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In order to seal the gases in the top of the cylinder and prevent their leaking down the side of the piston, piston rings installed in the grooves turned in the piston crown are used. It is desirable that the ring gap should be as small as possible, and it is equally important that it should never close completely.
In modern designs, we make use of the piston cleaning ring which is incorporated in the top of the cylinder liner. The piston cleaning ring has a slightly smaller inner diameter than the liner and hence scrapes off ash and carbon deposits built up on the piston top land.
Without such a ring, contact between the top-land and liner wall could wipe off the injected cylinder lubrication oil, preventing the lubricant from performing its optimized role. In some cases, deposits formation on the top land could cause bore polishing of the liner wall, contributing to deterioration of the cylinder condition. Introducing the piston cleaning ring eliminates contact between deposits on the top land and the liner, promoting an enhanced cylinder condition and lube oil performance.
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Reasons for Piston Ring Replacement
- If piston rings wear rate is such that it will be unsafe up to the next maintenance schedule.
- Is a piston ring is found struck in the ring groove and if it wears more on one side than another.
- If the axial height of the ring is reduced so that a large clearance is found.
- If performance piston rings have pressure variations around their circumference that are fitted in any groove and micrometer measurements show variation in the radial thickness of the ring.
- If the chrome layer on chromium plated piston ring is worn through or worn very thin.
Reason for Piston Ring Failure
- Insufficient piston ring and groove clearance, which causes the ring to jam in the groove at working temperatures, as a result blow-by occurs and the ring may break.
- Insufficient cylinder lubrication.
- Large amount of wear in cylinder liner.
- Excessive diametrical clearance between the piston and cylinder liner.
- Excessive wear on piston ring lading face in the piston ring groove.
- Ring gap too small, this usually leads to ring breakage, but could ultimately lead to disastrous consequences.
- Incorrect preparation of ends of piston ring adjusting gap.
- Radius at top and bottom of exhaust and scavenge ports in the cylinder liner inadequate, the ring then receiving a shock when sliding past the port edges.
- Wear on the port bars relative to cylinder liner working surface is such that surface of the bar is below the surface of the liner.
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Piston Ring Treatment
The pressure and temperature are greatest in the top rings, and these rings will have higher wear rates than others lower in the piston. The top ring which is subjected to the greatest load and pressure may be chrome plated or plasma coated. Plasma coating consists of a layer of very hard powered metals such as molybdenum and chromium fused to the rubbing surface of the rings.
Plasma coating is carried out with finely powered metal in a steam of gas so hot that the metal molecules become electrically charged causing fusion with the ring surface layer to form a very hard rubbing surface. Chrome or plasma coated piston rings must not be used in the chrome plated liners.
1. Chromium Plating
- On the running surfaces of compression ring and the sharp edges of scrapper rings.
- Used in four-stroke engines, but not used in chromium plated liners.
2. Plasma Coating
Spraying Elements at Plasma State are:
- Gas mixture – Nitrogen and Hydrogen or Argon gas at temperature 10,000 -15,000 C most materials and mixtures melt.
- Molybdenum – High resistance against scuffing but poor heat resistance,
- Copper – Suitable for running in period.
- Compounds of Cr, Ni, Mo, and Cr Carbide – High resistance to corrosion, wear, abrasion and scuffing.
3. Copper plating
4. Ferox coating and Phosphating
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In case of a short iron skirt of slightly larger diameter containing a lead bronze wearing ring is secured between the piston and piston rod.
- Wear rings are fitted into circumferential-machined grooves.
- They are usually fitted into two pieces with good clearance at the butts to alloy for thermal expansion.
- They provide a rubbing surface with low friction characteristics and they prevent the hot upper side-wall of the piston making contact with the working surface of the liner.
- Working clearance is maintained by wear ring.
- In trunk piston engines, the use of wear rings allow the detrimental effects of trunk distortion caused by the interference fit of gudgeon pins in the piston trunk. They are fitted above and below the gudgeon pin location.
Top Piston Ring Position and Working Condition
- Piston rings give the best result when their working temperature is the lowest practicable.
- So it is necessary for the top piston ring, which has the severest duty to perform, should be well clear of the hottest part of the piston.
- This then imposes a limit to the minimum distance from the top of the piston to the upper most grooves.
- The space formed between the side of the piston crown and the cylinder liner above the top piston ring is an area where carbon and ash from lubricating oil may encrust and build up.
- Working temperature should be as low as possible but too low temperature from carbon and ash deposits.
- If carbon and ash flake away causes rapid abrasive wear on landing face of piston ring as well as in liner.
- Chromium plated piston ring is fitted only in the top piston ring groove.
Which Material is Harder Between Piston Ring and Liner?
- Piston ring material is made harder than the material of cylinder liner in which they work.
- This advantage of giving the piston ring a long working life by reducing the radial wear rate.
How can the Life of Piston Ring and Liner be Increased?
Piston ring life can be increased with an increase in cylinder liner life by fitting the top piston ring groove with chromium plated piston ring. This increases the hardness of the surface that makes contact with the working surface of the cylinder, thus improves anti-friction conditions and consequently increasing piston ring and liner life.
Reasons for Failure and Replacement of Piston Rings
Looking for the Pistons used in Large Marine Diesel Engines , here in this series you will be able to get a clear picture and through knowledge on the piston cooling arrangements, operation troubles and piston ring problems.