The pistons must withstand very high temperatures and high pressures for all four strokes. Pistons are under high stress, especially in high performance and racing engines. turbocharged engine, mechanical superchargers or nitrous oxide injection, are more demanding on the strength of the piston. Add to that the possibility of an explosion and you're asking too much from these slugs. At high boost engine, where the task is to achieve maximum power, the use of cast pistons is not enough. All parts of the piston are shown in the figure below.
An example is a diesel engine piston.
Pistons production
Typically, OEM pistons are made from a precision cast eutectic alloy and are formulated with high content silicon dioxide. These pistons are much stronger and more stable than conventional cast pistons and can be used up to about 400 horsepower.
Forged pistons have more sophisticated technology production, but also have the best performance. In the first stage, a piece of hot aluminum alloy is forged, and then mechanical restoration to give shape. The piston workpiece enters the CNC machine, after which a high-precision part is obtained. Forged pistons are more expensive mainly due to a large number waste and CNC machining.
These mock-ups show the thickness of the piston for the turbocharger (left) and for the injection of nitrous oxide (right)
The construction of an engine designed for a high degree Compression or pressurization involves the use of forged pistons that are better able to withstand high temperatures and high pressures.
Vertical gas holes
These small, vertical holes in the piston crown all around allow combustion pressure to fuel mixture penetrate the first compression ring. This increases the tightness of the combustion chamber but also increases the wear of the ring (pressure strongly presses the ring against the cylinder walls). During operation, in addition to the stroke, the first compression ring is subjected to the usual pressure, as in a conventional piston, and, accordingly, a lower friction force; in fact, in these modes there is no need to strongly press the ring against the cylinder.
Such piston schemes are often used in drag racing.
The vertical holes, due to the pressure in the stroke mode, allow the upper compression ring to be pressed against the cylinder to provide a better seal.
Lateral gas holes in the grooves of the rings
These are very shallow indentations made in the upper part of the upper piston ring groove around the entire circumference of the piston, which allows gases to press the ring against the lower plane of the piston ring groove and thereby increase tightness.
This type is often used in circuit racing.
Small grooves from the top ring to the edge of the piston bottom - heat zone.
Large grooves, some make a barely noticeable top land.
Some pistons have a series of narrow grooves around the piston between the first compression ring and the bottom edge of the piston. These recesses are made to reduce the contact area with the cylinder when the piston is at top or bottom dead center. Also, these grooves serve to extinguish the flame on the way to the ring.
compensation groove
The compensation groove is made on the jumper between the compression rings. This recess creates additional volume for the gases to break through the first ring, thereby reducing the pressure between the rings and this provides less fluctuation of the first ring, it is better kept at the bottom of its groove while maintaining the tightness of the combustion chamber.
See also the following materials design features
IN crank mechanism The piston performs several functions, including the perception of gas pressure and the transfer of forces to the connecting rod, the sealing of the combustion chamber and the removal of heat from it. The piston is the most characteristic part of an internal combustion engine. it is with its help that the thermodynamic process of the engine is realized.
The conditions under which the piston operates are extreme and are characterized by high pressure, temperature and inertial loads. Therefore, the pistons modern engines are made of light, durable and heat-resistant material - aluminum alloy, less often from steel. Pistons are made in two ways - injection molding or stamping, the so-called. forged pistons.
The piston is a one-piece structural element, which is conditionally divided into a head (in some sources it is called a bottom) and a skirt. The shape and design of the piston is largely determined by the type of engine, the shape of the combustion chamber and the combustion process that takes place in it. The piston of a gasoline engine has a flat or close to flat head surface. Grooves can be made in it to fully open the valves. The pistons of engines with direct fuel injection have a more complex shape. The combustion chamber of a certain shape is made in the piston head of a diesel engine, which provides good swirl and improves mixture formation.
Below the piston head, grooves are made for installing piston rings. Piston skirt has a conical or curvilinear ( barrel-shaped) form. This shape of the skirt compensates for the thermal expansion of the piston when heated. Upon reaching operating temperature engine piston takes a cylindrical shape. To reduce friction losses on side surface piston, a layer of anti-friction material is applied ( molybdenum disulfide, graphite). In the piston skirt there are holes with tides ( bosses) for attaching the piston pin.
Piston cooling carried out from the inner surface different ways:
- oil mist in the cylinder;
- splashing oil through a hole in the connecting rod;
- spraying oil with a special nozzle;
- oil injection into a special annular channel in the zone of the rings;
- oil circulation through a 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 engine. internal combustion. 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 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 difficult 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 strength and allows for the maximum compression ratio. Compression rings can have a trapezoidal, barrel-shaped, conical shape, some are made with a cut (cut).
Oil scraper ring removes excess oil from the surface of the cylinder and prevents oil from entering the combustion chamber. The ring has many drainage holes. Some ring designs have a spring expander.
The connection of the piston to the connecting rod is carried out using a piston pin, which has a tubular shape and is 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 piston head of the connecting rod during operation. To prevent displacement of the finger, it is fixed with retaining rings. Much less often, 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.
Piston, piston rings and the piston pin are the well-established name of the piston group.
The piston is one of the elements of the crank mechanism, on which the principle of operation of many internal combustion engines is based. This article discusses the design and features of these parts.
Definition
The piston is a part that performs reciprocating movements in the cylinder and ensures the conversion of changes in gas pressure into mechanical work.
Purpose
With the participation of these parts, the thermodynamic process of the motor is realized. Since the piston is one of the elements of the crank mechanism, it perceives the pressure produced by the gases and transfers the force to the connecting rod. In addition, it provides sealing of the combustion chamber and removal of heat from it.
Design
The piston is a three-piece part, that is, its design includes three components that perform different functions, and two parts: the head, which combines the bottom and the sealing part, and the guide part, represented by the skirt.
Bottom
May have different shape depending on many factors. For example, the configuration of the bottom of the pistons of an internal combustion engine is determined by the location of other structural elements, such as nozzles, candles, valves, the shape of the combustion chamber, the features of the processes occurring in it, general design engine, etc. In any case, it determines the features of functioning.
There are two main types of piston bottom configuration: convex and concave. The first provides greater strength, but worsens the configuration of the combustion chamber. With a concave bottom, the combustion chamber, on the contrary, has an optimal shape, but carbon deposits are deposited more intensively. Less often (at two-stroke engines) there are pistons with a bottom represented by a reflector protrusion. This is necessary when blowing for the directed movement of combustion products. Parts of gasoline engines usually have a flat or almost flat bottom. Sometimes they have grooves to fully open the valves. For motors with direct injection pistons are characterized by a more complex configuration. In diesel engines, they are distinguished by the presence of a combustion chamber in the bottom, which provides good swirl and improves mixture formation.
Most pistons are single-sided, although there are also double-sided versions that have two bottoms.
The distance between the groove of the first compression ring and the bottom is called the firing zone of the piston. The value of its height is very important, which is different for parts from different materials. In any case, the height of the ring of fire goes beyond the minimum allowable value can lead to piston burnout and deformation seat top compression ring.
Sealing part
Here are the oil scraper and compression rings. For parts of the first type, the channels have through holes for the oil removed from the surface of the cylinder to enter the piston, from where it enters the oil pan. Some of them have a stainless cast iron rim with a groove for the top compression ring.
Consisting of cast iron, they serve to create a snug fit between the piston and the cylinder. Therefore, they are the source of the greatest friction in the motor, the losses from which are 25% of the total mechanical losses in the motor. The number and location of rings are determined by the type and purpose of the engine. Most often, 2 compression and 1 oil scraper rings are used.
Compression rings perform the task of preventing gases from entering the crankcase from the combustion chamber. The greatest loads fall on the first of them, therefore, in some engines, its groove is reinforced with a steel insert. Compression rings can be trapezoidal, conical, barrel-shaped. Some of them have a cutout.
Used to remove excess oil from the cylinder and prevents it from entering the combustion chamber. It has holes for this. Some options have a spring expander.
Guide part (skirt)
It has a barrel-shaped (curvilinear) or conical shape for compensation. It has two tides for the piston pin. In these areas, the skirt has the greatest mass. In addition, there are observed the greatest temperature deformations during heating. Various measures are used to reduce them. An oil scraper ring may be located at the bottom of the skirt.
To transfer force from the piston or to it, a crank or a rod is most often used. piston pin serves to connect this part with them. It is made of steel, has a tubular shape and can be installed in several ways. Most often, a floating finger is used, which can be rotated during operation. To prevent displacement, it is fixed with retaining rings. Rigid fastening is used much less often. The rod in some cases acts as a guiding device, replacing the piston skirt.
materials
The engine piston may be various materials. In any case, they must have such qualities as high strength, good thermal conductivity, corrosion resistance and low coefficient of linear expansion and density. For the production of pistons, aluminum alloys and cast iron are used.
Cast iron
Is different great strength, wear resistance and low The latter property allows such pistons to work with small gaps, due to which a good cylinder seal is achieved. However, due to the significant specific gravity, cast-iron parts are used only in those engines where the reciprocating masses have inertia forces that are no more than a sixth of the pressure forces on the bottom of the gas piston. In addition, due to low thermal conductivity, the heating of the bottom of cast iron parts during engine operation reaches 350-450 ° C, which is especially undesirable for carburetor options, as it leads to glow ignition. 
Aluminum
This material is used for pistons most often. This is due to the low specific gravity (aluminum parts are 30% lighter than cast iron parts), high thermal conductivity (3-4 times higher than that of cast iron), which ensures that the bottom is heated to no more than 250 ° C, which allows increase in degrees and compression and provides better filling of the cylinders, and high anti-friction properties. At the same time, aluminum has a 2 times greater than that of cast iron,linear expansion coefficient, which forces us to make big gaps with cylinder walls i.e. piston sizes less aluminum than cast iron A, for identical cylinders.
Moreover, such details and have a lower strength, especially when heated (at 300 ° C, it decreases by 50-55%, while cast iron nyh - on 10%). 
To reduce the degree of friction, the walls of the pistons are coated with graphite and molybdenum disulfide.
Heat
As mentioned, during operation they can warm up to 250-450 °C. Therefore, it is necessary to take measures aimed at both reducing heating and compensating for the thermal expansion of parts caused by it.
To cool the pistons, oil is used, which is fed into them in various ways: they create an oil mist in the cylinder, spray it through a hole in the connecting rod or with a nozzle, inject it into the annular channel, and circulate through the tubular coil in the piston bottom.
To compensate for temperature deformationsin tidal areas skirts turn on both sides metal 0.5-1.5 mm deepin the form of U- or T-shaped slots. This measure improves its lubrication and prevents the appearance of e from temperature deformations of scoring, so the data e recesses are called refrigerators. Their used in combination with a conical or barrel-shaped skirt.It makes up for it linear expansion due to the fact that when heatedthe skirt takes a cylindrical shape. In addition, compensating inserts are usedso that the piston diameter experiences a limited thermal expansion in the plane of swing of the connecting rod. It is also possible to isolate the guide part from the head that experiences the most heat. Finally, the walls of the skirt are given springy properties bymaking an oblique incision along its entire length.
Production technology
According to the manufacturing method, pistons are divided into cast and forged (stamped).Details of the first type used on most cars, and replacing pistons with forged ones is used in tuning. Forged options are characterized by increased strength and durability, as well as lower weight. Therefore, the installation of pistons of this type increases the reliability and performance of the engine.This is especially important for motors operating under increased loads, while for daily operation enough cast parts. 
Application
The piston is a multifunctional part. Therefore, it is used not only in engines. For example, there is a brake caliper piston,because it functions in the same way. Also the crank mechanism is used on some models of compressors, pumps and other equipment.
I will not stretch the introduction, I will briefly tell you what this one will be about. great post. And so we are talking about the types of pistons, four-stroke gasoline, diesel and two-stroke. The main task of all considered piston types, it is to control thermal expansion and withstand a certain load, below we will figure out how this is solved.
Pistons for four-stroke petrolengines
In modern gasoline engines use pistons with a symmetrical or asymmetric skirt
with different thicknesses of the bottom and piston skirt.
Controlled Expansion Pistons
Pistons with an annular insert that controls thermal expansion.
The inserts are made of gray cast iron. The main purpose of this ring is to reduce the thermal expansion of the aluminum alloy piston, since cast iron has a relatively small expansion and low thermal conductivity, the insert thereby holds the metal in shape. The production of such pistons is more expensive, and, accordingly, the price of the finished product is higher. The main disadvantage is the impossibility of manufacturing a forged piston, which is so necessary for turbocharged engines, a large mass of the piston. This type of piston is more of a distant past.
Auto thermal pistons
Auto thermal pistons, have a split between the annular belt and the skirt in the groove oil scraper ring, the skirt is held in the area of the bosses. This reduces heat transfer from the piston ring to the piston skirt, thereby achieving a more stable skirt shape. Steel insert in the boss area controls thermal expansion and increases strength. Such pistons are not able to withstand huge loads due to “cutting”, they are distinguished by low noise in operation and are more modern types.
Pistons
They operate in the same way as a car.
thermal pistons, but do not have a cut in the oil scraper groove. They also have steel plates in the area of \u200b\u200bthe bosses. More durable due to the integrity of the annular belt and skirt, they better withstand lateral loads compared to the first option. They are used both in gasoline and partially in diesel engines.
They are somewhat similar to auto thermal ones, but instead of a cut in the skirt they have a steel insert along the entire diameter. Thus limiting the temperature transition from the annular belt to the skirt and controlling the shape around the entire circumference.
This type of piston has a large cooler and a narrow, often oval skirt shape. The piston is designed so that during thermal expansion it changes its shape from oval to regular round.
In addition to this type of piston, there is also an option with a sloping skirt to the top of the piston. has a wider part of the skirt from below, tapering to the annular belt.
For pistons for engines with very high power output (greater than 100 kW/l) a cooling channel can be provided.
The biggest potential for reducing piston weight in four-stroke gasoline engines is EVOTEC® pistons, which primarily feature trapezoidal boss supports that allow the pin to be positioned particularly deep, close to the bottom, reducing the overall length and weight of the piston. In post The mass of the piston, we have already talked about the dignity of such an arrangement of the finger. This arrangement of the walls of the skirt allows you to very well strengthen the upper part of the bosses with a small thickness of the partitions and lighten the bottom by making the piston of an asymmetric shape. The skirt is quite narrow and has strong partitions at the edges, passing to the bosses, this is also a big plus. This arrangement of the piston resists lateral loads very well, there is little chance of skirt deformation, while the thickness of the skirt is much less than in a conventional piston, which also reduces the overall weight. Against all the background of the advantages noted above, the piston has significantly lost weight, this allows you to make the bosses thinner, since the inertial load on the bottom walls of the bosses has become less.
Forged aluminum pistons
In engines with very high specific loads, such as turbocharging or nitrous oxide injection, forged pistons. The advantage is undoubtedly the strength of the forged aluminum alloy. Withstand more high temperature and better resist detonation. Among the shortcomings, a higher price is noted, the impossibility of using some technologies, for example, some of those described above due to technological process manufacturing.
Forged piston for Formula 1
In the next post, we will talk about pistons for two-stroke and diesel engines, where loads and temperatures are even greater.
The most famous and widely used around the world mechanical devices- These are internal combustion engines (hereinafter referred to as internal combustion engines). Their range is extensive, and they differ in a number of features, for example, the number of cylinders, the number of which can vary from 1 to 24, the fuel used.
The operation of a piston internal combustion engine
Single cylinder internal combustion engine can be considered the most primitive, unbalanced and uneven course, despite the fact that it is the starting point in the creation multi-cylinder engines new generation. Today they are used in aircraft modeling, in the production of agricultural, household and garden tools. Widely used in the automotive industry four-cylinder engines and more solid devices.
How does it work and what does it consist of?
Reciprocating internal combustion engine has a complex structure and consists of:
- Housing, including a cylinder block, a cylinder head;
- gas distribution mechanism;
- Crank mechanism (hereinafter KShM);
- A number of auxiliary systems.
KShM is a link between the energy released during combustion fuel-air mixture(hereinafter referred to as TVS) in the cylinder and the crankshaft, which ensures the movement of the car. The gas distribution system is responsible for gas exchange during the operation of the unit: the access of atmospheric oxygen and fuel assemblies to the engine, and the timely removal of gases formed during combustion.
The device of the simplest piston engine
Auxiliary systems are presented:
- Inlet, providing oxygen to the engine;
- Fuel, represented by a fuel injection system;
- Ignition, which provides a spark and ignition of fuel assemblies for engines running on gasoline (diesel engines are characterized by self-ignition of the mixture from high temperature);
- A lubrication system that reduces friction and wear of contacting metal parts using machine oil;
- Cooling system, which prevents overheating of the working parts of the engine, providing circulation special liquids antifreeze type;
- An exhaust system that ensures the removal of gases into the corresponding mechanism, consisting of exhaust valves;
- A control system that provides monitoring of the operation of the internal combustion engine at the electronic level.
The main working element in the described node is considered internal combustion engine piston, which itself is a prefabricated part.
ICE piston device
Step-by-step operation diagram
The operation of an internal combustion engine is based on the energy of expanding gases. They are the result of combustion of fuel assemblies inside the mechanism. This physical process forces the piston to move in the cylinder. The fuel in this case can be:
- Liquids (gasoline, diesel fuel);
- gases;
- Carbon monoxide as a result of burning solid fuels.
Engine operation is a continuous closed cycle consisting of a certain number of cycles. The most common internal combustion engines are of two types, differing in the number of cycles:
- Two-stroke, producing compression and stroke;
- Four-stroke - are characterized by four stages of the same duration: intake, compression, working stroke, and the final - release, this indicates a four-fold change in the position of the main working element.
The beginning of the stroke is determined by the location of the piston directly in the cylinder:
- Top dead center (hereinafter referred to as TDC);
- Bottom dead center (hereinafter BDC).
By studying the algorithm of the four-stroke sample, you can thoroughly understand working principle of a car engine.
The principle of operation of a car engine
The intake occurs by passing from the top dead center through the entire cavity of the cylinder of the working piston with simultaneous retraction of the fuel assembly. Based on structural features, the mixing of incoming gases can occur:
- In the collector intake system, this is true if the engine is gasoline with distributed or central injection;
- In the combustion chamber, when it comes to diesel engine, as well as an engine running on gasoline, but with direct injection.
First measure runs with open intake valves of the gas distribution mechanism. The number of intake and exhaust valves, their open time, their size, and their state of wear are factors that affect engine power. The piston at the initial stage of compression is placed at BDC. Subsequently, it begins to move upward and compress the accumulated fuel assembly to the dimensions determined by the combustion chamber. The combustion chamber is the free space in the cylinder remaining between its top and the piston in top dead point.
Second measure involves closing all the valves of the engine. The density of their fit directly affects the quality of fuel assembly compression and its subsequent ignition. Also, the compression quality of the TVS has big influence the level of wear of engine components. It is expressed in terms of the size of the space between the piston and the cylinder, in the tightness of the valves. The compression level of an engine is the main factor influencing its power. It is measured with a special device compression gauge.
working stroke starts when it is connected to the process ignition system that generates a spark. The piston is in the maximum upper position. The mixture explodes, gases are released, creating high blood pressure and the piston is set in motion. The crank mechanism, in turn, activates the rotation of the crankshaft, which ensures the movement of the car. All system valves are in the closed position at this time.
graduation stroke is the final one in the considered cycle. All exhaust valves are in the open position, allowing the engine to "exhale" the products of combustion. The piston returns to its starting point and is ready to start a new cycle. This movement contributes to the release of exhaust system and then in environment, waste gases.
Scheme of operation of an internal combustion engine, as mentioned above, is based on cyclicity. Considering in detail, How does it work piston engine , it can be summarized that the efficiency of such a mechanism is not more than 60%. This percentage is due to the fact that at a given moment, the working cycle is performed in only one cylinder.
Not all the energy received at this time is directed to the movement of the car. Part of it is spent on keeping the flywheel in motion, which, by inertia, ensures the operation of the car during the other three cycles.
A certain amount of thermal energy is involuntarily spent on heating the housing and exhaust gases. That is why the engine power of a car is determined by the number of cylinders, and as a result, the so-called engine size, calculated according to a certain formula as the total volume of all working cylinders.
