RE:Desmodric valves etc

["John Fielding" <johnf@domain.elided>]

James Montebello wrote thus:

<<These days, metallurgy has improved to the point where a conventional
spring design can perform at least as well as a desmo system, with a
far less complicated (and much more compact) mechanical arrangement.
For hyper-revving engines, like F1 engines, pneumatic systems are better
than both.  Ducati still uses the desmo system exclusively on all of
it's bikes, including production street machines, but it's basically a
marketing exercise, not for any technical superiority.  Mercedes used
the system only once, on the straight-8 found in both the 300SLR sports
racer, and the W196 F1 car, in 1955>>.

Actually the Ilmor engine used by McLaren (under the MB name) uses a
conventional camshaft to open the valves and a high pressure air system to
close them, as I suspect quite a few other teams do now.  The air acts like
a spring but has none of the elasticity problems that valve springs have.  I
believe their is also a light spring to intially close the valves whilst the
motor is being started, as until the motor fires there is no air pressure
built up in the reservoir.  That is why the motor once it has stalled is
almost impossible to restart even by bumping it whilst the car is moving.

On a similar note:  The changes in the 2002 F1 regulations require that the
air pressure be at least at the working pressure for at minimum of 10
minutes after the motor has stopped.  This is because the recent
introduction of traction control and semi-automatic transmissions by some
teams uses air and not hydraulic pressure to operate them.  Hence a car
stopped on the circuit has to be able to be put into neutral by the marshals
or into gear to prevent it from rolling.  Remember there are no parking
brakes on an F1 car!

John Fielding
Durban
South Africa

Alfetta 1.8L turbo

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