Is there a reason it would not be better, at least in some instances to use the camshaft profile to close the valves and the spring pressure to open them?? I have been able to find some numbers on cam profile design for pushrod engines but not for f1 aor any pneumatic valvetrain. It seems like negative acceleration as the valve comes to the top of the lift curve is the greatest force on the spring that needs to be controlled and this far outweighs any of the positive acceleration numbers. Would it not make more sense to use a reverse cam profile and a spring to force the valve open?
I would also think that the deceleration force as the valve comes back to the seat is pretty high, but once it's closed, bounce would seem to be a non issue if the cam is holding it closed
Obviously spring vs demo is leaning heavily toward springs recently, especially if you can utilize pneumatic springs.
Cam closing valve train would obviously have less tolerance for imprecise adjustment of closed position or wear.
I just want to know if spring rate could be reduced or area under the curve could be increased with the spring open setup.
Also, uncontrolled movement near full open is not much concern as a 3/4 open valve and a full open valve are essentially the same, where as the difference between even a small opening and a closed valve is vastly different to engine operations. With a spring open, you have more spring pressure and thus more control at open and close area of the movement, rather than a normal system that has the most spring pressure and ability to control movement near peak lift. Essentially, agreeing with grunt guru' s assessment would mean a spring open would be able to get more area under the curve.
On one of the cars we had years ago (Ford Falcon EA in Australia) it had tiny little hydraulic adjusters on the end of the rocker arms. I have often wondered if such a device could be adapted to the closing rocker on a desmodromnic system, so that it would make the valve touching down onto the seat an easier task overall.
A hydraulic adjuster can take up any slack, but would not be good at holding actual pressure under load
For cam closed valve, you need some kind of heavy spring force. In desmo the rocker that closes the valve usually acts as that spring
A hydraulic adjuster can take up any slack, but would not be good at holding actual pressure under load
For cam closed valve, you need some kind of heavy spring force. In desmo the rocker that closes the valve usually acts as that spring
But they have been for decades.
The closing force required is quite a lot less than the force needed when the valve is moving.
. . and of course the beauty of the poppet valve is cylinder pressure during compression and combustion adds to the closing force. Some desmo systems use a very light spring to fully close the valve with the closing rocker having a small clearance.
Ah, yes, the hydraulic lifter/adjuster can support a great force, but it has to get a chance each cycle when the valve closes to take up the slack and return to max position. It does not need to precisely maintain that max position the whole time it accelerates the valve open which is when the forces are applied. Small amounts of lifter collapse as well as rocker and pushrod flex will just be relieved as the cam noses over and the valve wants to keep opening
Poppet valves still need a lot of spring closed pressure on a turbo engine though. I guess we don't have any desmo turbo applications. Might be for a reason. But desmo is so limited it's hard to say it couldn't be developed to do so. Most desmo styles I have looked at do have an interference type fit at valve closed and no spring is actually holding the valve closed separate from the natural spring rate of the rocker arm which would be a very, very high spring rate. The force may not be that high, but there is not much give there if it is adjusted tight
