1. Failure Symptoms, Cause Analysis, and Countermeasures for Piston Rings

The main faults that occur with piston rings include breakage, sticking, and abnormal wear. When such failures happen, they typically lead to combustion chamber leakage, increased exhaust temperature, black smoke in the exhaust, contamination of the stuffing box oil, and elevated cylinder liner water temperatures. During my tenure on the "Solar Ace" vessel, which had a MAN B&W 6L60MC diesel engine with direct air scavenging and had been in operation for 14 years, we encountered such issues.

While performing regular maintenance on cylinders No. 1 and No. 3 at Vancouver anchorage, we replaced the piston rings, even though the measured wear on them was not yet at the limit and the condition of the pistons and piston ring grooves was good. This replacement was precautionary, as the next maintenance was expected to be after 8,000–10,000 hours. About three days after departing from Vancouver, and shortly after switching to fuel supplied at Lianyungang (RMG380, 795 tons, with lab results showing impurities above acceptable levels, including aluminum and silicon content up to 60, with a maximum of 80), we noticed the fuel intake temperature for the main and auxiliary engines began to drop gradually and couldn’t be raised. Additionally, the pressure differential across the secondary fuel filters was abnormally high, requiring continuous backflushing.

After manually cleaning the secondary filters for both the main and auxiliary engines, the pressure differential and temperature returned to normal, only to experience similar issues again within a day. Consequently, we activated another purifier to operate in parallel, reducing the load on each purifier and shortening the sludge discharge interval to one hour, but the situation did not improve.

Later, we observed that the exhaust temperatures of cylinders No. 1 and No. 3 began to rise significantly above those of the other four cylinders, and reducing the engine load did not alleviate the problem. The exhaust from the main engine became increasingly dense, sometimes even causing slight surging in the turbocharger under rough sea conditions. Indicator diagrams revealed that the compression pressures in cylinders No. 1 and No. 3 were only 5.6 MPa and 5.4 MPa, respectively, compared to 6.1–6.2 MPa in the other cylinders, and their explosion pressures were also lower.

These symptoms indicated a sealing issue with cylinders No. 1 and No. 3, either due to the exhaust valve or the piston rings. Over the next few days, the situation stabilized slightly, although we continued manually cleaning the secondary fuel filters once daily until the ship arrived in Incheon, Korea.

Upon docking, we inspected the pistons, piston rings, and cylinder liners of each cylinder by opening the scavenge box door. The pistons were very dirty, with some piston rings in cylinders No. 1 and No. 3 broken or stuck, and the cylinder liners showing minor scuffing. The cylinder oil in cylinders No. 1 and No. 3 seemed to be on the low side, while the others were simply dirty with noticeable deposits at the base of the liners. We decided to overhaul cylinders No. 1 and No. 3 again. During this process, we found that, in cylinder No. 3, all but the bottom piston ring were broken into three or four pieces. In cylinder No. 1, the first piston ring was severely stuck in its groove, and the other rings had various forms of breakage. Fortunately, there was no significant erosion or ridging in the ring grooves, and the cylinder liners had only minor scuffing.

Our analysis indicated that the immediate causes of the breakage and sticking of the piston rings in cylinders No. 1 and No. 3 were:

1) Severe quality defects in the piston rings, as only the newly replaced rings in cylinders No. 1 and No. 3 had issues, while those in the other cylinders were essentially normal.

2) Fuel quality issues. Despite various measures, such as frequent draining of sediment from the settling and service tanks, parallel operation of purifiers to reduce throughput, and increasing heating temperatures in the settling and service tanks and purifiers, traditional purifiers cannot completely remove fine aluminum, silicon, and ash particles, leading to higher levels of impurities in the fuel.

During combustion, silicon particles adhered to the cylinder liner walls, accelerating wear between the piston rings and cylinder liners, causing the rings to stick and eventually break. Additionally, aluminum in the fuel led to high-temperature corrosion, which further exacerbated the wear of piston rings and liners. With the company’s approval, we cleaned and reinstalled the old piston rings after minor repairs to the ring grooves and cylinder liners. We also adjusted the oil injection quantity for cylinders No. 1 and No. 3 due to the reinstallation of used rings, which required a lower oil amount. For the ten days of the voyage from Korea to Australia, the main engine operated smoothly, with consistent exhaust temperatures, and the compression and explosion pressures from the indicator diagrams were stable. However, due to the excessive impurities in the fuel, we still had to clean the secondary fuel filters daily.

The above incidents demonstrate that when a piston ring’s sealing function is compromised by breakage or sticking, it can cause varying degrees of blow-by in the cylinder. This reduces the fresh air intake into the cylinder, leading to lower compression pressures, higher exhaust temperatures, increased cooling water temperatures, and poor combustion under low-speed, high-load conditions. With less air available, the exhaust energy increases significantly, raising the turbocharger speed and scavenging pressure.

If the engine speed remains constant, the air consumption of the engine will stay relatively stable, but the blow-by caused by broken or stuck rings can lead to backflow of gases into the scavenging box, potentially causing a fire in severe cases. This raises the backpressure in the turbocharger, causing it to operate under conditions of low flow and high backpressure, thereby disturbing the matching of the turbocharger and triggering surging.

These symptoms may all be caused by piston ring failures.

2. Other Causes of Piston Ring Failure

Apart from material quality, manufacturing processes, and dimensional accuracy of the piston rings, several other factors can also affect performance.

1) Influence of the Piston Ring End Gap

The end gap of a piston ring allows for thermal expansion during operation and enables a degree of circumferential movement under normal working conditions. When the gap is too small, thermal expansion can cause excessive pressure at the end, leading to breakage on the opposite side of the gap.

As the piston ring and cylinder liner wear during movement, the end gap gradually increases. If this gap becomes too large, it can lead to significant radial force imbalance. The radial force mainly arises from the ring's own elasticity and the gas pressure acting on the back of the ring. The end gap causes these forces to act unevenly on the opposite side, resulting in uneven wear and potentially leading to ring breakage. Additionally, reduced radial thickness and elasticity of the ring, along with severe carbon buildup in the ring groove, can lead to ring sticking.

The end gap size serves as a key indicator of piston ring wear. Thus, when the main engine is shut down, the condition of the piston rings should be inspected regularly through the scavenging air box for any signs of sticking or breakage. If issues are found, cylinder repair should be performed promptly to prevent further damage. The end gap value should be compared to the specifications in the manual to determine if it exceeds the limit and should be monitored against previous measurements to assess the wear rate. If wear rate suddenly increases within a given time period, factors like injector malfunctions, degraded cylinder oil, or excessive ash and impurities in the fuel should be investigated. By recording each ring’s data, systematic analysis of faults can be conducted, enabling comparisons of spare part quality and assessing the effects of fuel and lubricating oil on wear rate.

2) Influence of the Piston Ring and Cylinder Liner Fit

Achieving ideal full-film lubrication depends on factors such as the movement, speed, and performance of the "motion pair." The speed of the piston ring in the cylinder liner varies throughout the stroke, reaching zero at the top and bottom dead centers, where it is subject to high-temperature gas impact. As a result, optimal lubrication is hard to achieve, especially near the top dead center, where lubrication may be in the boundary or even dry friction state.

Typically, no friction occurs between the piston head and the cylinder liner. However, carbon deposits form around the piston head over time, which are hard and difficult to remove. As the deposits increase, the diameter of the piston head expands, causing friction with the cylinder liner and seriously disrupting the oil film, thus increasing wear on both the cylinder liner and piston ring.

Uneven wear occurs on the cylinder liner, causing out-of-roundness and cylindrical distortion in both the circumferential and axial directions. During piston ring movement in the cylinder, periodic expansion and contraction occurs. This deformation may cause sealing issues, particularly with new piston rings. Generally, new rings are expected to exhibit a total light leakage below 90° and no continuous leakage greater than 30°, with no leakage in the 30° area on either side of the ring’s end gap. High-quality rings can meet these requirements; however, to reduce costs, some companies have started using low-cost rings instead of original parts, resulting in increased light leakage due to differences in material, manufacturing process, and larger size errors. 

When the piston ring elasticity decreases significantly at high temperatures and carbon build-up in the ring groove causes sticking, the high-pressure gas leaks through the light gaps, exerting force on the outer working surface of the ring. This can wedge the ring into the groove, potentially causing sticking and seizure in the groove. Over time, such periodic cycles can lead to fatigue fractures in weak areas of the ring.

Practice shows that piston ring wedging is a primary cause of ring breakage. Additionally, cases of rings breaking due to hitting cylinder liner projections or hanging at the port end have also been observed in some diesel engines.

3) Influence of Fuel and Cylinder Oil

The quality of fuel used on ships varies widely, given the different countries and manufacturers, as well as the changing fuel ports. Since the production and refining processes differ, the fuel characteristics can vary significantly. When purifying fuel, the correct density ring should be selected based on the fuel type, and processing should occur at high temperatures (e.g., 95–98°C) with parallel or series separator configurations as needed. If not, purification effectiveness may suffer. High levels of aluminum, silicon, and ash in the fuel can accelerate wear between the piston ring and cylinder liner, impacting the diesel engine's combustion process and raising the combustion chamber temperature. Furthermore, diesel engines often run at low speeds or low loads, especially when entering and leaving port, causing excess cylinder oil injection without adjustment. This excess oil accumulates at the ring groove due to the pumping action of the rings, leading to carbon build-up when high cylinder temperatures cause the oil to combust. This carbon buildup can contribute to piston ring sticking and breakage.

4) Influence of Routine Maintenance and Management

Effective daily maintenance practices are crucial in preventing piston ring sticking, breakage, and other faults.

i) After replacing a cylinder liner or piston ring in a diesel engine, maintain low speed and low load for sufficient break-in time. During this period, increase the cylinder oil injection rate to avoid overheating and scuffing due to incomplete surface matching between the ring and liner, which may cause scoring or ring breakage.

ii) During regular operation, control the cooling water and oil temperatures and pressures within the normal range. Monitor parameter changes closely, especially the rate of change for each parameter. Periodically measure the indicator diagram to analyze parameters like compression pressure, explosion pressure, combustion start, and load distribution to assess combustion quality and the condition of the cylinder and piston rings. Detecting early signs of faults allows for timely interventions.

iii) Regularly open the scavenging air box to measure the piston ring end gap, maintain a record of each piston ring’s usage, and inspect conditions through the scavenging port. If any signs of sticking or breakage are observed, perform cylinder repair immediately.