Piston
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A piston is a component of
Piston engines
Internal combustion engines
An
The typical piston design is on the picture. This type of piston is widely used in car
High-power diesel engines work in difficult conditions. Maximum pressure in the combustion chamber can reach 20 MPa and the maximum temperature of some piston surfaces can exceed 450 °C. It is possible to improve piston cooling by creating a special cooling cavity. Injector supplies this cooling cavity «A» with oil through oil supply channel «B». For better temperature reduction construction should be carefully calculated and analysed. Oil flow in the cooling cavity should be not less than 80% of the oil flow through the injector.
The pin itself is of hardened steel and is fixed in the piston, but free to move in the connecting rod. A few designs use a 'fully floating' design that is loose in both components. All pins must be prevented from moving sideways and the ends of the pin digging into the cylinder wall, usually by circlips.
Gas sealing is achieved by the use of piston rings. These are a number of narrow iron rings, fitted loosely into grooves in the piston, just below the crown. The rings are split at a point in the rim, allowing them to press against the cylinder with a light spring pressure. Two types of ring are used: the upper rings have solid faces and provide gas sealing; lower rings have narrow edges and a U-shaped profile, to act as oil scrapers. There are many proprietary and detail design features associated with piston rings.
Pistons are usually
Early pistons were of cast iron, but there were obvious benefits for engine balancing if a lighter alloy could be used. To produce pistons that could survive engine combustion temperatures, it was necessary to develop new alloys such as Y alloy and Hiduminium, specifically for use as pistons.
A few early
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Trunk pistons
Trunk pistons are long relative to their diameter. They act both as a piston and cylindrical crosshead. As the connecting rod is angled for much of its rotation, there is also a side force that reacts along the side of the piston against the cylinder wall. A longer piston helps to support this.
Trunk pistons have been a common design of piston since the early days of the reciprocating internal combustion engine. They were used for both petrol and diesel engines, although high speed engines have now adopted the lighter weight slipper piston.
A characteristic of most trunk pistons, particularly for diesel engines, is that they have a groove for an oil ring below the gudgeon pin, in addition to the rings between the gudgeon pin and crown.
The name 'trunk piston' derives from the '
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Crosshead pistons
Large slow-speed Diesel engines may require additional support for the side forces on the piston. These engines typically use crosshead pistons. The main piston has a large piston rod extending downwards from the piston to what is effectively a second smaller-diameter piston. The main piston is responsible for gas sealing and carries the piston rings. The smaller piston is purely a mechanical guide. It runs within a small cylinder as a trunk guide and also carries the gudgeon pin.
Lubrication of the crosshead has advantages over the trunk piston as its lubricating oil is not subject to the heat of combustion: the oil is not contaminated by combustion soot particles, it does not break down owing to the heat and a thinner, less viscous oil may be used. The friction of both piston and crosshead may be only half of that for a trunk piston.[3]
Because of the additional weight of these pistons, they are not used for high-speed engines.
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Slipper pistons
A slipper piston is a piston for a petrol engine that has been reduced in size and weight as much as possible. In the extreme case, they are reduced to the piston crown, support for the piston rings, and just enough of the piston skirt remaining to leave two lands so as to stop the piston rocking in the bore. The sides of the piston skirt around the gudgeon pin are reduced away from the cylinder wall. The purpose is mostly to reduce the reciprocating mass, thus making it easier to balance the engine and so permit high speeds.
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Deflector pistons
Deflector pistons are used in
Much effort, and many different designs of piston crown, went into developing improved scavenging. The crowns developed from a simple rib to a large asymmetric bulge, usually with a steep face on the inlet side and a gentle curve on the exhaust. Despite this, cross scavenging was never as effective as hoped. Most engines today use Schnuerle porting instead. This places a pair of transfer ports in the sides of the cylinder and encourages gas flow to rotate around a vertical axis, rather than a horizontal axis.[8]
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Racing Pistons
In racing engines, piston strength and stiffness is typically much higher than that of a passenger car engine, while the weight is much less, to achieve the high engine RPM necessary in racing.[9]
Hydraulic cylinders
Hydraulic cylinders can be both
Steam engines
Pumps
Piston pumps can be used to move liquids or compress gases.
For liquids
For gases
Air cannons
There are two special type of pistons used in air cannons: close tolerance pistons and double pistons. In close tolerance pistons O-rings serve as a valve, but O-rings are not used in double piston types.[citation needed]
See also
- Air gun
- Fire piston
- Fruit press
- Gas-operated reloading, using a gas piston
- Hydraulic cylinder
- List of auto parts
- Piston motion equations
- Shock absorber
- Slide whistle
- Steam locomotive components
- Syringe
- Wankel engine, an internal combustion engine design with a rotor instead of pistons
Notes
References
- ^ Magda, Mike. "What Makes A Racing Piston?". Retrieved 2018-04-22.
- ^ Bailey, Kevin. "Full-Round vs. Strutted: Piston Forging Designs and Skirt Styles Explained". Retrieved 2018-07-15.
- ^ Ricardo (1922), p. 116.
- ^ Ricardo (1922), p. 149.
- ^ Piston with improved side loading resistance, 2009-10-12, retrieved 2018-04-22
- ^ Ricardo (1922), pp. 119–120, 122.
- ^ Irving, Two stroke power units, pp. 13–15.
- ^ Irving, Two stroke power units, pp. 15–16.
- ^ "Racing Piston Technology – Piston Weight And Design – Circle Track Magazine". Hot Rod Network. 2007-05-31. Retrieved 2018-04-22.
Bibliography
- Irving, P.E. (1967). Two-Stroke Power Units. Newnes.
- Blackie.