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This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2007) |
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.
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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 a fluid or a gas 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 (previously 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 upon various 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.How Poppet Valves Work. lexairinc.com (2007). Retrieved on 2007-06-28.
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:How Poppet Valves Work. lexairinc.com (2007). Retrieved on 2007-06-28.
Specific applications include:
Components of a typical, four stroke cycle, DOHC piston engine:
(E) Exhaust camshaft,
(I) Intake camshaft,
(S) Spark plug,
(V) Valves,
(P) Piston,
(R) Connecting rod,
(C) Crankshaft,
(W) Water jacket for coolant flow.
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. used in 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.
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.
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 making leaded fuel unnecessary.
Poppet valves have also been used on steam locomotives, often in conjunction with Lentz or Caprotti valve gear. British examples include:
Sentinel Waggon Works used poppet valves in their steam wagons and steam locomotives. Reversing was achieved by a simple sliding camshaft system.
| Heat engines | |
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| Stroke cycles | One-stroke · Two-stroke · Four-stroke · Six-stroke |
| Engine types | Controlled combustion · Jet · Orbital · Piston · Quasiturbine · Rocket · Swing-piston · Tschudi · Twingle · Wankel Rotary (Britalus · Coomber · Pistonless · Swing-piston) |
| Valves | Cylinder head porting · D slide · Four-stroke · Manifold · Multi · Piston · Poppet · Rocket engine nozzle · Sleeve |
| Piston layouts | Bourke · Delta · Double acting/differential cylinder · Opposed · Radial · Rotary · Single cylinder · Stelzer · Straight |
| Motion mechanisms | Cam · Connecting rod · Coomber rotary · Crank · Crank substitute · Crankshaft · Parallel motion · Piston rod · Pistons back-and-forth · QRMC Stirling/HydraLink · Revolving cylinder · Rhombic drive · Scotch yoke · Sun and planet gear · Swashplate Linkages (Evans · Peaucellier-Lipkin · Sector straight-line · Watt) |
| Thermodynamic cycle | |
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