I've been busy with some assignments on valve bounce and came across the following issue. The valve system in question had a natural frequency of approx 5500 rpm. Hence exceeding an engine speed of 5500 rpm wouldn't be impossible as the valves and springs would oscillate uncontrollably as the natural frequency was approached (resonance). So my question is : In an F1 engine, with pneumatic springs (which in theory cannot cause valve bounce) how does one find the natural frequency of the valve system and how does this frequency compare to the frequency range of the engine?
Not an expert... but I know modern FEM codes can calculate the natural frequency of a system... or one could do a "rap" test, as is done on aircraft and other vehicle structures to determine the natural frequencies without needing to excite the structure using actual operational loads.
Nothing conceptually different from the valvetrain with metal spring, you have air instead of the spring so you have just a different mass of the spring (very very small) and you have to be a bit careful with the spring rate (think about it). Obviously the almost non-existent spring mass makes the first natural frequency lot higher.
Question. Whats the natural frequency of a non-existant spring. In other words if you remove the root cause of the problem (valve bounce) "the valve" Problem solved.
All dynamic systems, like valvetrains or crankshafts, have multiple natural (or resonant) frequencies. The lower orders are typically more damaging than the higher orders, due to their greater amplitudes. These critical natural system frequencies are not normally a problem, since the engine will not normally be operated at the speed they occur at. They are simply passed through as the engine rev's up and down.
Even a pneumatic valve spring system has natural frequencies, but they tend to be less of an issue due to the inherent dampening characteristics of the system. Dampening is also used in metal coil valve spring systems. The dampening is achieved by using dual concentric (inner & outer) valve springs for each valve, that operate with a slight rubbing interference. Thus creating a crude form of friction dampener.
As for valve bounce, it is usually a result of the instantaneous acceleration forces imparted by the cam profile to the follower exceeding the instantaneous resisting force produced by the spring.
"Q: How do you make a small fortune in racing?
A: Start with a large one!"
I know this is an oldish thread but if anyone's still interested I found this website scanning the internet for valve stuff - it's quite interesting and has some interesting graphics of wire and pneumatic springs. There are also some really interesting graphs on valve behaviour.
