Why not sliding valves instead of poppets?
Why not sliding valves instead of poppets?
(OP)
Forgive me if this is a newbie question. "Piston valves" were the preferred type in steam engines. Why were they unsuitable for internal combustion? Can modern materials make them usable?
I can think of several advantages of sliding piston valves:
1. The valve can remain fully closed while the slider accelerates, allowing it to go from fully closed to fully open very quickly. Same for the reverse.
2. They should be much gentler on the cams and associated parts.
3. One valve can be made to do both intake and exhaust.
Any comments? Or direct me to an article, perhaps?
Thanks,
Mike Ackerman
I can think of several advantages of sliding piston valves:
1. The valve can remain fully closed while the slider accelerates, allowing it to go from fully closed to fully open very quickly. Same for the reverse.
2. They should be much gentler on the cams and associated parts.
3. One valve can be made to do both intake and exhaust.
Any comments? Or direct me to an article, perhaps?
Thanks,
Mike Ackerman





RE: Why not sliding valves instead of poppets?
Pancholin
RE: Why not sliding valves instead of poppets?
If you want to get away from poppet valves, why wouldn't rotaries be first choice?
RE: Why not sliding valves instead of poppets?
I'd prefer a piston valve to a rotary because pistons are so well established, and because lubrication, cooling, and sealing should be similar to power pistons.
Also, you wouldn't want a continuously rotating valve; you'd want one which would pause at the fully opened and fully closed positions, and quickly spin past the half-open positions.
Michael Ackerman
RE: Why not sliding valves instead of poppets?
The drawback to piston-valve type systems would be complexity. For a piston-valve to flow adequate amounts of gas, we would have to assume a piston diameter of at least the width of the intake (or exhaust) port the valve was used to meter. This can be a very substantial size. Even if the piston were hollow, it would seem that some weight penalty could be expected. Furthermore, each piston-valve would have to ride within a cylinder that was provided with sufficient lubricating oil. So now we need to add oil squirters to the mix. Also the piston-valve would have to seal its own cylinder, so we need to add some form of oil control ring.
The total package would seem to have a total mass that exceeds a poppet type valve. The increased mass would limit the rate at which the proposed piston-valve could reciprocate. Although it was mentioned that the same valve could be used to meter both an intake and an exhaust port. The reduction in valve train speed this could afford may negate the negative effects of increased mass.
I’m sure that there are some benefits to be had with alternative valve trains, but I think those benefits may be limited to specific applications. Unfortunately, I find that poppet-style valves will fit the bill for the vast majority of engine designs in the foreseeable future.
Bryan Carter
RE: Why not sliding valves instead of poppets?
http://www.geocities.com/kiwiengineer2002/sleeve.html
http://www.eagle.ca/~harry/aircraft/tempest/sabre/
http://www.enginehistory.org/buckel_galleries.htm
http://www.enginehistory.org/coventry.htm
http://www-g.eng.cam.ac.uk/125/achievements/ricardo/page41.htm
(see also pages 38 to 43)
Cheers
Aorangi
RE: Why not sliding valves instead of poppets?
Thread71-36446
(endof thread, post of january 1st)
RE: Why not sliding valves instead of poppets?
Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)
RE: Why not sliding valves instead of poppets?
Isn't one reason for having two exhaust valves to help cool them? A piston valve, having constant contact with its housing would seem to run cooler, reducing premature ignition and allowing higher compression ratios. This could overcome any added friction in the valves.
I found a company designing an engine with this type of valve. Have a look: http://alu-x.com/autocar.html
Mike Ackerman
RE: Why not sliding valves instead of poppets?
Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)
RE: Why not sliding valves instead of poppets?
Pancholin- the rotary valve you mentioned above, isn't that a 2 stroke engine?
John Woodward
RE: Why not sliding valves instead of poppets?
RE: Why not sliding valves instead of poppets?
This page has photos of a related engine, where the piston moves along the propeller's axis: http://www.wildcatfuel.com/engines_rcv120.html
RE: Why not sliding valves instead of poppets?
There is a nother example of a piston valve to examine
http://www.sixstroke.com/pageone.htm
Please comment
Malbeare
(1)The 6-stroke engine is fundamentally superior to the 4- stroke because the head is a net contributor to, and an integral part of the power generation within the engine.
(2)The 6stroke is thermodynamically more efficient because the change in volume of the power stroke is greater than the intake, compression, & exhaust strokes.
(3)The compression ratio can be increased because of the absence of hot spots.
(4)The rate of change in volume during the critical combustion period is less than in a 4stroke.
(5)The absence of valves within the combustion chamber allows design freedom.
(6) A one-piece engine from crankshaft to upper shaft becomes feasible. No head gasket.
(7)Fewer components, 15 per cylinder compared to 40 for a 4-stroke. Therefore the cost of manufacture is much less.
(8)Can be fitted to standard engine blocks so the market is much larger than the OEM sector , also includes the retrofit aftermarket sector.
The engine has proven to be robust on the race track, & have significant advantages over 4-strokes
(1)The valving is desmodromic
(2)There are no valves to drop or bounce.
(3)The rev limit is only what the bottom end can stand.
(4)Gas flow on intake increase of 20%.
(5)No possibility of engine damage if the timing belt slips or snaps
(6)The reed valves are so close to the intake ports that their tips become the virtual port opening. This achieves variable port area & variable engine demand valve timing. The tips open late & small amounts with low throttle settings & open early & fully at full throttle
RE: Why not sliding valves instead of poppets?
Thread71-25694, but it's better you did it yourself because you know it much better than me!
As it works on a 4-stroke cycle I find the name "6-stroke" misleading, but otherwise the idea looks fine.
Cheers,
Aorangi
RE: Why not sliding valves instead of poppets?
It is an enticing, elegant design, and I'd also like to know why it hasn't caught on. I like it better than the Alu-X engine's approach, where the piston valve does no work.
Mike Ackerman
RE: Why not sliding valves instead of poppets?
jlwoodward and NielRoshier: The engine I mentioned is a Webra 80. The shaft with a pair of holes that line up with a single oblong hole in the combustion chamber. I think that some of the more difficult to seal and lubricate valve ideas are easier on model engines because about 25% of the fuel is bean oil and there are no concerns about emissions yet. The miracle as I see it is that the bottom end is lubricated only by oil that gets past the rings. YS makes some air chamber four strokes that use crankcase supercharging that first pass the mixture through the crankcase though a rotary disk then to a chamber then into the cylinder. This would only work with single cylinders or multi's with the crankcase made to separate them.
After I wrote the first note, I remembered another thing. With poppet valves combustion pressures assist in holding the valves shut against the seat without adding any pressure to the cam lobes, but a piston valve and its operating mechanism would have to be strong enough to take the heat and pressure of combustion.
Pancholin