Piston ring’s shape design plays a key role in engine performance and efficiency. In addition to differences in appearance features, each shape is also unique in its functionality. The following will introduce the characteristics and working principles of each form one by one.
Rectangular piston ring’s cross-section is rectangular. The advantage is that the shape is simple, without any special curves or concavities, and it is easy to process. And because the contact surface with the cylinder wall is large, it is conducive to the cooling of the piston. However, the breaking-in performance is slightly poor, and its strong oil pumping effect will increase the oil consumption and increase the carbon deposits on the top of the piston and the combustion chamber. Therefore, this type of piston ring is relatively rarely used now.
Taper-faced piston ring’s outer circular surface is a cone surface with a very small cone angle (generally 30°~60°). Unlike the rectangular ring, which is in surface contact with the cylinder wall, the taper-faced ring is in line contact with the cylinder wall. It has good breaking-in performance and can increase the contact pressure and adaptability of the shape of the cylinder wall; however, with increasing use time, the contact surface will gradually become larger and eventually become an ordinary rectangular ring. Another factor of the tapered ring is that it can play the role of scraping oil during the downward stroke of the piston and can distribute oil during the upward stroke. It should be noted that since the contact area between the taper-faced ring and the cylinder wall is small, resulting in poor heat transfer, this shape is usually not used in the first ring.
Barrel-faced piston ring’s surface contacts the cylinder wall in a circular arc, which can effectively avoid corner loads and has excellent adaptability, sealing, and running-in properties to piston deflection. It can form a wedge-shaped oil film regardless of whether it moves up or down in the cylinder, thereby reducing the wear of the ring and cylinder wall. Its main disadvantage is that it is difficult to process convex-arc surfaces. This shape is often used in the first ring.
Inner cut, Under hook/Napier and Under cut/Scraper piston rings are collectively referred to as twisted ring. The reason for its name is that after being installed into the cylinder, due to the asymmetric cross-section, an unbalanced force is generated, causing the piston ring to twist. The twisted ring has the advantages of a taper-faced ring. It has an oil-spreading effect when the piston moves upward and an oil-scraping effect when the piston moves downward. At the same time, the shortened stroke of the twisted ring jumping up and down in the ring groove can reduce pump oil, slow down impact, and reduce wear; the ring is no longer twisted during the power stroke, and the two sealing surfaces will reach complete contact, which is beneficial to heat dissipation. The excellent characteristics of the twist ring make it widely used, but when installing, special attention must be paid to the cross-sectional shape and its designed installation direction. If installed backwards, it will increase engine oil consumption. The exceptional oil-scraping function of the twisted ring makes it often appear as the second ring in the combination.
Cutting a right-angled or beveled shape on the cross-section or the inside of the lower part of the piston ring is collectively called inner cut; the main function of inside-step-top cutting is to increase sealing, and inside-bevel-bottom cutting is to scrape oil.
An undercut is a right-angled notch cut under the outside of the ring, similar to an underhook piston ring, which is a sharp-angled notch under the outside of the ring.
Double keystone piston ring’s cross-section is trapezoidal, and its main feature is good adhesion resistance. It is common in diesel engines with large loads because the first ring easily gets stuck, causing it to lose its function in the ring groove. The double keystone shape will continuously change its position according to the direction of the piston's side pressure. This will squeeze out the carbon deposits in the ring groove, preventing the ring from being stuck in the ring groove and breaking, thereby extending the service life. There is also a half keystone piston ring, which has the characteristics of an inside-bevel-top cutting piston ring. The processing of keystone rings is difficult and requires precise processing; therefore, the yield is low, causing a rise in price. They are generally used as the first ring of diesel engines.
Overlapped joint piston ring is a right-angled shape cut out of the upper half of one side and a right-angled shape cut out of the lower half of the other side at the joint of the ring. Hook joint piston ring uses a more complex cutting process in the shape of an overlapped joint piston ring so that the two sides of the hook can hook each other. Both are more commonly used in compressors or agricultural equipment.
Joint with internal notch piston ring is a trapezoid-like angle cut from the inside at the joint of the ring. More commonly used on jet skis. Joint with side notch piston ring is a semicircle cut out from the side at the joint of the ring.
The shape design of the piston ring not only reflects its functionality, but also demonstrates its craftsmanship. By carefully studying the different forms of piston rings, we can better understand the various design considerations and make choices based on the actual situation.
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