Answer EKM Question 20

Q. With reference to piston rings;
A. Analyze the causes of breakage;
B. How maintenance and engine operation can minimize breakage;
C. Explain the possible consequences with respect to performance and safety of operating the engine with broken or severely worn rings.
Answer: A. Causes of Breakage: 

 

(1) Piston ring collapse:
(a) It is the 'collapse' i.e. inward push of the ring against the piston body due to gas pressure build up against the 'running face' of the ring.
(b) It is caused by the pressure build up against ring running face and liner wall due to
(i) Reduced axial clearance;
(ii) Poor ring and groove sealing;
(iii) Rings not free to move in the groove;
(iv) Poor lubrication on sealing surfaces.
In Fig. A, the reduced-axial clearance reduces the gas pressure P1, building up behind the ring to form a reduced P2 ring pressure.
In Fig. B as P2 increases slowly P1 gets between the liner and the ring.
In Fig. C,,the ring collapses against the piston groove body.
(2) Flutter
(a) flutter is the oscillation movement of the piston ring along its own plane.

 

(b) It is caused by a radially worn ring leading to a reduction in radial areas, or pounding of piston rings in the grooves when the piston changes its direction.
(c) In Fig. A, pressure P1 decreases at the same rate as the cylinder pressure, while ring pressure P2 falls at a slower rate than the cylinder pressure.
(d) In Fig. B, when P2 suddenly becomes more than P1, movement occurs since P2 changes and this causes a flutter.
(e) In both figures, observe the first piston ring fluttering and moving up' and down in its own place.
(3) Excess wear: This is due to poor clearances, corrosion, abrasion, scuffing or improper lubrication.
(4) Jammed or sticking piston rings: This is due to the build-up of carbon deposits or poor clearances.
(5) Scuffing: It is the overall damage on the sliding contact surfaces, caused by the formation of local welds. These welds occur due to high local temperature (800 deg C +) which harden the base metal forming hardened particles at that point.
Scuffing depends on:
(a) Oil film quantity, oil retention and countered rings lo promote oil film generation.
(b) Rotating pistons moving around any of the dry hot spots which are prone to welds.
(c) High temperatures due to poor sealing or poor heat transfer by bore cooling.
(6) Other operational reasons:
(a) Fatigue loading of rings due to excessive differential pressure across the ring.
(b) Ridge formation on liner, ridge formation on scavenge ports, may also result in ring breakage.
(c) If the scavenge ports do not have adequate radius at the top and the bottom, the rings experience a shock load as it passes past the ports.
(d) Overloading of engine i.e excessive peak pressure, i.e (Pmax — Pcom > 35 bar) can lead to fatigue loading and ring collapse.
(e) Sub standard piston rings will lead to early failure.
(f) In-sufficient cylinder lubrication.
(g) Corrosion caused due to the sulphur content in the fuel.
(h) Abrasive particle, poor quality fuel etc

B. State how maintenance and engine operation can minimize breakage.
Maintenance care:
(a) Following the manufacturer recommendation and maintenance routines according to the PMS and frequent inspection helps prevent piston ring failures.
(b) Most of the piston rings damaged while installing it. Proper installation procedure shall be followed while installing.
(c) During overhaul, piston ring grooves shall be thoroughly cleaned and groove gap clearance to be checked.
(d) The condition of the rings and groves should be noted; location of any wear shall also be noted.
(e) Ring gaps should be checked by fitting them alternately 180° apart.
(f) The condition of the liner shall be checked, if the liner is oval it shall be changed to avoid causing damages to the piston rings.
(g) If the liner is glazed or polished it shall be honed properly.
(h) In new engines the top ring is CPR (Controlled pressure relief) ring, it releases the high pressure in a controlled manner avoiding shock loading & fatigue.
(i) Proper running-in procedure shall be followed after installing a new piston and piston rings.
(j) Ring clearances are to be taken and ensured in proper allowance.
Operational Care:
(a) T/C Suction filter, air cooler and water mist catcher must be kept clean in-order to avoid dust and other abrasive particles to enter the engine.
(b) Maintain proper fuel pump timing.
(c) Fuel injectors to be overhauled at regular intervals
(d) Avoid over loading of the engine
(e) Avoid inferior quality fuel or mixing of sludge or by passing the fuel oil purifier etc.
(f) Peak pressure to be in optimum limit, adjust VIT properly.
(g) Maintain correct fuel oil viscosity and temperature.
(h) Sudden change of engine load and drastic change of cooling water temperature (High variation in JCW temperature.) to be avoided.

C. Explain the possible consequences With respect to performance and safety of operating the engine with broken or severely worn rings.
(a) Low compression and peak pressure.
(b) High exhaust temperature.
(c) Blow past, increase in scavenge temperature, accumulation of sludge and scavenge fire.
(d) Crankcase explosion due to blow past and leaky crosshead glands.
(e) Scuffing, cracks development on the liner surface.
(f) Drop in power developed.
(g) Drop in thermal & overall efficiency.
(h) T/C surge due to excessive blow past.
(i) Safety & fire hazard.
(j) 4-stroke engine is adversely affected by L.O quality deterioration, crankcase explosion etc
(k) Decreased turbocharger efficiency and scavenge air pressure
(l) Poor fuel injection
(m) Insufficient cylinder lubrication