How is the piston. Car engine piston: a detail worthy of praise

30.04.2019

1. List the elements of the piston and explain their purpose, explain the working conditions of the piston.

In the design of the piston, it is customary to distinguish the following elements:

head 1 and skirt 2. The head includes the bottom Z, fire (flame) 4 and

sealing 5 belts. The piston skirt consists of bosses b and a guide part.

The complex configuration of the piston, rapidly changing in magnitude and direction of heat flows acting on its elements, lead to an uneven distribution of temperatures over its volume and, as a result, to significant time-varying local thermal stresses and deformations

The heat supplied to the piston through its head, which is in contact with the working fluid in the engine cylinder, is removed to the cooling system through its individual elements in the following ratio, %: into the cooled cylinder wall through compression rings - 60 ... 70, through the piston skirt - 20 ...30, into the lubrication system through the inner surface of the piston bottom - 5...10. The piston also perceives part of the heat released as a result of friction between the cylinder and the piston group.

The main elements of the piston design

Groove for the first compression ring

Groove for second compression ring

Interring jumpers

Groove under oil scraper ring

Oil drain sample

"Fridge"

Piston skirt

Finger hole boss

Unloading fetch

Retaining ring groove

Finger hole

Piston skirt

Piston head

Ni-resist insert

Oil cooled cavity

The combustion chamber

cone displacer

piston crown

The piston is one of the most important engine parts. internal combustion. It transfers the energy of fuel combustion through the pin and connecting rod to the crankshaft. Together with the rings, it seals the cylinder from the ingress of combustion products into the crankcase. During operation, the piston is subjected to high mechanical and thermal loads.

The maximum pressure in the cylinder that occurs during the combustion of the fuel-air mixture can reach 65-80 bar in a gasoline engine and 80-160 bar in a diesel engine. This is equivalent to a force of several tons acting on the piston of a car engine and tens of tons on the piston of a heavy diesel engine.

During operation, the piston reciprocates, periodically accelerating to speeds of more than 100 km / h, and then decelerating to zero. Such a cycle occurs with a double crankshaft speed, i.e. at 6000 rpm, the acceleration-deceleration cycle occurs at a frequency of 200 Hz.

The maximum value of accelerations attributable to the upper and lower dead spots, can reach 15000-20000 m/s 2 , which corresponds to an overload of 1500-2000g. When launching a rocket into space, an astronaut briefly experiences overloads 150 times less. From the action of accelerations, inertial forces arise in magnitude commensurate with those that act from the pressure during combustion.

Combustion of the air-fuel mixture occurs at a temperature of 1800-2600°C. This temperature is much higher than the melting point of the aluminum-based piston alloy (~700°C). In order not to melt, the piston must be effectively cooled by transferring heat from the combustion chamber through the rings, skirt, cylinder walls, pin, and the inner surface of the coolant and oil. When the piston is heated, the tensile strength of the material decreases, thermal stresses arise from temperature drops along its body, which are superimposed on stresses from gas pressure forces and inertial forces. Thus, the operating conditions of the piston can be defined as very difficult.

In order for the piston to withstand these influences, it must be light, durable, wear-resistant, and conduct heat well. All of these conditions must be taken into account when designing. The shape of the internal surfaces and structural elements of the piston must provide the specified strength and performance due to the rational distribution and use of the material.

Particular attention is paid to the shape of the outer surface. The outer profile of the piston side surface is formed taking into account deformations from mechanical loads (gas pressure and inertial forces) and thermal effect from the combustion of the air-fuel mixture in such a way that, under no circumstances, jamming in the cylinder, breakthrough of hot gases into the crankcase, burning of the combustion chamber does not occur.

The temperature of the piston in the area of the combustion chamber (on the bottom) is higher than on the skirt, the thermal expansion of the head is greater than the skirt, therefore the piston in the cold state is barrel-shaped, with a decrease in diameter from the skirt to the head.

The gas pressure force, inertial forces and lateral force deform the piston so that the skirt ovalizes. To compensate for this deformation, the piston is initially made with an "anti-ellipse", the major axis of which is located perpendicular to the axis of the pin hole.

Clearances between piston and cylinder should be kept to a minimum to prevent noise, especially in a cold engine. But they should be sufficient to prevent jamming when the engine is running warm.

The barrel-shaped and oval shape of the outer surface, in addition to compensating for the corresponding deformations from force and thermal effects, ensures the formation of an oil film between the piston and the cylinder (hydrodynamic lubrication)

Design features of the piston

Details related to the structural elements of the pistons will provide a deeper understanding of the complexity of the challenges facing manufacturers.

The piston head is its upper part, which includes the bottom and the area of the grooves for the piston rings. Together with the cylinder head, the piston crown forms the combustion chamber. The combustion chamber can also be made in the head. Gas pressure and heat from fuel combustion act on the bottom. The piston head must:

Ensure good mixture formation and completeness of fuel combustion;

Maintain strength at high temperature;

Provide heat removal from the bottom;

Transfer force to the piston pin and connecting rod through the bosses;

Provide a specified resource for the wear of the grooves for the piston rings.

In diesel engines with direct injection, the combustion chamber, as a rule, is carried out in the piston and has a great influence on the processes of carburetion and combustion.

In diesel engines with pre-chamber injection and gasoline engines, the piston crown is flat or has small recesses.

The head of aluminum pistons can be anodized (protective oxide coating applied). In diesel engines, the combustion chamber can be strengthened by sintered fiber reinforcement in an injection molding process.

The piston ring grooves are located on the side surface of the piston head. Usually there are three of them: two for compression and one for oil scraper rings. The piston rings form a seal between the piston and the cylinder wall, preventing hot gases from escaping into the crankcase and oil from entering the combustion chamber.

The bridges between the grooves (especially between the first and second for compression rings) are subjected to high mechanical and thermal loads - 50-60% of the heat is removed to the cylinder through the compression rings.

Uneven heating and thermal expansion of the head can cause the grooves to become out of shape. This negatively affects oil consumption and causes wear on the cylinder wall and the groove itself. To eliminate this phenomenon, the annular grooves are made at a slight angle so that the outer edges are higher than the inner ones. This prevents an undesirable downward inclination of the groove cross-section during operating conditions.

The grooves of the upper compression rings are subject to particularly stringent requirements, especially in diesel engines with a high degree compression. To harden these grooves, they are often reinforced with special inserts made of ni-resist (nickel-alloyed cast iron), or the groove area is hardened by plasma remelting with the addition of alloying components. These measures increase wear resistance and reduce noise in the diesel engine.

The most common types of inserts are available with parallel sides and inserts with tapered sides. Ni-resist inserts are available with a single groove or, in some high-powered diesel engines, with two grooves for compression rings. Sometimes a strip of stainless steel is attached to the lower end surface of the groove of the first compression ring, which performs the same function as the ni-resist insert.

Significant variable forces and heat flows are transmitted through the piston pin during operation. Therefore, the surfaces of the pin holes in the piston must be machined with high precision, while the surface roughness can reach 0.1 µm. To reduce stress on the edges of the bosses and in the pin with inside holes are sometimes made with a small cone angle (less than 1 degree).

An important design technique for reducing the noise that occurs when the piston is moved near top dead point, is the displacement of the pin hole from the piston axis in the direction of that side of the piston skirt, which perceives the lateral force during the working stroke. In this case, a mark must be applied to the piston for correct installation into the engine.

Coatings

To improve the performance of pistons in an engine, their surface is often subjected to various types of processing, in particular, coatings are applied to it. These coatings perform two main functions:

Piston run-in improvement. Usually they are applied to the skirt, and they wear out after a certain time during the engine break-in stage;

Improving the mechanical properties of the piston surface (hardness, wear resistance). Some coatings remain on the piston for the duration of its operation, preventing erosion, cracking and improving anti-friction properties.

Piston head diesel engines sometimes anodized (coated with aluminum oxide) to reduce the temperature of the base material and the risk of head cracking caused by high thermal loads during operation.

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So our first task is to understand what is engine. The result of the engine is the presence of torque on it crankshaft.

The engine consists of two mechanisms:

1- crank- connecting rod mechanism (KShM, Crank mechanism) designed to convert the reciprocating motion of the piston in the cylinder into the rotational motion of the crankshaft of the engine.

2 — Gas distribution mechanism (timing, gas distribution mechanism) designed for timely supply of the engine combustible mixture, as well as for the release of exhaust gases.

In this part, we will analyze those engine parts that relate to the crankshaft. Looking ahead, I will announce the entire list of those details that make up KShM.

So, The crank mechanism consists of:

Flywheel
Pistons with rings and pins
Cylinder block with crankcase
cylinder heads,
engine oil pan

If the result of the work is the presence of torque on the crankshaft, then one of the engine parts is the Crankshaft.

1. Crankshaft(crankshaft)

The crankshaft is shown in the figure below:

The engine crankshaft with flywheel consists of:
1 - crankshaft of the engine; 2 - flywheel with a ring gear;
3 - connecting rod neck; 4 - root (support) neck; 5 - counterweight

flywheel- This is a massive metal disk that is mounted on the crankshaft of the engine. the flywheel always tries to maintain the state from which it is taken out. He gains momentum for a long time, thereby smoothing out the jumps. It also slows down for a long time. In short, due to its inertia, it creates smooth transitions from one speed to another. In addition, its inertness plays the role of an energy accumulator. Well, if you spin the flywheel, while spending work, it is able to do the same work until it stops. Roughly speaking, this is a kind of stabilizer that protects the engine from surges and shocks.

Now let's pay attention crankpin. It has this name because a rod is attached to it.

2. Connecting rod

connecting rod- a movable part of the crank mechanism of the engine, connecting the piston and crankshaft and transmitting force from the piston to the crankshaft of the Internal Combustion Engine (ICE), converting the translational movement of the piston into rotational movement of the crankshaft.

Crankshaft and connecting rod parts piston group shown in the picture below:

1 - crankshaft; 2 - insert connecting rod bearing; 3 – a bolt of fastening of a cover of a rod; 4 - piston pin; 5 - retaining ring; 6 - bushing of the connecting rod head; 7 - connecting rod; 8 - connecting rod cover; 9 – a nut of fastening of a cover of a rod

So, it means that a connecting rod is attached to the crankshaft. And the connecting rod, in turn, is connected to the piston.

3. Piston

The piston is a part of the crank mechanism of the engine that directly perceives pressure from the working mixture burning in the cylinder

The piston is shown in the figure below:

The cylinder head has combustion chambers, intake and exhaust ports, threaded holes for installing spark plugs and coolant passages. Seats and valve guides, made of special heat-resistant cast iron, are inserted into the preheated head while cooled, which ensures high tension in the connection after temperature equalization.

So we learned what the part of the heart of the car, called crank mechanism. Now we know that the engine consists of a crankcase in which a crankshaft with a flywheel is installed. Connecting rods are attached to the crankshaft, and pistons are attached to the connecting rods. The pistons, in turn, run in cylinder liners. This whole structure is covered by the cylinder head. The latter serves as the beginning for a story about the other side of the engine - the gas distribution mechanism. I will write about it in my next post.

I recommend a video for fixing:

P.S. I look forward to your wishes, suggestions, opinions and comments.

When we get behind the wheel of a car, turn the key in the ignition and press the gas pedal, a lot of very complex mechanisms that produce the movement. All these mechanisms do not interest us at all, the main thing is that the car is moving. But when a breakdown occurs, we begin to puzzle over what is the reason and we have to master all the necessary information about the structure and functioning of each individual part. But in order not to waste time on this, when you don’t have this time, before you get behind the wheel, you should have a good understanding of the features of automotive parts.

In particular, today we will talk with you about the piston. After all, this detail is central in the process of processing fuel energy into thermal and mechanical energy. We will deal with you what a piston is, its purpose, the basic requirements for it and the features of its design.

1. Engine piston and its main characteristics

We certainly hope that experienced motorists do not need to explain for a long time what an engine piston is. However, if there are “beginners” among our readers, then especially for them we will explain that the piston is a car part that converts changes in the pressure of gas, vapor and liquid inside the engine into mechanical force. The piston has the shape of a cylinder, inside which reciprocating movements are constantly made, due to which a mechanical force is formed.

The duty of this part is very responsible and its effectiveness depends on how well he copes with it. In fact, he is the most intricate detail car, to understand the features and contradictory properties of which is quite difficult for an unprepared mind. Few people know, but almost none automobile concern not engaged self-manufacturing pistons for their cars, and order them specifically for their engines. Complicates the situation for ordinary motorists and the fact that today there is a large number of different shapes and sizes of pistons. Therefore, the maintenance and repair of this part can always be carried out in different ways.

What requirements must a reliable piston meet?

Since the piston is a rather complicated part, a great many requirements are set for it. Due to the complexity of production, there are not so many manufacturers of engine pistons, and this part costs quite a lot on the car market. And so, let's see what requirements a good piston must meet:

1. Moving inside the cylinder, it is the engine piston that provides the expansion of compressed gases, which are the product of fuel combustion. This allows gases to mechanical work- actuate all other mechanisms of the car. As a result, the main requirement for pistons is the ability to resist the high temperature at which all these processes take place, the high gas pressure and seal the cylinder bore well (otherwise it will not be able to influence the gas pressure).

2. The piston is not a single device, it works together with the cylinder and piston rings. Together these parts form a linear plain bearing. In this regard, the bearing must necessarily meet all the requirements and features of the friction pair. If all requirements are taken into account with the highest accuracy, then this will not only help to minimize mechanical losses during fuel combustion, but also the wear of all parts.

3. The piston is constantly under heavy loads, the strongest of which are the loads from the combustion chamber of the fuel and the reactions from it. Its design must necessarily take into account all these factors and withstand such a strong mechanical impact.

4. Despite the fact that the piston moves during operation with quite high speed, it should not heavily load inertial forces crank mechanism vehicle, otherwise it may cause damage.

2. Appointment of pistons or their functional duties

We have repeatedly mentioned that the piston performs very important role throughout the operation of an automobile engine. So, the main purpose of pistons is to:

- to receive gas pressure from the combustion chamber and transfer these pressures to the engine in the form of mechanical force;

Seal the engine cylinder cavity above the piston. Thus, it protects the entire automotive mechanism from blowing gases into the crater and from penetrating lubricating oil.

Moreover, the second function is more important, since it is thanks to this that the piston provides itself normal conditions for work. Even about the technical condition of the engine, experts make a conclusion only after examining the piston group and checking its sealing ability. After all, if the oil consumption exceeds 3% of the fuel consumption (and this happens due to its waste when it penetrates into the combustion chamber), then the entire car engine it must be urgently sent for repair, or it can be taken out of service altogether. You can understand that something is wrong with your engine by looking at the smoke in the exhaust gases. But it's better not to let that happen.

Probably, reading that the piston and its elements work in conditions with very high temperatures, you are surprised how this device itself does not fail? We add to this that, in addition to difficult temperature conditions, the operation of the piston is constantly accompanied by cyclic, sharply changing loads. With all this, the elements of the described part do not even always have enough lubrication. But of course, all the designers and developers of pistons thought about this.

Firstly, they are designed taking into account the purpose and type of engine on which they will be installed (stationary, diesel, two-stroke, forced or transport), so only the most stable materials are used for this.

Secondly, there are several ways in which the cooling of this part is carried out. But first, a little about how and where heat (or even heat) flows from the combustion chamber. It goes out into the environment cold air, which washes the radiator and engine, as well as the cylinder block. But in what ways does the piston give heat to the block and antifreeze?

1. through piston rings. The most important of these is the first, since it is located closest to the bottom of the piston. Since the rings are simultaneously pressed against both the piston grooves and the cylinder wall, about 50% of the total heat flow from the piston is given off due to them.

2. Thanks to the second "coolant", the role of which is played by engine oil. Since the oil approaches the hottest parts of the engine, it is he who manages to carry a very large amount of heat from the most heated points into the crankcase. However, in order for the oil to be able to cool the pistons, it must also be cooled, otherwise it will have to be changed very soon.

3. Heat travels through the lugs to the pin, to the connecting rod, and to the oil. The less efficient way, however, and it plays its important role.

4. Oddly enough, but the fuel also helps to cool the piston and the engine as a whole. So, when a fresh mixture of fuel and air enters the combustion chamber, it draws quite a lot of heat onto itself, although then it gives it back in even larger quantities. However, the amount of the mixture and the heat it can absorb is directly related to how the car is running and how open the throttle is. The advantage of this path is that the mixture absorbs heat exactly from the side from which the piston heats up the most.

However, we are getting ahead of ourselves a little, as we started talking about the functioning of the piston without fully understanding design features this detail. This is what we will devote the next section to.

3. Piston design: everything the average car enthusiast needs to know about the part

In general, talking about the piston alone is like talking about bread, discussing only the properties of flour. It is more logical to get acquainted with the entire piston group of the engine, which is represented by such details:

- the piston itself;

Piston rings;

Piston pin.

This design of the piston group has been unchanged since the advent of the very first internal combustion engines. That's why, given description will be common to almost all engines.

Naturally, the most important features performs a piston whose design has not changed for 150 years. If you do not want to become a professional mechanic, then you only need to know about such important areas of the piston and their functional purposes:

1. Piston head. The surface of a part that directly faces the combustion chamber of an engine. With its profile, the bottom determines the lower surface of this very chamber. This form can depend on: the shape of the combustion chamber, on its volume, the features of the supply of fuel-air mass into it, on the location of the valves. There are cases when there is a recess on the bottom due to which the volume of the combustion chamber increases. But, since this is not desirable, then to reduce the volume of the chamber, it is necessary to use special displacers - a certain amount of metal located above the bottom plane.

2. "Heat (fire) belt." This term refers to the distance that runs from the bottom of the piston to its first ring. It is important to know that the smaller the distance from the bottom to the rings, the higher the thermal load will fall on these same elements, and the more they will wear out.

3. Sealing area. We are talking about the grooves that are located on the side surface of the cylinder-shaped piston. These grooves are a direct way to install the rings, which in turn allow the seal to move. Also, in the groove for the oil scraper ring, there must be a hole through which excess oil can be discharged into the internal cavity of the piston.

Another function of the sealing section is to remove part of the heat from the engine piston using, as we have already mentioned, piston rings. However, for effective heat dissipation, it is very important that the piston rings fit snugly into both the grooves and the cylinder surface. So, the end gap of the first compression ring should be about 0.045 to 0.070 mm, for the second - from 0.035 to 0.06 mm, and for the oil scraper - from 0.025 to 0.005 mm. But between the rings and grooves, the radial clearance indicator can be from 1.2 to 0.3 millimeters. But these indicators are not significant for the human eye, they can only be determined with the help of special equipment.

4. Piston head. This is a generalized section, which includes the bottom and the sealing part already described above.

5. Compression height of the piston. Distance that is calculated from the axis piston pin to the bottom of the piston.

6. "Skirt". The bottom of the piston. Includes lugs with holes for the piston pin to fit into. The outer surface of this section is the supporting and guiding surface for the piston. Thanks to it, the correct ratio of the piston axis and the engine cylinder axis is ensured. An equally important role is played by the side surface of the "skirt", due to which the transverse forces that periodically occur in the piston group of the engine are transmitted to the cylinder. And especially in order to improve the workability of the surface of the skirt and reduce friction, it is coated with a special protective coating of tin (graphite and molybdenum disulfide can also be used as the basis of the coating. Or, instead of coating, grooves of a special profile can be applied to the skirt, which retain oil and create hydrodynamic force preventing contact with the cylinder walls.

How and from what: features of the manufacture of automotive pistons

It is clear that in order to perform such functions that the piston performs, a sufficiently “hardy” metal is required. However, it is far from steel. Pistons are made from aluminum alloys, to which silicon is always added. This is done in order to reduce the expansion coefficient under the influence of high temperatures and increase the wear resistance of the part.

However, for the manufacture of pistons, an alloy with a different percentage of silicon content can be used. For example, 13% silicon alloys are most often used for this purpose, which are called eutectic. There are alloys with more high content silicon, which are called hypereutectic. And the higher this percentage, the higher the heat-conducting characteristics of the alloy. But this does not make such a material ideal for the manufacture of pistons.

The fact is that when cooled, such a material begins to release silicon grains, ranging in size from 0.5 to 1 millimeter. It is obvious that such a process is reflected in foundries and mechanical properties both the material and the part that is made of it. For this reason, in addition to silicon, the following list of regulating additives is introduced into such alloys:

- manganese;

How is the main part of the automobile piston made? There are even two ways in which you can get the workpiece of this part. The first of these involves pouring a hot alloy into special form called "kokil". This method is the most common. The second option for manufacturing a workpiece is hot stamping. But after machining the mold, the future piston is also subjected to various heat treatments, which makes it possible to increase the hardness of the metal, strength and wear resistance. Also, such procedures allow you to remove the residual stress in the metal.

Although the use of forged metal increases the strength of the part, they also have their drawbacks. Such products are usually made in the classic version with a high “skirt”, which makes them too heavy. Also, such products do not allow the use of thermocompensating rings or plates with them. Due to the increased weight of such a piston, its thermal deformation also increases, as a result, it is necessary to increase the size of the gap between the piston and the cylinder.

The consequences of this will not please the driver at all, since they are increased noise engine work, rapid wear cylinders and high flow oils. The use of forged pistons justifies itself only in cases where the car is regularly operated in the most extreme conditions.

To date, designers and physicists are making every effort to make the design of pistons as ideal and accurate as possible. In particular, the most important trends are directed to the following list:

- reducing the weight of the part;

The use of only “thin” rings on the piston;

Reducing the compression height of the piston;

Reducing piston pins and using only the shortest pistons in the piston design;

improvement protective coatings and applying them to all surfaces of the part.

A similar achievement today can be seen on the T-shaped design of the pistons. latest generation. this design is called T-shaped precisely due to the external similarity of the part with the letter "T". The main difference between such pistons is the reduced height of the skirt and the area of its guide part. Such pistons are made from a hypereutectic alloy, which contains a fairly large amount of silicon. And they are made mainly by hot stamping.

However, what kind of engine piston design will want to put on the car, its developers will depend on many factors. This decision is always preceded by a long period calculations and analysis of the behavior of all nodes of the connecting rod and piston group under the influence new part. The calculation of all parts is carried out on their most limiting capabilities of their designs and the materials from which they are made. However, sadly, but in this case, the manufacturer will not overpay. He will choose the option that provides the necessary resource just “in time” and will not spend money on increasing it.

Be that as it may, but an ordinary motorist has to understand and operate what has already been installed on his car. We hope that our article has helped you to better understand how pistons function and what is the purpose of them. We wish you that you never have problems with this part, for which it is necessary to provide it right conditions operation - do not “drive” too much and change the engine oil on time.

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The piston of the engine is a part that has a cylindrical shape and performs reciprocating movements inside the cylinder. It is one of the most characteristic parts for the engine, since the implementation of the thermodynamic process occurring in the internal combustion engine occurs precisely with its help. Piston:

perceiving the pressure of gases, transfers the resulting force to;
seals the combustion chamber;
removes excess heat from it.

The photo above shows four strokes of the engine piston.

Extreme conditions dictate piston material

The piston is operated extreme conditions, characteristic features which are high: pressure, inertial loads and temperatures. That is why the main requirements for materials for its manufacture include:

high mechanical strength;
good thermal conductivity;
low density;
insignificant coefficient of linear expansion, antifriction properties;
good corrosion resistance.

The required parameters correspond to special aluminum alloys, which are distinguished by strength, heat resistance and lightness. Less commonly, gray cast irons and steel alloys are used in the manufacture of pistons.

Pistons can be:

cast;
forged.

In the first version, they are made by injection molding. Forged ones are made by stamping from an aluminum alloy with a small addition of silicon (on average, about 15%), which significantly increases their strength and reduces the degree of expansion of the piston in the operating temperature range.

The design features of the piston are determined by its purpose

The main conditions that determine the design of the piston are the type of engine and the shape of the combustion chamber, the features of the combustion process taking place in it. Structurally, the piston is a one-piece element, consisting of:

heads (bottoms);
sealing part;
skirts (guide part).

Is the piston of a gasoline engine different from a diesel engine? The surfaces of the piston heads of gasoline and diesel engines are structurally different. IN petrol engine head surface - flat or close to it. Sometimes grooves are made in it, contributing to the full opening of the valves. For engine pistons equipped with direct injection fuel (SNVT), a more complex form is characteristic. The piston head in a diesel engine is significantly different from a gasoline engine - due to the execution of a combustion chamber of a given shape in it, better swirl and mixture formation are provided.

The photo shows the engine piston diagram.

Piston rings: types and composition

The sealing part of the piston includes piston rings that provide a tight connection between the piston and the cylinder. Technical condition engine is determined by its sealing ability. Depending on the type and purpose of the engine, the number of rings and their location are selected. The most common scheme is a scheme of two compression and one oil scraper rings.

Piston rings are made mainly from special gray ductile iron, which has:

high stable indicators of strength and elasticity at operating temperatures throughout the entire service life of the ring;
high wear resistance under conditions of intense friction;
good antifriction properties;
the ability to quickly and effectively break in to the surface of the cylinder.

Due to the alloying additives of chromium, molybdenum, nickel and tungsten, the heat resistance of the rings is significantly increased. By applying special coatings of porous chromium and molybdenum, tinning or phosphating the working surfaces of the rings, they improve their run-in, increase wear resistance and corrosion protection.

The main purpose of the compression ring is to prevent gases from the combustion chamber from entering the engine crankcase. Especially heavy loads fall on the first compression ring. Therefore, in the manufacture of rings for the pistons of some forced gasoline and all diesel engines, a steel insert is installed, which increases the strength of the rings and allows for maximum compression. The shape of the compression rings can be:

trapezoidal;
barrel-shaped;
tconical.

In the manufacture of some rings, a cut (cut) is performed.

The oil scraper ring is responsible for removing excess oil from the cylinder walls and preventing it from entering the combustion chamber. It is distinguished by the presence of many drainage holes. Some rings are designed with spring expanders.

The shape of the piston guide (otherwise, the skirt) can be cone-shaped or barrel-shaped, which allows compensating for its expansion when high operating temperatures are reached. Under their influence, the shape of the piston becomes cylindrical. Side surface In order to reduce losses caused by friction, the piston is coated with a layer of anti-friction material; for this purpose, graphite or molybdenum disulfide is used. Lug holes in the piston skirt allow the piston pin to be secured.

A unit consisting of a piston, compression, oil scraper rings, as well as a piston pin is commonly called a piston group. The function of its connection with the connecting rod is assigned to a steel piston pin, which has a tubular shape. It has requirements for:

minimal deformation during operation;
high strength under variable load and wear resistance;
good impact resistance;
small mass.

According to the installation method, piston pins can be:

fixed in the piston bosses, but rotate in the connecting rod head;
fixed in the connecting rod head and rotate in the piston bosses;
freely rotating in the piston bosses and in the connecting rod head.

The fingers installed according to the third option are called floating. They are the most popular because their length and circumference wear is negligible and uniform. With their use, the risk of seizing is minimized. In addition, they are easy to install.

Removal of excess heat from the piston

In addition to significant mechanical stresses, the piston is also subjected to the negative effects of extremely high temperatures. Heat is removed from the piston group:

cooling system from the cylinder walls;
the internal cavity of the piston, then - the piston pin and connecting rod, as well as the oil circulating in the lubrication system;
partially cold air-fuel mixture supplied to the cylinders.

From the inner surface of the piston, its cooling is carried out using:

splashing oil through a special nozzle or hole in the connecting rod;
oil mist in the cylinder cavity;
injection of oil into the zone of the rings, into a special channel;
oil circulation in the piston head through a tubular coil.

Video - operation of an internal combustion engine (strokes, piston, mixture, spark):

Video about four stroke engine- principle of operation:

IN cylinder-piston group(CPG) occurs one of the main processes due to which the internal combustion engine functions: the release of energy as a result of the combustion of the air-fuel mixture, which is subsequently converted into mechanical action- 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 engine piston has a cylindrical shape. It is located in the cylinder liner of the engine, it is a movable element - in the process of operation it performs reciprocating movements and performs two functions.

As the piston moves forward, it reduces the volume of the combustion chamber by compressing fuel mixture, which is necessary for the combustion process (in diesel engines ignition of the mixture does occur from its strong compression).
After the 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 performs return movement transmitted through the connecting rod to the crankshaft.

What is a car internal combustion engine piston?

The device of the part includes three components:

Bottom.
Sealing part.
Skirt.

These components are available both in solid pistons (the most common option) and in composite parts.

Bottom

Bottom - main working surface, since it, the walls of the sleeve 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, in which the bottom of a spherical shape is the protrusion of the bottom, this increases the efficiency of filling the combustion chamber with a mixture and the removal of exhaust gases.

In four-stroke gasoline engines the bottom is flat or concave. Additionally, technical recesses are made on the surface - recesses for valve plates (eliminate the possibility of a collision between the piston and the valve), recesses to improve mixture formation.

In diesel engines, the recesses in the bottom are the most dimensional and have a different shape. Such recesses are called piston combustion chambers and they are designed to create turbulence when air and fuel are supplied to the cylinder to ensure better mixing.

The sealing part is designed to install 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 under-piston space and lubricants into the combustion chamber (these factors reduce the efficiency of the motor). This ensures that heat is removed from the piston to the sleeve.

Sealing part

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

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

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

In the groove for the oil scraper ring, holes are made for the stack of oil, which is removed by the ring from the wall of the sleeve.

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

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Skirt

The skirt acts as a guide for the piston, preventing it from changing its 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 strength at the point of contact of the finger, special massive influxes, called bosses, are made on the inside of the skirt.

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

Piston types

There are two types of pistons used in internal combustion engines. constructive device- integral and composite.

One-piece 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 metalworking machines in the resulting workpiece, working surfaces are processed, grooves are cut for rings, and technological holes and recesses.

IN constituent elements the head and skirt are separated, and they are assembled into a single structure during installation on the engine. Moreover, the assembly in one piece is carried out by connecting the piston to the connecting rod. For this, in addition to holes for the finger in the skirt, there are special eyelets on the head.

The advantage of composite pistons is the possibility of combining materials of manufacture, which increases the performance of the part.

Manufacturing materials

Aluminum alloys are used as the manufacturing material 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 from it.

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

The design of composite pistons allows you to combine these materials with each other. In such elements, the skirt is made of aluminum alloys, which ensures good thermal conductivity, and the head is made of heat-resistant steel or cast iron.

But the elements composite type there are disadvantages, including: