Piston rings are open rings that are seated with a small gap in the piston grooves. Let's talk about the piston rings of a car engine, what are their main purpose.
What are there?
Compression rings- prevent a rush of gases from the combustion chamber to the crankcase. The outer diameter of the ring in the free state is larger than the inner diameter of the cylinder, so part of the ring is cut out. The cutout in the piston ring is called the lock.Oil scraper rings- prevent the penetration of oil from the crankcase into the combustion chamber, removing excess oil from the cylinder wall. They are installed below the compression level. They, unlike compression rings, have through slots.
Some manufacturers initially design engines with increased oil consumption due to the special design of the piston rings. This is done, firstly, in order to reduce friction losses; secondly, for the sake of less wear of the cylinder-piston group; thirdly, the oil is refreshed within a large service interval.
What are they made of?
One of the materials used for piston rings is cast iron. Its structure allows it to retain oil, reducing wear. A derivative of ductile iron, ductile iron, is also widely used. It has most of the qualities of cast iron and can be resiliently deformed, making it easier to install rings.Piston rings made from stainless steel are an improvement on chrome-plated cast iron rings. In fact, stainless steel is a material that contains a large amount of chromium. And such rings have properties similar to those of chrome-plated rings. Stainless steel also has the ability to withstand high temperatures, superior to chrome cast iron.
To increase the service life of the rings and ensure their fast running-in, molybdenum rings have been created. Its base is molybdenum coated cast iron. Molybdenum has many of the anti-wear properties of chromium, and in some cases may have greater wear resistance. Over time, molybdenum rings have become a staple in engines because they are durable, relatively easy to run in, and more reliable.
Top compression rings
There are many configurations of the top compression ring and the differences are hard to tell. For example, the ring may have an intentional slight twist. In other words, the top and bottom surfaces of the ring do not lie flat in the ring groove, but are slightly inclined, with only the top or bottom edge of the face making contact with the cylinder bore.
The rings are designed to accelerate the running-in of piston ring surfaces and cylinder walls and help seal the ring at the top and bottom of the ring groove. The amount of ring twist is very small and is usually done by chamfering the inside edge of the ring.
Second compression and oil scraper rings
The main task of the second compression ring- providing additional sealing after the upper oil scraper ring. Because of this, the second ring usually "follows" only the gases that pass the top ring, and the pressure and temperature are different from the values for the top compression ring. Accordingly, the materials and design of the second ring are less critical.The second ring has an important additional function: it helps the oil scraper ring, acting as a "scraper", preventing excess oil from entering the combustion chamber and causing detonation.
Some second compression rings are purposely bevelled to assist the oil scraper ring, with the bevel being smallest at the top edge of the ring. In doing so, it tends to move on top of the oil as it moves up in the cylinder and will remove oil as it moves down. If oil removal is a problem, then this type of ring will forcibly remove oil, although a second flat face ring along with a "normal" force wiper ring is all that is needed.
The second gapless compression ring is a new design. The term “no gap” used here is somewhat incorrect, because it is impossible to make a ring completely without a gap - it will be impossible to install it on the piston, and the ring will be unregulated even with the smallest deviations in the shape of the cylinder bore from the circle. Despite this, the ring can be made without a visible gap for gases passing by the ring.
When using these rings, the engine breaks in faster during the break-in process, and it produces slightly more power when tested on the bench.
The need for gapless rings depends on how other rings perform. While the top compression ring provides a good seal, a backlash-free second compression ring is less important. In reality, this is not the case and a second gapless compression ring can be a means of getting more power.
Oil scraper rings are important for the operation of engines, especially when using low-octane gasoline. Engine oil contaminates the combustion chambers and piston heads, which will cause power loss.
Considering the principle of operation of an internal combustion engine, one can understand that the main processes take place in the cylinders. Moreover, this requires the creation of certain conditions, one of which is to ensure the tightness of the combustion chamber - the space above the piston. In this case, the piston itself is a movable element that moves inside the cylinder, that is, there is a sliding connection between them.
It is worth noting that the piston diameter must be less than the internal dimensions of the cylinder. And all because the processes taking place in the cylinders are accompanied by the release of a significant amount of heat. Metals expand due to exposure to high temperatures. If the piston diameter were equal to the cylinder, then when heated, jamming would occur. It turns out that there is a gap between these elements, that is, there will be no tightness. To solve this problem, another element has been added to the design of the CPG - special rings mounted on the pistons.
Purpose, types, features
Piston device
These elements of the CPG have a number of important functions:
- Provide tightness of the combustion chamber.
- The amount of lubricant used to lubricate the cylinder walls is adjusted, and it is also prevented from entering the over-piston space.
- Perform heat removal from the piston to the cylinder.
The functioning of the piston rings takes place in rather difficult conditions - high-temperature exposure, significant mechanical loads arising not only from the constant exposure to gases, but also from increased friction due to a lack of lubricant in the area of the piston bottom. 
One ring would not have coped with the tasks set, so several elements are installed on the piston, each of which performs certain functions. All piston rings are divided into two types:
- compression (designed to ensure tightness);
- oil scraper (perform the adjustment of the amount of lubricant in the CPG).
The total number may be different and depends on the design features of the power plant. The most widely used three-ring layout (2 - compression, 1 - oil scraper). But there are engines in which their number can reach 7 pieces. And for example, on two-stroke engines, only two compression ones are installed, and the oil scraper is not used.
All rings used are of the open type. That is, they are not solid (it would simply be impossible to install it in the piston groove), and it has a cutout, by the way, which also plays an important role.
In the expanded state, the rings are made in the form of an oval, while the distance between the ends is significant. This makes it possible to put it on the piston without any problems and install it in a special groove in it. When planted in a cylinder, it takes the correct round shape, which ensures a fit around the entire circumference, while the cutout (lock) is reduced, and this gap is only 0.15-0.5 mm. This gap is thermal, and its task is to compensate for the dimensions as a result of thermal expansion.
Since there is a gap, gases can pass through it into the under-piston space. To eliminate this factor, two compression rings are installed. They create the so-called labyrinth-type seal, for which the lock of the first ring is turned 180 degrees. regarding the second. But even such a solution does not provide complete sealing of the over-piston space and part of the gases penetrates into the crankcase.
Video: ICE Theory: Piston Rings (Part 2)
Note that the installation of an additional third compression ring, although it allows to reduce leakage, but at the same time the friction force in the CPG increases greatly, so this solution is impractical.
Compression rings
The main load falls on the first compression ring, located closest to the piston crown. Its main task is to ensure the tightness of the combustion chamber. It is he who accounts for the most high-temperature exposure and gas pressure, and all this in conditions of a shortage of lubricant. To minimize friction between the wall and the ring, the latter has a rounded working surface. Also, the wear of the upper ring during operation in difficult conditions can be reduced by a molybdenum or chromium insert sprayed onto the surface, but it itself is made of flexible high-strength cast iron, but sometimes steel is also used.
Video: 2.0 ICE Theory: Installation error of the oil scraper piston ring
It is noteworthy that the working gases take part in creating the tightness of the combustion chamber. For this, the height of the ring is slightly less than the height of the groove. Through the formed gap, gases penetrate into the groove and begin to put pressure on the inner surface of the ring, additionally pressing it against the wall.
Some manufacturers are engaged in the production of so-called "one-piece" compression rings. In fact, it consists of two flat rings, which, after landing on the piston, turn 180 ° relative to each other with locks. In fact, this design makes it possible to complicate the labyrinth seal, thereby reducing the amount of gases passing through.
The second compression ring performs two tasks. Firstly, it is an element of the labyrinth seal and prevents the penetration of gases that have broken through the upper ring into the under-piston cavity. And secondly, it takes part in adjusting the amount of lubricant on the cylinder walls. This element has a specific shape of the working surface (conical or L-shaped). Such a surface plays the role of a scraper that removes excess lubricant from the walls and dumps it to the oil scraper ring. Therefore, it is also called scraper.
Since it perceives significantly lower loads than the first, high-strength coatings are not used in its design, it is made entirely of ductile iron.
Oil scraper rings
The task of oil scraper rings is to adjust the thickness of the oil film on the cylinder walls, namely adjustment, and not complete removal of the lubricant. If there is not enough oil, then the friction force will be increased, which will lead to rapid wear of the rings, as well as the possible appearance of scoring on the cylinder walls. A large amount of it during combustion in the combustion chamber will settle on all surfaces inside it.
Structurally, this element is the most complex and it is the only one that has drainage holes to drain the removed oil. Two types can be used on cars:
- U-shaped.
- Composite.
The working elements of the U-ring are two edges that scrape the lubricant from the walls. Moreover, the oil removed by the upper edge passes through the drainage holes and flows down through the channels made in the piston. Lubrication, scraped off by the lower edge, goes down the walls of the piston skirt and cylinder.
Video: We insert the pistons into the cylinder block
To ensure the necessary pressing to the surface, special tangential expanders are used:
- spiral;
- lamellar;
These expanders are installed in the piston groove under the ring. For a spiral expander, a special groove is made on the inner surface of the ring.
Composite oil scraper rings are distinguished by a collapsible design, which includes several elements, namely two flat annular plates (made of steel and chrome-plated), between which two expanders are placed - tangential and axial. In some cases, only one expander is used, allowing expansion in both directions.
Main malfunctions
Since these elements of the CPG are in constant contact with the cylinder wall, their main malfunction is the wear of the working surfaces. The resource of these elements largely depends on the material of manufacture and operating conditions, and it can vary from 150 thousand to 1 million km.
But non-compliance with the rules of operation can significantly reduce their service life. Resources can be affected by:
- Untimely replacement of lubricant in the power plant.
- Use of low quality fuel.
- Frequent operation of the car in traffic jams or short trips.
- Creating excessively large loads on the power plant.
- Motor overheating.
The main signs of severe wear of the piston rings are a strong drop in compression, as a result of which the power and dynamic performance of the car decreases and fuel consumption increases, as well as a significant increase in lubricant consumption.
A good car can cover hundreds of thousands of kilometers without any complaints, if you follow it, regularly change consumables and repair parts that are close to failure in time. The engine does not have a specific service life, but after 150,000 kilometers, most engines have various serious problems. One of these problems is the occurrence of piston rings. If it is noticed and eliminated in time, you can save the engine from the need for major repairs.
Table of contents:Why piston rings are needed
Pistons are key elements in internal combustion engines. They must be kept lubricated so that they can operate normally without overheating or chafing. In this case, the oil should not enter the combustion chamber, otherwise carbon deposits will occur, which will lead to a deterioration in engine performance.
Piston rings are open. They are put on the piston and come in two types - compression and oil scraper. On modern pistons, most often the distribution of rings is as follows: 2 compression and 1 oil scraper.
The purpose of the compression rings is to pressurize the combustion chamber. There is a small gap between the pistons and cylinders, due to which it is not possible to create the necessary pressure in the combustion chamber. If there were no rings, gases from the combustion chamber would go into the crankcase through the gap. The rings are made open so that they can be compressed and unclenched within the limits predetermined by the designers, maintaining the required pressure.
At the same time, the very presence of compression rings does not completely eliminate the problem with the penetration of exhaust gases into the crankcase. To avoid this, designers use a number of other tricks, at a minimum, breaks in open compression rings are positioned so that they are 180 degrees relative to each other.
Why piston rings stick
The main reason for the occurrence of piston rings is the formation of a large amount of carbon deposits on the engine elements. There are 4 main reasons for this:
To avoid the occurrence of piston rings, it is necessary to exclude the formation of carbon deposits on the engine elements as much as possible.
Signs of the occurrence of piston rings
Symptoms of the occurrence of compression and oil scraper rings are as follows:
- From the exhaust pipe of the car comes out;
- Decreased;
- The car began to pick up speed more slowly due to a deterioration in the quality of the engine;
- The motor starts worse in the cold season.
In especially neglected situations, it can smell burning in the cabin when the engine is running.
How to get rid of the occurrence of piston rings
We recommend reading:To eliminate the occurrence of piston rings, it is necessary to perform decoking of the engine. This procedure can help if the problem is not running, and the resulting carbon deposits have not caused damage to the piston or combustion chamber. In fact, the decarbonization procedure allows you to remove carbon deposits from engine elements, including piston rings.
Engine decoking is performed as follows:
Decoking will remove the resulting carbon deposits and eliminate the occurrence of piston rings. For decoking, you can use both specialized tools and "folk" options, such as kerosene and a mixture of kerosene and acetone.
The piston and cylinder of an internal combustion engine have close dimensions, however, no matter how high precision they are made, there is still a gap between them, through which gases resulting from the combustion of the working mixture could freely pass into the crankcase, and from the crankcase into the chamber combustion engine oil would get in. Piston rings are used to prevent these extremely undesirable phenomena.
They are open rings, set with a small gap in the piston grooves intended for this purpose. According to their purpose, they are divided into two groups:
- compression, the task of which is to prevent the breakthrough of gases from the combustion chamber into the crankcase;
- oil scraper, designed to remove excess engine oil from the cylinder walls.
Compression piston rings have an outer diameter slightly larger than the cylinder diameter. In order for the part to fit inside, a cutout called a lock is made in it. The surface of the compression ring is made even, without any grooves. Oil scraper rings, unlike them, have through slots designed to drain oil.
The number of rings installed on one piston may vary. At the dawn of the automotive era, when engines were slow to cope with compression losses as best as possible, their number reached seven. In modern engines, as a rule, three are used for each piston: two compression and one oil scraper. In sports cars with high-speed forced motors, engineers quite often limit themselves to two at all.
What materials are used to make piston rings
Various materials are used in the manufacture of rings, such as cast iron, stainless steel, chromium and molybdenum. Cast iron combines low cost and fairly high performance, which caused its wide distribution.
Stainless steel contains a large amount of chromium to increase heat resistance and wear resistance. Thanks to this, stainless steel rings, like chrome ones, resist high temperatures better than cast iron ones. Since the first compression ring operates under conditions of lack of lubrication, this factor must also be taken into account when choosing a material for it. As well as possible, the same chrome is suitable for coating. Chrome-plated products resist abrasion much better.
Molybdenum is used as a coating for the working part of the rings in order to increase their resource. In addition, molybdenum piston rings run in faster to the cylinder walls, which is especially important if they are replaced during an engine overhaul.
Piston ring anatomy
You should not think that the same metal is used for all the rings in the kit. The material used is mainly influenced by the position of the piston rings on the piston. Since all piston rings operate under different conditions, the requirements for the alloys from which they are made are different.
First compression ring
The first compression rings are subjected to the most severe conditions, so they are subject to the highest requirements for heat resistance and wear resistance. Most often they are made of cast iron with a molybdenum anti-wear insert. 
At first glance, all compression rings look the same, but their configuration can vary significantly. For example, the top may be slightly twisted. As a result, only its edge is in contact with the surface of the cylinder, and not the entire surface. As a result, friction losses are reduced and run-in time is shortened.
Another type is compression rings with an L-shaped section. Their distinguishing feature is the ability to change the degree of compaction depending on the pressure exerted by the working gases on the back of the large L-shaped protrusion. Under pressure, they expand, improving compression (during the compression stroke), when the pressure decreases, the diameter, on the contrary, decreases, while reducing friction and wear on parts. Which of the rings is better, everyone decides for himself, since everyone has their own advantages.
Second compression ring
Its working conditions are less stressful, so the requirements for the material from which it is made are less stringent. The second compression ring performs a dual task:
- provides additional sealing, trapping gases that have broken through the first;
- works like a scraper, preventing engine oil from entering the combustion chamber.
Often the second compression rings are truncated cone shaped, i.e. The diameter of the top is smaller than the diameter of the bottom. Thanks to this design, when the piston moves down from the cylinder walls, oil is removed. 
Both compression rings have only one correct position for installation, they must not be turned over in any case, otherwise they will not work correctly. To prevent errors during installation, they are marked on their upper side, for example, "T" or "TOP".
Oil scraper rings
They are installed under compression. Unlike the latter, their surface is not continuous, it has windows designed to drain engine oil. In modern engines, oil scraper rings are installed one for each piston, in the past they were installed several, especially in engines designed for stationary use. 
Selection of spare parts
When performing an engine overhaul, it is necessary to choose the right piston rings, since an incorrect selection will either lead to a lack of the necessary compression, or to jamming of the pistons in the cylinders, followed by the destruction of the rings and the formation of numerous scoring on the pistons and cylinder walls. Consider, using the example of the VAZ-2111 engine, how to choose the right parts.
There is a table of repair sizes for pistons and rings, with which it is quite easy to choose the appropriate diameter of parts. The base diameter of the cylinders of the VAZ-2111 engine is 82 mm, the repair pistons have an increased diameter: by 0.4 mm for the first repair and by 0.8 mm for the second. Rings for repair dimensions also have an increased diameter, and they are marked accordingly.
In general terms, the overhaul process of the VAZ-2111 engine looks like this: the cylinders are bored to the first repair size, while leaving a small allowance for honing. Then the old pistons are replaced with new ones of increased diameter, and piston rings of the first repair size are mounted on them.
A repair size table exists for all engines, and it will not be difficult to choose the right diameter for any engine, whether it is a VAZ or Subaru engine.
Which rings are best
The question of which piston rings is better to purchase for replacement haunts many car owners. With so many to choose from, it's not easy to make a choice. You can answer this: if the purpose of the replacement is to restore the normal operation of the motor, regular ones are enough, but if the owner wants to improve engine performance, it is better to pay attention to more “fancy” products, for example, chrome or molybdenum.
Function:
Oil scraper piston rings were designed only to distribute oil on the cylinder wall and remove excess oil. To improve the sealing and oil scraping function, oil scraper rings usually have two working belts. Each of these belts removes excess oil from the cylinder wall. Thus, a certain amount of oil appears both on the lower edge of the oil scraper ring and between the belts, which must be removed from the ring area. When the piston is tilted within the inner bore of the cylinder, the seal functions the better, the closer to each other are both annular working belts.
First of all, the oil that is removed from the upper oil scraper belt and appears between the annular working belts must be eliminated from this area, since otherwise it will fall outside the oil scraper piston ring and then it will have to be eliminated by the second compression piston ring. For this purpose, oil scraper piston rings, one-piece or two-piece, have either oblong slots or holes between the annular working belts. Through these holes in the ring itself, the oil removed from the upper working belt is discharged to its reverse side.
Now the further removal of the removed oil can take place in various ways. One method is to deliver oil through the holes in the oil scraper groove to the inside of the piston so that it can drip into the oil pan (fig. 1). With so-called slits in the cover "Coversiots" (fig. 2 and fig. 3), the skimmed oil is delivered through a recess around the boss to the outside of the piston. A combination of both designs also finds its application.
Both designs proved to be suitable for draining the removed oil. Depending on the shape of the piston, the method of combustion or the purpose of the application, either one or the other version of the ring is used. Theoretically, one of these structures cannot be preferred. The decision which method is best suited for a particular piston is therefore determined by various tests in practice.
Important note:
In two-stroke engines, the piston is lubricated with a fuel mixture. Therefore, due to the design, the use of an oil scraper piston ring can be omitted.
Design Types: One-Piece Oil Scraper Piston Rings
One-piece oil scraper rings are no longer used in modern engine building. They derive their elasticity only from the cross section of the piston ring. Therefore, these rings are relatively rigid and cannot adhere well to the cylinder surface around the entire perimeter and, at the same time, do not have such good sealing properties as oil scraper piston rings, which consist of separate parts. One-piece oil scraper piston rings are made of gray cast iron.
Slotted oil scraper box piston ring
The simplest design with rectangular cross-section oil scraper belts and slots for oil drainage.
Oil scraper box-shaped piston ring with converging chamfers
Compared to the slotted oil scraper box piston ring, the edges of the wear bands are chamfered. This is done in order to achieve improved surface pressure.
Oil scraper box-shaped piston ring with parallel chamfers
For this ring, the working belts have a chamfer only on the side of the combustion chamber. As a result, when the piston moves down, the removal of oil from the cylinder walls is more efficient.
Two-Piece Oil Scraper Piston Rings (Helical Coiled Expander Designs)
Oil scraper piston rings, consisting of two parts, consist of the ring itself and behind it a coil spring. The ring itself has a significantly smaller cross section compared to the one-piece oil scraper piston ring. As a consequence, these rings are relatively flexible and can adhere very well to the surface of the cylinder all around. The groove for the twisted spring expander in the inner side of the ring itself is either semicircular or V-shaped.
The tension itself comes from a helical pressure spring made of heat-resistant spring steel. She lies behind the ring and presses it against the wall of the cylinder. During operation, the springs fit snugly against the back of the ring itself and together form one whole. Although the spring in the ring does not scroll, the entire annular block, like other rings, also rotates freely in the annular groove. The distribution of radial pressure for oil scraper piston rings, consisting of two parts, is always symmetrical, since the contact pressure is evenly distributed throughout the volume of the helical spring (see also chapter 1.6.2 Radial pressure distribution).
To increase the service life, the outer diameters of the springs are ground, wound more tightly on the piston ring lock, or a Teflon hose is pulled over them. By taking these measures, frictional wear between ring and coil spring is reduced.
The two-piece rings are made from either gray cast iron or steel.
Important note: The gap in the lock of an unstressed piston ring, i.e., the distance of the butt ends of the ring in the dismantled state without an expander spring located behind it, is insignificant for composite oil scraper piston rings consisting of several parts. Especially with steel rings, this gap can be almost zero. This is not a defect or a reason for a complaint.
Slotted oil scraper box piston ring with helical coil spring expander
The simplest type of design with better sealing than a one-piece slotted oil scraper piston ring.
Oil scraper box-shaped piston ring with parallel chamfers and coil spring expander
The same surface shape as the Parallel Chamfered Box Ring Piston Ring, however with better sealing.
Oil scraper box-shaped piston ring with converging chamfers and coiled spring expander
The same surface shape as the oil scraper box piston ring with converging chamfers, with better sealing.
This is the most common oil scraper piston ring. It can be built into every engine model.
Oil scraper box-shaped piston ring with converging chamfers, with coiled spring expander and chrome-plated working bands
The same properties as the oil scraper box piston ring with converging chamfers and coiled spring expander, however, with increased wear resistance and, at the same time, with a longer service life. This ring is therefore particularly suitable for diesel engines.
Oil scraper box-shaped piston ring with converging chamfers and coiled spring expander made of nitrided steel
This ring is wound from profile steel tape and covered on all sides with a protective layer. It is very
flexible and breaks less frequently than the gray cast iron rings mentioned above. Oil is drained between the slats through cut round holes. This type of oil scraper piston ring is mainly used in diesel engines.
Three-piece oil scraper piston rings
Three-piece oil scraper piston rings consist of two thin steel plates that are pressed against the cylinder wall with a spacer spring and an expander spring. Oil scraper piston rings with steel plates are available either with chrome-plated running surfaces or with nitrided coating. The latter serve to improve wear resistance not only on the running surface, but also between the expander spring and the plates (secondary wear). Three-piece oil scraper piston rings have the ability to adhere very well to the surface of the cylinder around the entire perimeter. These rings are mainly used in gasoline engines of passenger cars.
Installation situation
Various models of expander spring
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