Poppet valve


Poppet valve

A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide. In most applications a pressure differential helps to seal the valve and in some applications also open it.

Presta and Schrader valves used on air-filled tires are examples of poppet valves. The Presta valve has no spring and relies on a pressure differential for opening and closing while being inflated.

Poppet valve operation

Poppet valves are a very robust and resilient construction for use in industrial directional control valves. They are usually very tolerant of typical air line contaminants (rust, scale, etc) when used in compressed air service. This type of valve construction is typically characterized as being a high flow, fast acting design due to the large flow paths through the body that can be opened quickly. Think of a poppet valve very much like a stopper or plug in a bath tub drain. When the plug is pulled, the flow path opens quickly and the area that opens is quite large. The large opening of a poppet allows particulate to pass through the valve easily.

Poppets are only one of several different types of construction used in the manufacture of industrial directional control valves. Poppet valves are characterized by having a movable element (the poppet) that is used to direct the flow of fluid through the valve body. The poppet inside is moved via a stem that pushes the poppet off its seat allowing a flow path (in the case of a two-way, normally closed valve), or closing off a flow path by pushing the poppet onto a seat (in the case of a two-way normally open valve). The stem is moved by some sort of actuator (typically a pilot, manual, mechanical or solenoid operator). In the case of a pilot actuator, a piston chamber is pressurized by a fluid or gas, causing a piston to push down on the stem. In the case of manual operation, some sort of device such as a knob, lever, or pedal is actuating the stem via human force. Valves actuated by manual force are often referred to as "human interface" devices. Mechanical operators such as a stem extension, roller, or a cam roller are actuated by the actual process in which the valve is installed. Actuation of the stem via a solenoid can be achieved in one of two ways. When using a direct solenoid, the actual electro-mechanical force pushes directly on the stem to open the poppet. In the case of a solenoid/pilot actuator, the solenoid only controls the flow of a gas (typically compressed air) or liquid into and out of a pilot chamber (discussed above) which moves the stem.

When the operator force is removed from the stem on a normally closed valve, a spring pushes the poppet towards the seat in the body and is assisted by the flow through the valve. Once the poppet reaches the seat, the inlet pressure assists in keeping the poppet seated bubble tight. On normally open valve models, the flow through the valve assists a spring in pushing the valve off its seat to return it to the open condition.

Poppet valves such as illustrated here feature a design that incorporates a seal that is crimped into the poppet's sealing face. The seal materials that are used include various types of rubber, plastics or other exotic polymers which are chosen based on type of medium and operating conditions. Parameters that affect seal material choices would include operating pressure, temperature extremes, chemical composition of the gas or liquid passing through the device, environmental concerns, etc. In some cases, the entire poppet may be made from exotic polymers rather than just the seal insert.

Typical models of poppet valves would include two-way (either in a normally closed or normally open configuration) and three-way operation for filling and exhausting functions from one device. [cite web | url=http://www.lexairinc.com/valves/learning/poppet.html | title=How Poppet Valves Work | year=2007 | publisher=lexairinc.com | accessdate=2007-06-28]

Poppet Valve Applications

Poppet valves are used in many industrial process from controlling the flow of rocket fuel to controlling the flow of milk.

Poppet valve applications include: [cite web | url=http://www.lexairinc.com/valves/learning/poppet.html | title=How Poppet Valves Work | year=2007 | publisher=lexairinc.com | accessdate=2007-06-28]

  • Car wash equipment
  • Laundry equipment
  • Industrial liquid or air controls
  • Water/waste water treatment
  • Air compressors and controls
  • Industrial air dryers and controls
  • Paper and pulp processing
  • Foundry equipment for high flow air, water or other liquids for cooling and processing
  • Utility facilities - for controlling the flow of liquids, gases, etc.
  • Textile industry - flow on bleaching, dyeing and drying equipment
  • Machine tool coolant flow regulation
  • Cooling or refrigeration heat exchanger controls
  • Injection molding machine cooling water controls
  • Induction heating equipment - controlling quench or cooling water flow
  • Resistance welding equipment - controlling cooling water flow
  • Test equipment requiring a fast acting or high pressure, bubble tight valve - includes air, vacuum, liquids, etc.
  • Mining and construction equipment (dust suppression)
  • Coolant flow in vehicles
  • Paintball markers
Specific applications include:
  • Three-way poppet valves have additional applications including:
    • processes that require "rinsing cycles" - cycles where fresh liquid is pumped in then emptied and refilled with fresh liquid
    • pressurization/dumping applications
  • DIN Solenoid Pilot Operated valves equipped with Intrinsically Safe Solenoids work well in hazardous locations
  • Stainless steel poppet valves are designed for pressure applications with highly corrosive or ultra-pure liquid systems, including:
    • Dairy processing
    • Food and Beverage filling, packaging and dispensing
    • Chemical dispensing and processing
    • Breweries and distilleries - water, pasteurization, glycol solutions for cooling, deaeration processes, blending, carbonation, etc.
    • Fertilizer production
    • Pharmaceutical and cosmetic mixing, blending, and dispensing
    • Bottling and bottlewashing equipment

Internal combustion engine

"Poppet valves" are used in most piston engines to open and close the intake and exhaust ports in the cylinder head. The valve is usually a flat disk of metal with a long rod known as the "valve stem" out one end. The stem is used to push down on the valve and open it, with a spring generally used to close it when the stem is not being pushed on. Desmodromic valves are closed by positive mechanical action instead of by a spring, and are used in some high speed motorcycle and auto racing engines, eliminating 'valve float' at high RPM.

For certain applications the valve stem and disk are made of different steel alloys, or the valve stems may be hollow and filled with sodium to improve heat transport and transfer.

The engine normally operates the valves by pushing on the stems with cams and cam followers. The shape and position of the cam determines the valve lift and when and how quickly (or slowly) the valve is opened. The cams are normally placed on a fixed camshaft which is then geared to the crankshaft, running at half crankshaft speed in a four-stroke engine. On high performance engines (e.g., Ferrari cars), the camshaft is movable and the cams have a varying height, so by axially moving the camshaft in relation with the engine RPM, also the valve lift varies. See variable valve timing.

Although better heat conductors, aluminum cylinder heads require steel valve seat inserts while cast iron cylinder heads often used integral valve seats in the past.

Because the valve stem extends into lubrication in the cam chamber it must be sealed against blow-by to prevent cylinder gases from escaping into the mechanical part of the engine. A rubber lip-type seal ensures that excessive amounts of oil are not drawn in from the crankcase on the induction stroke and that exhaust gas does not enter the crankcase on the exhaust stroke. Worn valve seals are characterised by a puff of blue smoke from the exhaust when pressing back down on the accelerator pedal after allowing the engine to over-run, such as when changing gears.

Desmodromic valve drive

Before the days when valve drive dynamics could be analyzed by computer, desmodromic drive seemed to offer solutions for problems that were worsening with increasing engine speed. Famous examples of successful desmodromic engines were Mercedes-Benz W196 and Mercedes-Benz 300 SLR racing cars. Since those days, lift, velocity, acceleration, and jerk curves for cams have been modeled by computer [ [http://www.profblairandassociates.com/pdfs/4sthead-Insight.pdf] ] to reveal that cam dynamics are not what they seemed. With proper analysis, valve adjustment, hydraulic tappets, push rods, rocker arms, and above all, valve float, became things of the past, even without desmodromic drive.Today most automotive engines use overhead cams, as shown in the adjoining dynamic illustration, driving a flat tappet to achieve the shortest and most inelastic path from cam to valve, thereby avoiding elastic elements such as pushrod and rocker arm. Computers enabled accurately designing acceleration profiles, the most important dynamic of valve motion, because it defines forces from F=Ma.

Before numerical computing methods were readily available, acceleration was only attainable by differentiating cam lift profiles twice, once for velocity and again for acceleration. This generates so much hash (noise) that the second derivative (acceleration) was uselessly inaccurate. Computers permitted integration from the jerk curve, the third derivative of lift, that is conveniently a series of contiguous straight lines whose vertices can be adjusted to give any desired lift profile.

Integration of the jerk curves produces a smooth acceleration curves while the third integral gives an essentially ideal lift curve (cam profile). With such cams, that mostly do not look like the ones "artists" formerly designed, valve noise (lift-off) went away and valve train elasticity came under scrutiny.

Today's cams have mirror image (symmetric) profiles with identical positive and negative acceleration while opening and closing valves. An asymmetric cam either opens or closes valves more slowly than it could, speed being limited by Hertzian contact stress between curved cam and flat tappet from accelerating the mass of valve, tappet and spring.

In contrast, desmodromic drive uses two cams per valve, each with separate rocker arm (lever tappets) whose mass and bending elasticity cancel supposed advantages. Maximum valve acceleration being limited by cam-to-tappet galling stress, is governed by moving mass and cam contact area. Rigidity and contact stress are best achieved with conventional flat tappets and springs whose lift and closure stress is unaffected by spring force, both occurring at the base circle [ [http://www.webcamshafts.com/pages/cam_glossary.html] ] where spring load is minimum and contact radius is largest. Curved (lever) tappets [ [http://www.usq.edu.au/users/grantd/motorcycle/ducati/DESMO.HTM] ] of desmodromic cams cause higher contact stress than flat tappets for the same lift profile, thereby limiting rate of lift and closure.

With conventional cams, stress is highest at full lift, when turning at zero speed (engine cranking), and diminishes with increasing speed as inertial force of the valve counter spring pressure, while a desmodromic cam has essentially no load at zero speed (in the absence of springs), its load being entirely inertial, and therefore increasing with speed. However, its greatest inertial stress bears on its smallest radius. Acceleration forces for either method increase with the square of velocity resulting from kinetic energy. [ [http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/energy/u5l1c.html] ]

Desmodromic valve drive was often justified by claims that springs could not close valves reliably at high speed and that the forces caused by suitably strong springs exceeded what cams could withstand. Since then, valve float was analyzed and found to be caused largely by resonance in valve springs that generated oscillating compression waves among coils, much like a Slinky. High speed photography showed that at specific resonant speeds, valve springs were no longer making contact at one or both ends, leaving the valve floating [ [http://www.engr.colostate.edu/~dga/high_speed_video/mechanisms/MERC_valve_spring_tests_1000-6000rpm_1000fps.wmv] ] before crashing into the cam on closure.

For this reason as many as three concentric valve springs, press fit into each other, were often used, not for more force (the inner ones having no significant spring constant), but to act as dampers to reduce oscillations in the outer spring.

An early solution to oscillating spring mass was the "mousetrap", or "hairpin", spring [ [http://www.enginehistory.org/ACEvolution/ACLawrancePenguin.jpg] ] used on Norton Manx [ [http://members.shaw.ca/nortonmanx/engine.htm] ] engines. These avoided resonance but were ungainly to locate inside cylinder heads. Today, formula-one racing engines use gas springs that have no resonant parts, their working parts having an insignificant mass compared to the force of their compressed gas. These springs are expensive and short lived, therefore, offering no benefit for most motors.

Valve springs that do not resonate are progressive, wound with varying pitch or varying diameter called beehive springs [ [http://www.wmr1.com/tipscont.htm] ] from their shape. The number of active coils in these springs varies during the stroke, the more closely wound coils being on the static end, becoming inactive as the spring compresses or as in the beehive spring, where the small diameter coils at the top are stiffer. Both mechanisms reduce resonance because spring force and its moving mass vary with stroke. This advance in spring design removed valve float, the initial impetus for desmodromic valve drive.

Today desmodromic valve drive is an anachronism that, with diligence, can be made to work but at significant cost and design effort, for example, Ducati motorcycles. That overhead cams using flat tappets and springs offer advantages over desmodromic cams is apparent in current automobile engines, none of which use desmodromic drive. Why other motor companies are not using desmodromic valve drive is mentioned in "Motorcycle designs."

Valve position

In very early engine designs the valves were 'upside down' in the block, parallel to the cylinders - the so called L-head engine because of the shape of the cylinder and combustion chamber, also called 'flathead engine' as the top of the cylinder head is flat. Although this design makes for simplified and cheap construction, it has two major drawbacks; the tortuous path followed by the intake charge limits air flow and effectively prevents speeds greater than 2,000-2,500 RPM, and the travels of the exhaust through the block lead to excessive overheating under sustained heavy load. This design therefore evolved into 'Intake Over Exhaust', IOE or F-head, where the intake valve was in the block and the exhaust valve was in the head; later both valves moved to the head.

In most such designs the camshaft remained relatively near the crankshaft and the valves were operated through pushrods and rocker arms. This led to significant energy losses in the engine, but was simpler, especially in a V engine where one camshaft can actuate the valves for both cylinder banks; for this reason, pushrod engine designs persisted longer in these configurations than others.

More modern designs have the camshaft on top of the cylinder head, pushing directly on the valve stem (again through cam followers, also known as tappets), a system known as "overhead camshaft"; if there is just one camshaft, this is a single overhead cam or "SOHC" engine. Often there are two camshafts, one for the intake and one for exhaust valves, creating the dual overhead cam, or "DOHC". The camshaft is driven by the crankshaft - through gears, a chain or a rubber belt.

Valve wear

In the early days of engine building, the poppet valve was a major problem. Metallurgy was not what it is today, and the rapid opening and closing of the valves against the cylinder heads led to rapid wear. They would need to be re-ground every two years or so, in an expensive and time consuming process known as a "valve job". Adding tetra-ethyl lead to the petrol reduced this problem to some degree as the lead would coat the valve seats, in effect lubricating the metal. Valve seats made of improved alloys such as stellite have generally made this problem disappear completely and made leaded fuel unnecessary.

team engine

Poppet valves have also been used on steam locomotives, often in conjunction with Lentz or Caprotti valve gear. British examples include:

* LNER Class B12
* LNER Class D49
* LNER Class P2
* LMS Stanier Class 5 4-6-0
* BR standard class 5
* BR standard class 8 71000 Duke of Gloucester.

Sentinel Waggon Works used poppet valves in their steam wagons and steam locomotives. Reversing was achieved by a simple sliding camshaft system.

References

ee also

* D slide valve
* Piston valve
* Crank Piston Valve [http://www.new4stroke.com]
* Sleeve valve
* Safety valve
* Relief valve
* Pressure relief valve
* Corliss valve


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