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Nine Reasons For Broken Piston Rings

2021-10-29

Piston ring breakage is one of the common forms of damage to piston rings. Generally, the first and second-way piston rings are prone to breakage, and the broken parts are mostly near the overlap. The piston ring may be broken into several pieces, or it may be shattered or even missing. The broken piston ring will increase the wear of the cylinder. The broken ring of the two-stroke diesel engine may be blown into the exhaust pipe or the scavenging box, or even blown into the turbo end of the turbocharger to damage the turbine blades, causing serious accidents. Today I will take you to see what is the reason for the broken piston ring~
Common causes of broken piston rings:

There are many reasons for the broken piston ring. In addition to material defects and low processing quality, it is mainly caused by poor maintenance and management during use and poor assembly quality.

1. The gap between the mouth is too small

When the overlap gap is smaller than the assembly gap, the heating temperature of the piston ring increases during operation, so that the metal at the overlap does not have sufficient room for expansion, and the two ends of the overlap are bent and broken near the overlap.

2. Ring groove carbon deposit

Poor combustion, overheating of the cylinder wall to oxidize or burn the lubricating oil will cause serious carbon deposits in the cylinder. When the coke deposits are severe, the movement of the ring is blocked, and the ring and the cylinder wall act strongly, and the scraped oil and metal chips are mixed, and under the action of the leaking gas, a local hard coke deposit is formed on the lower end of the ring groove. There are local hard carbon deposits under the piston ring, and the upper part is subjected to periodic gas pressure, causing the piston ring to break due to bending fatigue.

3. Cylinder liner grinding table

The long-term relative movement of the piston assembly and the cylinder liner causes the cylinder liner to wear out and a grinding platform appears on the upper part of the cylinder liner. When the piston moves up to the top dead center, the first piston ring hits the grinding table and is impacted and breaks.

4. Excessive wear of the ring groove

The lower end surface of the ring groove is inclined (trumpet-shaped) after excessive wear. When the piston is near the top dead center, the gas pressure causes the ring to stick to the lower end surface of the inclined ring groove, the piston ring is twisted and deformed, and the piston ring groove is excessively worn and fatigued and broken. 

5. The piston ring hangs the air port

In two-stroke diesel engines, the piston ring is often caught in the sweep and the exhaust port causes the ring to break. Because the piston ring has the largest tension at the opening part, the heat deformation is large, and the ribs between the air ports on the cylinder liner are easily deformed by heat. When the piston moves, the ring meets the air port, and the ring will be broken as long as the ring opening slightly catches the air port.

6. Radial expansion and contraction fatigue of piston ring

When the elastic force of the piston ring is insufficient or the cylinder liner is excessively worn, the piston ring and the cylinder wall cannot be closely attached, that is, the air tightness cannot be maintained, so that the high pressure gas leaks the ring into the ring groove. When the piston descends, the gas pressure in the cylinder decreases, and the piston ring is ejected from the ring groove. The piston ring continuously expands and contracts radially, causing fatigue and fracture.

7. Excessive wear of the piston ring makes the strength of the ring unable to meet the requirements and the ring is broken

The current piston ring materials are mostly brittle materials such as gray cast iron, alloy cast iron, ductile iron, and the internal structure may have porosity, cracks or segregation of components during the casting process, which may cause local strength to drop or stress concentration at the cracks, causing shocks during work Fracture or fatigue fracture.

8. Serious wear of the cylinder liner

Stepped wear at the top and bottom dead center positions causes shoulders, large wear at the large end of the connecting rod or changes in the original dead center position after the large and small ends of the connecting rod are repaired, resulting in an impact and broken ring under the action of inertial force .

9. The problem of cooperation between lubricating oil and fuel

From the analysis of the nature of the fuel, some fuels contain higher sulfur content, such as Reye's 1000s fuel. Therefore, when the fuel burns, more SO2 and SO3 are produced. However, SO2 and SO3 are gaseous at high temperatures, and their direct interaction with metals can cause gas corrosion. It also promotes the hardening of carbon deposits and colloidal deposits, which increases the wear of cylinder liners and piston rings. According to requirements, when diesel engines use high-sulfur fuels, strong or high-alkaline special lubricating oil should be used. If the main engine does not use the cylinder oil that matches the fuel or replaces the cylinder oil with engine oil. In this way, the ability of the lubricating oil to neutralize the acid generated by combustion is reduced, causing the acidic substance to corrode the piston and piston ring more seriously, resulting in frequent piston ring fracture.
Emergency treatment method for broken piston ring:

The breaking of the piston ring mainly occurs in the upper piston ring, which is often exposed to high temperature and high pressure, especially when the surface of the cylinder is worn. If the ring breaks during operation, the fracture of the ring will scratch the inner surface of the cylinder and make a sound; or the lower part of the cylinder will overheat due to gas leakage, which will then leak into the crankcase and produce a large amount of oil mist. On the other hand, after the piston ring is broken, the compression pressure is also reduced, resulting in poor combustion and black smoke in the exhaust gas.