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model steam engine: Comments on the design I have in mind?

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rleete:
jonogt, that's an interesting idea, but probably not too practical.  Sealing it without excessive friction is probably going to be a real issue, as Bogs said.

What you've done is very similar to the way a sliding valve works.  The key to the sliding valve, however, is that because it's in a sealed box, the incoming steam (or air) seals the valve against the mating surface.  If the valve is lapped in, it seals very effectively with nothing else incorporated.  In the case of our air operated engines, sealing the valve chest (where the operating rod goes in) isn't as critical.

If you were to try to build your design, I'd recommend that instead of a deep slot, the outside piece was separate.  You then might be able to get a decent seal by lapping both sides of the valve, and attaching the outside plate in such a way as to be able to tweak in how tightly it pressed against the valve.  Might be fiddly to get it to work, but it would provide a visual valve system, rather than be hidden in a valve chest.

NickG:
John,

Yours is an interesting design, and the mechanism is feasible, but as John (Bogstandard) said, the first thing I noticed was the difficulties you would have sealing the valve down that deep slot. Once the slot is cut, you can do little with the surface finish inside and it is a finite width so it would be too difficult to get a good seal between the faces and the valve.

The only way I could see that working was if you made the cylinder in 2 separate pieces - the cylinder with a lapped port face, then a cover with the exhaust and inlet pipes, again lapped on the inside. This would have to be spring loaded equally around the valve (which would have to be lapped on both sides also). Again, as Bogs pointed out, you would need only 1 hole in the valve itself, which would oscillate between the inlet and exhaust ports.

In essence, all you would have done different from a standard oscillating engine is to immobilise the cylinder and create a separate valve that oscillates driven from another eccentric or crank.

Having said that, nobody will try to put you off, only help you, I still think it's quite a novel design and well done for grasping the concept and designing a mechanism that would work.

I would echo what Bogs said though, don't have any quarms about using ideas from other peoples engines, I've based all of my engines on other designs I've seen too, I haven't tried to re-invent the wheel, because that's too difficult!

I would recommend doing a simple oscillating engine first too, the link that Stew posted is one I designed recently specifically with beginners such as yourself in mind. I tried to make operations simple and keep parts and materials to a minimum. Doing an engine like this can instill a lot of confidence as well as give invaluable experience of the sorts of limits and fits you need to get engines to run before you go on to make one to your own design.

However, if you decide to try your own design straight off there are lots of people willing to help and offer advice so don't hesitate to ask!

Nick

NickG:
It seems rleete was typing the same thing at the same time as me!  :doh:  :)

John,

Here is a pic and video of my engine incase you're interested. By no means am I saying it's the best one to do, it is just one example. If you have a trawl aroud the forums there are probably better designs than this one, but I know it works, it's made from sensible materials and has the right parameters in terms of port spacing etc. There aren't many parts and if you mess it up it's not going to break the bank. I know at least one other person has sucessfully made a running engine to my plans too.

bogstandard:
Hi Jon,

Had a little think about your valve design, and have come up with a C-o-C for you to look at.

I hope it explains my thinking to you. The sliding faces would need to be very flat. The unneeded areas could be shaped away to reduce mass.

I have also shown a basic cam operating system, the ramps on the cam would have to have a good profile to allow the follower a smooth action between inlet and exhaust.

John

NickG:
Bogs,

That is similar to what I was thinking, is the width of the cut away on your cylinder exactly the same width as the valve or slightly smaller? If smaller, will having the springs at the bottom not tend to try and tip that plate? Probably not enough to pull it away from one side of the valve as it will only need light spring stiffness. I was wondering, is could some transparant plastic be used on that so it's possible to see the valve working?

Nick

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