Author Topic: A Simple Uniflow Engine  (Read 13589 times)

Offline gary.a.ayres

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Re: A Simple Uniflow Engine
« Reply #270 on: January 12, 2021, 11:07:21 PM »
MJM -

Good point re the thickness of the pin and how much room there is around it for the air/steam to flow. Again possibly something to play with and although I have now drilled and tapped a hole M3 in the piston, there's no reason why the projecting part of the pin couldn't be turned down a bit if needs be, leaving the part inside the hole M3.

I had thought of the potential problem of the pin winding in or out in use, effectively changing its length. That would inevitably happen, and the current setup is not a final one. My thinking was around some kind of adhesive but there are potential problems with that I'm sure, especially as yes - I do aim to run it on steam eventually. I hadn't thought of a lock nut, which would be better. Don't know if there's room for one - I'll check, hopefully tomorrow evening. However, the current setup is very temporary. As soon as the engine runs I'll be taking the test rig apart and moving on with finishing the engine, so the pin won't be subject to a huge amount of stress in its present form. A proper version can be developed later.

On the steam ports: apart from making fabricating an exhaust manifold more tricky, is there any other reason for not drilling more? In other words, is it necessary to make sure that there isn't too much exhaust/loss of pressure at that part of the cycle? Or is it a case of the more steam that is exhausted, the better? If the latter, I'm sure I could drill one or more extra ports and still be able to make a manifold of some kind. But I don't want to go making holes that I'd have to then plug up again!

Meanwhile, though - the pin!

Online crueby

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Re: A Simple Uniflow Engine
« Reply #271 on: January 12, 2021, 11:13:42 PM »
Hi Gary,
Are there any diagrams around for how this type of engine works? I'm baffled at the moment...  :headscratch:
Thanks,Chris

Offline MJM460

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Re: A Simple Uniflow Engine
« Reply #272 on: January 13, 2021, 02:54:45 AM »
Hi Gary, it looks like you have the pin issues under control.

With regards to the exhaust ports, again there are the two functions, timing, and flow area. 

The exhaust ports must not be uncovered by the piston too early, as that reduces the useful work done, and you need all you can get.  But you want enough port area to get the pressure in the cylinder close as possible to atmospheric pressure before the piston closes them off on the return stroke.  So more holes is the way to go, so long as you leave enough metal for the cylinder to hold together.  Just how much is enough is a question more difficult to answer. 

In principal you can put the ports lower in the cylinder if you have more holes.  However, there is very little work done in the bottom few degrees of crankshaft rotation, as you can see if you look at sine tables near 90 degrees or closing near 0.  And you do have to uncover the port for long enough for the air to escape.  Not easy to tell of you have too little.  I suspect you would need to get it running, then drill an extra hole to see if it runs better and if so, drill another and so on.

Looking forward to the next tests,

MJM460

The more I learn, the more I find that I still have to learn!

Offline Jasonb

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Re: A Simple Uniflow Engine
« Reply #273 on: January 13, 2021, 07:39:15 AM »
Having had a google about I'm a bit clearer how this engine works now, must say it's crying out for an exhaust like we use on the CHUK engines which stops the rising piston from having to compress any trapped gasses

There is a good explanation over on HMEM of the fine line between too much and too little pressure if you search "tappet/clapper engine".

Offline gary.a.ayres

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Re: A Simple Uniflow Engine
« Reply #274 on: January 13, 2021, 01:55:41 PM »
@ Chris -

Here's an overview of it, copied from Stan Braye's book:



Pretty simple concept. The pin in the end of the piston opens the valve, and the incoming air or steam pressure closes it again. It exhausts through a port drilled through the cylinder wall (not shown in drawing). Interestingly though, apropos of MJM's point above, the diameter of the pin in the drawing allows no space around it for steam flow.

@ Jason -

Thanks for that - I'll check out the source you refer to. I'll be interested to see what the CHUK exhaust is and how it works, with a view to possibly incorporating similar on this engine.

@ MJM -

As always, there are parameters in mutual influence! I have already drilled two exhaust ports (positioned as per plans) so I'll go with them for now, and probably drill one or two extra in the same plane if it appears to be required later. Unless of course the exhaust that Jason suggests leads in a different direction altogether ...

Online crueby

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Re: A Simple Uniflow Engine
« Reply #275 on: January 13, 2021, 02:15:35 PM »
Thanks for the drawing Gary, now it makes sense to me. The length and clearance around the pin look like critical dimensions for air flow.

Offline gary.a.ayres

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Re: A Simple Uniflow Engine
« Reply #276 on: January 13, 2021, 02:56:00 PM »
You're welcome Chris.

Thank you for your interest and encouragement.

gary

Offline gary.a.ayres

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Re: A Simple Uniflow Engine
« Reply #277 on: January 13, 2021, 06:25:44 PM »
Having had a google about I'm a bit clearer how this engine works now, must say it's crying out for an exhaust like we use on the CHUK engines which stops the rising piston from having to compress any trapped gasses

There is a good explanation over on HMEM of the fine line between too much and too little pressure if you search "tappet/clapper engine".

Jason - if you look in again on this here thread - I found the thread on HMEM. Thanks:

https://www.homemodelenginemachinist.com/threads/tappet-clapper-valve-steam-engine.26288/

I also googled for the exhaust you refer to on the CHUK engines, but without success. Can you by any chance suggest where I might find info on this?

Offline Jasonb

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Re: A Simple Uniflow Engine
« Reply #278 on: January 13, 2021, 06:49:32 PM »
Thinking about it some more I don't think the exhaust will work as you will loose expansion as the cylinder fills.

What size hole is in the head where the pin goes in to meet the ball, does not look to be much clearance on that drawing.

Offline gary.a.ayres

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Re: A Simple Uniflow Engine
« Reply #279 on: January 13, 2021, 07:10:35 PM »
Ah ok... I'll stick with the original arrangement then.

Yes, in the drawing above it looks like there is no clearance at all. In the actual plans he gives the pin as 1.5mm diameter and it looks like the passage is 1.8mm diameter. So pretty close.

My engine is 3.5 times the size. I didn't measure for the valve - I just put an off-the-shelf clack valve in the end of the cylinder. Will need to check the size later, but it's large-ish. The temporary pin in mine is a piece of M3 threaded bar. Again, I'll have a look at the clearance when I go out to the shop shortly.

But suffice to say, while I have pretty much built the rest of the engine to plan, there was quite a bit of guesswork with the valve. I appreciate that this may come back to haunt me, but I'll start with other parameters, i.e air pressure and pin length, and see how I get on.

Cheers.

 :ThumbsUp:

Online crueby

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Re: A Simple Uniflow Engine
« Reply #280 on: January 13, 2021, 07:59:23 PM »
Two tests that I would suggest to narrow things down, going from your description that it does not seem to have much power on the piston.
 - Remove the piston but leave the clack valve in place, turn on the compressed air feed, and push a length of the rod in through the cylinder to open the valve, and see if you are getting a decent flow of air. If not, then something in the valve or rod passage itself is the issue, too small an opening around the valve or rod most likely.

- Opposite case - remove the clack valve but have the piston/conrod installed. When air is applied, it should push the piston down all the way and be hard to manually push it back. If not, then the rod may be closing up the passage too much. Also in this scenario with the air line disconnected the engine should turn over by hand relatively easy.

Hope drawing the fence on the problem like that will help narrow down where the issue is! 
 :cheers:

Offline gary.a.ayres

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Re: A Simple Uniflow Engine
« Reply #281 on: January 13, 2021, 10:47:02 PM »
Chris -

Yes. In fact I tried opening the valve manually with the piston out tonight using a couple of different long poky things, and there was a good airflow with the valve opening and closing as it should. This was with the compressor end of the air tube disconnected and me blowing through it with my mouth, in the interests of sensitivity. I didn't test this with a length of the pin material though because it was not to hand and the hour was late, so I will do so tomorrow night as priority. However, I think it will be ok because the pin appears to have a fair bit of clearance - the pin is M3 and I'd guess the valve bore at 4 or 4.5 mm.

I'll also try your second idea if I don't get it running soon. I'm fairly optimistic about that because the crankshaft and big end are running nice and free and air even at low pressure easily has the piston sliding. So yes - great ways to test the flow around the pin; thanks for that.

In addition to the above, I tried various lengths of pin but no joy. Then - at the end of the evening - it occurred to me that I have been stabbing wildly in the dark over this. The recommended valve lift is only 1/32 inch so the sweet spot (outside of which the engine probably won't run at all) is very small and I probably haven't been near it yet. Instead of wild guesswork, measurement is the key. This had me thinking that a digital depth micrometer would be great for this but a quick look at the prices put me off. Rather than spend a hundred quid or more (at least for now) I'll try to rig up another way of measuring to where the end of that pin should be at top dead centre. Digital caliper and some kind of jig...

Feeling fairly optimistic about it though. I don't think the engine is too shabbily made, so at this point I see no reason why it shouldn't run once the relevant parameters are in balance...

 :cheers:

Offline MJM460

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Re: A Simple Uniflow Engine
« Reply #282 on: January 14, 2021, 02:13:08 AM »
Hi Gary, looks like the pin has enough clearance for flow to be a minor issue at most.

Donít worry about a depth mike, put some degree marks on your flywheel near top and bottom dead centre, and use them rather than the actual lift measurement.  You can calculate the height from the degree marks by geometry if you wish, but as degrees of rotation is a proxy for time open, it is probably more accurate than lift, as the lift varies so little near top and bottom dead centre, while the time of opening gives time for fluid flow.

Once you have a mark for the valve being just touched by the pin, an M3 thread is 0.5 mm pitch, so one turn changes the pin height by 0.5 mm.  A mark on the top of the piston will help you keep track of how far it has been turned.  I guess you have to grip it with pliers as you donít want a slot in the top to be hammered by that valve.  Or machine some spanner flats on the pin.

Does that check valve have a spring to hold the ball in place when there is no air pressure?  Just a little point to be aware of, if it does not appear to close properly when starting.

I am still thinking about whether the pressure can be too high.  Not necessarily a given.  But it will make it necessary to get the flywheel moving much faster to have the energy to drive the engine over top dead centre against the incoming air.  I have a vague memory of reading about starting the really high performance versions of the engine with a pull cord like starting a model marine diesel or glo plug.  But given a really energetic flick to start, it goes like the clappers.

Hmmmm...  Goes like the clappers, eh?  Sounds like there is something catching in the air at the moment.

MJM460


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Offline steam guy willy

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Re: A Simple Uniflow Engine
« Reply #283 on: January 14, 2021, 03:00:16 AM »
Hi ..could you make the push rod a more triangular shape   a bit like a safety valve ? this would give you a lot of air entering the cylinder ?? just a thought

Willy

Offline Jasonb

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Re: A Simple Uniflow Engine
« Reply #284 on: January 14, 2021, 07:13:00 AM »
If you put a small bit of rod into the air inlet and hold it against the ball you should be able to see how much it moves when the piston's pin moves the ball.

Have you tried the same test of the valve that you mention with it under air pressure, would give an idea of how much force the piston needs to open the valve.