Piston Rings and Piston Ring Maintenance
The primary function of the piston ring set is to seal the combustion chamber against leakage of compressed air and combustion gases. The effectiveness of the leak-free seal determines the cylinder overhaul period. In many cases, the piston rings’ function limits the engine’s power output. The piston ring surface must be in complete contact with the liner surface and the bottom of the groove throughout its depth and circumference.
The majority of diesel engine problems are due to poor piston ring operation. In the manufacture of piston rings, gaps are cut into rings machined from oversized cast barrels. When the ring is pushed into the hole in the liner, the opening is reduced to the calculated small dimension.
The ring applies a slight contact pressure to the surface of the liner. The spring action of the ring is the primary seal, encircling most of the circumference but leaving the area adjacent to the open end exposed with minimal bending moment. The surface of the inner ring is artificially stressed towards the edge of the aperture to bring the free edge closer to the surface of the liner. A secondary seal is created by increasing gas pressure at the back of the ring, allowing high-pressure gas to pass through a small gap and into the space behind the ring.
The gas pressure also flexes the ring, creating mating contact between the surface of the cylinder liner and the shoulder at the bottom of the groove, where a sealing surface is formed. The operation of a ring set can be compared to a labyrinthine seal consisting of a series of openings followed by expansion chambers. The opening is the small clearance of the rings in the groove and the expansion space behind each ring.
Due to the design process of piston rings, it is difficult to make them perfectly circular to match the bore of the cylinder liner. So coating the outer surface of the ring is not a new idea for the break-in process. Perfect sealing is achieved after the coating has run under a light load for some time only.
Types of piston rings
Compression Rings
The primary function of the upper compression ring is to block most of the combustion gases, allowing maximum power output from the engine.
These are fitted in grooves on the top. It also transfers heat from the piston to the cylinder wall. These rings have different cross-sections. The following types of compression rings are used:
• rectangular ring
• Tapering
• Barrel ring
• Bevel ring
• Keystone ring
Wiper ring
A wiper ring is a piston ring with a tapered surface that fits in an annular groove below the compression. Scraper rings seal the combustion chamber and wipe excess oil off the cylinder walls. The scraper ring stops combustion gas that has passed through the compression ring.
Oil scraper ring
The oil scraper ring is located in the lower piston groove. The primary purpose of an oil control ring is to scrap the excess oil from the liner and drain it back to the oil sump during the downward movement of the piston. It prevents the oil from reaching the combustion chamber.
These rings are also used to evenly distribute the oil around the liner. Oil is sprayed onto the cylinder wall.
Drainage holes or slots are furnished to save oil from gathering among the 2 outer lands.
Scratching of the ring faces will cause incomplete sealing of the power cylinder and can cause excessive oil consumption. Possible causes of this type of condition: Leaks in the air intake system. Abrasives in the engine oil
Single-Piece Oil Control Rings
These rings are designed to prevent oil from leaking through the space between the ring face and the cylinder.
Slotted Oil Control Ring:
A slot in the radial center of the ring allows excess oil to flow back into the reservoir, controlling the amount of lube up and down the cylinder wall. The width of the parallel ridges is smaller than the overall width, so the unit pressure is higher.
Chamfered Oil Scraper Ring
In this ring shape, the outerland is chamfered (or chamfered) and the contact pressure between the ring and liner is higher than the slotted oil scraper ring.
Oil scraper ring with double bevel edge
The ridges on the edge connecting the burn champ are both chamfered in this type of ring. Chamfered edges reduce oil consumption by improving chipping of chamfered edges.
Two-part oil scraper ring:
Multi-piece oil scraper rings are used more often than single-piece scraper rings in modern engines to meet today’s higher performance requirements for ring flexibility. The two-piece oil control ring is manufactured with heat resistant spring steel around the ring to maintain greater pressure and contact with the liner while maintaining oil economy. Place the coil spring in a semi-circular or V-shaped groove to reduce the moment of inertia.
Slotted oil control ring with coil spring
Like the slotted oil control ring, the coil spring oil control ring has two parallel outer lands. This ring also has a groove inside the diameter to accommodate the coil spring. There are many variations of coil spring oil control rings.
Piston Ring Lubrication
Lubrication of the cylinder liner surface and the piston ring in the ring groove is essential. The position of the top piston ring should be in the liner’s cooling zone up to the top ring’s maximum stroke so that all rings are sufficiently lubricated. The top ring should be protected from direct heat flow. Even with a thin layer, lubricating oil can lubricate the ring surface and seal effectively. As the piston reciprocates, the piston rings slide into the grooves. The ring also flaps under dynamic conditions caused by a pressure differential across the back surface of the ring.
MAN Piston Rings for Operation on Low-Sulphur Fuels
Cermet-Coated Piston Rings for Operation on Low-Sulphur Fuels (0.5% S or lower)
To increase the margin against damage to the piston rings and cylinder liners, which will lead to overhaul on short notice, M.A.N. have introduced Cermet-Coated Piston Rings for Operation on Low-Sulphur Fuels for two-stroke marine diesel engines. Type: MC/MC-C, ME/ME-C, ME-B, MEGI, ME-GIE and ME-LGI.
Configuration of these piston rings incorporates the hard cermet coating on the running side of all piston rings. Cermet-coating on the piston ring running surface increases the reliability of the main engine. In many cases, the cermet-coated rings continue to perform after an episode with momentary oil film breakdown, provided the oil film is restored. This ensures that the engine can continue in normal operation until the next planned overhaul.
Mounting and disassembling piston rings
- Gently removes stubborn dirt from used pistons. Take special care to keep the ring groove free of carbon and dirt. Use a drill or another suitable tool to clean the oil drain hole.
- Be careful not to damage the sides of the groove when removing the carbon. The bottom groove side is the sealing area. Scratch damage can lead to increased oil consumption and blow-by gas emissions during engine operation.
- Be sure to use piston ring pliers to install and remove piston rings. Other tools, such as wire loops and screwdrivers, will damage piston rings and pistons.
- Do not pull the ring by hand (Exception:Double bevel oil scraper ring made of steel shell segments). There are risks of ring breakage, bending and over-stressing, and the risk of ring breakage and injury from sharp ring edges.
- Opening the ring more than necessary risks overstressing it. Overstressing the ring will permanently be distorted and no longer conforms to the inner cylinder running surface.
- Rotate the piston rings so that the ring gap is 180° away from him to prevent gas leakage due to ring gap collapse.
- Top” mark of piston rings should be assembled in upper side of groove.
- If piston rings exceed the wear limit and/or if the piston has any abnormal mark, then replace by new one.
Life of piston rings
For large 2-stroke piston rings with a bore of about 900mm, the total ring life is up to 24,000 hours, and for smaller 500mm bore engines, up to 16,000 hours. High-speed 4-stroke auxiliary marine engines typically have shorter piston ring life than 2-stroke engines.
Reasons for Piston Ring failure
1) Cylinder oil: A low feed rate of lube oil to causes piston ring seizure, excessive wear, worst case piston seizure, too high feed rate causes hard carbon deposits and piston ring sticking in the groove. If the feed rate is too high, do not reduce it immediately. While watching the condition of the piston rings, gradually reduce the amount.
2) Charge air temperature under the piston: If a particular cylinder temperature is 3°C or higher than the average of the other cylinders, that cylinder is considered blown through. In these cases, blow-by and ring failure have occurred, and immediate inspection should be performed to positively assess the piston rings’ condition.
3) Excessive ring groove wear: This increases the stress on the piston rings and is a significant cause of failure. Once the piston is rebuilt, measure the groove and replace it with a replacement crown if necessary.
4) Remove the wear collar: The upper limit of the liner surface, the step just above the top piston ring’s top edge, is called the “wear ridge or collar.” Remove worn burrs before pulling out the piston with a special tool. Note:
Do not use hand sanding without a positioning device, as you risk damaging the surface of the liner.
5) Precautions when installing new rings: Use special tools that minimize the widening of the annular gap. Suppose the annular gap is too large during installation. In that case, the stresses associated with the annular gap become too high, and plastic deformation in this area can lead to piston ring failure.
6) Running-in after overhaul: Manufacturers recommend following the RUNNING-IN process when replacing piston rings and liners.
7) Good Combustion: Knocking, improper atomization, insufficient combustion charge, and carbon deposits lead to premature piston ring failure.
8) Cylinder liner wear pattern: When the wear limit of the liner is approaching and replacement of the liner is warranted to improve performance.
9) Pre-lubrication: At the departure port, the engine is slowed down for manual lubrication. Excessive lubrication fills the grooves in the ring, preventing it from moving freely and causing fatigue cracks.
10) Regular contamination checks: Debris deposited between the grooves of the ring encourages cracking of the ring each time the ring is checked for contamination.
11) After every long run: You should check the condition of the piston rings/cylinder liners through the flushing port and replace them immediately if any cracks or deformations are found on the piston rings. If the error recurs, repeat the steps.
12) Temperature and pressure: The cooling temperature of the jacket was always maintained above 80°C.
13) Difference between scavenge air temperature and air cooler: The scavenging /supply air temperature should be 500 C, and if equipped, the heater should be used effectively during maneuvers. Intercooler drop, max allowed 300mm wp. Rinse chamber Keep the condensate outlet clean at all times.
14) Cylinder Liner wall temperature recorder (if equipped): If the cylinder liner wall temperature rises, the piston rings should be inspected and replaced as soon as possible.
15) Liner wears: Wear can occur for many reasons. Some reasons are High fuel sulfur content due to loss of lubricating film and sludge formation, the temperature rising to the melting point, leading to ring, liner, and even crown cracking.
16) High firing pressure or knock while maneuvering or reversing: The reason for this occurrence may be part of the following reasons: Fuel injected too early before the top dead center, fuel oil temperature too low, fuel limiter start and control error, accumulation of water or fuel oil in the combustion chamber, or faulty fuel injectors can cause this. The reason is due to such a situation. This severe ignition slap is another cause of crushed or broken piston rings, a problem that must be rectified immediately.
17) Piston ring gap: The piston ring gap is too narrow. Less ring clearance usually causes the ring to break, which can have serious consequences.
18) Piston and liner clearance: Excessive or uneven diametral clearance between piston crown and cylinder.
19) Landing of Piston Rings: Sections of the piston ring can experience high localized wall pressure on the liner during the early stages of bedding, resulting in oil film breakdown in the region of the highly stressed portion of the piston ring and subsequent seizure. can occur.
20) Piston ring butt end: Improper preparation of piston ring ends where the connection point is adjacent to a relatively wide gap.
21) Radius at Scavenge Port: Due to short radii at the top and bottom of the cylinder’s exhaust and scavenging ports, the rings are impacted as they pass the ends of the ports.
Other reasons for ring failure
- Maintenance of parameters such as fuel quality, fuel separation criteria (treatment, centrifugation, filtration), viscosity, pressure, temperature, and, in older containers, steam systems associated with high-pressure lines to maintain fuel temperature; factors must be considered.
- Excessive fuel attrition caused by catalyst particulate matter can be isolated mainly by washing and microfiltration. If water mixes with fuel or seawater, it won’t be easy to separate the catalyst fines during centrifugation. These fine catalytic particles are embedded and appear as white specks of metallic sheen that promote uneven high wear at the TDC location on the top piston ring. Because of these solid vertical scratches, newly installed piston rings tend to wear and fail quickly. Before installing the new ring, you must remove any cat particles embedded in the liner with a honing machine.
- Asphalt burns poorly and promotes poor combustion, sticking or breaking piston rings, increasing deposits in the combustion chamber, exhaust manifold, and turbocharger pump contamination. Additives and intensive fuel injection maintenance are the only solutions available.
Blog Conclusion
We hope you enjoyed our blog post about Piston Rings and Piston Ring Maintenance! This blog post gives a very simple overview of piston rings and piston ring maintenance, as well as do’s and don’ts that we hope will help you learn more about piston rings and piston ring maintenance. Our blog post also touches on some key terminology around piston rings and piston ring maintenance so the readers are able to fully understand the topic. If you have any questions or comments, please let us know by visiting http://sh057.global.temp.domains/~arinedie/. Thank you for reading, we hope this blog post has helped you learn more about Piston Rings and Piston Ring Maintenance.
