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Engine piston

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In its broadest sense, a piston is a part that slides in a slideway within a connecting rod-crank system,the crankshaft of a reciprocating engine, in this case the cylinder. The piston is used in driven machinery as well as prime movers, internal combustion engines and steam engines, reciprocating hydraulic pumps, compressors.

It moves in a manner resembling sinusoidal motion. In each of these machines, the piston either transfers forces to a gas or liquid in the form of pressure or receives forces from them. Its beginnings can be found in canon. Gunpowder was the fuel utilized in the earliest combustion engine experiments, which were made in the 16th

Due to American Brayton, the piston’s design changed from cylindrical to something more complex and resembling its current form in 1873. He added elastic segments that were contained in grooves carved out of the piston, and he also added inner reinforcement zones to the axis bores (bosses).

For a very long time, the base material was cast iron. The production of the first aluminum pistons by Hispano-Suiza in 1911 gave them a notable weight-related advantage.

However, due to the three times larger thermal expansion of aluminum compared to cast iron and the subsequent seizure danger, nearly all other engine manufacturers decided to continue using cast iron pistons for an additional ten years.
limiting themselves to limiting their weight by reducing the thickness of the material.

From 1920, thanks to the new light alloys and the improvement of casting and machining techniques, the aluminum piston began to impose itself, although in the 1930s there was a return in the United States. back for mainly economic reasons. but also partly technical.

The shape of the plunger

Four parts can be distinguished: the head, which receives the thrust and the heat from the combustion gases; the upper part of the barrel which, by means of the segments, seals against gases and lubricating oil and at the same time dissipates part of the heat received;

the bosses receiving the pin by which the piston is coupled to the connecting rod; and finally the skirt which guides the piston in its movement and still transfers heat to the cooling fluid (air or water).

The dimensions of a piston are defined by the following dimensions:

A = bore;

L = total height;

B = compression dimension and

D = axis diameter.

The so-called compression dimension, which determines the position of the plane of the head at top dead center and, consequently, the useful volume of the combustion chamber, has a direct effect on the volumetric ratio , quotient of the displacement increased by the volume of the combustion chamber of combustion by the volume of the latter.

The number of segments was formerly quite high; one could count up to five grooves in the piston. This number is reduced today to three on the pistons of gasoline engines (passenger car) and to four on the pistons of Diesel engines.

In the latter, one will sometimes find a fifth groove at the lower part of the skirt, for an additional scraper segment; but this is a dying characteristic.

The first groove from the head receives the so-called shotgun sealing segment, the second receives a radeur segment; the third still a sealing ring; the fourth is pierced with holes intended to recover a certain part of the oil projected on the walls of the cylinder.

In its downward stroke, the scraper segment stops only part of the oil; the other part escapes and will serve to improve the friction conditions of the other segments.The firing segment blocks the residual part of the oil that reaches it.

Regarding lubrication, it should be noted that the last cylindrical drape (the one between the last two segments) has a diameter approximately 1 mm less than that of the other cylindrical drapes, in order to create a buffer space in which will form a ring of liquid which, slowing down the progress of the oil towards the cylinder head, will provide additional sealing.

The zone of the bosses, corresponding to the point of attachment of the piston to the connecting rod, is very delicate, given the forces in play. A poorly executed coupling has catastrophic consequences (rupture of the bosses, seizing and complete deterioration of the cylinder).

The axle housing bore is machined with diamond tools with machining tolerances of 4 to 7 microns. Similarly, the pin (in treated steel with a hardened outer surface) is ground with tolerances of 5 to 7 microns. In general, three types of joints are used:

– Shaft fixed in relation to the connecting rod and floating in the bosses; – Shaft fixed in relation to the piston and free in relation to the connecting rod; Free axis both in the bosses and in the “full-floating” connecting rod

When the shaft is mounted free in the bosses, it is prevented from sliding by placing snap rings. The aluminum alloys commonly used for the manufacture of pistons can be classified into three categories:


aluminium-copper-nickel (or iron) and


This last category is the most used because it offers excellent characteristics of mechanical resistance, a low coefficient of expansion and high coefficients of thermal conduction.

There are also aluminum alloys with copper, silicon and magnesium which are suitable for press-forged pistons of high mechanical strength.

These alloys mainly concern pistons for competition engines and for aviation engines. In any internal combustion engine, the piston must meet the following conditions:

– transmit to the crankshaft via the connecting rod, the forces due to the combustion gases;  ensure gas and lubricating oil tightness and transfer the heat received from the gases to the cylinders.

The first function is essentially linked to the mechanical resistance of the piston (dimensioning of thicknesses and choice of material).

The gas seal allows the use of all the energy produced during combustion and prevents the gases, leaking into the crankcase, from burning the oil and causing the segments to seize or stick.

The oil seal limits consumption, but also prevents the formation of carbonaceous deposits (calamine) between the cylindrical fields and in the combustion chamber (these deposits are generators of early ignition by hot spots)

The sizing of the skirt and of the part carrying the segments contributes to a certain extent to ensuring sealing because it is not possible, under pain of seizing, to exceed certain values ​​of the coupling clearances between piston and cylinder.

The third function (heat removal) promotes the maintenance of the mechanical characteristics of the material, it reduces the risk of sticking of the segments and the wear of the grooves.

The range of piston types differing in form, function and size is unlimited. We can nevertheless admit a division into two large classes, each of which brings together subtypes with well diversified characteristics.

Pistons for spark ignition engines

These are most commonly used in four-stroke and two-stroke engines. Bore values ​​range from 30 to 70 mm for motorcycles and from 52 to 110 mm for automobiles.

The shapes are very varied (flat, domed or hollowed head) and reflect the continuous search for complete combustion, therefore a lower content of unburned matter in the exhaust gases.

The area of ​​the skirt adjacent to the bosses is sometimes lightened by indentations to reduce the weight of the piston without compromising its resistance.

Apart from the distinction by the shapes, we can also take into consideration the various techniques devised to control thermal expansion. A four-valve Ferrari 312 B piston, Lancia Stratos flathead piston, Aermacchi motorcycle piston.

A four-valve Ferrari 312 B piston, Lancia Stratos flathead piston, Aermacchi motorcycle piston. D.R.
We will then distinguish:

– Full-skirted monometallic pistons. These are the simplest and most used; the thermal expansion of the skirt is relatively large, which requires large clearances and high compensating ovalizations. In the past, this type presented a notch, vertical or oblique, with the function of making the skirt more elastic and absorbing expansions.

However, this solution resulted in a reduction in the rigidity of the skirt, which was often subject to breakage. In this category, we can classify competition forged pistons, with domed heads. They have a maximum of three segments, sometimes two, and their very reduced skirt assumes only a single guiding function.

The indentations are very pronounced to reduce the weight as much as possible which, due to the high number of revolutions (9,000 to 11,000 per minute), strongly influences the magnitude of the inertial forces exerted.

– Pistons with controlled thermal expansion. They receive during casting, incorporated into the constituent material, steel plates which allow them high performance with regard to thermal expansion.

In 1925, A.L. Nelson, taking advantage of the fact that invar metal has a low coefficient of expansion compared to that of aluminum alloys, built a piston with invar pads.

Subsequently, we saw the appearance in Germany of the Autothermik and Autothermatik pistons from the house of Mahle and the pistons with expansion rings from the house of Karl Schmidt.

The Autothermik piston is characterized both by the type of pads and by the fact that it has a notch in the groove of the scraper ring intended to cool it, therefore to make it easier to control.

The Autothermatik piston differs from the previous one only by cooling holes (instead of the notch), which promote heat transfer from the head to the skirt. This arrangement allows for greater rigidity: the head now rests with its entire circumference on the rest of the piston body.

Pistons of this type offer the same advantages as uncut monometallic models and Autothermik, for this reason they are intended for particularly fast and highly stressed engines.

Pistons with expansion rings are, for their part, characterized by the incorporation of a circular steel part whose outer part, smooth or toothed, is embedded in the piston during casting.

The assembly of the piston

The assembly of the piston in the cylinder must be done after a thorough cleaning of the latter, with petrol or a very fluid lubricating oil, in order to eliminate all the abrasive residues which would have remained after deglazing.

The assembly of the piston in the cylinder is carried out using a collar which compresses the rings, pushing the piston by hand, without hitting it on the head because this could cause the breaking or cracking of the rings.

Each manufacturer indicates in the repair manual the dimension, that is to say the diameter of the piston which ensures the correct operating clearance for a given cylinder bore.

In this way, the repairer is spared the difficult task of measuring the diameter of the piston which is variable from one point to another.

The segments are arranged in the following order, from top to bottom:

– A compression ring with a rectangular section, the surface of which is chromed or coated with molybdenum;
– A segment with a trapezoidal section, that is to say with a sliding surface inclined by approximately half a degree with respect to the surface of the cylinder, which functions both as a compression segment and as an oil scraper ;

– A stepped scraper ring;
– A scraper ring with holes distributed around the circumference to evacuate the oil inwards.
A flat-bottomed piston two-stroke engine.
A flat-bottomed piston two-stroke engine. D.R.
A lightweight piston for small diesel engines.
A lightweight piston for small diesel engines. D.R.

The indication (above) placed in the vicinity of the cut of the segment must always face upwards. The reversal of direction, even of a single segment, can lead to high oil consumption.

The segment must not be excessively open because permanent deformations can easily occur which would compromise its operation by modifying the bearing pressure against the walls.

Pistons for Diesel Engines

The heads of these pistons have very varied shapes, which all meet the same requirements: high combustion efficiency, dissipation of heat from the combustion chamber, transmission of the thrust of the gases to the connecting rod via the axis.

Running-in, lubrication and wear

Faulty lubrication of the piston causes it to seize in the cylinder. To remedy this incident. relatively common during the running-in period, the walls of the cylinders are hollowed out with a series of grooves crossed at 120°.

which will always create the conditions for wet friction. In the center: a shirt after running in; the upper part corresponds to the zone of action of the segments while the base is covered only by the skirt. To the right ; the profile recorded with the roughness gauge of the upper part.

A piston is a part of, among other related devices, reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders, and pneumatic cylinders.

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