Pneumatic valve springs

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.

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 valvetrain 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, and 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 bore and thus forces the retainer upwards. This ensures that no crown-to-valve contact occurs when shut down.

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 to begin a winning streak.

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