Valve float

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Valve float is an adverse condition which can occur at high engine speeds [1] when the poppet valves in an internal combustion engine valvetrain do not properly follow the closure phase of the cam lobe profile. This reduces engine efficiency and performance. There is also a significant risk of severe engine damage that can include valve spring failure, pistons contacting the valves, or catastrophic lifter and cam lobe failure, especially with roller lifters. [2]

Contents

Similar conditions

'Valve lift' or "loft" is intentional, using controlled valve float to increase lift [3] and duration of the valve open cycle. In some motorsports there are rules that limit camshaft lift, preventing this type of exploitation. Properly optimizing the valve duration avoids undue stresses to the camshaft lobes and tappets.

'Valve bounce' is a related condition where the valve does not stay seated because of the combined effects of the valve's inertia and resonance 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 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 racing. Since the 1960s, Italian motorcycle manufacturer Ducati has used a desmodromic 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 to push the valve closed.

Related Research Articles

<span class="mw-page-title-main">Poppet valve</span> Type of valve

A poppet valve is a valve typically used to control the timing and quantity of petrol (gas) or vapour flow into or out of an engine, but with many other applications.

<span class="mw-page-title-main">Camshaft</span> Mechanical component that converts rotational motion to reciprocal motion

A camshaft is a shaft that contains a row of pointed cams in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines, mechanically controlled ignition systems and early electric motor speed controllers.

<span class="mw-page-title-main">Desmodromic valve</span> Reciprocating engine valve actuation mechanism

A desmodromic valve is a reciprocating engine poppet valve that is positively closed by a cam and leverage system, rather than by a more conventional spring.

<span class="mw-page-title-main">Ford CVH engine</span> Reciprocating internal combustion engine

The Ford CVH engine is a straight-four automobile engine produced by the Ford Motor Company. The engine's name is an acronym for either Compound Valve-angle Hemispherical or Canted Valve Hemispherical, where "Hemispherical" describes the shape of the combustion chamber. The CVH was introduced in 1980 in the third generation European Escort and in 1981 in the first generation North American Escort.

<span class="mw-page-title-main">VTEC</span> Automobile variable valve timing technology

VTEC is a system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine, resulting in higher performance at high RPM, and lower fuel consumption at low RPM. The VTEC system uses two camshaft profiles and hydraulically selects between profiles. It was invented by Honda engineer Ikuo Kajitani. It is distinctly different from standard VVT systems which change only the valve timings and do not change the camshaft profile or valve lift in any way.

<span class="mw-page-title-main">Variable valve timing</span> Process of altering the timing of a valve lift event

Variable valve timing (VVT) is the process of altering the timing of a valve lift event in an internal combustion engine, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems.

VarioCam is an automobile variable valve timing technology developed by Porsche. VarioCam varies the timing of the intake valves by adjusting the tension on the timing chain connecting the intake and exhaust camshafts. VarioCam was first used on the 1992 3.0 L engine in the Porsche 968.

<span class="mw-page-title-main">Overhead camshaft engine</span> Valvetrain configuration

An overhead camshaft (OHC) engine is a piston engine in which the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion chamber in the engine block.

<span class="mw-page-title-main">Tappet</span> Internal combustion engine part

A tappet or valve lifter is a valve train component which converts rotational motion into linear motion in activating a valve. It is most commonly found in internal combustion engines, where it converts the rotational motion of the camshaft into linear motion of intake and exhaust valves, either directly or indirectly.

A camless or free-valve piston engine is an engine that has poppet valves operated by means of electromagnetic, hydraulic, or pneumatic actuators instead of conventional cams. Actuators can be used to both open and close valves, or to open valves closed by springs or other means.

<span class="mw-page-title-main">Rocker arm</span> Oscillating lever in engine

A rocker arm is a valvetrain component that typically transfers the motion of a pushrod in an overhead valve internal combustion engine to the corresponding intake/exhaust valve.

<span class="mw-page-title-main">Valvetrain</span> Mechanical system in an internal combustion engine

A valvetrain 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 into the combustion chamber, while the exhaust valves control the flow of spent exhaust gases out of the combustion chamber once combustion is completed.

A hydraulic tappet, also known as a hydraulic valve lifter or hydraulic lash adjuster, is a device for maintaining zero valve clearance in an internal combustion engine. Conventional solid valve lifters require regular adjusting to maintain a small clearance between the valve and its rocker or cam follower. This space prevents the parts from binding as they expand with the engine's heat, but can also lead to noisy operation and increased wear as the parts rattle against one another until they reach operating temperature. The hydraulic lifter was designed to compensate for this small tolerance, allowing the valve train to operate with zero clearance—leading to quieter operation, longer engine life, and eliminating the need for periodic adjustment of valve clearance.

<span class="mw-page-title-main">IOE engine</span> Type of combustion engines

The intake/inlet over exhaust, or "IOE" engine, known in the US as F-head, is a four-stroke internal combustion engine whose valvetrain comprises OHV inlet valves within the cylinder head and exhaust side-valves within the engine block.

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 Formula One in 1986 with the Renault EF-Type.

A helical camshaft is a type of mechanical variable valve actuation (VVA) system. More specifically, it is a camshaft that allows the valve opening duration to be varied over a wide, continuous, step-less range, with all of the added duration being at full valve lift.

<span class="mw-page-title-main">MultiAir</span> Automobile variable valve timing technology

MultiAir or Multiair is a hydraulically-actuated variable valve timing (VVT) and variable valve lift (VVL) engine technology enabling "cylinder by cylinder, stroke by stroke" control of intake air directly via a gasoline engine's inlet valves. Developed by Fiat Powertrain Technologies, the technology addresses a primary engine inefficiency: pumping losses caused by restricting intake passage by the throttle plate that regulates air feeding the cylinders.

Variable valve lift (VVL) is an automotive piston engine technology which varies the height a valve opens in order to improve performance, fuel economy or emissions. There are two main types of VVL: discrete, which employs fixed valve lift amounts, and continuous, which is able to vary the amount of lift. Continuous valve lift systems typically allow for the elimination of the throttle.

The I-Active Valve Lift System or i-AVLS is a valvetrain technology implemented by Subaru in the 2.5L naturally aspirated engines SOHC to improve emissions, efficiency and performance. Note that AVLS is different from AVCS used on other Subaru engines. AVLS improves performance and efficiency by changing which camshaft is operating which of the two intake valves. The camshafts on all AVLS Subaru engines have specially designed lobes for intake valves. They feature two different cam profiles: a low/mid lift profile or a high lift profile. The two intake valves in each cylinder are operated by a rocker arm with its own cam lobe. The cam utilized is selected by the Engine Control Unit (ECU). To select different valve lift modes, oil pressure generated by the engine moves a pin which locks the two lobes together. At low engine speeds the low/mid lift camshafts increases the speed of air rushing into the engine thereby increasing torque and efficiency. At higher engine speeds the high lift camshafts fully open the intake valves, reducing resistance to incoming air and improving power. AVLS only operates one of the intake valves in each cylinder as the other is always open to promote swirl.

<span class="mw-page-title-main">Maybach VL I</span>

The Maybach VL I was an Otto cycle V-12 engine, made from 1924 in Germany. The airship LZ 126 was powered by five VL I engines which emphasised reliability and low fuel consumption.

References

  1. ↑ Emanuel, Dave (1995). Big-Block Chevy Performance. p. 94. ISBN   9781557882165.
  2. ↑ Lingenfelter, John (1996). John Lingenfelter on Modifying Small-Block Chevy Engines. Penguin Group. p. 111. ISBN   9781557882387.
  3. ↑ Forst, Sarah (2008). How to Build Performance Nissan Sport Compacts, 1991-2006. pp. 28–29. ISBN   9781557885418.
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