Piston (technology)
In mechanical engineering, pistons are movable components that, together with the surrounding housing, form a closed cavity, the volume of which changes due to movement . This principle can be implemented in various designs: with reciprocating pistons moving up and down, rotating rotary pistons or something else.
A simple, widespread design that was already known in antiquity is a round disc that is immersed in a tubular housing , the cylinder . The position of the piston in the housing determines the size of the cavity.
Usage
In engines , part of its energy is taken from a gaseous or liquid working medium in these variable cavities created in this way , which can then be used in a working machine . In pumps and compressors, on the other hand, energy is supplied, whereby the pressure and, in the case of gases, also the temperature of the medium increases.
Types
Machines in which pistons transmit the energy belong to the piston machines . For energy transfer in the machine, the piston and housing can interact with inlet and outlet pipes and valves .
The piston, and rarely also the housing, is the component with which the energy exchange takes place. With reciprocating pistons, forces are usually introduced into the piston via crank drives with connecting rods or via piston rods . In free piston machines, the piston typically works on gas springs.
The shape of a piston, the material it is made of, its path and the speed at which it moves are subject to a variety of factors. The fluid used, the forces and temperatures that occur and the type of machine lead to very different designs.
Section through the cylinder of a double-acting steam engine with disc piston
Plunger piston with connecting rod of a gasoline engine
Section through a pump with a plunger piston
Roots blower with two rotary pistons
Rotary piston of a screw compressor
Reciprocating pistons are available as disc pistons, plunger pistons and plunger pistons:
- Disc pistons are thin plates that seal against the cylinder at the edge. They usually work via a piston rod that is guided outside and prevents tilting. In the case of crank drives, the connecting rod is connected via a cross head that keeps the side force away from the piston rod.
- In the case of plungers, the sealing plate and the cross head are combined into one component. The lateral forces are transferred to the cylinder.
- The slim and cylindrical shaped plungers move in and out of the working area through a fixed seal (typically a stuffing box ). The shape of the working space can largely be freely selected for pumps.
Application example internal combustion engine
The piston engines that are most widely used to date (2016) are the Otto engine (petrol) and the diesel engine . The use of the principle described above is explained here using this machine.
General
The piston shows the following functional components:
- the piston crown, which is in contact with the medium. The piston crown is also referred to as the top land edge.
- the top land. It extends from the piston crown to the upper piston ring groove. It protects the first piston ring from overheating.
- the ring belt: Together with the other grooves and ring lands, the top land forms the so-called ring belt .
- the piston skirt or piston skirt or piston wall: the cylindrical component that fits into the cylinder bore with a small amount of play,
- the piston pin with its bearing that connects the piston with the connecting rod .
Ring belt of a piston
Top land, piston roof and piston skirt on one piston.
Pistons in internal combustion engines
Pistons for reciprocating engines are now predominantly made of cast aluminum alloys , previously often made of cast iron . The blanks are cast in permanent molds , for some - especially for powerful engines - they are forged. The outer surface, valve pockets , piston ring grooves and the piston pin bore are then mechanically machined.
Technological differences exist between diesel and gasoline engine pistons due to the different combustion processes:
Diesel pistons are subject to higher thermal and mechanical loads and must therefore be reinforced in the first piston ring groove with a cast-in ring carrier made of austenitic cast iron ("Niresist") in order to prevent the groove from knocking out and material transfer to the ring through micro-welds. In the case of very heavily loaded pistons, brass bushings are stretched in the pin bore. Another characteristic feature of the pistons of direct injection diesel engines is the floor cavity in which the injected fuel is swirled and mixed with the air. Pistons subject to high thermal loads (especially racing, aviation or turbo- diesel engines) are often implemented with spray nozzles (for engine oil) to cool the piston crown. The piston can be provided with a circumferential oil channel or can only be cooled by injection molding on the bottom. In the case of large, slow-running engines, the piston can also be cooled by circulating cooling. The medium is fed to the piston through a telescopic tube.
Pistons in gasoline engines are characterized by their significantly lower wall thickness, which allows higher engine speeds due to their lower weight. In the area of the first piston ring groove, hard anodizing can be used to reduce wear and tear and micro-welds.
The piston crown partially carries flat pockets to accommodate the valves protruding into the combustion chamber .
The piston skirt is used to guide the piston in the cylinder barrel and is coated with a lubricating varnish on most pistons. In older designs, it often has a cast steel strip on the inside ( control piston , “control plate”, “autothermal piston”) to control the increase in diameter when heated. To save weight, many high-speed four-stroke engines have the piston skirt on the sides (at the piston pin openings) offset inwards ("box" piston).
In articulated pistons, the sealing piston crown with the piston rings and the piston end, which transfers the side forces from the connecting rod to the cylinder, are two components that are articulated via the piston pin. The gap between the two parts reduces the heat loss from the combustion chamber.
The piston has one or more grooves for the piston rings, the uppermost of which are the compression rings and at least one lower one serves as an oil control ring. Most car pistons have two compression rings and one oil control ring. So-called two-ring pistons with only one compression ring are also used for racing engines. With two-stroke engines , the piston skirts can also have windows. In addition, most two-stroke pistons have locking pins in the piston ring grooves to prevent the piston ring joints from twisting and jamming in the cylinder's control windows. Up until the 1950s there were two-stroke engines with nose pistons that were supposed to improve the gas exchange during transverse scavenging. Since the 1930s, however, two-stroke engines with reverse scavenging according to Adolf Schnürle have generally had a flat piston crown.
The power transmission from the piston to the connecting rod takes place via the piston pin . This is stored in a hole in the piston in the inwardly thickened part of the shirt. This bore often has grooves at the end for retaining rings ("Seeger rings") in order to limit the lateral movement of the piston pin.
Displacement of the piston pin
In the automotive sector, the axis of the piston pin is offset by approx. 0.5–1.5 mm from the center of the piston to the side under pressure. Without this dislocation, the piston would change the contact side after top dead center (TDC) under full combustion pressure. As a result of the offset, the piston changes the contact side before TDC when the compression pressure is only being built up. This reduces wear on the piston and reduces engine noise.
An incision from the
socket + piston
Combustion chamber located in the piston crown in a pre-chamberless diesel engine , see also M method
Piston of a two-stroke diesel engine with a mass of 570 kg
Major suppliers of pistons
- Mahle GmbH
- KSPG AG
- Wössner GmbH
- Federal Mogul Ltd.
- Pankl Engine Systems
- Wiseco Pistons Company Inc.
- Vertex Pistons Inc.
See also
literature
- Wilfried Staudt: Handbook Vehicle Technology Volume 2. 1st edition, Bildungsverlag EINS, Troisdorf, 2005, ISBN 3-427-04522-6 .
- Jan Drummans: The car and its technology. 1st edition, Motorbuchverlag, Stuttgart, 1992, ISBN 3-613-01288-X .
Web links
Individual evidence
- ↑ Ralf Peters: Pistons and piston rings: structure and designation . In: Maxima Asbirg Bernert (Hrsg.): Waspenblech - Archive . Recklinghausen / NRW August 20, 2016 ( square7.ch [accessed July 27, 2017]).
- ↑ Ralf Peters: Pistons and piston rings: structure and designation . In: Maxima Asbirg Bernert (Hrsg.): Waspenblech - Archive . Recklinghausen / NRW August 20, 2016 ( square7.ch [accessed July 27, 2017]).
- ↑ Ralf Peters: Pistons and piston rings: structure and designation . In: Maxima Asbirg Bernert (Hrsg.): Waspenblech - Archive . Recklinghausen / NRW August 20, 2016 ( square7.ch [accessed July 27, 2017]).
- ↑ Ralf Peters: Pistons and piston rings: structure and designation . In: Maxima Asbirg Bernert (Hrsg.): Waspenblech - Archive . Recklinghausen / NRW August 20, 2016 ( square7.ch [accessed July 27, 2017]). Pistons and piston rings: structure and designation ( Memento from December 30, 2017 in the Internet Archive )
- ↑ Patent EP0028287 .
- ↑ Patent DE1998002488 .
- ↑ kfz-tech.de: Deaching .