Pneumatic Valve Springs !

[Chris Howard]

Hi,

>
>What about valve float at 'higher' rpm's? I realize the pneumatic system
>probably has less inertia than metal springs, but I still can't see how
>they could effectively curb this problem without some type of Desmo system
>(like that used on Ducati's).  Any insight?
>

Valve float is caused by resonance of the valve spring when it is excited at
its natural frequency or a harmonic thereof. A typical natural frequency of
a valve spring is around 700 Hz (42000 rpm). So at 7000 camshaft rpm, if
there is a strong sixth harmonic in the cam lift profile, the spring will be
in resonance and the effective spring force will be diminished or
eliminated, unless there is adequate damping of the springs. This damping is
generally provided by friction between the inner and outer springs, but is
not a total solution. What is desired is that the natural frequency be much
higher such that it corresponds to at least the ninth harmonic of cam speed
as the higher harmonics have smaller components.

A pneumatic valve spring is just a enclosed column of gas within the space
previouusly occupied by the coil springs. The natural frequency of a typical
such system is around the 5kHz mark (300000 rpm). So with a pneumatic valve
spring at 7000 camshaft rpm, it will be the about the 40th harmonic which
will excite the pneumatic spring. These higher harmonics have negligible
effect, so the pnuematic spring is virtually "float free".

If anyone wants any more info, reply by email as i'm not quite sure valve
springs are a part of the FI system :)

Chris Howard
Computer Science
University of New South Wales
email: s2184002 at cse.unsw.edu.au
Web: http://www.usyd.edu.au/~choward