Question

Give a reasoned opinion as to the accuracy of the following assertions:

(a) Absence of oil grooves in the liner wall between the oil supply points results in increased wear of liner and rings.

(b) 'Timed lubrication' has little merit,

(c) The most suitable position of the all supply point is immediately below the bottom piston ring with the piston at top dead centre

Answer 1

Part (a)

Absence of oil groove:

If there are no oil distribution grooves at the oil feed point, the oil tends to be scraped, locally upward and downward, which cannot spread evenly.
By this effect, in a narrow vertical band leading upwards and downwards from the feed point, there is more alkalinity excessive than required to nuetralise any sulphuric acid in the local area.
The surplus metallic salt, such as calcium carbonate, exposed to high temperature and mixed with other thermal decomposition compounds, tends to form abrasive compounds, such as calcium oxide.
In some cases this has result in serious vertical grooving, on the cylinder liner or piston rings or both, in line with oil feed point.
If there absence of oil grooves, the undesirable effect is, the area remote from the feed point is starved of oil, so that there is both corrosive and abrasive wear can be excessive.
The similar problem can be arise when operating with residual fuel with high carbon and asphaltene contents but with a very low sulphur content.
For the proper distribution of lubrication, around the cylinder liner circumference may be aided by provision of oil grooves, adjacent to the oil feed point.
The suitable downward sloping distribution grooves should linking up each other and to form one continuous groove, around the liner.
With oil grooves, they change the oil from intermittent flow into more continuous flow.

Part (b)

Timed lubrication is little merit:

For cylinder lubrication, to ensure that it is not wasted in the cylinder, the lubricant should be injected into the cylinder in carefully metered quantities, at the point between the piston rings as the piston passes over the feed points.
This involves carefully timed lubrication, which is most difficult to achieve in practice and required very rapid injection of oil at a pressure appreciably above that existing in the area of injection.
The best place to supply the cylinder oil is between the first and second compression rings. Using a conventional mechanical lubricator, the amount of oil supplied per stroke per quill is only 6.8cu.mm and the time available is equivalent to only 2 degree of crank angle.
Conventional mechanical lubricators of fairly simple design discharging through small bore pipes to the various feed points and various lengths, which could not be expected to be accurate enough to achieve the degree of fine timing required.
In some design, the non-return valve, usually of ball type, is located at the top of the lubricator, so the lubricator discharge pressure must exceed the cylinder gas pressure before oil enters the cylinder. The hot combustion gases tend to carbonize the oil and block the inlet holes and pipe.
In most suitable design, the non-return valve is located as near as possible to the feedhole in the liner. Due to air in solution and also possibly in suspension, the problem is the compressibility of oil at pressure, so timing of oil inlet is impossible.

Therefore timed lubrication has little merit.

Part (c)

By positioning the oil supply point immediately below bottom piston rings with the piston at top dead centre.

The place less likely affected by exposure to hot gases and to extreme pressure.
This also ensures delivery of lubricants at colder region of the cylinder liner, where away from the extreme temperature and pressure region.
If near the hot gases the oil tends to form hard deposits and eventually block the holes.
All the piston rings will pass through the feed points, improve oil distribution circumferentially, which lead to better vertically distribution and well lubrication.
The place not near the ports, so oil cannot be scraped, over the edge of the ports and cannot be blown away.
Proper gas sealing effect is achieved also achieve boundary lubrication.
At that time, piston speed is lower, therefore the lubricant should be delivered during this period. It is the most suitable position, in order to achieve following circumstances.
The lubricants should not be carried away to the combustion side by the movement of the piston.

✓ Verified Model Answer (Text Solution)

Absence of oil groove:

Timed lubrication is little merit:

By positioning the oil supply point immediately below bottom piston rings with the piston at top dead centre.