Piston

In Mécanique the piston is a rigid part sliding in a complementary shirt of form ensuring the variation of the volume of the room, and the conversion of a Pression into force in the case of an Engine spark-ignition and of a Vérin, or the reverse in the case of a positive-displacement pump with pistons and of a Seringue.

In the jargon of the engineers, a Piston (with a capital letter) is a former student of the Central School of Paris. In the common jargon, a “piston” is a person of knowledge which introduces somebody in a company favorably, with an aim of making integrate this applicant in the aforementioned company.

General mechanics

  • connection with the shirt (mobilities)

  • connection with the transmission: stem, rod, Positive plate, unilateral or .
  • play and sealing
  • law of conversion (pressure/force)
  • oscillatory movement (cubic capacity and race).
  • rectilinear motion
  • shirt sits valve

Thermal engine

Role of the piston in a thermal engine

The piston is the variable component ensuring the variation of the room of a cylinder. Generally related to a rod, it ensures the Compression combustion gases and undergoes to them relaxation source of the movement of the engine. When the room is opened by a Soupape, it expels the gases flarings or aspires the mixture of the cycle according to.

The piston is a cylindrical part , sometimes slightly conical, and in certain cases in the shape of barrel; these forms and the play in its adjustment with the shirt confer on the unit a mechanical Liaison less constraining for the assembly and operation.

The piston has other roles quite as important for the good performance of the engine:

More precisely let us see the various parts which compose the piston.

The head of the piston

For more detailed analysis, let us begin side of the head of piston. This one ensures the compression part/evacuation of gases. Its form is related to that of the cylinder. There exist several shapes of head of piston:

  • the heads punts, especially present on the engines 2 times and less on the engines 4 times, in our case we chose a piston has head punt.
  • convex heads, with prints in glance with the valves. The convex part makes it possible to have more powerful combustion chambers (better ignition of gases, evacuation easier and fast, better cooling of the candle) and higher compressions. The prints are slightly larger than the diameter of the heads of valves, which avoids with the piston and the valves to touch itself (what could be the case at the time of a panic of valve or a light maladjustment of the distribution).
To support the explosions, the heads of piston undergo surface treatment more and more in order to reinforce them (ex: treatment with nickel, graphite…); moreover, to obtain a better dissipation of the heat, the heads are improved by veins on their back and thus the thermal heat-transferring surface increases and makes it possible to better dissipate a surplus of heat quickly.

The segmentation

The segments are rings " élastiques" opened which is placed in throats machined in the head of the piston. The opening of the segment is called cut, because the segments are manufactured starting from a spring cut then in sections; the width of the opening once in place in the cylinder is called play with the cut . On the engines four times generally used in the car, one generally finds three segments (four on the old engines, two on the engines of competition). They ensure the sealing between the combustion chamber (the hot gases) and oils it in the casing of the crankshaft. They ensure also the dissipation of the heat of combustion towards the cylinder. The 3 types of segments are:

  • the segment of fire is the segment in contact with gases. During the explosion, it is plated against the piston (in its throat) and against the cylinder, which ensures almost all the sealing.
  • the ring seal or of compression ensures the total sealing of gases by stopping those which would have passed by the cut of the segment of fire. Its cut is shifted or ploughed for the third time compared to that of the segment of fire.
  • the oil scraper ensures the sealing at the level of oil, it owes " racler" the oil of the walls of the cylinder to prevent that it is burned with the following cycle.

A failure of the segments of fire or compression results by a loss of compression and performances of the engine, and in the pressurization of the casing by gases of escape. A failure of the oil scraper results in a consumption of oil and smoke blue with acceleration.

The skirt of the piston

The skirt of the piston is the part which is in lower part of the last segment and is used for guidance of the piston in the cylinder. It can be complete or reduced.

The goal of this reduction is to reduce the weight of the piston and frictions of the skirt on the cylinder in order to improve the performances of the engine with high mode. The surface quality of the skirt is thus paramount to ensure a good lubrication, sometimes a surface treatment can be applied to the piston or only on the skirt, this one will take a coloring dark gray then even black.

In a two-stroke engine, it is the skirt of the piston which given the diagram of opening/closing of the lights by sealing or décrouvrant these last with each movement.

The axis of the piston

The axis of the piston makes it possible to connect the piston to the rod. The axis must be extremely resistant from its dimensions and the materials used, because it undergoes and transmits the mechanical efforts due to the explosions. It is polished also perfectly to turn in the rod or the piston (sometimes both). Most of the time, the axis of the piston is hollow to reduce the weight of the moving element without decreasing his resistance. It is generally maintained laterally by these circlips or rings snap in the piston, and can be gone up free or tight in the rod; in this last case it is necessary to heat the rod, to cool the axis or to associate the two methods for the assembly.

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