What is the piston crown? Engine pistons: device, purpose, types

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

The following elements are usually distinguished in the design of the piston:

head 1 and skirt 2. The head includes a bottom 3, a 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 affecting its elements, lead to an uneven temperature distribution throughout its volume and, as a consequence, 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 absorbs part of the heat released as a result of friction between the cylinder and the piston group.

Main piston design elements

Groove for the first compression ring

Groove for the second compression ring

Inter-ring jumpers

Groove for oil scraper ring

Oil drain sample

"Fridge"

Piston skirt

Pin hole boss

Offload sampling

Groove for snap ring

Finger hole

Piston skirt

Piston head

Niresist insert

Oil-cooled cavity

The combustion chamber

Cone displacer

Piston crown

The piston is one of the most important engine parts internal combustion. It transmits the energy of fuel combustion through the pin and connecting rod crankshaft. It, together with the rings, seals the cylinder from combustion products entering the crankcase. During operation, the piston is subject 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 passenger car engine and tens of tons acting on the piston of a heavy diesel engine.

During operation, the piston moves back and forth, periodically accelerating to a speed of more than 100 km/h, and then slowing down to zero. This cycle occurs at double the crankshaft speed, i.e. at 6000 rpm, the acceleration-deceleration cycle occurs at a frequency of 200 Hz.

The maximum acceleration value 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 that are 150 times less. From the action of accelerations, inertial forces arise in magnitude comparable to those that act from pressure during combustion.

Combustion of the air-fuel mixture occurs at a temperature of 1800-2600°C. This temperature significantly exceeds the melting point of the aluminum-based piston alloy (~700°C). To avoid melting, the piston must cool effectively by transferring heat from the combustion chamber through the rings, skirt, cylinder walls, pin and inner surface to the coolant and oil. When the piston is heated, the tensile strength of the material decreases, thermal stresses arise from temperature changes throughout its body, which are superimposed on the 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 listed conditions must be taken into account when designing. The shape of the internal surfaces and structural elements of the piston must ensure the specified strength and performance through rational distribution and use of the material.

Particular attention is paid to the shape of the outer surface. The external profile of the piston side surface is formed taking into account deformations from mechanical loads (gas pressure and inertial forces) and thermal effects from combustion of the air-fuel mixture in such a way that under no circumstances does jamming in the cylinder, breakthrough of hot gases into the crankcase, or burning out of the combustion chamber occur.

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

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

The clearances between the 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 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 design components of the pistons will provide further insight into the complexity of the challenges facing manufacturers.

The piston head is its upper part, which includes the bottom and the area of ​​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 complete fuel combustion;

Maintain strength at high temperatures;

Provide heat removal from the bottom;

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

Ensure the specified wear life of the grooves for the piston rings.

In diesel engines with direct injection, the combustion chamber is usually located in the piston and has a great influence on the processes of mixture formation 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 (a protective oxide coating is applied). In diesel engines, the combustion chamber can be strengthened by reinforcement with sintered metal fibers during the injection molding process.

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

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

Uneven heating and thermal expansion of the head can lead to irregularities in the shape of the grooves. 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 the appearance of an undesirable downward inclination of the groove cross-section during operating conditions.

Particularly stringent requirements are placed on the grooves of the upper compression rings, especially in diesel engines with high degree compression. To strengthen these grooves, they are often reinforced with special inserts made of niresist (nickel-alloyed cast iron), or the groove area is strengthened by plasma remelting with the addition of alloying components. These measures increase wear resistance and reduce noise in a diesel engine.

The most common types of inserts are available with parallel sides and inserts with tapered sides. There are ni-resist inserts with one groove or, in some high-performance diesel engines, with two grooves for compression rings. Sometimes to the lower end surface of the groove of the first compression ring a strip of stainless steel is attached, which performs the same function as the nyresist 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, and the surface roughness can reach 0.1 microns. To reduce stress on the edges of the bosses and in the pin inside The holes are sometimes made into a cone with a small angle (less than 1 degree).

An important design technique for reducing the noise that occurs when moving the piston near the top dead center is to shift the pin hole from the piston axis in the direction of the side of the piston skirt that absorbs 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 treatment, in particular, coatings are applied to it. These coatings perform two main functions:

Improved piston running-in. They are usually applied to the skirt and wear out after a certain period of time during the engine break-in phase;

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

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

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The piston takes central place in the process of converting fuel energy into thermal and mechanical energy. Let's talk about engine pistons, what they are and how they work.

What it is?

Piston - detail cylindrical, performing reciprocating motion inside the engine cylinder. Needed to change gas pressure in mechanical work, or vice versa - reciprocating movement in pressure change. Those. it transmits the force arising from gas pressure to the connecting rod and ensures the passage of all strokes of the working cycle. It looks like an inverted glass and consists of a bottom, a head, and a guide part (skirt).

IN gasoline engines Pistons with a flat bottom are used due to ease of manufacture and less heating during operation. Although on some modern cars make special recesses for the valves. This is necessary so that if the timing belt breaks, the pistons and valves do not meet and cause major renovation. The bottom of the diesel piston is made with a recess, which depends on the degree of mixture formation and the location of valves and injectors. With this shape of the bottom, air is better mixed with the fuel entering the cylinder.

The piston is exposed to high temperatures and pressures. He moves with high speed inside the cylinder. Therefore, initially for car engines they were cast from cast iron. With the development of technology, aluminum began to be used, because it provided the following advantages: increased speed and power, lower loads on parts, better heat transfer.

Since then, engine power has increased many times over. The temperature and pressure in the cylinders of modern automobile engines (especially diesel engines) have become such that aluminum has reached its strength limits. Therefore in last years Such motors are equipped with steel pistons that can confidently withstand increased loads. They are lighter than aluminum due to thinner walls and lower compression height, i.e. distance from the bottom to the axis of the aluminum pin. And the steel pistons are not cast, but prefabricated.

Among other things, reducing the vertical dimensions of the piston while keeping the cylinder block unchanged makes it possible to lengthen the connecting rods. This will reduce the lateral loads in the piston-cylinder pair, which will have a positive effect on fuel consumption and engine life. Or, without changing the connecting rods and crankshaft, you can shorten the cylinder block. This will make the engine lighter.

What are the requirements?

• The piston, moving in the cylinder, allows expansion compressed gases, a product of fuel combustion, and perform mechanical work. Therefore, it must be resistant to high temperature, gas pressure and reliably seal the cylinder bore.
• He must the best way meet the requirements of a friction pair in order to minimize mechanical losses and, as a result, wear.
• Experiencing loads from the combustion chamber and reaction from the connecting rod, it must withstand mechanical stress.
• When performing reciprocating motion at high speed, it should load as little as possible crank mechanism inertial forces.

Main purpose

Fuel, burning in the space above the piston, releases great amount heat in each engine operating cycle. The temperature of the burnt gases reaches 2000 degrees. They will transfer only part of the energy to the moving parts of the engine, the rest will heat the engine in the form of heat. What remains will fly away into the chimney along with the exhaust gases. Therefore, if we do not cool the piston, it will melt after some time. This important point to understand the operating conditions of the piston group.

Let us repeat once again the well-known fact that heat flow is directed from more heated bodies to less heated ones.

The hottest is working fluid, or, in other words, gases in the combustion chamber. It is absolutely clear that the heat will be transferred to the surrounding air - the coldest. The air, washing the radiator and engine housing, cools the coolant, cylinder block and head housing. All that remains is to find the bridge through which the piston transfers its heat to the block and antifreeze. There are four ways to do this.

So, the first path providing the greatest flow, are piston rings. Moreover, the first ring plays the main role, as it is located closer to the bottom. This is the shortest path to coolant through the cylinder wall. The rings are simultaneously pressed against both the piston grooves and the cylinder wall. They provide more than 50% of the heat flow.

The second way is less obvious. The second coolant in the engine is oil. Having access to the hottest parts of the engine, oil mist carries away and transfers a significant part of the heat from the hottest spots to the oil pan. In the case of using oil nozzles that direct the jet to the inner surface of the piston bottom, the share of oil in heat transfer can reach 30 - 40%. It is clear that when loading the oil with the function of a coolant, we must take care to cool it. Otherwise, overheated oil may lose its properties. Also, the higher the temperature of the oil, the less heat it can tolerate.

Third way. Some of the heat is taken away for heating by fresh air-fuel mixture, entered the cylinder. The amount of fresh mixture and the amount of heat it will take away depends on the operating mode and the degree of throttle opening. It should be noted that the heat obtained during combustion is also proportional to the charge. Therefore, this cooling path is pulsed in nature; It is fast and highly efficient due to the fact that heat is taken from the side from which the piston heats up.

Due to its greater significance, close attention must be paid to the heat transfer through the piston rings. It is clear that if we block this path, it is unlikely that the engine will withstand any long-term forced conditions. The temperature will rise, the piston material will “float”, and the engine will collapse.

Let's remember such a characteristic as compression. Let's imagine that the ring does not adhere along its entire length to the cylinder wall. Then the burnt gases, breaking through the gap, will create a barrier that prevents the transfer of heat from the piston through the ring to the cylinder wall. This is the same as if they closed part of the radiator and deprived it of the ability to be cooled by air.

The picture is more terrible if the ring does not have close contact with the groove. In those places where gases are able to flow past the ring through the groove, the piston section is deprived of the opportunity to cool. The result is burnout and chipping of the part adjacent to the leak.

How many rings does a piston need? From a mechanical point of view, the fewer rings, the better. The narrower they are, the lower the losses in the piston group. As their number and height decrease, the piston cooling conditions worsen, increasing the thermal resistance of the bottom - ring - cylinder wall. Therefore, the choice of design is always a compromise.

Rice. Piston diesel engine(A) truck and piston shapes different engines(b): 1 - groove of the lower oil scraper ring;
2 - groove for the piston pin retaining ring;
3 - inner surface bosses;
4 - hole for lubrication of the piston pin;
5 - groove of the upper oil scraper ring;
6 - grooves of compression rings;
7 - piston head;
8 - combustion chamber in the piston;
9 - piston bottom;
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 power stroke, the piston is exposed to high pressure from gases expanding at high temperatures. On the other hand, when the engine is running, especially at high speed, 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 sharply accelerates and moves with 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 blood pressure and heating upon contact with hot gases, followed by cooling when a cold fresh charge is supplied to the cylinder. Currently, pistons in gasoline and diesel automobile engines are made from aluminum alloys. When producing a piston, steel inserts are often placed into the casting during the manufacturing process, which increase its rigidity and prevent thermal expansion. Sometimes a steel insert is placed in a 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 special form. The piston skirt in the transverse plane has the shape of an oval rather than a circle. In the longitudinal plane, the piston skirt looks like a truncated cone. Parts of the piston with a high temperature or with a large volume of metal expand more (for example, the part of the skirt where the bosses are located), and when it reaches operating temperature In an engine, the piston takes the shape of a cylinder.
During their existence, 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 significantly shorter. Most of the skirt is cut off on each side, leaving only two small sections to prevent the piston from twisting in the cylinder. Thanks to the perfect design, the forces acting on the piston are balanced to minimize the tendency to turn. Distance from the piston crown to the upper groove under piston ring reduced in order to reduce the possibility of carbon formation in this part. By reducing the cross-sectional dimensions in the piston design, it was possible to significantly reduce its weight. To reduce friction losses and increase the durability of crankshaft parts on lateral surface The piston is coated with a layer of antifriction material containing molybdenum disulfide or graphite.
The piston bottom can be flat, convex, concave, or have grooves so that when the valves are fully opened they do not touch the piston. In a diesel engine, the combustion chamber can be made in the piston.
The pistons of engines with direct fuel injection have special shape necessary to ensure the fuel combustion process.
Piston rings are made from specially modified cast iron. In engines modern cars Several types of rings are used. The upper compression rings serve to prevent gases from breaking through into the engine crankcase, and the lower oil scraper ring controls the amount of oil on the cylinder walls (the walls are lubricated by oil coming from the crankcase in the form of oil mist). Oil is necessary to prevent wear of the CPG, but its excess is undesirable. Therefore, you should supply more of it than necessary, and remove the excess using an oil scraper ring that acts as a scraper. One of the ways to obtain more compact and lighter pistons is to make the rings narrower and smaller and placing them compactly in the upper part of the piston head. At the same time, increased demands are placed on the material from which they are made and on the accuracy of their manufacture.

IN cylinder-piston group(CPG) one of the main processes occurs due to which the internal combustion engine operates: the release of energy as a result of combustion of the fuel-air mixture, which is subsequently converted into mechanical action– rotation of the crankshaft. The main working component of the CPG is the piston. Thanks to it, the conditions necessary for combustion of the mixture are created. The piston is the first component involved in converting the resulting energy.

The engine piston is cylindrical in shape. It is located in the engine cylinder liner, it is a moving element - during operation it performs reciprocating movements and performs two functions.

1. At forward movement the piston 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 occurs entirely from its strong compression).
2. After the air-fuel mixture is ignited, the pressure in the combustion chamber increases sharply. In an effort to increase volume, it pushes the piston back, and it makes return movement, transmitted through the connecting rod to the crankshaft.

What is a piston in an internal combustion engine?

The design of the part includes three components:

1. Bottom.
2. Sealing part.
3. Skirt.

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

Bottom

The bottom is the main working surface, since it, the walls of the liner and the head of the block form the 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 protrusion of the bottom, this increases the efficiency of filling the combustion chamber with the 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 plates (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 largest and have different shapes. Such notches are called piston chamber combustion and 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 sub-piston space and lubricants into the combustion chamber (these factors reduce Motor efficiency). This ensures heat transfer from the piston to the liner.

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, into which the ring locks rest. These inserts are necessary to eliminate the possibility of the rings turning and their locks getting into the intake and exhaust windows, which can cause their destruction.


The bridge from the edge of the bottom to the first ring is called the fire belt. This belt takes on the greatest temperature impact, so its height is selected based on the operating conditions created inside the combustion chamber and the material used to make the piston.

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

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

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 position relative to the cylinder and providing only reciprocating movement of the part. Thanks to this component, a movable connection is made between the piston and the connecting rod.

For connection, holes are made in the skirt to install the piston pin. To increase the strength at the point of contact of the finger, special massive bulges 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

Internal combustion engines use two types of pistons, differing in structural device– integral and composite.

Solid parts are manufactured by casting followed by machining. The metal casting process creates a blank that is given the overall shape of the part. Next, on metalworking machines, the working surfaces in the resulting workpiece 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, assembly into one part is carried out by connecting the piston to the connecting rod. For this purpose, in addition to the finger holes in the skirt, there are special eyes on the head.

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

Manufacturing materials

Aluminum alloys are used as manufacturing materials for solid-cast pistons. Parts made from 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 from 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 heat causes glow ignition (the air-fuel mixture ignites from contact with heated surfaces, and not from a spark plug spark).

The design of composite pistons allows the above materials to be combined 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 elements of the composite type also have disadvantages, including:

• Can only be used in diesel engines;
• greater weight compared to cast aluminum;
• the need to use piston rings made of heat-resistant materials;
• higher price;

Due to these features, the scope of use of composite pistons is limited; they are used only on large-sized diesel engines.

Video: The principle of operation of the engine piston. Device

Piston

Piston- a cylindrical part that performs a reciprocating movement inside the cylinder and serves to convert changes in pressure of a gas, steam or liquid into mechanical work, or vice versa - reciprocating movement into a change in pressure. IN piston mechanism, unlike a plunger seal, the seal is located on the cylindrical surface of the piston, usually in the form of one or more piston rings.

Structure

The piston is divided into three parts that perform different functions

• bottom
• sealing part
• guide part (skirt)

To transmit force from the piston (or vice versa), a rod or crank can be used, which is connected to the piston using a pin. Other methods of transmitting force are used less frequently. In some cases, the rod can play the role of a guiding device, in which case a skirt is not needed.

The piston may be one-sided or two-way. In the latter case, the piston has two ends.

Bottom

The shape of the bottom depends on the function performed by the piston. For example, in internal combustion engines, the shape depends on the location of the spark plugs, injectors, valves, engine design and other factors. With a concave bottom shape, the most rational combustion chamber is formed, but carbon deposits occur more intensively in it. With a convex bottom shape, the strength of the piston increases, but the shape of the combustion chamber deteriorates. In some two-stroke engines, the piston bottom is made in the form of a protrusion-reflector for the directional movement of combustion products during purging. The distance from the piston crown to the groove of the first compression ring is called the piston fire zone. Depending on the material from which the piston is made, the fire belt has a minimum permissible height, a decrease in which can lead to burnout of the piston along the outer wall, as well as destruction seat upper compression ring.

The sealing functions performed by the piston group are of great importance for normal operation piston engines. ABOUT technical condition The engine is judged by the sealing ability of the piston group. For example, in automobile engines it is not allowed that oil consumption due to its waste due to excessive penetration (suction) into the combustion chamber exceeds 3% of fuel consumption. When oil burns out, it is observed increased smokiness exhaust gases and engines are removed from service, regardless of the satisfaction of power and other indicators.

Sealing part

The bottom and the sealing part form the piston head. Compression and oil scraper rings are located in the sealing part of the piston. In some designs of pistons made of aluminum alloys, a rim made of corrosion-resistant cast iron (niresist) is poured into its head, in which a groove is cut for the upper most loaded compression ring. In particular, the pistons of engines produced by . This significantly increases the wear resistance of the piston. Ring channels for oil scraper rings are made with through holes through which the oil removed from the cylinder mirror enters the piston and flows into the engine sump.

Guide part

The piston skirt (tronk) is its guiding part when moving in the cylinder and has two bosses (bosses) for installing the piston pin. Since the mass of the piston at the bosses turns out to be greater than in other parts of the skirt, thermal deformations during heating in the plane of the bosses will also be the greatest. To reduce the temperature stress of the piston, metal is removed to a depth of 0.5-1.5 mm from the surface of the skirt on both sides where the bosses are located. These recesses, which improve lubrication of the piston in the cylinder and prevent the formation of scuffing from temperature deformations, are called “coolers”. An oil scraper ring may also be located at the bottom of the skirt.

Materials

The following requirements apply to materials used for the manufacture of pistons for automotive engines:

• high mechanical strength;
• low density;
• good thermal conductivity;
• low linear expansion coefficient;
• high corrosion resistance;

For the manufacture of pistons, gray cast iron and aluminum alloys are used.

Cast iron

Advantages

• Cast iron pistons are durable and wear-resistant.
• Due to their low coefficient of linear expansion, they can operate with relatively small clearances, providing a good cylinder seal.

Flaws

• Cast iron has a fairly high specific gravity. In this regard, the scope of application of cast iron pistons is limited to relatively low-speed engines, in which the inertial forces of reciprocating masses do not exceed one sixth of the gas pressure force on the piston bottom.
• Cast iron has low thermal conductivity, so the heating of the bottom of cast iron pistons reaches 350-400 °C. Such heating is undesirable especially in carburetor engines, since it causes glow ignition.

Aluminum

The vast majority of modern car engines have aluminum pistons.

Advantages

Advantages of aluminum pistons:

• low weight (at least 30% less compared to cast iron);
• high thermal conductivity (3-4 times higher than the thermal conductivity of cast iron), ensuring heating of the piston bottom to no more than 250 °C, which contributes to better filling of the cylinders and allows increasing the compression ratio in gasoline engines;
• good anti-friction properties.

Flaws

The disadvantages of aluminum pistons are:

• large coefficient of linear expansion (about 2 times more than cast iron),
• a significant decrease in mechanical strength when heated (increasing the temperature to 300 °C leads to a decrease in the mechanical strength of aluminum by 50-55% versus 10% for cast iron).

Gaps between the cylinder walls and aluminum pistons that are unacceptable for normal engine operation are eliminated constructive measures, the main ones being:

• giving the piston skirt an oval or oval-conical shape;
• insulation of the trunk (guide) part of the piston from its most heated part (head);
• oblique cut of the skirt along the entire length, providing springy properties of the walls;
• T- and U-shaped slots in the piston skirt are not full length in combination with its ovality;
• compensation inserts limiting thermal expansion skirts in the plane of swing of the connecting rod.

Application

Two main problems solved when designing motors:

1. how to avoid increased wear piston
2. How to avoid piston burnout.

Both of these problems arise due to the desire of designers to lighten the piston as much as possible, since this improves the performance of motors and compressors.

see also

• Reciprocating external combustion engine

Links

Notes

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Synonyms:

See what “Piston” is in other dictionaries:

Husband. (from the dust, prue?) pistons, astrakh. porushni (destroy?), postsols, leather jackets, kaligi, a type of sandals; the pistons are not sewn at all, but are bent (bend the pistons, make them) from one piece of raw leather or skin (with wool), on a hold-up, spectacle, belt trim;... ... Dictionary Dahl- 1. PISTON1, piston, man. (those.). A movable cylindrical body that is tightly adjacent to the walls of the cylinder and serves to pump and expel liquids, gases, and steam from the cylinder. Pistons in pumps, compressors, engines. 2. PISTON2, pistons,... ... Ushakov's Explanatory Dictionary

PISTON, schna, pl. and, to her and to her, husband. A moving part, elongated or disk-shaped, moving tightly inside the cylinder and pumping or pumping out liquid, gas, steam. | decrease porshenyok, nka, husband. | adj. piston, oh, oh. P. pump. Intelligent... ... Ozhegov's Explanatory Dictionary

- (Piston) one of the parts steam engine, separating the working cavities of the cylinder, taking on steam pressure and moving in the cylinder. Samoilov K.I. Marine dictionary. M.L.: State Naval Publishing House of the NKVMF of the USSR, 1941 ... Marine Dictionary Technical Translator's Guide