Valve float is an adverse condition which can occur at high engine speeds when the

poppet valve A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into an engine. It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usua ...

s in an

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...

valvetrain A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) ...

do not properly follow the closure phase of the

cam lobe Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bind ...

profile. This reduces engine efficiency and performance. There is also a significant risk of severe engine damage due to valve spring damage, and/or pistons contacting the valves, and/or catastrophic lifter and cam lobe failure, especially with roller lifters.books.google.com
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Similar conditions

'Valve lift' is intentional, using a controlled valve float to increase lift and duration of the valve open cycle. In some

motorsport Motorsport, motorsports or motor sport is a global term used to encompass the group of competitive sporting events which primarily involve the use of motorized vehicles. The terminology can also be used to describe forms of competition of t ...

s there are rules that limit camshaft lift, provoking this type of exploitation. Properly optimizing the system avoids undue stresses to the camshaft lobes and

tappet A tappet is most commonly a component in an internal combustion engine which converts the rotating motion of the camshaft into linear motion of the valves, either directly or indirectly. An earlier use of the term was for part of the valve gea ...

s. 'Valve bounce' is a related condition where the valve does not stay seated because of the combined effects of the valve's

inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...

and

resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscil ...

of metallic valve springs that reduce the closing force and allow the valve to re-open partially.

Remedies

Stiffer valve springs can help prevent valve float and valve bounce but at the expense of increased friction losses, higher stresses, and more rapid wear in the valvetrain. Various techniques have been used to offset the effect of stiffer springs, such as dual-spring and progressive-sprung valves, roller-tipped tappets, and pneumatic valve springs. Valve float can also be prevented by using lighter valvetrain components.

Titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion i ...

valves, retainers, and pushrods are commonly used for this purpose. Undercut valves can both increase flow and decrease weight. Also, by using more than the traditional two valves per cylinder, smaller and lighter valves can be used. By reducing valvetrain mass, valves can close more rapidly with a given spring stiffness. Pneumatic valve springs have been used in

Formula One Formula One (also known as Formula 1 or F1) is the highest class of international racing for open-wheel single-seater formula racing cars sanctioned by the Fédération Internationale de l'Automobile (FIA). The World Drivers' Championship, ...

racing. Since the 1960s, Italian motorcycle manufacturer

Ducati Ducati Motor Holding S.p.A. () is the motorcycle-manufacturing division of Italian company Ducati, headquartered in Bologna, Italy. The company is directly owned by Italian automotive manufacturer Lamborghini, whose German parent company is Au ...

has used a

desmodromic :''In general mechanical terms, the word ''desmodromic'' is used to refer to mechanisms that have different controls for their actuation in different directions.'' A desmodromic valve is a reciprocating engine poppet valve that is positively clo ...

valvetrain to counter this problem and allow for higher engine speeds by using positive closing as well as opening of the valves, without springs. The system consists of a mechanical lifter mechanism that uses a second

rocker arm In the context of an internal combustion engine, a rocker arm is a valvetrain component that typically transfers the motion of a pushrod to the corresponding intake/exhaust valve. Rocker arms in automobiles are typically made from stamped steel ...

to push the valve closed.

References

Engine valves Motorcycle engines {{Automotive engine , collapsed