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The Info List - Piston


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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

Contents

1 Piston
Piston
engines

1.1 Internal combustion engines

1.1.1 Trunk pistons 1.1.2 Crosshead
Crosshead
pistons 1.1.3 Slipper pistons 1.1.4 Deflector pistons

1.2 Steam engines

2 Pumps

2.1 For liquids 2.2 For gases

3 Air cannons 4 See also 5 References 6 Bibliography 7 External links

Piston
Piston
engines[edit] Main article: Reciprocating engine Internal combustion engines[edit]

Internal combustion engine
Internal combustion engine
piston, sectioned to show the gudgeon pin.

An internal combustion engine is acted upon by the pressure of the expanding combustion gases in the combustion chamber space at the top of the cylinder. This force then acts downwards through the connecting rod and onto the crankshaft. The connecting rod is attached to the piston by a swivelling gudgeon pin (US: wrist pin). This pin is mounted within the piston: unlike the steam engine, there is no piston rod or crosshead (except big two stroke engines). 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
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 cast from aluminium alloys. For better strength and fatigue life, some racing pistons may be forged instead. 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 gas engines[i] had double-acting cylinders, but otherwise effectively all internal combustion engine pistons are single-acting. During World War II, the US submarine Pompano[ii] was fitted with a prototype of the infamously unreliable H.O.R. double-acting two-stroke diesel engine. Although compact, for use in a cramped submarine, this design of engine was not repeated. Media related to Internal combustion engine
Internal combustion engine
pistons at Wikimedia Commons Trunk pistons[edit] 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 'trunk engine', an early design of marine steam engine. To make these more compact, they avoided the steam engine's usual piston rod with separate crosshead and were instead the first engine design to place the gudgeon pin directly within the piston. Otherwise these trunk engine pistons bore little resemblance to the trunk piston; they were extremely large diameter and double-acting. Their 'trunk' was a narrow cylinder mounted in the centre of the piston. Media related to Trunk pistons at Wikimedia Commons Crosshead
Crosshead
pistons[edit] 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.[1] Because of the additional weight of these pistons, they are not used for high-speed engines. Media related to Crosshead
Crosshead
pistons at Wikimedia Commons Slipper pistons[edit]

Slipper piston

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.[2] Reduced inertia also improves mechanical efficiency of the engine: the forces required to accelerate and decelerate the reciprocating parts cause more piston friction with the cylinder wall than the fluid pressure on the piston head.[3] A secondary benefit may be some reduction in friction with the cylinder wall, since the area of the skirt, which slides up and down in the cylinder is reduced by half. However, most friction is due to the piston rings, which are the parts which actually fit the tightest in the bore and the bearing surfaces of the wrist pin, and thus the benefit is reduced. Media related to Slipper pistons at Wikimedia Commons

Deflector pistons[edit]

Two-stroke deflector piston

Deflector pistons are used in two-stroke engines with crankcase compression, where the gas flow within the cylinder must be carefully directed in order to provide efficient scavenging. With cross scavenging, the transfer (inlet to the cylinder) and exhaust ports are on directly facing sides of the cylinder wall. To prevent the incoming mixture passing straight across from one port to the other, the piston has a raised rib on its crown. This is intended to deflect the incoming mixture upwards, around the combustion chamber.[4] 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.[5] Media related to Deflector pistons at Wikimedia Commons

Steam engines[edit]

Cast-iron steam engine piston, with a metal piston ring spring-loaded against the cylinder wall.

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.) Another factor is that since almost all steam engines use crossheads to translate the force to the drive rod, there are few lateral forces acting to try and "rock" the piston, so a cylinder-shaped piston skirt isn't necessary.

Early (c. 1830) piston for a beam engine. The piston seal is made by turns of wrapped rope.

Pumps[edit] Piston
Piston
pumps can be used to move liquids or compress gases. For liquids[edit] Main article: Reciprocating pump For gases[edit] Main article: Reciprocating compressor Air cannons[edit] 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. See also[edit]

Air gun Fire piston Fruit press Hydraulic cylinder Slide whistle Wankel engine
Wankel engine
(an internal combustion engine with a rotary 'piston') Steam locomotive components A gas operated firearm uses a gas piston to operate the action of the weapon Many everyday objects contain pistons of one form or another; for example, a syringe is a rubber piston inside of a cylinder; shock absorbers and pneumatic door closers contain pistons.

References[edit]

^ 'Gas' here refers to a fuel gas, not gasoline. ^ A handful of submarines in the following class used a similar engine, with almost equally poor results.

^ Ricardo (1922), p. 116. ^ Ricardo (1922), p. 149. ^ Ricardo (1922), pp. 119–120,122. ^ Irving, Two stroke power units, pp. 13–15. ^ Irving, Two stroke power units, pp. 15–16.

Bibliography[edit]

Irving, P.E. (1967). Two-Stroke Power Units. Newnes.  Ricardo, Harry (1922). The Internal Combustion Engine. Vol I: Slow-Speed Engines (1st ed.). London: Blackie. 

External links[edit]

Wikimedia Commons has media related to Pistons.

Piston
Piston
Engines Essay How Stuff Works - Basic Engine Parts

v t e

Automotive engine

Part of the Automobile series

Basic terminology

Bore Compression ratio Crank Cylinder Dead centre Diesel engine Dry sump Engine balance Engine configuration Engine displacement Engine knocking Firing order Hydrolock Petrol engine Power band Redline Spark-ignition engine Stroke Stroke ratio Wet sump

Main components

Connecting rod Crankcase Crankpin Crankshaft Crossplane Cylinder bank Cylinder block Cylinder head
Cylinder head
(crossflow, reverse-flow) Flywheel Head gasket Hypereutectic piston Main bearing Piston Piston
Piston
ring Starter ring gear Sump

Valvetrain

Cam Cam
Cam
follower Camshaft Desmodromic valve Hydraulic tappet Multi-valve Overhead camshaft Overhead valve Pneumatic valve springs Poppet valve Pushrod Rocker arm Sleeve valve Tappet Timing belt Timing mark Valve
Valve
float Variable valve timing

Aspiration

Air filter Blowoff valve Boost controller Butterfly valve Centrifugal-type supercharger Cold air intake Dump valve Electronic throttle control Forced induction Inlet manifold Intake Intercooler Manifold vacuum Naturally aspirated engine Ram-air intake Scroll-type supercharger Short ram air intake Supercharger Throttle Throttle
Throttle
body Turbocharger Twin-turbo Variable-geometry turbocharger Variable-length intake manifold Warm air intake

Fuel system

Carburetor Common rail Direct injection Fuel filter Fuel injection Fuel pump Fuel tank Gasoline
Gasoline
direct injection Indirect injection Injection pump Lean-burn Stratified charge engine Turbo fuel stratified injection Unit injector

Ignition

Contact breaker Magneto Distributor Electrical ballast High tension leads Ignition coil Spark plug Wasted spark

Electrics and engine management

Air–fuel ratio meter Alternator Automatic Performance Control Car
Car
battery (lead–acid battery) Crankshaft
Crankshaft
position sensor Dynamo Drive by wire Electronic control unit Engine control unit Engine coolant temperature sensor Glow plug Idle air control actuator MAP sensor Mass flow sensor Oxygen sensor Starter motor Throttle
Throttle
position sensor

Exhaust system

Automobile emissions control Catalytic converter Diesel particulate filter Exhaust manifold Glasspack Muffler

Engine cooling

Air cooling Antifreeze
Antifreeze
(ethylene glycol) Core plug Electric fan Fan belt Radiator Thermostat Water cooling Viscous fan (fan clutch)

Other components

Balance shaft Block heater Combustion chamber Cylinder head
Cylinder head
porting Gasket Motor oil Oil filter Oil pump Oil sludge PCV valve Seal Synthetic oil Underdrive pulleys

Portal Category

v t e

Aircraft piston engine components, systems and terminology

Piston
Piston
engines

Mechanical components

Camshaft Connecting rod Crankpin Crankshaft Cylinder Cylinder head Gudgeon pin Hydraulic tappet Main bearing Obturator ring Oil pump Piston Piston
Piston
ring Poppet valve Pushrod Rocker arm Sleeve valve Tappet

Electrical components

Alternator Capacitor discharge ignition Dual ignition Electronic fuel injection Generator Ignition system Magneto Spark plug Starter

Terminology

Air-cooled Aircraft engine
Aircraft engine
starting Bore Compression ratio Dead centre Engine displacement Four-stroke engine Horsepower Ignition timing Manifold pressure Mean effective pressure Naturally aspirated Monosoupape Overhead camshaft Overhead valve engine Rotary engine Shock cooling Stroke Time between overhaul Two-stroke engine Valve
Valve
timing Volumetric efficiency

Propellers

Components

Propeller governor Propeller speed reduction unit Spinner

Terminology

Autofeather Blade pitch Constant-speed Contra-rotating Counter-rotating Scimitar Single-blade Variable-pitch

Engine instruments

Annunciator panel EFIS EICAS Flight data recorder Glass cockpit Hobbs meter Tachometer

Engine controls

Carburetor
Carburetor
heat Throttle

Fuel and induction system

Avgas Carburetor Fuel injection Gascolator Inlet manifold Intercooler Pressure carburetor Supercharger Turbocharger Updraft carburetor

Other systems

Auxiliary power unit Coffman starter Hydraulic system Ice protection system Recoil start

v t e

Steam engines

Operating cycle

Atmospheric Watt Cornish Compound Uniflow

Valves

Valves

Slide

D slide

Piston Drop Corliss Poppet Sleeve Bash

Valve
Valve
gear

Gab Stephenson link Joy Walschaerts Allan Baker Corliss Lentz Caprotti Gresley conjugated Southern

Mechanisms

Beam Cataract Centrifugal governor Connecting rod Crank Crankshaft Hypocycloidal gear Link chain Parallel motion Plate chain Rotative beam Sun and planet gear Watt's linkage

Boilers

Simple boilers

Haystack Wagon Egg-ended Box Flued Cornish Lancashire

Fire-tube boilers

Locomotive Scotch Launch

Water-tube boilers

Babcock & Wilcox Field-tube Sentinel Stirling Thimble tube Three-drum Yarrow

Boiler
Boiler
feed

Feedwater heater Feedwater pump Injector

Cylinder

Locomotive Oscillating Single- and double-acting

Condenser

Condensing steam locomotive Jet Kirchweger Watt's separate "Pickle-pot" Surface

Other

Crosshead Cutoff Expansion valve Hydrolock Piston Reciprocating engine Return connecting rod engine Six-column beam engine Steeple engine Safety valve Steeple compound engine Stroke Working fluid

History

Precursors

Savery Engine (1698)

Newcomen engine

Newcomen Memorial Engine
Newcomen Memorial Engine
(1725) Fairbottom Bobs
Fairbottom Bobs
(1760) Elsecar Engine
Elsecar Engine
(1795)

Watt engine

Beam

Kinneil Engine
Kinneil Engine
(1768) Old Bess (1777) Chacewater Mine engine (1778) Smethwick Engine
Smethwick Engine
(1779) Resolution (1781)

Rotative beam

Soho Manufactory engine (1782) Bradley Works engine (1783) Whitbread Engine
Whitbread Engine
(1785) National Museum of Scotland engine (1786) Lap Engine
Lap Engine
(1788)

High-pressure

Richard Trevithick

Puffing Devil (1801) London Steam Carriage (1803) "Coalbrookdale Locomotive" (1803) "Pen-y-Darren" locomotive (1804)

Compound

Woolf's compound engine (1803)

Murray

Murray's Hypocycloidal Engine
Murray's Hypocycloidal Engine
(1805) Salamanca (1812)

High-speed

Porter-Allen (1862) Ljungström (1908)

See also

Glossary of steam locomotive components History of steam road vehicles

Cugnot's fardier à vapeur (1769) Murdoch's model steam carriage (1784)

Lean's Engine Reporter List of steam technology patents Modern steam Stationary steam engine Timeline of steam power Water-returning engine

Authority control

.