Hello Everyone
I have been thinking a lot about uniflow engines and I have some questions. In the literature, it is argued that uniflow engines derive efficiency from the fact that the exhaust exits at BCD and so does not cool the cylinder head/inlet ports.
But - whatever 'exhaust' is not exhausted close to BDC is recompressed, and by the time the piston is at TDC, the pressure is/can be greater than supply steam pressure, which means that when the inlet valve opens, compressed exhaust steam is pushed back down the inlet passage, until piston has dropped far enough to draw the exhaust back in to the cylinder and then be followed by fresh steam. this is clearly inefficient in two ways - recompressing exhaust takes energy, and the delay in getting fresh stream also seems undesirable.
Is my understanding correct?
I know some Uniflow engines have/had relief valves in the cylinder head - this would allow the remaining compressed exhaust gas to be exhausted (but only after it had been compressed and at the cost of cooling the head (?).
What kind of valves were these - passive spring loaded or cam actuated...?
I've seen some double acting single cylinder designs - are these preferable to single acting types?
thanks for any input!
that is not possible. To have a return pressure greater than the steam pressure would be going against the law of conservation of energy. (caveats with that of course.) If you couldn't exhaust at least 90% of the used steam, my guess, is that any machine would not work at all or at least very poorly. However, examine the exhaust pports of other types of engines, particularly the slide valve, on so many of the little engines: the steam enters the slide and the valve moves in such a way that it goes into the cylinder body thru a tiny orifice almost at the center of the part! (This is really a poor design.) then the steam follows a pathway to the end of that body to the inside top (or bottom) of the actual cylinder where the piston resides. The steam pushes the piston. When the piston reaches near or at BDC, all that steam, that is, the same amount that went in weight wise, has to return thru the same pathway that allowed it in. Now that pathway has not enlarged but the volume of steam is now about 10 or more times as large under lower pressure--how does it escape quickly enough to not cause a similar problem to what you are talking about? Well, inside the valve, the escape port is a bit bigger, which helps, and the time for the valve to be open is a bit longer.
The steam manages to get out thru the same passageway it came in! This causes serious problems many to do with the size and shape of the tunnel system in these little engines: turbulence is the bane of moving any fluid quickly and with out loss of velocity thru friction. The least amount of friction caused by any sort of conveyance is a round, polished surface. The reason? a circle has the least "surface" for any given "volume" and I don't have to explain why to polish. So imagine a square thru hole for a moment, a square is the shape that has four sides and has the least amount of "surface" for it's volume, approximating a circle (that is better than a rectangular form). Now when that form is a tube, the corners get terrible turbulence, sukking up a lot of energy and slowing down the fluid.(This is particularly important in foundry work in the green sand.) It is, however, easier to make a round hole thru metal than a square one. (I had some square drill bits but they all broke, dang!) but there is more to it than that. There are size constraints and possibly the need to have a minimum "volume" which means one has to squish that round hole down to an oval. Well, ovals are harder to drill than square holes are. (I was hoping some clever manufacturer would come up with oval drills.) So what happens is a series of round holes are drilled in beside each other till you get an approximation of the size you needs. YOu can chip out the remaining peices or mill them. So after a "square hole" the next best thing is a pentagon hole which is really next to impossible to do, and then a hexagon. The hexagon may not be as difficult, ultimately as the square. But this should be noticed: That even-sided holes are far easier to do than odd number of sides. (I mean look, guys, can you even build a hole with ONE side? how would you do a triangle?) The point is that as you get more and more sides, you approach a circle which is really an infinite sided, closed geometric design.
The next feature that causes turbulence is: Corners. Every corner is going to slow your fluid down. Engineers typically have tools to round the corners of piping and whatnot for large projects, the larger the radius, the less turbulence. But worse than turbulence (or maybe the grand caliph of turbulence) is the "bounce". This is when a fluid is let directly into a flat, perpendicular to the flow, surface. it strikes that surface ded on and bounces back blocking the fluid from coming in. This sets up a vibrational type of turbulence. (Imagine a theatre pakt full of popkorn popping people when the theatre catches fire--the people jam the exits. If even ONE person stands picking his pickel nosed protuberance just outside the doors of the theatre, somebody dies! the same at soccer games in Brazil when the fans get in fights and then rush the gates.) Anyway, perpendicular hole/interfaces are the worst of all. MUCH better is to have a 45deg. angled ramp which would reflect the incoming fluid at 90 deg. to the march of the flow. You can approach the circular method again over a ramp, but in this case, the best shape is not EXACTLY circular, it is a hyperbola or maybe parabola. Well, try to make THAT with what we amateurs have! A ball ended mill end would be just fine and the amount of difference is like the tiny difference between Einsteins theory of gravitation and Newtons theory for a ball dropt 10 feet from the surface of the earth.
With all that said, the little steam engines that use this little valving system are VERY poorly designed. So, along comes the Corliss which overcame some of these problems and created others. (I just made that up.) Later in the 1880's or later (does anybody know the date of inception for the uni-flo?) the uniflo idea was hit upon. This is where there IS NO VALVE! They are not spring loader or cams--they are HOLES in the cylinder wall that allow the steam to exhaust when the piston passes the holes. Clearly, there is going to be VERY LITTLE turbulence in exhausting! There are no square or round tubes or corners to let the steam out, it is simply holes!
My understanding of the problem about why these uniflos never really took off is that the port exhaust causes some kind of trouble with the piston and rings as they pass.
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