Types of internal combustion engine pistons. Piston engine

Piston is one of the parts of the engine crank mechanism and is an integral element conditionally divided into a head and a skirt. It is the basis for the process of converting fuel combustion energy into thermal energy, and then into mechanical energy. The performance of the engine, as well as its reliability and durability, directly depends on the quality of the work of this part.

Purpose and types of pistons

In the engine, the engine piston performs a number of functions, in particular, these are:

transformation of gas pressure into the force transmitted to the connecting rod;
ensuring the tightness of the combustion chamber;
heat sink.

The piston operates under extreme conditions under consistently high mechanical stress. Therefore, for modern engines, they are made from special aluminum alloys, which are light and durable with sufficient heat resistance. Steel pistons are somewhat less common. Previously, they were mainly made from cast iron. The piston markings that must be present on each product will tell you what it is made of. These parts are made by two methods - casting and stamping. Forged pistons, common in tuning, are made precisely by stamping, and not forged by hand.

Piston design

The piston structure is not complicated. This is a one-piece piece, which, for convenience of definition, is conventionally divided into a skirt and a head. Specific form and design features pistons are determined by the type and model of the engine. In common forms gasoline internal combustion engines you can see only pistons with flat or very close to this shape heads. They often have grooves designed to maximize valve opening. In engines with direct fuel injection, the pistons are made in a slightly more complex shape. The piston of the diesel engine has a specific configuration of the piston to provide optimal turbulence for a good mixture formation.

Engine piston diagram.

Under the head on the piston there are grooves in which the piston rings are installed. The skirts of different pistons are also different: with a shape similar to a cone or a barrel. This configuration makes it possible to compensate for the expansion of the piston that exists when it heats up in operation. It should be noted that the piston acquires a full working volume only after the engine warms up to normal temperature.

To minimize the effect of constant lateral friction of the piston against the cylinder, a special anti-friction material is applied to its side surface, the type of which also depends on the type of engine. Also in the piston skirt there are special holes with lugs, designed for mounting the piston pin.

The operation of the piston assumes its intense heating. It is cooled, and in different motors different ways... The most common ones are:

by supplying oil mist to the cylinder;
by splashing oil through a connecting rod or a special nozzle;
through the injection of oil through the annular channel;
with the help of constant circulation of oil through a coil located directly in the piston head.

It is not the piston itself that is in close contact with the cylinder walls, but its rings. To ensure the highest wear resistance, they are manufactured from a special grade of cast iron. The number and exact position of these rings depends on the type of motor. Most often, the piston has a pair of compression rings and another oil scraper.

Compression Rings are designed to prevent gases from the combustion chamber from breaking through into the crankcase. The first ring has the most serious load, therefore, in all diesel and powerful gasoline engines, a steel insert is additionally present in the groove of the first ring, which increases the strength of the structure. There are many types of compression rings that are unique to almost every independent manufacturer.

Oil scraper rings- to remove excess oil from the cylinder and prevent its penetration into the combustion chamber. Such rings are made with a large number of drain holes, as well as with spring expanders, although not in all engine models.

Piston device

The engine piston is connected to the connecting rod through the piston pin, a tubular steel part. The most common way of attaching a pin is floating, thanks to which the part can rotate during operation. Special retaining rings prevent the finger from moving to the sides. Rigid finger grip on this moment practically not widespread due to the obvious greater vulnerability of such constructions.

Failure of the piston and related parts

In the course of intensive or just long-term operation, the piston may fail due to the presence of a foreign body in the cylinder, which the piston constantly encounters during movement. A particle of the connecting rod, or something else, flying off the part, can become such an object. The surfaces of such a fracture are gray in color, they are not characterized by abrasion, cracks or other visual signs. The piston disintegrates quickly and suddenly.

A fracture caused by metal fatigue is characterized by the formation of raster lines in the problem area. This allows early detection of a breakdown and replacement of the piston. In addition to aging, this fracture can be caused by knocking ignition, increased vibration of the piston due to the collision of the piston head with the cylinder head, or excessive skirt clearance. In any case, cracks form on the part, indicating its imminent failure.

After ring wear, piston head damage is most common.

In addition to metal wear and aging, piston-related breakdowns can occur for a variety of reasons, including:

violation of the combustion mode, for example, due to ignition delay;
improper organization of starting a cold engine;
filling the cylinder with oil or water with the engine off, as they say;
unreasonable increase in power as a result of electronics reconfiguration;
use of unsuitable parts;
other reasons.

Most often, the repair is carried out by replacing the piston, rings or the entire piston group.

Related terms

The engine piston is one of the most important parts and, of course, the successful operation of the engine and its long service life depend on the material and quality of the pistons. In this article, more designed for beginners, everything (well, or almost everything) that is connected with the piston will be described, namely: the purpose of the piston, its structure, materials and piston manufacturing technology and other nuances.

I want to warn dear readers right away that if some important nuance associated with pistons, or with the technology of their manufacture, I have already written in more detail in another article, then of course it makes no sense for me to repeat myself in this article. I will simply put the appropriate link, by clicking on which the dear reader, if desired, will be able to go to another more detailed article and in it to get acquainted with the necessary information about pistons in more detail.

At first glance, many beginners may think that the piston is a fairly simple part and it is impossible to come up with something more perfect in its production technology, shape and design. But in reality, everything is not so simple and despite the outward simplicity of the shape, pistons and their manufacturing technologies are still being improved, especially on the most modern (serial or sports) higher-revving forced engines. But let's not get ahead of ourselves and start from simple to complex.

To begin with, let's analyze what a piston (pistons) is in an engine for, how it works, what forms of pistons are for different engines, and then we will smoothly move on to manufacturing technologies.

What is the engine piston for?

The piston, due to the crank mechanism (and - see the figure below), moving reciprocally in the engine cylinder, for example, moving upwards - to suck in the cylinder and compress the working mixture in the combustion chamber, as well as due to the expansion of combustible gases moving downward in the cylinder, performs work, converting the thermal energy of the combusted fuel into motion energy, which contributes (through the transmission) to the rotation of the driving wheels vehicle.

The engine piston and the forces acting on it: A - the force pressing the piston against the cylinder walls; B - force moving the piston down; B is the force transmitted from the piston to the connecting rod and vice versa, G is the pressure force of the combustion gases that moves the piston down.

That is, in fact, without a piston in a single-cylinder engine, or without pistons in a multi-cylinder engine, the movement of the vehicle on which the engine is installed is impossible.

In addition, as can be seen from the figure, several forces act on the piston (also the opposite forces are not shown in the same figure, pressing on the piston from the bottom up).

And based on the fact that several forces are pressed on the piston and quite strongly, the piston must have some important properties, namely:

the ability of the engine piston to withstand the enormous pressure of gases expanding in the combustion chamber.
the ability to compress and withstand the high pressure of the compressed fuel (especially on).
the ability to resist the breakthrough of gases between the walls of the cylinder and its walls.
the ability to transfer enormous pressure to the connecting rod, through the piston pin, without breakage.
the ability not to wear out for a long time from friction against the cylinder walls.
the ability not to wedge in the cylinder due to thermal expansion of the material from which it is made.
the engine piston must be able to withstand the high combustion temperature of the fuel.
have high strength with low weight to eliminate vibration and inertia.

And these are not all the requirements for pistons, especially on modern high-revving engines. O useful properties and the requirements of modern pistons we will talk more about, but first, let's look at the device of a modern piston.

As you can see in the figure, a modern piston can be divided into several parts, each of which is important and has its own functions. But below the main most important parts of the engine piston will be described and we will start with the most important and critical part - from the piston bottom.

The bottom (bottom) of the engine piston.

This is the highest and most loaded piston surface, which faces directly into the engine's combustion chamber. And the bottom of any piston is loaded not only with a large pressing force from gases expanding at a tremendous speed, but also with a high combustion temperature of the working mixture.

In addition, the bottom of the piston with its profile defines the lower surface of the combustion chamber itself and also determines such an important parameter as. By the way, the shape of the bottom of the piston may depend on some parameters, for example, on the location of candles or injectors in the combustion chamber, on the location and size of the valve opening, on the diameter of the valve plates - in the photo on the left, you can clearly see the recesses for the valve plates in the bottom of the piston, which exclude the meeting valves with a bottom.

Also, the shape and size of the bottom of the piston depends on the volume and shape of the combustion chamber of the engine, or on the peculiarities of feeding the fuel-air mixture into it - for example, on some old two-stroke engines on the bottom of the piston, a characteristic ridge protrusion was made, which plays the role of a reflector and guides the flow of combustion products when purging. This protrusion is shown in figure 2 (the protrusion on the bottom is also visible in the figure above, where the piston arrangement is shown). By the way, Figure 2 also shows the workflow of the ancient two-stroke engine and how the protrusion on the bottom of the piston affects the filling with the working mixture and the release of exhaust gases (that is, to improve the blowdown).

Two-stroke motorcycle engine - workflow

But on some engines (for example, on some diesel engines), on the bottom of the piston in the center, on the contrary, there is a circular notch, due to which the volume of the combustion chamber increases and, accordingly, the compression ratio decreases.

But, since a recess of a small diameter in the center of the bottom is not desirable for favorable filling with the working mixture (unwanted swirls appear), then on many engines, recesses in the center of the piston bottoms have ceased to be made.

And to reduce the volume of the combustion chamber, it is necessary to make so-called displacers, that is, to make a bottom with a certain volume of material, which is located slightly above the main plane of the piston bottom.

Well, another important indicator is the thickness of the piston bottom. The thicker it is, the stronger the piston is and the greater the heat and power load it can withstand for quite a long time. And the thinner the thickness of the bottom of the piston, the greater the likelihood of burnout, or physical destruction of the bottom.

But with an increase in the thickness of the bottom of the piston, the mass of the piston correspondingly increases, which is very undesirable for forced high-revving motors. And therefore, the designers make a compromise, that is, they "catch" the golden mean between strength and weight, and of course they are constantly trying to improve the technologies for the production of pistons for modern engines (more on technologies later).

Piston top land.

As can be seen in the figure above, which shows the design of the engine piston, the head land is the distance from the bottom of the piston to its uppermost compression ring. It should be noted that the smaller the distance from the bottom of the piston to the upper ring, that is, the thinner the top zone, the higher the thermal stress will be experienced by the lower elements of the piston, and the faster they will wear out.

Therefore, for highly stressed forced engines, it is desirable to make the top zone thicker, but this is not always done, since this can also increase the height and mass of the piston, which is undesirable for forced and high-revving engines. Here, as well as with the thickness of the piston bottom, it is important to find a middle ground.

Sealing section of the piston.

This section starts from the bottom of the top land to where the groove of the lowest piston ring ends. The grooves of the piston rings are located on the sealing section of the piston and the rings themselves (compression and oil-removable) are inserted.

The grooves of the rings not only hold the piston rings in place, but also ensure their mobility (due to certain gaps between the rings and grooves), which allows the piston rings to be freely compressed and expanded due to their elasticity (which is very important if the cylinder is worn out and has the shape of a barrel) ... This also contributes to the pressing of the piston rings against the cylinder walls, which eliminates gas blowout and contributes to good, even if the cylinder is slightly worn.

As can be seen in the figure with the piston device, in the groove (s) intended for the oil scraper ring there are holes for the return flow of engine oil, which the oil scraper ring (or rings) removes from the cylinder walls when the piston moves in the cylinder.

In addition to the main function (to prevent the breakthrough of gases) of the sealing area, it has another important property - it is the removal (more precisely, distribution) of part of the heat from the piston to the cylinder and the entire engine. Of course, for efficient distribution (removal) of heat and to prevent gas breakthrough, it is important that the piston rings fit fairly tightly to their grooves, but especially to the surface of the cylinder wall.

Engine piston head.

The piston head is a common section, which includes the piston bottom and its and the sealing section already described by me above. The larger and more powerful the piston head, the higher its strength, better heat dissipation and, accordingly, the longer the resource, but the mass is also greater, which, as mentioned above, is undesirable for high-revving engines. And to reduce the weight, without reducing the resource, it is possible to increase the strength of the piston by improving the manufacturing technology, but I will write about this in more detail later.

By the way, I almost forgot to say that in some designs of modern pistons made of aluminum alloys, a nirezist insert is made in the piston head, that is, a rim made of nirezist (special durable and corrosion-resistant cast iron) is poured into the piston head.

A groove is cut in this rim for the uppermost and most loaded compression piston ring. And although thanks to the insert, the mass of the piston slightly increases, but its strength and wear resistance significantly increases (for example, our domestic Tutaev pistons, manufactured at TMZ, have a ni-resist insert).

Compression piston height.

Compression height is the distance in millimeters from the bottom of the piston to the axis of the piston pin (or vice versa). Different pistons have different compression heights and, of course, the greater the distance from the axis of the pin to the bottom, the larger it is, and the larger it is, the better the compression and the less probability of gas breakthrough, but also the greater the friction force and heating of the piston.

On older low-speed and low-revving engines, the compression height of the piston was higher, and on modern, higher-revving engines, it became less. Here, too, it is important to find a middle ground, which depends on the forcing of the motor (the higher the rpm, the less friction and a lower compression height should be).

Engine piston skirt.

The skirt is called the lower part of the piston (it is also called the guide part). The skirt includes bore piston bosses into which the piston pin is inserted. The outer surface of the piston skirt is the guiding (supporting) surface of the piston and this surface, like the piston rings, rubs against the cylinder walls.

There are lugs around the middle of the piston skirt that have holes for the piston pin. And since the weight of the piston material at the tides is heavier than in other parts of the skirt, the deformations from the effect of temperature in the plane of the bosses will be greater than in other parts of the piston.

Therefore, to reduce the temperature effects (and stresses) on the piston, a part of the material is removed from both sides of the skirt surface, approximately to a depth of 0.5-1.5 mm, and small depressions are obtained. These recesses, called refrigerators, not only help to eliminate temperature effects and deformations, but also prevent the formation of scoring, and also improve the lubrication of the piston when it moves in the cylinder.

It should also be noted that the piston skirt has the shape of a cone (at the top at the bottom it is narrower, at the bottom it is wider), and in a plane perpendicular to the axis of the piston pin it has the shape of an oval. These deviations from the ideal cylindrical shape are minimal, that is, they have only a few hundred mm (these values are different - the larger the diameter, the greater the deviation).

The cone is needed so that the piston expands evenly from heating, because at the top the temperature of the piston is higher, and the
The thermal expansion is also greater. And since the diameter of the piston at the bottom is slightly smaller than at the bottom, then when expanding from heating, the piston will take a shape close to an ideal cylinder.

Well, the oval is designed to compensate for the rapid wear on the skirt walls, which wear out faster where the friction is higher, and higher in the plane of the connecting rod movement.

Thanks to the piston skirt (more precisely, its lateral surface), the desired and correct position of the piston axis to the axis of the engine cylinder is ensured. With the help of the side surface of the skirt, lateral forces are transmitted to the engine cylinder from the action of the lateral force A (see the uppermost figure in the text, as well as the figure on the right), which periodically acts on the pistons and cylinders, when the pistons are shifted during the rotation of the crankshaft (crank connecting rod mechanism).

Also, thanks to the side surface of the skirt, heat is removed from the piston to the cylinder (as well as from the piston rings). The larger the side surface of the skirt, the better the heat dissipation, less gas leakage, less piston knocking with some wear of the bushing of the upper connecting rod head (or with inaccurate processing of the bushing - see the figure on the left), however, as with three compression rings, not two (I wrote about this in more detail).

But if the piston skirt is too long, its mass is greater, more friction arises against the cylinder walls (on modern pistons, to reduce friction and wear, they began to apply an anti-friction coating on the skirt), and excess weight and friction are very undesirable in high-revving forced modern (or sports) motors and therefore, on such engines, the skirt was gradually made very short (the so-called miniskirt) and gradually got rid of it almost - and this is how the T-shaped piston, shown in the photo on the right, appeared.

But T-shaped pistons also have drawbacks, for example, they again may have problems with friction against the cylinder walls, due to the insufficient lubricated surface of a very short skirt (and at low revs).

In more detail about these problems, as well as in which cases T-shaped pistons with a miniskirt are needed in some engines, and in which they are not, I wrote a separate detailed article. It also says about the evolution of the shape of the engine piston - I advise you to read it. Well, I think we have already figured out the piston device and are smoothly moving on to piston manufacturing technologies in order to understand which pistons are made different ways better and which are worse (less durable).

Pistons for engines - materials of manufacture.

When choosing a material for the manufacture of pistons, strict requirements are imposed, namely:

the piston material must have excellent anti-friction (anti-seize) properties.
the material of the engine piston must have a fairly high mechanical strength.
the piston material must have a low density and good thermal conductivity.
the piston material must be resistant to corrosion.
the material of the piston should have a low coefficient of linear expansion and be as close as possible to or equal to the coefficient of expansion of the material of the cylinder walls.

Cast iron.

Earlier, at the dawn of engine building, since the days of the very first cars, motorcycles and airplanes (airplanes), gray cast iron was used for the piston material (by the way, for compressor pistons too). Of course, like any material, cast iron has both advantages and disadvantages.

Of the advantages, it should be noted good wear resistance and sufficient strength. But the most important advantage of cast iron pistons installed in engines with cast iron blocks (or liners) is the same coefficient of thermal expansion as a cast iron engine cylinder. This means that the thermal clearances can be minimized, that is, much less than that of an aluminum piston operating in a cast iron cylinder. This made it possible to significantly increase the compression and the resource of the piston group.

Another significant plus of cast iron pistons is a small (only 10%) decrease in mechanical strength when the piston is heated. For an aluminum piston, the decrease in mechanical strength during heating is noticeably greater, but more on that below.

But with the advent of more revving engines, when using cast iron pistons, their main drawback began to come to light at high speeds - a rather large mass, compared to aluminum pistons. And they gradually switched to the manufacture of pistons from aluminum alloys, even in engines with a cast iron block, or a liner, although they had to make aluminum pistons with much larger thermal clearances in order to eliminate the wedge of the aluminum piston in the cast iron cylinder.

By the way, earlier on the pistons of some engines, an oblique cut of the skirt was made, which provided the spring properties of the skirt of the aluminum piston and excluded its jamming in the cast-iron cylinder - an example of such a piston can be seen on the engine of the IZH-49 motorcycle).

And with the advent of modern cylinders, or cylinder blocks, completely made of aluminum, in which there are no longer cast iron liners (that is, coated with nikasil or), it became possible to produce aluminum pistons with minimal thermal clearances, because the thermal expansion of an alloy cylinder has become almost the same as and on the alloy piston.

Aluminum alloys. Almost all modern pistons on serial engines are now made of aluminum (except for plastic pistons on cheap Chinese compressors).

Pistons made of aluminum alloys also have both advantages and disadvantages. Of the main advantages, it should be noted the low weight of the alloy piston, which is very important for modern high-speed engines. The weight of an aluminum piston, of course, depends on the alloy composition and on the piston manufacturing technology, because a forged piston weighs much less than one made of the same alloy by casting, but I will write about the technologies a little later.

Another advantage of light-alloy pistons, which few people know about, is a rather high thermal conductivity, which is about 3-4 times higher than the thermal conductivity of gray cast iron. But why is it a dignity, because with high thermal conductivity and thermal expansion is not quite small and you will have to and will have to make more thermal clearances, unless of course the cast-iron cylinder (but with modern aluminum cylinders this is no longer necessary).

And the fact is that high thermal conductivity does not allow the bottom of the piston to heat up to more than 250 ° C, and this contributes to a much better filling of engine cylinders and, of course, allows to further increase the compression ratio in gasoline engines and thereby increase their power.

By the way, in order to somehow strengthen the pistons cast from light alloy, engineers add various reinforcing elements to their design - for example, they make the walls and the bottom of the piston thicker, and the bosses under the piston pin are cast more massive. Well, or they make inserts from the same cast iron, I already wrote about this above. And of course, all these reinforcements increase the mass of the piston, and as a result, it turns out that the more ancient and durable piston made of cast iron loses in weight to the light-alloy piston quite a bit, about 10 - 15 percent.

And here the question begs anyone, is it worth the candle? It is worth it, because aluminum alloys have one more excellent property - they dissipate heat three times better than the same cast iron. And this important property is indispensable in modern high-revving (forced and hot) engines, which have a fairly high compression ratio.

Besides modern technologies the production of forged pistons (about them a little later) significantly increase the strength and reduce the weight of the parts and it is no longer necessary to reinforce such pistons with various inserts, or more massive castings.

The disadvantages of pistons made of aluminum alloys are such as: a rather large coefficient of linear expansion of aluminum alloys, in which it is approximately two times greater than that of pistons made of cast iron.

Another significant drawback of aluminum pistons is a rather large decrease in mechanical strength when the temperature of the piston rises. For example: if an alloy piston is heated to three hundred degrees, this will lead to a decrease in its strength by as much as half (by about 55 - 50 percent). And for a cast-iron piston, when it heats up, the strength decreases significantly less - by only 10 - 15%. Although modern pistons made of aluminum alloys by forging rather than casting, they lose much less strength when heated.

On many modern aluminum pistons, a decrease in mechanical strength and too large thermal expansion is eliminated by more advanced production technologies that have replaced traditional casting (more on this below), as well as special compensating inserts (for example, the inserts from ni-resist that I mentioned above), which not only increase strength, but also significantly reduce the thermal expansion of the piston skirt walls.

Engine piston - manufacturing technology.

It's no secret that over time, in order to increase the power of the engines, they gradually began to increase the compression ratio and engine speed. And in order to increase the power without much damage to the life of the pistons, the technologies of their manufacture were gradually improved. But let's start in order - with conventional cast pistons.

Pistons made by conventional casting.

This technology is the simplest and most ancient, it has been applied from the very beginning of the history of auto and engine building, even from the time of the ne loose cast iron pistons.

The technology of producing pistons for the most modern engines by conventional casting is almost no longer used. After all, the output is a product that has flaws (pores, etc.) that significantly reduce the strength of the part. And the technology of ordinary casting into a mold (chill mold) is quite ancient, it was borrowed from our ancient ancestors, who cast bronze axes many centuries ago.

And the aluminum alloy poured into the chill mold repeats the shape of the chill mold (matrix), and then the part still needs to be processed thermally and on machines, removing excess material, which takes a lot of time (even on CNC machines).

Injection molding.

The strength of a piston made by a simple casting method is not high, due to the porosity of the part, and gradually many firms moved away from this method and began to cast pistons under pressure, which significantly improved strength, since there is almost no porosity.

The technology of injection molding differs significantly from the technology of conventional casting of Bronze Age axes and, of course, the result is a more accurate and durable part with a slightly better structure. By the way, by casting aluminum alloys under pressure into a mold (this technology is also called liquid stamping), not only pistons are cast, but also the frames of some modern motorcycles and cars.

But still, this technology is not ideal, and even if you pick up a pressure-molded piston and examine it, you will not find anything on its surface, but this does not mean that everything is perfect inside. Indeed, in the casting process, even under pressure, it is possible that internal voids and cavities (tiny bubbles) appear, which reduce the strength of the part.

But still, piston injection molding (liquid stamping) is much better than conventional casting, and this technology is still used in many factories in the manufacture of pistons, frames, chassis parts and other parts of cars and motorcycles. And who is interested in reading in more detail about how liquid-stamped pistons are made and about their advantages, then we read about them.

Forged pistons of a car (motorcycle).

Forged pistons for domestic cars.

This is the most advanced technology at the moment for the production of modern light-alloy pistons, which have many advantages over cast ones and which are installed on the most modern high-revving engines with a high compression ratio. Forged pistons made by reputable firms have virtually no downsides.

But it makes no sense for me to write about forged pistons in detail in this article, since I wrote two very detailed articles about them that everyone can read by clicking on the links below.

That seems to be all, if I remember anything else about this important detail, like an engine piston, I will definitely add, success to everyone.

Rice. The piston of a diesel engine (a) of a truck and the shape of pistons of different engines (b): 1 - groove of the lower oil scraper ring;
2 - groove for the piston pin retaining ring;
3 - inner surface of the boss;
4 - hole for piston pin lubrication;
5 - groove of the upper oil scraper ring;
6 - grooves of compression rings;
7 - piston head;
8 - combustion chamber in the piston;
9 - the bottom of the piston;
10 - holes for oil drainage;
11 - skirt

The piston has a rather complex design, because it is subjected to very large and variable loads.
The outer surface of the guide part is called skirts... During the working stroke, the piston is exposed to the high pressure of the gases expanding at high temperatures. On the other hand, when the engine is running, especially at high revs, the piston is subjected to large alternating inertial loads. When the piston is at TDC and BDC, its acceleration is zero, and then the piston accelerates sharply and moves at high speed, and the direction of movement changes hundreds of times per second. To reduce inertial loads, it is necessary to reduce the mass of the piston as much as possible. At the same time, it must have high strength to withstand high pressure and heat when in contact with hot gases, followed by cooling when a cold fresh charge is fed into the cylinder. Currently, the pistons of gasoline and diesel automobile engines are made from aluminum alloys. In the manufacture of a piston, steel inserts are often inserted into the casting during the manufacturing process, which increase its rigidity and prevent thermal expansion. Sometimes the steel insert is placed in the groove under the upper compression (most loaded) piston ring.
When heated, the piston expands. To compensate for the thermal expansion of the piston when heated, it is given a special shape. The piston skirt in the transverse plane has the shape of an oval, not a circle. In the longitudinal plane, the piston skirt looks like a truncated cone. Piston parts with high temperature or with a large volume of metal, they expand more (for example, the part of the skirt where the bosses are located), and when the operating temperature in the engine is reached, the piston takes the shape of a cylinder.
During its existence, the pistons have undergone significant design changes. If you compare the piston of a modern car engine with its predecessor, you will notice that the pistons have become much shorter. Most of the skirt is cut on each side, leaving only two small sections to prevent the piston from skewing in the cylinder. Due to the sophisticated design, the forces acting on the piston are balanced in such a way as to minimize the tendency to turn. The distance from the piston crown to the upper piston ring groove is reduced in order to reduce the possibility of carbon deposits in this part. By reducing the size of the sections in the design of the piston, it was possible to significantly reduce its weight. To reduce friction losses and increase the durability of the KShM parts, a layer of antifriction material containing molybdenum disulfide or graphite is applied to the side surface of the piston.
The bottom of the piston can be flat, convex, concave, have grooves so that when the valves are fully open, they do not touch the piston. In a diesel engine, the combustion chamber can be made in the piston.
The pistons of direct injection engines are specially shaped to support the combustion process.
The piston rings are made from specially modified cast iron. Several types of rings are used in modern car engines. The upper compression rings are used to prevent the breakthrough of gases into the crankcase of the engine, and the lower oil scraper - controls the amount of oil on the cylinder walls (the walls are lubricated with oil coming from the crankcase in the form of oil mist). Oil is necessary to prevent wear on the CPG, but excess oil is undesirable. Therefore, it should be fed more than necessary, and the excess should be removed using an oil scraper ring that works like a scraper. One way to get more compact and lighter pistons is to make the rings narrower and smaller and compactly fit in the top of the piston head. At the same time, increased requirements are imposed on the material from which they are made and on the accuracy of their manufacture.

IN crank mechanism The piston performs several functions, including the perception of gas pressure and the transfer of forces to the connecting rod, sealing the combustion chamber and removing heat from it. The piston is the most characteristic part of an internal combustion engine, because it is with its help that the thermodynamic process of the engine is realized.

The conditions in which the piston operates are extreme and are characterized by high pressures, temperatures and inertial loads. Therefore, pistons on modern engines are made of lightweight, durable and heat-resistant material - aluminum alloy, less often steel. Pistons are manufactured in two ways - injection molding or stamping, the so-called. forged pistons.

The piston is a one-piece structural element, which is conventionally divided into a head (in some sources it is called a bottom) and a skirt. The shape and design of a piston is largely determined by the type of engine, the shape of the combustion chamber and the combustion process taking place in it. The piston of a gasoline engine has a flat or near-flat head surface. It can be grooved for full opening of the valves. The pistons of direct injection engines are more complex in shape. A combustion chamber is made in the piston head of a diesel engine a certain form which provides good swirl and improves mixture formation.

Below the piston head, grooves are made for installing the piston rings. Piston skirt has a tapered or curved ( barrel-shaped) form. This shape of the skirt compensates for the thermal expansion of the piston when heated. When the engine reaches its operating temperature, the piston takes on a cylindrical shape. To reduce frictional losses, a layer of antifriction material ( molybdenum disulfide, graphite). The piston skirt has holes with lugs ( lugs) for attaching the piston pin.

Piston cooling carried out from the side of the inner surface in various ways:

oil mist in the cylinder;
splashing oil through the hole in the connecting rod;
spraying oil with a special nozzle;
injection of oil into a special annular channel in the area of the rings;
oil circulation through the tubular coil in the piston head.

Piston rings form a tight connection between the piston and the cylinder walls. They are made from modified cast iron. Piston rings are the main source of friction in an internal combustion engine. Friction losses in the rings reach up to 25% of all mechanical losses in the engine.

The number and arrangement of rings depends on the type and purpose of the engine. The most common scheme is two compression rings and one oil scraper ring. Compression rings prevent the breakthrough of gases from the combustion chamber into the crankcase. The first compression ring works in the most severe conditions. Therefore, on the pistons of diesel and a number of forced gasoline engines, a steel insert is installed in the ring groove, which increases the strength and allows to realize the maximum compression ratio. Compression rings can be trapezoidal, barrel-shaped, conical, some are cut (cut).

Oil scraper ring removes excess oil from the cylinder surface and prevents oil from entering the combustion chamber. The ring has many drainage holes. Some ring designs have a spring-loaded expander.

The connection of the piston to the connecting rod is carried out using a piston pin, which is tubular and made of steel. There are several ways to install the piston pin. The most popular so-called. floating finger, which has the ability to rotate in the bosses and the piston head of the connecting rod during operation. To prevent the pin from dislodging, it is secured with retaining rings. Rigid fastening of the ends of the pin in the piston or rigid fastening of the pin in the piston head of the connecting rod is used much less frequently.

The piston, piston rings and piston pin bear the well-established name of the piston group.

In the cylinder-piston group (CPG), one of the main processes takes place, due to which the internal combustion engine functions: the release of energy as a result of combustion of the air-fuel mixture, which is subsequently converted into a mechanical action - the rotation of the crankshaft. The main working component of the CPG is the piston. Thanks to him, the conditions necessary for the combustion of the mixture are created. The piston is the first component involved in the conversion of the received energy.

The piston of the engine is cylindrical. It is located in the cylinder liner of the engine, it is a moving element - during operation, it reciprocates, because of which the piston performs two functions.

When moving forward, the piston reduces the volume of the combustion chamber, compressing the fuel mixture, which is necessary for the combustion process (in diesel engines, the mixture is ignited by its strong compression).
After ignition of the air-fuel mixture in the combustion chamber, the pressure rises sharply. In an effort to increase the volume, it pushes the piston back, and it makes a return movement, which is transmitted through the connecting rod to the crankshaft.

DESIGN

The device of the part includes three components:

Bottom.
Sealing part.
Skirt.

These components are available both in one-piece pistons (the most common option) and in component parts.

BOTTOM

Bottom - main working surface, since it, the walls of the liner and the head of the block form a combustion chamber, in which the fuel mixture is burned.

The main parameter of the bottom is the shape, which depends on the type of internal combustion engine (ICE) and its design features.

In two-stroke engines, pistons are used with a spherical bottom - a bottom protrusion, this increases the efficiency of filling the combustion chamber with a mixture and removing exhaust gases.

In four-stroke gasoline engines, the bottom is flat or concave. Additionally, technical recesses are made on the surface - recesses for valve discs (eliminate the likelihood of a piston colliding with the valve), recesses to improve mixture formation.

In diesel engines, the recesses in the bottom are the most overall and have different shape... These recesses are called a piston combustion chamber and are designed to create turbulence in the flow of air and fuel into the cylinder for better mixing.

The sealing part is designed for the installation of special rings (compression and oil scraper), the task of which is to eliminate the gap between the piston and the liner wall, preventing the breakthrough of working gases into the sub-piston space and lubricants into the combustion chamber (these factors reduce the efficiency of the motor). This ensures heat transfer from the piston to the liner.

SEALING PART

The sealing part includes grooves in cylindrical surface piston - grooves located behind the bottom, and bridges between the grooves. In two-stroke engines, special inserts are additionally placed in the grooves, into which the ring locks abut. These inserts are necessary to eliminate the possibility of the rings turning and getting their locks into the inlet and outlet ports, which can cause them to collapse.

The jumper from the bottom edge to the first ring is called the head land. This belt takes on the greatest temperature effect, so its height is selected based on the operating conditions created inside the combustion chamber and the material of the piston.

The number of grooves made on the sealing part corresponds to the number of piston rings (and they can be used 2 - 6). The most common design is with three rings - two compression rings and one oil scraper.

In the groove for the oil scraper ring, holes are made for the oil drain, which is removed by the ring from the liner wall.

Together with the bottom, the sealing part forms the piston head.

SKIRT

The skirt acts as a guide for the piston, preventing it from changing position relative to the cylinder and providing only the reciprocating movement of the part. Thanks to this component, a movable connection of the piston with the connecting rod is carried out.

For connection, holes are made in the skirt for installing the piston pin. To increase the strength at the point of contact of the finger, special massive beads, called bosses, are made on the inside of the skirt.

To fix the piston pin in the piston, grooves for retaining rings are provided in the mounting holes for it.

PISTON TYPES

In internal combustion engines, two types of pistons are used, differing in design - one-piece and composite.

Solid parts are made by casting followed by machining. In the process of casting, a blank is created from metal, which is given the general shape of the part. Further, on metal-working machines in the resulting workpiece, the working surfaces are processed, grooves for rings are cut, technological holes and grooves are made.

In the components, the head and skirt are separated, and they are assembled into a single structure during installation on the engine. Moreover, the assembly into one piece is carried out when the piston is connected to the connecting rod. For this, in addition to the piston pin holes in the skirt, there are special lugs on the head.

The advantage of composite pistons is the ability to combine materials of manufacture, which increases the performance of the part.

MANUFACTURING MATERIALS

Aluminum alloys are used as the material of manufacture for solid pistons. Parts made of such alloys are characterized by low weight and good thermal conductivity. But at the same time, aluminum is not a high-strength and heat-resistant material, which limits the use of pistons made of it.

Cast pistons are also made of cast iron. This material is durable and resistant to high temperatures. Their disadvantage is their significant mass and poor thermal conductivity, which leads to strong heating of the pistons during engine operation. Because of this, they are not used on gasoline engines, since the high temperature causes glow ignition (the air-fuel mixture ignites from contact with hot surfaces, and not from the spark of the spark plug).

The design of the compound pistons allows the specified materials to be combined with each other. In such elements, the skirt is made of aluminum alloys, which provides good thermal conductivity, and the head is made of heat-resistant steel or cast iron.

But elements of a composite type also have disadvantages, including: