a valve seating theory question.

[henry roberts]

I have read that a valve doesn't start to flow until it is +/- .040" off the seat. following on from that it would make sense, if possible, to never seat the valve, instead stop it at say .010 to .030" off the seat, so you loose less time and energy moving the valve from .000" to the point it starts to flow.

as none of the manufactures do this already, there has to be a, or a number of, damn good reasons why...


my question is: what are the reasons?


I'll take a shot that heat transfer out of the valve head is one of the reasons, but would love to know more about that too.

[Howard 111]

If I understand the question, if the valves never seated, during the compression stroke, it would blow out of both the intake and exhaust.
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[MacLeod Willy]

Even if you could somewhere get enough compression to fire.

You would have 0 compression unless it was at tremendous rpm

[busdaddy]

And after a while this happens:

Not only does it need to sit on the seat for compression, it also needs to transfer it's heat into the head.
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[Boolean]

The short answer to the question is that the valve starts to flow the moment it cracks open. It is not much in comparison to the rest of the volume, but it is there.
The total duration doesn't matter as much as the duration from a bit open to a bit closed.
Hence the 050 figure on cams.
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[raul arrese]

Are you serious ??? wow
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[modok]

[Dan Ruddock]

Raul ya think?

Kind of what a VZ cam does. Gets the valve to a useable opening very quickly. The problem is the parts get beat to death.

There is always a pro and con to every idea.

Dan

[mark tucker]

wow.hooda thunk it. I reckon a lot of time and money have been wasted on low lift head work. the sooner and faster the fill starts the more it will fill/exit the chamber. no there isant a lot of flow at low lift,but a lot can be done to imporve it.and the @.050" cam has nothing to do with flow.

[henry roberts]

[andy198712]

The main thing is, if the valve doesn't seal, they'll be no compression and all that heat and expansion energy that would normally push the piston down will be lost and thus no power made.
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[FreeBug]

Found it!

"Exhaust valves have presented a difficult cooling problem ever since the advent of the internal combustion engine.The heat-receiving surface is much larger than the heat-rejecting surface, which consists only of the stem and seat. Furthermore, both the mean gas temperature and the gas velocity relative to the surface are higher than at any other point in the engine.
The difficulty of cooling poppet exhaust valves has been the principal stimulant for the developpment of other types of valve, and the solution of this problem has been the principal factor in making other valve types obsolete. This solution has been due both to improved design and to the developpment of valve materialsthat can withstand high temperatures without failure due to breakage, warping, burning, or excessive wear. [...] However, even with the materials, the question of cooling and heat flow must receive the designer's careful attention. While the valve is open, the only path for heat flow to the engine coolant is through the stem and valve guide,but this path is obviously a poor one. The most important path for heat flow (75% according to ref. 12.21) is through the valve seat while the valve is closed."

The Internal Combustion Engine in Theory and Practice, Vol.2: Combustion, Materials, Design

CHARLES Fayette Taylor, MIT press.

Nothing hasn't been thought of before, well, almost nothing, it's just a question of the right materials being invented, rather than the applications. People, really really smart people have been scratching their heads for over a hundred years about this stuff, but you're right, one has to occasionally ask "what if?".

[Anvil]

If you want to see the future of valve train technology, look to F1 and MotoGP. They do have the knowledge, time, and finances and they spend a lot of those resources figuring out how to keep the valves on the seats.
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[nsracing]

When someone tells you that a rattlesnake does not strike if you dont move is a LIE!

If you have a valve not seating is like pulling a plugwire out. That is how it runs. Now why would you do that?

When the combustion happens, you want both valves to be seated shut to direct all available hot gases to push the piston down. if you have valves hanging open while in the burn, they will erode shortly.

YOu have too much time on your hands.

[Northof49]

Try setting your exhaust valve clearance to zero or negative and see how long the valve lasts. Not to mention how it would run.
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[mark tucker]

just think if your oil didnt start dripping untill it has a .040" clearance....no more oil leeks!!!!! and since oil is quite thicker it possiably wouldent drip untill the gap was closer to 1/4".....and if frogs had wings they could teach the butt monkeys how to land in a banana tree.... (somebody must be an engineer...a certified one....or in training to be one...Im not saying whoo... but somebody)

[raul arrese]

[FreeBug]

Pneumatic and desmodronic are just variations on the poppet valve. Sleeve valves, now that was an idea! Worked fine till we started becoming demanding as regards RPM...

[johnr156]

Heny...I like how you think.

[VWCOOL]

Henry... what you read is wrong, or it was a misinterpretation. It might be better said that valves don't flow meaningful volumes until they are off the seat about 40-thou

It's a bit like a draft coming in under a door, compared to the door being open