market dynamics
Understand Piston Ring
Release date:2019.07.17
Piston rings are mainly divided into gas rings and oil rings.
The action of piston ring
        The function of the gas ring is to ensure the sealing between the cylinder and the piston, prevent air leakage, and transfer most of the heat absorbed by the top of the piston to the cylinder wall, which is taken away by the cooling water. The oil ring plays the role of oil distribution and oil scraping, scraping the excess oil on the cylinder wall when going down, and spreading a uniform oil film on the cylinder wall when going up. This not only prevents the oil from entering the cylinder and burning out, but also reduces the friction resistance between the piston and the cylinder wall. In addition, the oil ring can also play an auxiliary role in sealing gas.
        Working Conditions and Performance Requirements of Piston Ring
When the piston ring works, it is affected by high temperature and high pressure gas in the cylinder, and the temperature is higher (especially the first ring, the temperature can reach 600K). The piston ring moves at high speed in the cylinder, and some oil deteriorates at high temperature, which makes the lubrication condition of the piston ring worse. It is difficult to ensure liquid lubrication and serious wear. Therefore, piston rings are required to have good elasticity, high strength and wear resistance.
Clearance of piston ring
        The piston ring will expand in contact with high temperature gas in the process of engine operation, and the periodic reciprocating motion will cause its radial expansion and contraction deformation. Therefore, in order to ensure normal operation, the piston ring should have the following clearance in the cylinder.
D-Piston Ring Internal Diameter; B-Piston Ring Width
        End clearance, also known as open clearance, refers to the clearance between the two ends of the piston ring when it is loaded into the cylinder in cold condition. Generally 0.25-0.50 mm.
Side clearance, also known as side clearance, refers to the clearance between the side of the piston ring and the groove of the piston ring after it is loaded into the piston. The first ring is usually 0.04-0.10 mm because of its high working temperature and large clearance, while the other rings are 0.03-0.07 mm. The side clearance of oil ring is smaller than that of air ring.
Back clearance refers to the clearance between the cylindrical surface of the piston ring and the bottom of the piston ring groove after the piston ring is loaded into the cylinder, which is generally 0.50-1.00mm. The back clearance of the oil ring is larger than that of the gas ring, which is helpful to increase the oil storage clearance and facilitate the decompression and oil relief.
Pumping Oil Action of Piston Ring
        Because of the existence of backlash and backlash, when the engine works, the piston ring produces the function of pumping oil. The reason is that when the piston goes down, the piston ring rests on the top of the ring groove, and the oil scraped from the cylinder wall fills the bottom of the ring groove; when the piston goes up, the piston ring rests on the bottom of the ring groove and squeezes the oil above the ring groove. Such repeated movement will pump the oil on the cylinder wall into the combustion chamber. Because of the pumping effect of piston rings, the oil channeling into the combustion chamber will cause the formation of carbon deposits and increase oil consumption in the combustion chamber, and may also form carbon deposits in the ring groove (especially in the first air ring groove), which will make the ring stuck, lose its sealing effect and even break the piston ring.
Gas ring
Sealing mechanism of gas ring
        The piston ring has an incision, and is not circular in the free state. Its shape size is larger than the inner diameter of the cylinder. Therefore, when it is loaded into the cylinder together with the piston, it produces elastic force and sticks to the cylinder wall.
Under the action of gas pressure, the piston ring is pressed on the lower end surface of the ring groove, so the gas flows around the back of the ring and expands, and the pressure decreases. At the same time, the force of gas pressure on the back of the ring makes the piston ring more closely attached to the cylinder wall. When the pressure has been reduced, the gas leaks from the incision of the first gas ring to the upper plane of the second gas ring, then presses the gas ring on the lower end of the second gas ring groove. As a result, the gas flows around the back of the ring, expands again, and the pressure decreases further.
        As this goes on, the pressure and velocity of the gas leaking from the last gas ring have been greatly reduced, so the amount of gas leaking is very small. Therefore, the "labyrinth" sealing device consisting of a few gas rings with staggered incisions is sufficient to effectively seal the high-pressure gas in the cylinder.
        Section Shape of Gas Ring and Gas Pressure at Clearance of each Ring
Incision of gas ring
        The main way of gas leaking into crankcase in cylinder is the incision of piston ring. Therefore, the shape of incision and the clearance after loading into cylinder have certain influence on the amount of gas leaking into crankcase. If the incision clearance is too large, the leakage will be serious and the engine power will be reduced; if the clearance is too small, the piston ring will expand when heated. Can jam or break. The value of incision clearance is generally 0.25-0.8 mm. The temperature of the first air ring is the highest, so the notch clearance is the largest.
Incision shape of air ring
        The right-angle incision has good workmanship; the ladder incision has good sealability, but its workmanship is poor; the oblique incision generally has a bevel angle of 30 degrees or 45 degrees, and its sealability and workmanship are between the first two kinds, but its sharp-angle position is easy to be damaged when the piston is packed into the piston; the figure (d) is a two-stroke engine piston ring with anti-turning pin groove. The notch of the piston ring and the pin pressed in the piston ring groove are used to prevent the piston ring from rotating around the piston center line in operation.
Section shape of gas ring
Section shape of gas ring
        The advantages of rectangular ring are simple structure, convenient manufacture, good heat dissipation and low waste rate. The main drawbacks of rectangular ring are pumping oil, which easily leads to excessive oil consumption and carbon deposition in combustion chamber. In addition, the scraping, running-in and sealing properties of Rectangular rings are poor, and they are basically not used in modern automobiles.
        The advantages of conical ring are linear contact with cylinder wall, good sealing and running-in performance, obvious oil scraping effect, easy formation of oil film to improve lubrication, and poor heat transfer performance. Conical rings are mainly used in rings other than the first ring.
The twisted ring is a kind of piston ring widely used in modern automotive engine. The main reason is that the twisted ring has the advantages of conical ring, but also can reduce the effect of pump oil, reduce wear and improve heat dissipation performance. Special attention should be paid when installing twisted rings: the inner circle groove is upward, the outer circle groove is downward, and the reverse can not be installed.
        The main advantage of trapezoidal ring is that it can extrude the coke deposited in the ring groove, thus avoiding the breakage of the piston ring due to bonding. At the same time, its sealing performance is superior and its service life is long. The disadvantage is that the grinding process of upper and lower ends is more complex. Trapezoidal rings are widely used in diesel engines with large heat loads.
        The advantage of the bucket face ring is that the wedge-shaped oil film can be formed in the upstream and downstream of the piston to improve lubrication. It has good adaptability to the swing of the piston in the cylinder, small contact area and is conducive to sealing. The disadvantage is that it is difficult to process the convex arc surface, which is mostly used to strengthen the first ring of the diesel engine.
Oil ring
        Oil rings can be divided into two types: common oil rings and combined oil rings.
Ordinary oil rings are made of alloy cast iron. A groove is cut in the middle of the outer circular surface, and many perforated oil discharge holes or slits are machined at the bottom of the groove. The outer edge of the upper lip of the oil ring is generally chamfered, which can form an oil wedge when the oil ring moves upward. The oil can push the oil ring away from the cylinder wall, which makes it easy to enter the groove of the oil ring. The outer edge of the lower lip is not chamfered, so the ability to scrape oil downward is stronger. The oil scraping ability of nose oil ring and double nose oil ring is stronger, but the processing is more difficult.
Oil Ring and Its Oil Scraping Function
Section Shape of Oil Ring
        For the combined oil ring consisting of three scraper plates and two elastic lining rings, the axial lining rings are clamped between the second and third scrapers, and the radial lining rings press the three scrapers on the cylinder wall. The advantages of this kind of oil ring are that the ring is thin and the specific pressure on the cylinder wall (pressure per unit area) is large, so the oil scraping effect is strong; the three oil scrapers are independent, so they have good adaptability to the cylinder; light weight; and large oil return path. Therefore, the combined oil ring has been widely used in high-speed engines. The disadvantage is the high manufacturing cost (the outer surface of the ring must be chromium-plated, otherwise the sliding property is not good).

Content comes from
Automobile Principle Construction and Mapping