During scavenge space inspection, following items especially related to piston rings must be inspected:
- Check condition of sludge for any metal particle
- Check whether flap valves through which air enters scavenge space is free or not
- Check Piston Rings ( for good piston ring Condition)
When good and steady service conditions have been achieved, the running surfaces of the piston rings and cylinder liner will be worn bright (this also applying to the ring undersides and the “floor” of the ring grooves, which, however, cannot be seen). In addition, the rings will move freely in the grooves and also be well oiled, intact, and not unduly worn.
The ring edges will be sharp when the original roundings have been worn away, but should be without burrs.
Check Piston Rings for Micro-Seizure
Causes and symptoms of piston ring seizure
lf, over a period of time, the oil film partially disappears, SO that dry areas are formed on the cylinder wall, these areas and the piston ring surfaces will, by frictional interaction, become finely scuffed and hardened, i.e. the good “mirror surface” will have deteriorated.
In case of extensive seizures, sharp burrs may form on the edges of the piston rings.
Effect of piston ring seizure
A seized surface, which has a characteristic vertically-striped appearance, will be relatively hard, and may cause excessive cylinder wear.
Due to this hardness, the damaged areas will only slowly disappear (run-in again) if and when the oil film is restored. As long as the seizure is allowed to Continue, the local wear will tend to be excessive.
Seizure may initially be limited to part of the ring circumference, but, since the rings are free to “turn’ in their grooves, it may eventually spread over the entire running face of the ring.
The fact that the rings move in their grooves will also tend to transmit the local Seizure all the Way around the liner surface.
What to do in case of piston ring seizure?
lf seizures have been observed, then it is recommended that the cyl. oil dosage is temporarily increased.
- Check Piston Rings for Scratching
Causes or reasons of piston ring scratching:
Scratching is caused by hard abrasive particles originating from the ring itself, or, usually, from the fuel oil. As regards liner and ring wear, the scratching is not always serious, but the particles can have serious consequences elsewhere.
- Check Piston Rings for sticking
lf, due to thick and hard deposits of carbon, the piston rings cannot move freely in their grooves, dark areas will often appear on the upper part of the cylinder wall (may not be visible at port inspection). This indicates lack of sealing, i.e. combustion gas blow-by between piston rings and cylinder liner.
The blow-by will promote oil film breakdown, which in turn will increase cylinder Wear. Sticking piston rings will often lead to broken piston rings.
The free movement of the rings in the grooves is essential, and can be checked either by pressing them with a wooden stick (through the scavenge ports) or by turning the engine alternately ahead and astern, to check the free vertical movement.
- Check Piston Rings for Breakage/Collapse
Broken piston rings manifest themselves during the scavenge port inspection by:
– Lack of “elastic tension’, When
the rings are pressed into the groove by means of a stick – Blackish appearance – Fractured rings – Missing rings.
Piston ring breakage is mostly caused by a phenomenon known as “collapse”. However, breakage may also occur due to Continual striking against wear ridges, or other irregularities in the cylinder wall.
Collapse occurs if the gas pressure behind the ring is built up too slowly, and thereby exerts an inadequate outward pressure. In Such a case, the Combustion gas can penetrate between the liner and ring, and violently force the ring inwards, in the groove. This type of sudden “shock” loading will eventually lead to fracture — particularly if the ring ends “slam” against each other.
- Piston Rings: Blow-by.
Leakage of Combustion gas past the piston rings (blow-by) is a natural Consequence of sticking, collapse or breakage in the later stages, when blow-by becomes persistent, it is usually due to advanced ring breakage, caused by Collapse.
Blow-by is indicated by black, dry areas on the rings and also by larger black dry zones on the upper part of the liner wall which, however, can only be seen when overhauling the piston (or when exchanging the exhaust valve.
- Deposits on Pistons
Usually some deposits will have accumulated on the side of the piston Crown (top land). Carbon deposits on the ring lands indicate lack of gas sealing at the respective rings.
If the deposits are abnormally thick, their Surfaces may be Smooth and shiny from rubbing against the cylinder wall. Such Contact may locally wipe away the oil film, resulting in micro-seizure and increased wear of liner and rings.
- Lubricating Condition
Note whether the “oil film” on the cylinder wall and piston rings appears to be adequate. All piston rings should show oil at the edges.
White or brownish coloured areas may sometimes be seen on the liner surface. This indicates corrosive wear, usually from Sulphuric acid and should not be confused with grey-black areas, which indicates blow-by.
In such cases it should be decided whether, in order to stop such corrosive attack, a higher oil dosage should be introduced
Replacement of Piston Rings
When piston rings should be replaced?
It is recommended that the complete set of piston rings is replaced at each piston overhaul, to ensure that the rings always work under the optimum service conditions, thereby giving the best ring performance.