Pneumatic Valve Springs !

Chris Howard s2184002 at cse.unsw.edu.au
Mon Apr 1 12:00:09 GMT 1996


At 21:37 31/03/96 -0800, you wrote:
>"Valve float is caused by resonance of the valve spring when it is
>excited at its natural frequency or a harmonic thereof."
>-Chris Howard
>
>I always thought it was the inertia of the lifter/valve/partial mass of
>the spring.  In other words, the valve follows the correct path with
>respect to the cam until the region of the largest part of the cam lobe.
>But after that, the respective valve train components continue to travel
>(now in the wrong direction with respect to the cam) because the valve
>spring isn't strong enough to resist the inertia of the components.
>

Yes this is true. The valve spring must be strong enough to overcome the
inertia of the moving valve assembly. At high speeds the effective valve
spring force is reduced due to the phenomenon above. A obvious solution is
to increase the spring force but this increases the wear rate at the cam -
follower interface and indeed this can be a limiting factor as engine speeds
reach the stratosphere. For this reason, pneumatic valve springs allow lower
spring rates to be used for a given rpm ceiling, or alternatively allow
higher rpms to be used for a given allowable spring rate. I really should
have said valve float is indirectly caused by valve spring resonance. A
reference is an article in Racecar Engineering, Vol 3, No 4 Page 21.


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




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