23 How do pneumatic valve return systems work ?
Description
This article is from the Motorsport FAQ, by A H Henry bspahh@midge.bath.ac.uk with numerous contributions by others.
23 How do pneumatic valve return systems work ?
The technology of all the current pneumatic valve return systems is
that of simply replacing the valve spring with a pneumatic spring,
using an inert gas (nitrogen, which behaves substantially the same as
air) as the compressive fluid. The camshaft exercises its normal
precise control of the motion of each valve, and each spring
maintains the contact force between an individual valve assembly, cam
and tappet bucket, during operation. Pneumatic valve spring systems
are thus an improved replacement for mechanical springs. They are not
a complete valve control system like a desmodromic operation - which
uses no springs.
Pneumatic valve springs operate on a ring-main system with the
essential back-up of a compressed gas cylinder, pressure regulators,
one-way valves and an oil scavenging system. The principle reduction
in valve assembly mass is that of the upper one-third of each valve
spring. Although a nett small reduction in valve assembly mass is
possible, this is accompanied by added friction of the stem seal
ring.
The pneumatic spring is not subject to fatigue failure, or
diminished damping, with running time. Valve lift is not constrained
by spring wire maximum stress and stress range limits. Renault
reports that the rising rate characteristics of the pneumatic spring
assists in matching spring force to valve assembly inertia force
requirements in the particular case of its V10. The fundamental
pneumatic spring advantage for very high speed engines is that the
natural frequency of the compressed gas column is in the order of
eight times that of a steel wire coil spring.
It was Jean-Pierre Boudy, the Chief Engineer of Engine Development
at Renault Sport during the turbocharged V6 race engine era, who
invented the first pneumatic valve spring system to be used in a
competitive racecar engine. The pneumatic spring equipped Renault
engine was first raced, in a Lotus chassis driven by Ayrton Senna, at
the beginning of the 1986 GP season.
The Honda RA122E/B (1992 V12 F1) ran at 6-8 bar (87-116 psi) with
the gas replenishment cylinder charged, initially, to approx 150 bar
(2175 psi). Honda claimed a reduction of valve assembly reciprocating
mass of 20 % with similar levels of valve gear friction(as compared
to conventional systems).
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