The main function of the piston ring is to seal the combustion chamber, ensuring that when the piston reaches the top dead center, the fresh air in the combustion chamber has sufficient temperature and pressure to meet the temperature of fuel combustion and make it burn rapidly and completely;
The working conditions of the piston, piston ring, and cylinder liner are extremely harsh, and the friction loss between them accounts for 55% to 60% of the total friction loss power of the diesel engine.
Therefore, the working state of the piston ring plays a crucial role in the completeness of fuel combustion and the working condition of the diesel engine. It is particularly important to manage and maintain the operation of the piston ring to ensure the good operation of the diesel engine.
Fault phenomena, cause analysis and treatment measures of piston rings
The main faults of piston rings include breakage, adhesion, abnormal wear, etc.
When the above-mentioned faults occur, they generally cause phenomena such as combustion chamber leakage, exhaust temperature rise, black smoke emission from exhaust, contamination of oil in the packing box, and temperature rise of cylinder liner water.
During my tenure on the "Solace" ship, the main engine model was a MAN B&W6L60MC diesel engine with DC scavenging and operated for 14 years.
Normal maintenance and hoisting of main engine cylinders 1 # and 3 # were carried out at the Vancouver anchorage. Although the wear of the piston rings did not reach the limit and the piston and piston ring grooves were in good condition, considering that the next hoisting of the cylinders will take another 8000-10000 hours.
Therefore, all piston rings were replaced.
The exhaust valve of cylinder 1 was replaced for about 800 hours, and the exhaust valve of cylinder 3 was replaced for about 500 hours.
After coming out of Vancouver for more than 3 days, I switched to oil (RMG380795t) added from Lianyungang. The test result showed that the impurities exceeded the standard, with aluminum and silicon content reaching 60, and the maximum amount was 80. In less than a day, I found that the temperature of the fuel entering the main and auxiliary engines gradually decreased, and the oil temperature could not be adjusted. I also found that the pressure difference of the fuel secondary filter was large, and continuous backwashing was not stopped.
After manually disassembling and cleaning the secondary filters of the main and auxiliary machines, the pressure difference and temperature returned to normal. However, within less than a day, the automatic flushing continued without stopping. Therefore, another oil separator was started in parallel to reduce the oil separation volume of a single oil separator and shorten the slag discharge interval to 1 hour. However, the situation did not improve.
Later, it was found that the exhaust temperature of cylinders 1 and 3 gradually increased and was significantly higher than the other four cylinders. There was no significant improvement in deceleration operation, and the exhaust color of the main engine became heavier and more thick smoke was seen. When the sea conditions were not good, the turbine also had a slight surge.
Through measuring the dynamometer diagrams of each cylinder, it was found that the compression pressures of cylinder 1 # and cylinder 3 # were 5.6MP and 5.4MP, respectively, while the compression pressures of other cylinders were 6.1-6.2MP. The explosion pressure was also lower for cylinder 1 # and cylinder 3 #.
The phenomenon indicates that there is a problem with the sealing of cylinders 1 # and 3 #: one is the exhaust valve, and the other is the piston ring.
In the next few days, it will gradually stabilize, and the secondary fuel filters of the main and auxiliary engines need to be disassembled and cleaned once a day until the ship arrives in Incheon, South Korea.
After arriving at the port, the first thing to do was to open the scavenging box door and inspect the pistons, piston rings, and cylinder liners of each cylinder. It was found that the pistons were very dirty, and some of the piston rings of cylinders 1 and 3 had already been broken or adhered, and the cylinder liners were slightly pulled. The oil in cylinders 1 and 3 seemed to be insufficient, while the rest of the cylinders were relatively normal, only slightly dirty. There were obvious oil residues and sludge in the lower part of the cylinder liners.
We have decided to lift cylinders 1 # and 3 # again.
When lifting the cylinder, it was found that except for the bottom piston ring which was normal, all the other piston rings of cylinder 3 were folded into 3 or 4 sections, or the first piston ring of cylinder 1 was severely adhered to the piston ring groove, while the other piston rings had different forms of fracture.
Fortunately, there is no obvious erosion or protrusion in the piston ring groove, and the cylinder liner is only slightly pulled.
According to the analysis of the phenomenon:
The most direct cause of piston ring breakage and adhesion in cylinders 1 and 3 is:
One is that there are serious quality defects in the piston rings, as only the newly replaced piston rings of cylinder 1 and cylinder 3 are broken or stuck, while the piston rings of other cylinders are basically normal;
The second issue is the quality of the fuel. Although we have taken many measures to purify the fuel, such as frequently discharging residues in sedimentation tanks and daily use tanks, parallel operation to reduce the oil separation capacity of the oil separator, and increasing the heating temperature of sedimentation tanks, daily use tanks, and oil separators, traditional oil separators are difficult to completely separate small impurities such as aluminum, silicon, and ash, resulting in an increase in impurities in the fuel.
During the combustion process of the cylinder, silicon particles adhere to the cylinder liner wall, accelerating the wear between the piston ring and the cylinder liner, causing the piston ring to adhere and eventually break;
Aluminum in the oil can cause high-temperature corrosion and accelerate the wear of piston rings and cylinder liners.
After obtaining the company's approval, we cleaned the old ring and replaced it.
Of course, simple treatment has been done for the piston ring groove and cylinder liner strain before.
Furthermore, some adjustments have been made to the oil injection volume of the 1 # and 3 # cylinder injectors, as they have been replaced with old rings and the required oil volume needs to be appropriately reduced.
During the ten days from South Korea to Australia, the operation status of the host has been good, the exhaust temperature is basically the same, and the compression pressure and explosion pressure measured from the dynamometer are also basically the same. The host is working normally.
The reason why the secondary fuel filter is still disassembled and cleaned once a day is due to excessive impurities in the fuel.
From the above faults, it can be found that when the sealing performance of the piston ring is reduced or lost due to fracture or adhesion, different degrees of cylinder blowby will occur, resulting in a significant decrease in the amount of fresh air in the cylinder, causing a decrease in compression pressure, an increase in exhaust temperature, an increase in cooling water temperature, and the diesel engine working at low speed and high load. The decrease in air volume deteriorates combustion, and the energy of exhaust gas increases significantly compared to normal conditions. The speed of the turbocharger increases, and the scavenging pressure increases.
In the case where the speed of the diesel engine remains basically unchanged, the air consumption of the diesel engine is basically stable. At the same time, ring breakage and adhesion cause gas to flow back into the scavenging box, and in severe cases, the scavenging box may catch fire. These factors cause the back pressure of the turbocharger to increase, making it work in a low flow and high back pressure state, which destroys the normal matching of the turbocharger and leads to its wheezing.
The occurrence of the above phenomena may be caused by piston ring failure.
3, Analysis of Other Causes of Piston Ring Failure
In addition to the material, processing technology, and dimensional accuracy of the piston ring, it is also related to the following reasons.
1. The influence of clearance between piston rings
The overlap of the piston ring can ensure that there is room for thermal expansion of the piston ring during operation, and at the same time, it can allow the piston ring to have a certain circumferential movement during normal operation.
When the clearance between the piston rings is too small, it will limit the thermal expansion of the piston during operation and cause compression at the joint, making it easy for the rings to break on the opposite side of the joint.
Due to the wear and tear of piston rings and cylinder liners during movement, the clearance between the ring joints will gradually increase.
When the clearance between the joints is too large, it will cause serious imbalance in the radial force of the piston.
The radial force is mainly composed of the elastic force of the ring itself and the gas force acting on the back of the ring. The presence of the gap between the lap joints causes the combined force of the two to cause uneven wear on the opposite side of the lap joint, which can lead to ring fracture in severe cases.
At the same time, the radial thickness of the piston ring decreases, and the elasticity decreases. If the ring groove is severely clogged, it is easy to cause ring adhesion.
In actual operation, the size of the clearance between the piston rings is an important indicator for judging the wear condition of the piston rings.
Therefore, when the host is stopped, the working condition of the piston ring and whether there is adhesion or breakage should be checked regularly through the scavenging box.
If there is, the lifting cylinder should be inspected to avoid the expansion of the fault.
The value of the clearance between the piston ring and the joint should be compared with the requirements in the manual to determine whether it exceeds the limit value;
Secondly, the wear rate of the piston ring should be determined by comparing it with the gap measured last time.
If the wear rate of the ring suddenly increases within a certain time interval, the cause should be identified and resolved, such as fuel injection system failure, cylinder oil deterioration, excessive fuel ash and impurities, etc.
By recording the files of each ring in this way, not only can the fault phenomenon of the piston ring be systematically analyzed, but also the influence of the quality, fuel, and lubricating oil of different batches of spare parts on the wear rate can be compared.
2. The influence of the fit between piston rings and cylinder liners
Whether ideal full liquid lubrication can be formed mainly depends on factors such as the motion form, speed, and performance of the lubricating oil of the "motion pair".
The speed of the piston ring's movement inside the cylinder liner is constantly changing throughout the entire stroke, with zero movement speed at the top and bottom dead centers. At the same time, it is subjected to the impact of high-temperature gas, making it difficult to form good lubrication. This is an important reason for the poor performance of the piston ring, especially when the piston is near the top dead center, the lubrication state is in a boundary lubrication state, sometimes even in a dry friction state.
Under normal circumstances, there is no friction between the piston head and the cylinder liner. With the operation of the diesel engine, carbon deposits will form around the piston head, which are hard and difficult to clean. The increase in carbon deposits causes the diameter of the piston to increase. When the piston moves inside the cylinder, the carbon deposits at the head generate friction with the cylinder liner, seriously damaging the oil film and significantly increasing the wear rate of the cylinder liner and piston ring.
With the operation of the diesel engine, the cylinder liner will experience uneven wear, resulting in ovality and cylindricity in the circumferential and axial directions, respectively. When the piston ring moves inside the cylinder, it will undergo periodic opening and closing movements;
At the same time, there will inevitably be errors in the ellipticity of the piston ring and the cylinder liner.
Especially after replacing with a new piston ring, different degrees of light leakage will occur on the sealing mating surface between the piston ring and the cylinder liner. It is usually required that the total light leakage of the new piston ring installed in the cylinder should be less than 90 °, the continuous light leakage should be less than 30 °, and there should be no light leakage within the 30 ° range on both sides of the joint.
High quality new piston rings can meet the above requirements, but in recent years, many companies have used low-priced piston rings instead of original spare parts to reduce costs.
Due to differences in materials and processing techniques, as well as significant dimensional errors in manufacturing, there is excessive light leakage.
At the same time, the elasticity of the piston ring decreases too much when working at high temperatures. When encountering carbon accumulation and blockage in the ring groove, the leakage amount will increase, causing high-pressure gas to act on the outer working surface of the ring from the leakage point, causing the piston ring to wedge into the ring groove, which may cause the ring to get stuck in the ring groove and produce the phenomenon of holding the ring;
When the gas pressure decreases, the piston ring that was originally compressed will pop out again under its own elasticity.
This periodic repetitive action, over time, will result in fatigue fracture at weak strength points.
Practice has proven that the phenomenon of piston ring compression is the main cause of ring fracture.
In addition, it has been found in some diesel engines that the piston ring breaks when it touches the boss of the cylinder liner and the hanging air port at the ring end.
3. The influence of fuel and cylinder oil
The fuel used on board ships often changes due to changes in refueling ports, and the performance indicators of fuel from different countries and manufacturers vary greatly due to differences in origin and smelting processes.
If using a fuel separator for purification, corresponding specific gravity rings should be selected for different fuels, and the process should be carried out at higher temperatures (such as 95-98 ℃) as much as possible. Depending on the situation, the fuel separators should be connected in series or parallel, otherwise it will affect the fuel treatment effect.
If the aluminum, silicon, and ash content in the fuel is too high, it will cause increased wear on the piston rings and cylinder liners, which will inevitably affect the combustion process of the diesel engine and result in high temperatures inside the combustion chamber;
At the same time, due to some diesel engines operating at low speeds and low loads for a long time or frequently entering and exiting ports, without adjusting the cylinder oil volume, the cylinder oil injection volume increases. Excessive cylinder oil accumulates at the ring groove due to the pumping effect of the piston ring. When the temperature inside the cylinder is too high, the accumulated lubricating oil burns to form carbon deposits, which become the cause of piston ring adhesion and breakage.
4. The impact of daily maintenance and management work
Doing a good job in daily management plays an important role in avoiding faults such as piston ring adhesion and breakage.
1) After replacing the cylinder liner or piston ring in the diesel engine, it is necessary to maintain low speed.
Sufficient time for low load running in, while increasing the amount of cylinder oil injection during the running in period, can avoid overheating of the friction surface and causing cylinder pulling or piston ring breakage due to incomplete matching of the shape and roughness of the ring and cylinder liner surfaces during the running in period;
2) During normal operation, the temperature and pressure of the cooling water and cooling oil should be controlled and maintained within the normal range;
At the same time, pay attention to the changes in parameters, and the most important thing is the speed of change of each parameter;
Regularly measure the dynamometer chart, analyze the compression pressure, explosion pressure, combustion starting point, load distribution, etc., to determine whether the combustion process is good, judge the working status of the cylinder and piston ring, timely detect fault symptoms, and take corresponding measures based on the cause of the fault;
3) Regularly open the scavenging box to measure the clearance between the piston rings, establish a file for the use of each piston ring, and check the basic conditions of the piston rings, cylinder liners, cylinder oil injection, etc. through the scavenging port. If any adhesion or breakage of the piston rings is found, the cylinder should be lifted for maintenance in a timely manner.