Combined Piston/Valve

[PaulR]

Would a combined piston/valve like that in the attached image work or have I forgotten something fundamental?

The (long) piston has longitudinal passages with short connections to the circumference to admit and expel air. Air enters via the passage about halfway up the cylinder and is exhausted as the other passage begins to be exposed towards the top of the stroke. It seems like the engine would get a short blast of air and time to get much of the exhaust out.

[Jasonb]

My gut feeling says the short inlet burst may not be long enough. Also the inlet will start to open as the piston is still trying to go down

[PaulR]

...Also the inlet will start to open as the piston is still trying to go down

Oops yes hadn't realised that. Can't think of an obvious way to fix that so probably a non-starter then.

[Kim]

...Also the inlet will start to open as the piston is still trying to go down

Oops yes hadn't realised that. Can't think of an obvious way to fix that so probably a non-starter then.

But how is that really any different from your uniflow engine?    Doesn't the inlet open on that as the piston is still going up?  And it worked...

Kim

[PaulR]

...Also the inlet will start to open as the piston is still trying to go down

Oops yes hadn't realised that. Can't think of an obvious way to fix that so probably a non-starter then.

But how is that really any different from your uniflow engine?    Doesn't the inlet open on that as the piston is still going up?  And it worked...

Yes I actually typed in the original post that it seems to be a variant of the uniflow engine but edited that out before posting! I'll think a bit more on it, see if there's any sort of refinement that can be made. Changing the inlet from a circle to a slot springs to mind but not sure if that would make a positive difference 

[mklotz]

I built a valveless engine, Simon Junior, from plans in Model Engineer magazine.  You can see a description and video of it in action here...

https://www.homemadetools.net/forum/valveless-steam-engine-50201#post72033

It runs very nicely and no porting through the piston is required.

[PaulR]

I built a valveless engine, Simon Junior, from plans in Model Engineer magazine.  You can see a description and video of it in action here...

https://www.homemadetools.net/forum/valveless-steam-engine-50201#post72033

It runs very nicely and no porting through the piston is required.

Thanks Marv, you sent me the plans for that some time ago and it's still on my to-do list.

[PaulR]

I'm going to give this a try, off to the build log section we go...

[gipetto]

I don't think it would work efficiently or at all, because for the piston to return to top dead center it would need to compress air which would react against the power stroke.
I think a better design would be to make the piston rotate to give more configurability to the strokes. that is similar to how an orbital power steering unit operates.

[crueby]

I don't think it would work efficiently or at all, because for the piston to return to top dead center it would need to compress air which would react against the power stroke.
I think a better design would be to make the piston rotate to give more configurability to the strokes. that is similar to how an orbital power steering unit operates.

Orbital power steering  unit, I  got to look that one up! Google! Get over here!!

[PaulR]

I don't think it would work efficiently or at all, because for the piston to return to top dead center it would need to compress air which would react against the power stroke.

You may be right about this design but as Kim says and, my recent uniflow engine shows, a small amount of air left in the cylinder can be overcome. We'll soon find out! 

[bent]

Quote from: gipetto on October 06, 2025, 01:06:48 PM
I don't think it would work efficiently or at all, because for the piston to return to top dead center it would need to compress air which would react against the power stroke.

It will work as long as there is enough momentum in the flywheel to carry it past the compression resistance.

[PaulR]

Quote from: gipetto on October 06, 2025, 01:06:48 PM
I don't think it would work efficiently or at all, because for the piston to return to top dead center it would need to compress air which would react against the power stroke.

It will work as long as there is enough momentum in the flywheel to carry it past the compression resistance.

...and assuming I can make it well enough  I've already planned to make the flywheel a bit bigger than the last couple of engines to help with compression.

Opinion seems to be divided on whether this design will work, which suits me as I'm guaranteed to please at least half the people whatever the outcome