Apakah itu : "Piston Rings"

   "Piston Rings" 

Golden Points :

• Piston doesn't touch liner, only piston ring touches liner.
• Piston ring exerts small pressure against the liner.
• It rotates at very slow speed (with small angle), it gives good effect on circumferential distribution of lube oil.
• At TDC, rings are at slowest speed and highest temperature, that's why it should be of high tensile strength.
• Due to construction method, rings can not be perfectly circular, therefore coating is required.

Function :

• First and foremost function is to seal the combustion chamber i.e prevent leakage of gases & air.
• Make better heat transfer from the piston to cylinder wall.
• Regulates the oil used in engine by scraping. (oil scraper ring or control ring).

Material :

• Made of spheroidial graphite cast iron having higher strength and toughness together with a degree of malleability. Mo (for anti-creep) and Cr (for strength and anti-corrosive).

2-stroke & 4-stroke : Number of Rings ?

2-stroke piston - 4 rings

1. In 2-stroke, there are 4 rings - all are compression rings.
2. In 4-stroke, there are 3 rings - 2 rings are compression rings & 1 ring is oil scraper (oil control) ring. 

    Compression ring - provides the effective seal for combustion. Prevent blow-by. 

    Oil scraper/control ring - controls the how much oil goes up or goes down (scraps).

Ring Design :

(1) Compression Rings :

• Plain - simple & inexpensive
• Barrel - with chrome plated, better 
• Taper - faster bedding-in

(2) Oil scraper/control Rings :

• Single edge
• Double edge

Position of Top Rings:

Piston Ring, PCR, Top land

• Top land has been increased.
• Piston ring has gone lower.

Sulzer vs MAN B & W : 

• Sulzer uses coated piston rings but introduced SIPWA-TP (sulzer's integrated piston wear analysis- with trend processing).
• MAN B & W uses Controlled Pressure Relief (CPR) rings.

SIPWA : It's a method in which it monitors the actual wear on the top ring. It uses online feedback measurement of the piston ring wear condition.

CPR Ring :  (MAN B & W)

• Controlled pressure relief ring - top ring with relief grooves to maintain a regulated supply of gas pressure behind other rings.
• Fatigue loading decreased for other rings.
• Top piston ring is sealed and the relief grooves will supply gas to other piston rings.
• Other rings have coating and oblique cuts.

Special Rings :

1. Piston Cleaning Ring (PCR) : embeded in the top edge of the liner just below the cylinder head level. It is used for removing excessive carboneous deposits at top-land portion of the combustion chamber wall. (Deposits affects lubrication)
2. Anti-Polishing Ring (APR) : Polishing is harmful because it doesn't allow oil film retention. It reduces polished effects.

Ring Lubrication :

• Lubrication between liner and rings is essential.
• Either we use oil present in groove when engine reciprocates or use different liner lubrication methods.
• SIP (swirl injection principle) may used.

Clearance : 

Clearances

As piston ring is fitted into machined groove, it should be free to move. That's why clearance is provided.

• Butt clearance
• Axial/Groove/Gap clearance
• Back clearance

Problems :

1. Scuffing - (polishing - formation of local welding)
2. Flutter - (oscillation movement of piston ring along with its own plane)
3. Leakage 
4. Collapse