spool pistons for uniflow engine?
March 18, 2012 12:45PM
Hi again:

I'm just doing some thinking about steam engine details; here's another one and I'd like to hear some opinions on the subject, as usual:

It occurs to me that in traditional uniflow engines the long piston required to cover the centralized exhaust port ring in the cylinder walls is a source of excess weight and possible drag. Is it necessary to keep the ports covered during the midpoint of the stroke when there is no effect on the working steam in the ends of each cylinder, beyond each end of the piston in a double-acting engine?

The use of a spool-type piston design, with contact only at its ends where it would support the necessary sealing rings, would create a much lighter and lower-drag result. Assuming the exhaust port is connected to atmospheric or even vacuum values at its outlet, the dead space in the cylinder between the ends of the piston would appear to be harmless. The appearance of the piston would approximate that of many piston valves used in steam apps over the years.

Bill
Re: spool pistons for uniflow engine?
March 18, 2012 05:27PM
The issue has to do with exhaust steam leaking into the crankcase when the ports are uncovered. If there is any pressure in the condenser then there will be leakage. If a strong vacuum then it is not an issue. Toward the end John Wetz converted an in-line four IC engine to uniflow steam. He milled some of the water jacket away so that he could drill two holes through the cylinder, making it a four hole uniflow exhaust into the water jacket and when exposed by the piston, which was an original, the steam went deliberately into the crankcase along with about everything else. This was dealt with by having a very large oil-water separator column. The really clever way of doing things was illustrated by the Leslie engine, a 132 c.i.d. in-line six, that had really long pistons with rings on each end so they acted like hour-glass shaped pistons and there was an auxiliary set of ports below the uniflow exhaust ports to take away any pressure of any steam that leaked past. The wrist pin was very low in the piston making it act almost like a cross head. The ideas never end. Tom Kimmel
Re: spool pistons for uniflow engine?
March 18, 2012 06:13PM
tkimmel@locallink.net Wrote:
-------------------------------------------------------
> The issue has to do with exhaust steam leaking
> into the crankcase when the ports are uncovered.
> If there is any pressure in the condenser then
> there will be leakage. If a strong vacuum then it
> is not an issue. Toward the end John Wetz
> converted an in-line four IC engine to uniflow
> steam. He milled some of the water jacket away so
> that he could drill two holes through the
> cylinder, making it a four hole uniflow exhaust
> into the water jacket and when exposed by the
> piston, which was an original, the steam went
> deliberately into the crankcase along with about
> everything else. This was dealt with by having a
> very large oil-water separator column. The really
> clever way of doing things was illustrated by the
> Leslie engine, a 132 c.i.d. in-line six, that had
> really long pistons with rings on each end so they
> acted like hour-glass shaped pistons and there was
> an auxiliary set of ports below the uniflow
> exhaust ports to take away any pressure of any
> steam that leaked past. The wrist pin was very
> low in the piston making it act almost like a
> cross head.

Tom:

Thanks for your reply, with real-world examples. I believe your comments are related to single-acting engines--and I was referring to double-acting.

Please keep up with more replies, Tom and everybody else!

Bill
Re: spool pistons for uniflow engine?
March 19, 2012 09:45PM
I think that you'd be better off w/ designing a uniflow to use hollow pistons rather than a spool-style piston. I think this would result in less cooling of the cylinder walls. There's obviously no reason to run a tight clearance in the center section (between the rings at either end), but an extra .010" or so would seem plenty, and would cut drag.

Obviously, the hollow section should be well sealed, but a bit of silicone would take care of that.

I would be also inclined to fasten each end separately to the piston rod rather than attempting to place the
entire piston structure in compression if you're building a larger engine. Historically, hollow pistons were used.

- Bart

----
Bart Smaalders [smaalders.net]
Re: spool pistons for uniflow engine?
March 20, 2012 11:01PM
Hal Fuller, the last Cheif Engineer of Skinner Engine Company designed two spool type pistons for Skinner horizontal unaflows in the 90's- for a 500kW and an 800kW unit. Both ran succesfully for years and I inspected the rings on the 500kW which now has about 40,000 hours on it last year. Both units exhaust to backpressure, one about 10 psig and the other up to 18 psig, which is very high, and the steam sides of the piston were dished to compensate. In a horizontal application, the spool piston has the advantage of clearing foriegn debris much faster than the barrel type. A further advantage is that the piston can be made in three simple chunks, the loss of the pattern at Skinner being the reason in these cases. Hal thought this was the first application of a spool piston but, Interestingly enough, I ran into a drawing for a Filer & Stonwell four valve nonreleasing Corliss uniflow which has just that. The space does take a while to get used to the thought of, but really, whatever is in there at the time that area is exposed to a main or aulilliary exhaust port just stays bottled up and waits for to be reconnected to it to make any equalization changes. You can see pistures of it here:
Skinner Horizontal Spool Piston
Re: spool pistons for uniflow engine?
March 21, 2012 08:31PM
This sounds a lot like the una-flow I designed. (But will be a long time before making.) The two ends of the piston had two rings each separated by a spacer giving the 0.1 clearance to the wall between the two ends. The spacer is hollow to reduce weight. The push rod ran through the entire assembly holding the piston together. It has a shoulder on one end, and a nut on the other. The idea is ease to disassemble and reassemble.

Leakage could be prevented with form-a-gasket. Although I did not expect leakage as the spacer is always in the exhaust.

Wthout building it, it is just lines on paper. No experience here, just agree with the idea.

Charles
Re: spool pistons for uniflow engine?
March 22, 2012 06:51AM
Many valid reasons for using hollow pistons and/or hour glass imho. The little prototype hp piston was fab'd solid, mass well within spec. Much larger and it has to get hollow. So many possiblities, so little time. -Keith
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