Valve Float
Within
an internal combustion engine there is a condition known as "valve
float" which happens when the poppet valves lose contact with the
lobe in the camshaft during the closure phase of the valve lobe cycle.
The efficiency of the engine is reduced however the performance is increased
together with the potential for the adverse effect of higher exhaust ommissions.
There is another condition which is known as "valve bounce" in which the seating of the valve is negated due to the added together effects of inertia in the valve and metallic valve resonance. The closing force is then resulting reduced a partial re-opening of the valve.
Valve float can be prevented using stiffer valve springs. The accepted down-side are the loss of friction. Numerous techniques have been atempted to negate the effects of stiffer springs such as multiple (usualy dual) and sprung valves using the progressive technique. Also pneumatic valves and tappets (roller tipped type) are also used as an offset.
Ducati, the motorcycle designers based in Italy, uses a springless (desmodromic) valve spring system to enable increased engine speeds. Their system uses a mechanical lifter mechanismwhich pushes the valve closed using a secondary rocker arm as a pivot. Formula one engine designers use this system to close the valve cycle enabling higher maximum revolution per minute with minimal/negated valve float issues.