Pneumatic valve springs are metal bellows filled with compressed air used as an alternative
to the metal wire springs used to close valves in high-speed internal combustion
engines. This
system was introduced in the mid-1980s in Renault turbocharged 1.5 litre Formula One engines.
CONCEPT
Racing
engines often fail at high rotational speeds because mechanical springs are
unable to retract the valves quickly enough to provide clearance for the
piston. Renault's pneumatic valve technology replaced steel springs with light
weight compressed air bellows. These could retract
valves more quickly and reduce the possibility of piston-valve interference, as
long as pressure could be maintained. Additionally, the amount of seat tension
required to keep a coil sprung valve under control results in greater peak lift
loading, with added stress to the entire valve train as a result. Pneumatic systems, sharing a common reservoir of pressure retain
a more static level of force, controlling the valve effectively, without any
attendant peak lift load increase.
The actuation
mechanism is simply a piston and cylinder, similar to a small pneumatic ram.
The tappet bore where
a hydraulic tappet would normally reside, becomes
the cylinder, and the retainer assembly becomes the piston. Pressurized air
(nitrogen) is pumped into this cylinder which then causes the piston/retainer
to rise to the top of cylinder, causing the valve to form an airtight seal with
the seat. The compressed gas then becomes the spring, so to speak, but does not
have the same traits as springs do at elevated rpm. A small light spring is
sometimes fitted between the piston and retainer so that when the system is
switched off the spring forces the piston down against the bottom of the bore,
thus forcing the retainer upwards. This ensures that no crown-to-valve contact
occurs when shut down.
PNEUMATIC VALVE TECHNOLOGY IN RACING
Pneumatic
valve springs gave Renault an advantage with its turbocharged engines, often said to be one of the most powerful.
However, reliability and poor handling of their chassis kept the cars from
success until 1989, when Renault provided Williams with a new V10 engine that began a winning streak.
Pneumatic
valve springs are also found in several Moto GP motorcycle engines, debuting
in 2002 with the Aprilia RS Cube. In 2005, Team Roberts was the
first to use pneumatic valves full-time in their uncompetitive KTM powered bike. Today, almost
all of the MotoGP teams use pneumatic valve technology on their bikes,
including Yamaha, Suzuki and Honda. Ducati uses a desmodromic design.
FUTURE VALVE TECHNOLOGY
While
pneumatic valve springs have become standard in Formula One engines, Renault
has been researching computer-controlled electromagnetic valve actuation (EVA) using no camshaft, to reduce moving parts while improving valve control.
WORKING