wrong direction.

Dan

 

[Troutsqueezer]

I gotcha Dan. I'm heading out to the shed shortly. Right now, my brain's in a blender as to what all the symptoms are in the various cycles. I'll take better notes this time as I go along.

-T

 

[arnoldb]

There is no exhaust leakage with the valve fully right but there is when it is fully left and I don't think there should be.

That means the valve most likely seals properly on the left as well. It is possible that air could be leaking between the ports where the valve plate and the cylinder comes together...
As a test, you could put a folded piece of paper or thin cardboard between the valve plate and the cylinder - just poke a couple of holes with a pin into it through the port holes to open the holes. That should seal the bottom well and let you establish if there's a leak there...

;D I really want to see your engine running Thm:

 

[Dan Rowe]

expansion engines don't work on compressed air.

George,
It is possible with the addition of a reheater see:
http://www.dself.dsl.pipex.com/museum/transport/comprair/comprair.htm#p

Dan

 

[Blogwitch]

Trout,

It is my own fault for not studying close enough, and being half asleep when replying


Bogs

 

[gbritnell]

I studied the drawings again and with it drawn the way that it is I have discovered something else.
The bottom right section drawing shows pressure (steam/air) being admitted to what is called the HP side of the piston. This will force the piston to the left (head end). While this is happening the volume on the LP side of the piston is being allowed to exhaust thru the port and D valve.
Now the piston reaches the head end, the D valve shifts to the right as in the left lower sketch. The arrows are showing a flow from the HP end to the LP end but where is this pressure coming from? There is nothing left on this side of the piston.
The only air flow would be if the inertia of the flywheel/crankshaft kept the piston moving toward the HP end and thus transferred some of that volume to the LP side of the piston.
I'll say it again, it might work on air with higher pressure but it's certainly won't have the efficiency of what it's supposedly designed to have.
Again stating what I said about a double acting compound engine, once the high pressure piston reaches the end of it's stroke steam is admitted to the opposite side of the piston thus pushing it back the other way and in the course of doing so the lower pressure steam on the other side is then directed to the next LP cylinder.
If someone has one of these engine running it's only because it's turning fast enough to keep going, just like a single acting engine.
George

 

[Troutsqueezer]

OK, here's the fix: Build a new cylinder and port it for standard single-acting steam config. I think perhaps Monsieur Elmer should have kept this design in the closet.

First, I inspected the valve rod/shoe for even travel, that looked good but what I did see was that I made the shoe with an uneven bottom. It was not square to the cylinder so I thought "aha!" I squared it up, made a more accurate nut to place in it and checked the action. It was much better than before. But alas, nothing changed when I hooked up air to the engine.

Noting that there was heavy air flow coming from the exhaust hole when the valve was full left and now knowing it was not coming from a leaky valve, I pulled the cylinder head off and brought the piston forward. The closer it came to top dead center the more air was rushing into that space above the cylinder (now open to the air) There was a lot, and I do mean a lot of air coming through and it wasn't coming from the top port hole it was blowing by the piston!

Now, I assumed this piston was a good fit since I did all the tests; put my thumb over the end and watch it very slowly slide down or pull it out and witness the pretty loud popping sound as it broke free. But that fit apparently is not good enough for this setup. I tried wrapping some teflon tape around the oil grooves in the piston to simulate rings. Nada.

In conclusion, so much air blows by the piston, it is prevented from moving to top dead center by the counter pressure.

I give up. I'd rather make a new cylinder at this point and turn it into something more conventional. Now that this engine is my new avatar, I must bring it to life. That and I've already started making the video. :big:


George, you and I were typing at the same time. Thank you for examining this in such great detail. I do believe you are correct and my experiments seem to prove you out. Again, maybe Elmer should have picked a different design for that particular issue. A good learning experience none-the-less.


-Trout

 

[Troutsqueezer]

Thinking about this some more, it occurs to me that I should make the engine into a double-acting rather than single, keeping in mind my flywheel is aluminum. I should think (dangerous) the required weight of the flywheel can be less with two power strokes vs. one.

I should be able to swap the exhaust port with the low pressure port in this case and call it done, no? Also, I believe I can use all the existing parts, just change out the cylinder, unless there's something I'm missing.

-T

 

[Troutsqueezer]

Why re-make the cylinder? After all it was a lot of work. I think I'll just re-direct the ports on the existing one.

I really think they should let me fix the Gulf oil leak. ;D

-T

 

[Dan Rowe]

Trout, You need a better piston seal if it leaks now it will still leak as a normal steam cylinder.

George, I do not have the numbers only the drawing Trout posted but it looks to me that the HP or crank end lap is zero or very near zero as it looks to be the same width as the HP port. Zero lap will give us 100% admission or full stroke admission for the HP end. That way at the end of the stroke the Hp end is a mini air receiver. This air is nearly at full supply pressure so it will have some energy to move the piston the other way with a larger piston area in the LP end.

Dan