A Simple Uniflow Engine

[gary.a.ayres]

Lots of ideas there - thanks guys.

@MJM - Degree marks on the flywheel sounds like a good way of monitoring the valve. Will keep that in mind. Would like to see this engine going like the clappers (!) but a more sedate pace would be fine too. We shall see.

@Willy - good idea. I guess another way would just be to turn it down a bit. I still have to properly test the valve with a bit of pin material though - it may be ok as is. I should know by this evening...

@Jason - testing the valve lift with a bit of pin material pushed in from the other side. Absolutely pragmatic idea, and beautifully simple. Sometimes I despair of myself that I don't think of things like this. They seem obvious after I hear them from someone else! As for testing on the compressor - I made a start but a few air line issues arose so more testing to follow.

You have all given me much to consider and try - much appreciated.

Now, I'd better get back to my tax return... 

[crueby]

In looking at the diagrams, seems like the engine would be able to run the same in either direction, since it is just the piston movement controlling the valve directly??  Also, is this an engine that would run much better on steam, since it would be able to take a small charge of steam during a short valve opening which would then expand more during the stroke, more than compressed air would? A longer valve opening would seem to be counterproductive, since it would mean that pressure would be coming in longer during the piston-up travel. One of those designs that makes my head hurt!

[gary.a.ayres]

One of those designs that makes my head hurt!

Not as much as it's hurting mine!

To think I chose this because it's only my second engine (the first being an oscillator) and I thought it would be easier than something with a separate valve. Easier to build, perhaps, but...

I should probably have just gone for a Corliss or a triple expansion engine instead!   

Yes, it should indeed run equally well in either direction, for the reason you give.

I have had the same thought about steam, and how its expansiveness would help in this engine. Unfortunately I do not have a boiler that would power it. Yet... 

And yes, from all I have gleaned in this thread, I think a short sharp opening of the valve is what's required. In the book Stan Braye specifies a valve lift of 1/32" irrespective of whether or not the engine is scaled up.

I think it all points to it requiring very accurate adjustment to get everything in balance to get it to run at all. And I am a little haunted by the guy on youtube (same guy as in the thread Jason pointed me to) who says that he and his grandfather couldn't get theirs running for love nor money.

That said, they didn't have you guys to help them...   

[gary.a.ayres]

I don't think the engine is too shabbily made, so...

Even as I typed that I had an uneasy sense it would come back to haunt me, and sure enough...

Some good bits first though. With you guys' encouragement I have now established that the pin leaves room for a good flow of air around it. I have also homed in on the pin length and located the small range between the point at which the pin just touches the ball and the point at which the pin pushes the ball too far so that the latter is rammed against the back of the valve. Within that range there is a smaller range  - a sweet spot - in which the valve action feels 'alive' and the piston gets a satisfyingly sharp kick from the incoming air from the compressor. So some progress there in figuring things out.

But now the not so good bit. Even with the pin optimized, the piston doesn't have enough momentum to even reach bottom dead centre never mind start a new stroke. It became clear that there isn't enough pressure to drive it. I was about to blame my bottom-of-the-range puny compressor, but then noticed that when the piston reaches top dead centre and opens the valve, there is a sharp spurt of air from the exhaust ports. Not good, I think, because the ports are meant to be closed at that point. I guess that even in an ideal situation there may be a tiny bit of leakage but this feels like more than that. From the bottom end, the piston looks to be a nice snug fit in the cylinder. I can't remember what the top end looks like but during the weekend I'll dismantle the setup and check and do some measuring. I'm concerned that there may be a taper in the equation, with either the piston being narrower at the top or (probably more likely) the cylinder wider at the top.

If this is the case, I don't relish the thought of making a new piston, and I REALLY don't relish the thought of boring a new cylinder! I'm thinking that a couple of rings (is viton the kind of rubber that is used?) may be the answer, but on this I am no expert. You'll recall it's quite a hefty piston (cast iron, 35mm diameter, and long):



Will be most interested in your thoughts on whether piston rings may be the way forward here, and in any relevant issues around that...

I do have some graphite yarn which I could try but not sure I have enough of it for a piston this size. Also, given the choice I'd go for viton or some kind of rubber rings.

Trepidatiously,

gary

[Jasonb]

Your ball movement issues are no doubt due to using a reversed clack valve as these tend to have a way of limiting the amount the ball will lift off of it's seat and that is what you can feel when the pin pushes the ball too far. It may be possible to get the valve apart and modify the internals.

At TDC there should be no or very little seepage out of the exhaust, once running a bit of loss may not matter but if you are getting a lot of air loss then you are loosing a lot of energy. Adding a ring will help with sealing but will also introduce friction which will need enough energy stored in the flywheels to overcome as the piston is carried back to TDC. It should be able to work with no rings but if needs be a single one that is only lightly compressed may be an option and you can always remove it if it causes too much friction and run with an empty groove in the piston.

There is also a message waiting for you.

[crueby]

Seconding what Jason said, a viton ring will seal well, but does add friction. Sometimes just a layer of grease or oil on the piston will help a lot. A groove around the side of the piston will retain the oil better.

[Admiral_dk]

I just had a look back in this thread and it's a VERY heavy piston + rod and I can only spot a very small hole on the side of the cylinder - absolutely not big enough to be the exhaust ....

Some years back there was a build here of a record breaking Flash Steam boat (actually not the boat itself - but the boiler and engine) and the engine was extremely similar to a Nitro burning racing two stroke Airplane engine - with the exception of the feed pump for the boiler being part of the engine.
His piston and rod weighted a few grams and the piston were (if memory serves) lapped into the cylinder, to get an extremely tight tolerance.
The exhaust port was just as big as the Nitro engine .... so probably 25-30 % of the diameter in with and a height of 20-40 % of the stroke ....

Since you aren't going after this kind of power - I would consider two ports opposite each other - each taking up a fourth of the circumference and start with 20 % of the stroke. You can divide into 3 or 4 ports instead.
But you still have a very heavy piston and rod arrangement for a small intake off steam (or worse air) .... has anybody had a runner out of this design yet ?

Sorry if I sound like a spoil sport - just thinking out loud here ....

Best wishes

Per

[gary.a.ayres]

Gentlemen -

Thank you so much for all your input.

@ Jason - on your point about the valve: It may be that a longer lift than it currently allows would help. I'm not sure, as I wasn't looking for that today. I have a feeling that perhaps I'm getting enough lift but I can explore that a bit tomorrow. I'm sure you're right that the valve could be modified if needs be, or indeed I could have a go at making one.

On the viton ring - yes, that's the trouble. I'm not sure that the saving in pressure would compensate for the additional friction. However, as you say I can always experiment as even a groove may help if the ring doesn't. I guess I could always play around with graphite yarn too...

And thanks for the PM - will reply separately.


@ Chris - I have used some nice additive-free oil used to lubricate gardening machinery, and some thick, gloopy old steam cylinder oil in various combinations. Unfortunately without much success in terms of performance. In fact the steam oil adds drag and makes it worse! That's how bad it is. My sense is that the thinner oil is better for now, given this is a test situation. So yes, a groove will be good for that as well as to reduce steam escape.


@ Per - thanks for dropping by. This kind of design has produced at least this runner...

<a href="https://www.youtube.com/watch?v=895q7D6v_84" target="_blank">http://www.youtube.com/watch?v=895q7D6v_84</a>

... but it isn't scaled up like mine is. They say it's hard to scale nature.

The weight of the piston has been on my mind from the outset, having realised it would be heavy but - as a complete novice - not knowing what the tolerances in such things are or what to expect, I thought I'd try it as it is initially. I'm pretty sure I could cut a big slot in the back part of the piston (positioned so that it would not interfere with the exhaust events) which would reduce the weight while still keeping the piston long enough to avoid the need for a crosshead guide. I guess that might help a bit.

On the exhaust ports - there are actually two - one opposite the other. I'd probably go down the route of drilling additional ports rather than enlarge the existing - again something which I am aware I may need to do.

But you do not sound like a spoilsport! There's nothing wrong with a bit of realism. I will give this engine my best shot, but equally I don't want to spend the rest of my life struggling with it either. If it turns out to be a dead duck, I could always convert it to a mill engine - which would be another learning curve and another set of issues to grapple with!

Meanwhile, I need to have a closer look at that piston and cylinder and see if there are ways in which I can reduce that steam leak.


And finally guys - the garage where we take our van and car to be serviced is owned by a man who has a collection of model steam engines! So given he will be sympathetic I'm tempted to take this engine round there and ask him to hook it up to their airline and see what it does on some serious pressure...

Is that worth a try...?

[propforward]

I admire your perseverance Gary. I don't think there's any harm in attempting to run on a bit higher air pressure - within sensible limits of course.

[MJM460]

Hi Gary, good to see a successful engine on the design.  Interesting to note that despite the symmetry, it seems to run better in one direction than the other.  Manufacturing tolerances I suppose.  He did seem to be using a very small compressor, very encouraging.

So onward with the original design.  I know you have gone bigger but there is really nothing magic about the original size.  Start with the designers intended arrangement, and only change what doesn’t work.

Speaking of which, clearly the design intent is that the piston provides a reasonable seal before the exhaust ports open.  So that needs to be attended to if it is leaking badly.

I would be concerned about rings or packings of any kind as they will likely be damaged as they cross the exhaust ports.  So I would suggest grooves.  You have plenty of length for several for a good labyrinth effect, and they will collect some oil, so improve the seal one way or the other.

Clearly, simple is a misnomer when it comes to understanding this little beast.

MJM460

[gary.a.ayres]

@ Stuart -
Thank you sir. It's either perseverance or failure. If  I have to accept the latter I will, and call it a learning experience and lots of nice machined parts to customise for another engine.
But I'm not there yet, so perseverance it is...
I may well look to the garage airline if it doesn't work when I have tried everything else.

@ MJM -
Your point about scale is encouraging. 
I know I often make observations retrospectively. When one is new to an area of knowledge one often intuits something but struggles to articulate it until someone more experienced does. So yes, it had occurred to me that erosion from these ports could bugger up rings!
I don't think the leak is catastrophic. Significant maybe, but hopefully not irredeemable. So I googled ( actually Duck Duck Go'ed, which doesn't exactly trip off the tongue) 'labyrinth piston grooves' and what I found was a series of concentric grooves creating a 'labyrinth' effect, as opposed to  what I imagined, i.e. a maze-like system of intersecting grooves.   
Is concentric grooves near the head of the piston what you meant?

And yes - simple in terms of parts machining; not so simple in coming to terms with the dynamics...


You guys are amazing. Without you I'd be despairing right now.

[gary.a.ayres]

Sitting in a bar for an early evening libation with my lovely lady this evening and I had a realisation.   

If the steam leak around the piston is caused by a taper in the cylinder and can't be fixed by other means such as grooves in the piston, I could always try turning the cylinder round. All I'd need to do would be to drill and tap a bolt circle for the cap on the other end of the cylinder, and maybe plug the existing exhaust ports and drill new ones. If that worked, the rest would be cosmetic.

I'm a bit ahead of myself here and there are a couple of steps in between but at least it puts another option on the table to try if the other interventions don't fix things.

[crueby]

Great that the drink loosened up the brain cells, but how much of her conversation did you miss while your brain drifted off to the shop? Hope no bar-stool-to-the-head repercusions...!

[gary.a.ayres]

Glad to tell you that she extremely tolerant and indulgent of my meanderings, ramblings and obsessions.

Lucky for me, I have to say...

 

[Zephyrin]

Hi,
it is often said that we cannot reduce the laws of physics to the scale of models, but it is true in the other direction too! I do not see this enormous piston moving lightly in a breath of air, not only the weight, but the dragging force on such surfaces too.

And yes, i agree, the exhaust port is much too small.
it seems to me that a uniflow engine (flash steam engine) works best with a large intake advance...