Why not sliding valves instead of poppets? 1

[mackerm]

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

 

[Pancholin]

I believe the biggest problem would be lubrication, or the lack of it. Thermal expansion wouldn't help and it would be sad if the "valve" got hot quicker than its cylinder. I have an interesting four stroke model airplane engine that uses a rotary valve for both intake and exhaust, but the oil is in the fuel. And the oil carries away a lot of heat, and lubricates the valve shaft.

Pancholin

 

[Metalguy]

Mackerm,

If you want to get away from poppet valves, why wouldn't rotaries be first choice?

 

[mackerm]

Metalguy,

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

 

[Retracnic]

Well it would seem to me the problem with alternative valve trains is that you gain a significant amount of fictional area, with little corresponding gains in gas flow.

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

 

[aorangi]

The only system to ever successfully compete with poppet valve was the Burt Mc Collum sleeve valve. It was widely used in British WW II aircrafts engines of which the most famous were the Bristol Hercules 14 cylinders radials ( Bristol also produced the Perseus, Taurus and finally the 18 cylindres Centaurus with the same concept ) and H24 Napier Sabre. After the war Rolls-Royce built some prototypes of the H24 Eagle analogous to the Sabre but bigger and even more powerfull ( about 3500 bhp). The problem with such a system today would be oil consumption and emissions, but otherwise it'd be fine.

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

 

[aorangi]

Fieroman, we talked about the Coates engines in
thread71-36446
(endof thread, post of january 1st)

 

[ShaunT]

while I am not too familiar with most of the engines discussed in this particular thread, I am extreemly familiar with camshaft design as it is what I do. The valve can open quickly with a piston valve, but imagine what would happen to a poppet valve and the typical camshaft governed valve-trains with that kind of peak acceleration and jerk rate. Further more, I have cam designs that in some motors,(depending on a couple of variables) the motor can't keep up with. The valve out runs the piston.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[mackerm]

Thanks for the interesting comments, everybody. Retracnic makes a good point that piston valves could add friction and complexity. But I look at dual camshaft engines with four valves per cylinder, and can't help thinking they're pretty complicated too.

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

 

[ShaunT]

Beryillyum Copper works Great on Exhaust valve seats. They Really cool well.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[EngJW]

One car that used them was the Knight Sleeve Valve of around the late 1920s or early 1930s. They still turn up in antique car shows. Much information about sleeve valves rotary valves is in Ricardo, the father of all engine books, if you can ever find a copy.

Pancholin- the rotary valve you mentioned above, isn't that a 2 stroke engine?

John Woodward

 

[NeilRoshier]

I think Pancholin may be talking about the 'Weber' 90 four stroke model airplane engine that used a rotary valve in the head driven by a toothed belt?

 

[mackerm]

Hmmm. It seems that the truly insane engineers are all designing model airplane engines. This page has a diagram of a 4-stroke engine with a cylinder which doubles as a rotary valve:

http://www.wildcatfuel.com/engines_rcv58.html

This page has photos of a related engine, where the piston moves along the propeller's axis:

http://www.wildcatfuel.com/engines_rcv120.html

 

[malbeare]

Dear fellow members,
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

 

[aorangi]

Malcolm, I should have written previously something about your concept since we already spoke about it in
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

 

[mackerm]

The more I read about Beare's engine, the better it looks. I had a look at its US patent ( 5,713,314 ). I then looked at the oldest "prior art" patent it referenced (1,237,696 ) from the year 1917. Funny thing is, they look pretty much the same. All the major parts are there. Maybe the timed cover on the exhaust valve is Beare's new invention, but I'm no lawyer.

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

 

[Pancholin]

aorangi: Thanks for the excellent links. You deserve another star at least.
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