aussiegman wrote:As the pneumatic valve system shuts off as well and all valves subsequently close....
Expand on this, are you claiming that the pressure is dropped ?
A serious WTF moment has just been had!!
I needed to read your post twice to make sure I read it right the first time as it seemed more like you're trolling than asking a serious question, as exactly where did I say, infer or even give the slightest hint that the pneumatic valve pressures were dropped?? No where as far as I can tell.
The system should never drop the pressures as if they do drop below a predetermined point it would have the exact opposite effect of opening all the valves and then seeing the pistons crowns smash them in to tiny little pieces. This is apparently what happened to the Renault in Malaysia when the pneumatic system failed, the pressure dropped and a valve or valves impacted the piston crowns. I have been present for an engine being removed from a car and a remote gas cylinder was connected for exactly this reason, to stop the valves dropping and touching the piston crowns.
I know this because I asked "What is that gas cylinder used for??" when I saw it. Their reply is above..
Regardless, why would they drop the pneumatic chamber pressures anyway????? What would it possibly achieve?? If anything they could possibly increase the pressures as the nitrogen reservoir is usually at approximately 2500+psi at race start which would increase the pressures on the cam and so on...
As per my previous post, the system simply shuts off (not looses pressures) and the valves close. As they are following the cam lobes some cylinders will be undergoing their compression stroke and this will create resistance to the engines rotation.
As there is 100+psi in the pneumatic valve chambers, this in turn presses the followers against the cam lobes with some degree of force that causes resistance to the engine rotation however small. The magnitude was not the point.
So to complete the expansion you trolled for, sorry requested:
A pneumatic valve system does not use a standard coil spring, it uses compressed gas held in a chamber in its place. The cam follower on the valve tip is effectively a piston that moves against this pressurised chamber of gas to compress it resulting in what is effectively a "pneumatic spring".
The gas held in the chamber is compressed by the cam follower/valve tip piston as the cam lobe pushes the valve open. As the cam lobe moves past peak lift and along the downward ramp slope, the gas in the chamber acts as a spring by expanding to refill the chamber and close the valve.
The chambers are pressurised to approximately 100+psi as far as I know. The pneumatic systems suffer some pressure leakage during use that necessitates cars carry nitrogen reservoir at around 2500psi to top off the system as pressures drop through operation cycle. As with tyre inflation, nitrogen is used because of its stability under differing temperatures which help maintain cam lobe to follower clearances within tolerances. Regardless the high operating temp can see over pressurisation and chambers use a bleed valve to expel excess gas pressure. If ti drops to low it is replenished by the nitrogen reservoir.
The main benefits are light weigh, no springs to bind under compression and less impact from harmonic and resonance interferences. The seals are the worse aspect of the system and it biggest weakness as they preform two functions. As a seal and a type of "movement damper" for the valve. The seals press mainly against the chamber walls, though there is also some resistance from the valve stems seals, to create an amount of friction that is used to slow the valve at is peak travel so it does not jump off the cam lobe at peak lift or bounce off the valve seat when it returns. These seals have failed in the past dropping valves into the engine where they impact with the piston crowns up to 19,000 time a second.
Hows that????? Do I pass with a gold star???