Piston

A piston is known in engineering a movable component which, together with a stationary member, the cylinder having a closed cavity, whose volume can be changed.

Operation

The moving piston gives the system the ability to change this cavity.

A simple version of this arrangement, a piston, which plunges into a correspondingly shaped housing. The respective position of the piston in the housing is determined as the size of the cavity.

Usage

This variable cavity thus created makes it possible to refer to a fluid part of its energy (motion, pressure) and in a machine to make it usable (motor). Also, this arrangement allows the opposite way, ie to supply the fluid energy ( pump).

Conditions

However, the interaction between the moving piston and proof casing for energy transfer between the fluid and the machine is not possible without proper shutoff and inlet and outlet facilities for the fluid.

The piston is the component with which the energy exchange is carried out. Analogously, the forces must be introduced or derived by means of suitable devices to the flask.

The shape of a piston, the material from which it is made, his way and the speed at which it moves, subject to a variety of factors. The standing for use fluid, the forces and temperatures that occur as well as machine-side conditions lead to very different designs.

Machines in which access piston used is called piston engines. For more information on machines with different piston can be found in the relevant articles.

Example of internal combustion engine

Today (2012) most widely used piston engines are of the Otto engine (gasoline) and diesel engines. Here the use of the above-described principle will be explained using this machine.

General

The piston has the following functional components:

Pistons in internal combustion engines

Piston for reciprocating engines today are mostly made of cast aluminum alloys, early, however, often made ​​of cast iron. The production of the blank is in permanent mold casting. Because of performance improvements as well as fuel consumption and emission reductions through higher ignition pressures the piston for powerful turbo diesel engines are also forged. This is followed by mechanical machining of the outer diameter, of the valve bag, the piston ring grooves and the piston pin hole.

Technologically exist between diesel and gasoline engine piston due to the different loads through both combustion processes are significant differences:

Diesel pistons are both thermally and mechanically loaded higher and must therefore in the first piston ring groove with a cast ring carrier made ​​of an austenitic cast iron ( " Niresist " ) are reinforced to prevent deflection of the groove and material transfer to the ring by micro-welding. For very highly stressed piston pin bore in the brass bushings are overstretched. Another distinguishing feature of piston direct injection diesel engines, the bottom tray in which the injected fuel is swirled with the air and mixed. Thermally highly loaded piston (especially for racing, flight - or turbo-diesel engines) are often realized with spray nozzles for cooling the piston crown. The piston may have a circumferential oil channel or cooled only by Bodenanspritzung. In low-speed heavy-duty engines, the piston can also be cooled by circulating cooling. The medium is supplied to the piston by means of a telescopic rod.

Characteristic of petrol piston is significantly thinner walled design that allows higher engine speeds because of the lower weight. In the area of the first piston ring groove can be used in part a hard anodizing to reduce wear and micro-welding.

The piston head bears partially shallow pockets for receiving the protruding into the combustion chamber valves.

The piston skirt serves to guide the piston in the cylinder tube and is coated with most piston with a bonded coating. It helps older types often inside a cast steel strip ( " control plate ", " Autothermik - piston ") in order to control the diameter growth when heated. To save weight, is today in many high-speed four -stroke engines the piston skirt on the sides ( to the piston pin holes ) inwards ( "box" pistons).

A rare piston design is the articulated piston, which contributes both to the optimized combustion chamber design as thermal management of the engine.

The piston carries one or more grooves for the piston rings, the uppermost of the compression rings, and at least a lower oil control ring is used as. Car pistons have for the most part two compression and one oil scraper ring. For racing engines, so-called two-ring piston come with only one compression ring for use. In two-stroke engines the piston skirts can also be provided with windows. In addition, the fact that most two-stroke piston locking pins in the piston ring grooves, to prevent twisting or jamming of the piston ring gaps to control the windows of the cylinder. With old engines up in the 1950s with two-stroke engines a piston nose was customary to allow gas exchange or rinsing. More modern two-stroke engines have, however, generally a flat piston crown, as the flushing or the gas exchange than reverse circulation ( system Schnürle ) takes place with a change of direction in the cylinder.

The transmission of the piston to the connecting rod takes place via the piston bolt. This is stored in the flask in the thickened inward part of the shirt in a hole. This hole often wears at the end grooves for retaining rings ( " retaining rings " ) to limit the lateral excursion of the piston pin.

Desachsierung of the gudgeon pin

In the automotive sector, the axis of the piston pin is offset by about 0.5-1.5 mm from the piston center of pressure loaded side. Without this Desachsierung the piston would the investment side after the top dead center ( TDC) change under full combustion pressure. As a result of Desachsierung the piston changes the investment side already before TDC when the compression pressure is still under construction. This reduces wear of the piston and reduce the engine noise.

The piston head lying combustion chamber in a diesel engine vorkammerlosen

Piston of a two-stroke diesel with a mass of 570 kg

Main suppliers of piston

• KSPG AG
• Mahle GmbH
• Federal Mogul GmbH
• Pankl Engine Systems