Answer EKM Question 47
(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 recent developments in the piston rings to minimize breakage. Development of Piston Ring Packs:
The present design of piston ring packs and cylinder liners represents a development driven by environmental regulations, reliability issues and operating costs of our two-stroke engines.
The piston ring design has been developed from a simple oblique-cut piston ring into an advanced design with a gas-tight lock and controlled leakage grooves. This has led to a strong reduction of the maximum temperatures experienced by the rings. Thus the risk of ring collapse or ring breakage due to thermal overload has been eliminated. The mechanical load of the piston ring is determined by the forces acting on the ring. One key factor in this regard is the differential pressure over the rings. The ring pack seals against the combustion pressure. In order to share the load between the rings, a certain gas flow is required. With regard to the oblique cut ring, this gas flow results in a relatively high thermal load around the ring gap.
This has various negative effects:
a. Lube oil in the position of the ring gap experiences a high thermal input, resulting in a lack of oil locally on the liner surface.
b. High surface pressure between the liner and the ring at the ring gap due to uneven thermal expansion of the ring.
c. Gas jet from the top ring gap leads to local thermal overload of ring No.2.
The local effect of the above is strongly reduced by the CPR ring design, where the ring gap is gas tight and the gas flow is divided into 4 or 6 CL grooves.
The running-in properties were strongly improved with the implementation of a copper-based running-in layer, enhanced with aluminium oxides. The brand name is Alu-coat. In 2003, chrome-plated ring undersides on rings 1 and 2 were introduced. This finally concluded the ring groove wear issue. Up to 2003, we had experienced a number of cases with too short time between overhauls, due to the ring groove wear. The latest update of the ring pack relates to the running surface, which has been upgraded by applying a cermet coating. A cermet is a composite material consisting of ceramic and metal. This gives the optimal properties of ceramic, such as high temperature resistance and hardness, combined with the metal properties, such as the ability to undergo plastic deformation. The cermet used consists of chromium carbides as the ceramic part, and molybdenum, nickel and chrome as the metallic part. The high melting temperature and the high hardness of the chromium carbides leads to an increased margin against scuffing, and low wear of the coating. The low wear of the coating ensures long time between overhaul.
Comments
Post a Comment