Valve float

From Wikipedia, the free encyclopedia

Valve float is a condition which occurs when the valves on an internal combustion engine do not stay in contact with the camshaft lobe. Stiffer valve springs can help prevent this but only at the expense of increased friction losses.

Valve bounce is a related condition where the valve does not stay seated, due to the combined effects of the valve's inertia and resonance effects of metallic valve springs that effectively reduce the closing force, and allow the valve to re-open partially.

Various techniques have been used to offset the effect of stiffer springs, such as dual-spring and progressive-sprung valves, and pneumatic valves.

The motorcycle manufacturer Ducati uses the desmodromic (springless) valve system to counter this problem and allow for higher engine speeds. The system consists of a mechanical lifter mechanism that uses a second rocker arm to push the valve closed. Formula 1 engine manufacturers use a pneumatic system to close the valves to allow for very high engine speed without valve float.