Pneumatic valve system and stem lubrication

[rooj]

Hi Everyone
Some time ago I set out to build a working ¼ scale model of a Ferrari Type 049 V10.
Utter madness I know, and I was very naive as to what I was letting myself in for.
I’m currently looking into the pneumatic valve train and a few questions come to mind.
Firstly, how is the valve stem lubricated?
If ordinarily oil would be allowed in controlled quantities by the valve stem seal, in the case of pneumatic valves it would make sense that a seal was in place to prevent the escape of Nitrogen, and hence any oil from making its way into the valve guide.
Cross sectional drawings don’t seem to indicate that there are any other means of transferring oil to the valve guide.
Secondly, in the sectional drawing below, there are 3 valves (I believe) indicated? I suspect that there is a 4th that doesn’t get shown in this cross section.
I am I correct in thinking these are simply one way valves?
May understanding (probably incorrect) is that nitrogen at a regulated pressure is introduced to the area under the valve pistons, via a one way valve. The system works on a total loss basis. There must be a means via another regulator to reduce the pressure under the piston as factors such as temperature change.
In a nutshell, if when the valve piston is at the point where it is under least compression and the pressure is less than being provided by the inlet regulator, the one way valve on the inlet side opens to balance the pressure to the correct level?
If when the piston is at its point of maximum compression, the pressure is greater than set at the outlet regulator, some nitrogen is bled via the one way valve until the correct pressure is once again reached?
I have no idea if any of this is correct, it’s my interpretation from the limited comments I’ve read on the subject so far.
Any help on the subject or help in general would be very much appreciated.







One of my Ti Conrods

[strad]

Cute...hard to deal with titanium...
In real life aren't the bags that form the valve springs donut shaped which still allows oil to get to the stem?

[Onch]

There are several ways to design a pneumatic spring system.

From the attached image, it looks indeed like this particular one is working with a check valve (keep minimum pressure constant) and an over-pressure valve.
In practice there will always be some leak through the piston seal, and it works in both ways: oil can somehow get in, and air can get out. The seal design will allow some control of the flows of air and oil, but I am pretty sure no seal on earth will ever be able to seal perfectly a piston moving up and down 150 times per seconds...
The most dangerous thing is that oil can go in at a faster rate than it gets out (thus filling the pneumatic chamber and increasing the compression ratio), but careful positioning of the overpressure valve should expel the oil excess if need be.

And finally, I do not think it is worth the hassle going for nitrogen, good old compressed air is more than up to the job...!

[riff_raff]

rooj,

Those 1/4 scale Ti rods are awesome.

As for your 1/4 scale pneumatic valvetrain, that might be a bit of a challenge. The fit/clearance of the components in an F1 pneumatic valve spring are very precise, and shrinking the system down to 1/4 scale would make achieving the necessary fits/clearances much more difficult. Leakage, feed pressure, compression ratio, frictions, etc. are all important with pneumatic valve springs.

As for lubrication of the valve stems, there is not much side load on the valve stems, so you might be able to get away with something like a dry film lube or even a DLC (if it's in your budget), with a limited life model engine.

Good luck with your project. It looks impressive so far. Here's some motivation for you: http://www.enginehistory.org/ModelEngin ... e_22.shtml

riff_raff