Piston


Piston
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.

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall.

Contents

Piston engines

Internal combustion engines

There are two ways that an internal combustion piston engine can transform combustion into motive power: the two-stroke cycle and the four-stroke cycle. A single-cylinder two-stroke engine produces power every crankshaft revolution, while a single-cylinder four-stroke engine produces power once every two revolutions. Older designs of small two-stroke engines produced more pollution than four-stroke engines. However, modern two-stroke designs, like the Vespa ET2 Injection utilise fuel-injection and are as clean as four-strokes. Large diesel two-stroke engines, as used in ships and locomotives, have always used fuel-injection and produce low emissions. One of the biggest internal combustion engines in the world, the Wärtsilä-Sulzer RTA96-C is a two-stroke; it is bigger than most two-storey houses, has pistons nearly 1 metre in diameter and is one of the most efficient mobile engines in existence. In theory, a four-stroke engine has to be larger than a two-stroke engine to produce an equivalent amount of power. Two-stroke engines are becoming less common in developed countries these days, mainly due to manufacturer reluctance to invest in reducing two-stroke emissions. Traditionally, two-stroke engines were reputed to need more maintenance (despite exceptions like the Ricardo Dolphin engine, and the Twingle engines of the Trojan car and the Puch 250 motorcycle). Even though the simplest two-stroke engines have fewer moving parts, they could wear out faster than four-stroke engines. However fuel-injected two-strokes achieve better engine lubrication, also cooling and reliability should improve considerably

Steam engines

Steam engines are usually double-acting (i.e. steam pressure acts alternately on each side of the piston) and the admission and release of steam is controlled by slide valves, piston valves or poppet valves. Consequently, steam engine pistons are nearly always comparatively thin discs: their diameter is several times their thickness. (One exception is the trunk engine piston, shaped more like those in a modern internal-combustion engine.)

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. While in close tolerance pistons, O-rings serve as a valve, O-rings are not used in in double piston types.

Close-tolerance pistons have a number of disadvantages: They can swell and stick, their properties alter due to atmospheric changes, and they fit tightly in the cylinder with close tolerances. Backlash may suck some of the bin material into the valve which can cause the piston to stick.

Common features of double piston construction: They cannot swell and stick, they fit loosely in the cylinder (no tight tolerances), atmospheric changes do not affect them, and foreign material entering the cylinder doesn't cause sticking.

Drawbacks

Since the piston is the main reciprocating part of an engine, its movement creates an imbalance. This imbalance generally manifests itself as a vibration, which causes the engine to be perceivably harsh. The friction between the walls of the cylinder and the piston rings eventually results in wear, reducing the effective life of the mechanism.

The sound generated by a reciprocating engine can be intolerable and as a result, many reciprocating engines rely on heavy noise suppression equipment to diminish droning and loudness. To transmit the energy of the piston to the crank, the piston is connected to a connecting rod which is in turn connected to the crank. Because the linear movement of the piston must be converted to a rotational movement of the crank, mechanical loss is experienced as a consequence. Overall, this leads to a decrease in the overall efficiency of the combustion process. The motion of the crank shaft is not smooth, since energy supplied by the piston is not continuous and it is impulsive in nature. To address this, manufacturers fit heavy flywheels which supply constant inertia to the crank. Balance shafts are also fitted to some engines, and diminish the instability generated by the pistons movement. To supply the fuel and remove the exhaust fumes from the cylinder there is a need for valves and camshafts. During opening and closing of the valves, mechanical noise and vibrations may be encountered. A two-stroke engine does not require valves, meaning it doesn't need a camshaft, making these engines faster and more powerful.

See also

References

External links


Wikimedia Foundation. 2010.

Synonyms:

Look at other dictionaries:

  • piston — [ pistɔ̃ ] n. m. • 1648; « pilon » 1534; it. pistone, de pestare → piste 1 ♦ Pièce cylindrique qui se meut dans un tube (corps de pompe, cylindre de machine, de moteur), où elle reçoit et transmet une pression exercée par un fluide. Course,… …   Encyclopédie Universelle

  • Piston — Pis ton, n. [F. piston; cf. It. pistone piston, also pestone a large pestle; all fr. L. pinsere, pistum, to pound, to stamp. See {Pestle}, {Pistil}.] (Mach.) A sliding piece which either is moved by, or moves against, fluid pressure. It usually… …   The Collaborative International Dictionary of English

  • piston — PISTÓN, pistoane, s.n. 1. Organ de maşină care efectuează o mişcare alternativă de translaţie într un cilindru în care se găseşte un fluid sub presiune. 2. Instrument muzical de suflat din alamă, asemănător cu trompeta. ♦ Clapă mobilă care… …   Dicționar Român

  • pistón — (Del fr. piston). 1. m. émbolo. 2. Parte o pieza central de la cápsula, donde está colocado el fulminante. 3. Llave en forma de émbolo que tienen diversos instrumentos musicales de viento. ☛ V. escopeta de pistón, fusil de pistón, llave de pistón …   Diccionario de la lengua española

  • Piston — kann Folgendes bedeuten: eine umgangssprachliche Bezeichnung für die französische Ingenieursschule École Centrale Paris ein Blechblasinstrument, siehe Kornett (Instrument) die Befestigungsvorrichtung für Zündhütchen bei Perkussionswaffen, siehe… …   Deutsch Wikipedia

  • piston — PISTON. s. m. Cylindre de bois, de fer, ou de cuivre, qui est garni de cuir ou de feutre par le bout, & qui entre dans le corps d une pompe; & sert à eslever l eau. Piston usé. la pompe ne va pas, le piston est rompu. raccommoder un piston …   Dictionnaire de l'Académie française

  • piston — 1704, from Fr. piston, from M.Fr. piston large pestle, from O.It. pistone, variant of pestone a pestle, from pestare to pound, from L.L. pistare, frequentative of L. pinsere (pp. pistus) to pound (see PESTLE (Cf. pestle)) …   Etymology dictionary

  • Piston [1] — Piston (fr., spr. Pistong), 1) Pumpenstock, Kunststange; 2) bei Percussionsgewehren der mit einem Zündloche versehene Stahlcylinder, auf welchen das Zündhütchen aufgesetzt wird …   Pierer's Universal-Lexikon

  • Piston [2] — Piston, griechischer Bildhauer aus Kalauria, Schüler Amphions …   Pierer's Universal-Lexikon

  • Piston — (franz., spr. óng), 1) Pumpenstock, Ziehstange; bei Perkussionswaffen der das Zündloch enthaltende Zündstift, auf den das Zündhütchen aufgesetzt wird. S. auch Pistons. – 2) Blasinstrument, soviel wie Cornet à pistons (s. Kornett 3) …   Meyers Großes Konversations-Lexikon

  • Piston — (frz., spr. óng), Kolben, Pumpenstock; Zündkegel, zum Aufsetzen des Zündhütchens an Perkussionsgewehren; mechan. Vorrichtung an Blechblasinstrumenten, die deren Schallröhre verlängert; fälschlich auch für Cornet à pistons (s. Kornett) gebraucht …   Kleines Konversations-Lexikon


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.