Cylinder lubrication in a low-speed main propulsion diesel engine:
Cylinder lubrication For marine diesel engines operating on residual fuels containing sulphur, cylinder lubrication must generally serve the following purposes:
■ Create and maintain an oil film to prevent metal to metal contact between the cylinder liner and piston rings.
■ Neutralise sulphuric acid in order to control corrosion.
■ Clean the cylinder liner, and particularly the piston ring pack, to prevent malfunction and damage caused by combustion and neutralisation residues.
Cylinder lubricating oil for a low-speed main propulsion diesel engine is admitted to each cylinder during the compression stroke. Cylinder lubricating oil, for lubricating the piston rings and the liner, has to be admitted when the piston, piston rings and the liner are in cool condition and the piston is moving upward so that oil can be retained on the piston rings and sprayed by the piston rings on the liner walls. This is only possible during the compression stroke. Otherwise, the piston is hot and if the lubricating oil is sprayed on it, it will evaporate very fast and will not carry out any work of lubrication. At the same time, if lubricating oil is injected during the expansion stroke, i.e. when the piston is moving downwards, it will have a scrapping effect rather than lubrication.
Cylinder Lubrication in four-stroke trunk piston engine:
In four-stroke trunk piston engines, there are a number of different methods for lubricating the cylinder liners and piston rings, depending on engine size and make:
■ Splash from the revolving crankshaft
■ “Inner lubrication”, where the oil is supplied from the piston side
■ “Outer lubrication”, where the oil is supplied by an external, separate cylinder lubricating device from the cylinder liner side.
In a four-stroke trunk piston engine, the cylinder lubricating oil is identical to the engine system oil used for bearing lubrication and cooling purposes.
A small amount of the cylinder lubricating oil by-passes the piston rings and ends up in the combustion space,
where it is “consumed”. However, the piston in a four-stroke trunk piston engine has an oil scraper ring that scrapes most of the oil supplied to the cylinder liner back to the engine’s oil pan, from where it is drained, cleaned and recycled.
Normally, a large, modern, well maintained four-stroke trunk piston diesel
engine will consume some 0.3 to 0.5 g/kWh of lubricating oil.
Type of Oil Used in Cylinder Lubricating System
- The cylinder lubricant must be of a higher viscosity so that it can form a good lubricating film between the liner and the piston rings.
- It must also withstand the heat variations in the combustion area and must deal with the combustion products.
- Under normal running conditions this oil will typically be an alkaline cylinder lubricating oil of SAE 50 viscosity.
- The alkalinity is indicated by the TBN (Total Base Number ) rating of the lubricant. The TBN value most suitable for the cylinder lubricating oil depends largely on the sulphur content of the fuel used. Typical values for sulphur content of 0.5 to 1% may be between 20 to 25 TBN. For sulphur content over 1.5% the TBN number may be 70 or higher.
Using the Correct Feed Rate for Cylinder Lubrication
Once the correct lubricating oil is chosen the correct feed rate must be established in accordance with the engine builder’s recommendations.
- The feed rate has a critical effect on good engine operation apart from the question of oil consumption. With a too low feed rate the danger of the oil film breaking down causing blow by or additional wear is increased.
- Too high a feed rate is a waste of lubricant and money. The correct feed rate will allow the formation of the lubricating film between the liner and the rings and will give maximum protection at the piston reversal points.
The cylinder oil consumption burette is a useful means of checking the oil consumption of individual lubricator boxes to help ensure that the oil is distributed across the boxes as intended.
The volume between the two internal discs is 1/2 litres. Given the temperature density characteristics of the oil, the actual mass of the oil during
its use in engine calibration can be calculated from the oil temperature. Calibration time lies typically between 3 10 minutes depending on the oil consumption rates and the speed/power of the engine, (if the oil feed drive is speed/power dependent).
In slow speed operation, the use of heavy fuel oil with high sulphur content makes the job of the cylinder lubricant very difficult. Even high alkalinity oils cannot hope to neutralise all the sulphuric acids which are produced during combustion.
Effect of Under Lubrication and Over Lubrication of Cylinder:
A correct viscosity is important in order to ensure the spreadability of the cylinder oil, and the applied feed rate and injected amount of oil per stroke are key factors in the delicate balance between under- lubrication and over- lubrication:
■ Under-lubrication
If too little cylinder oil is supplied, starvation will occur which might result in corrosion, accumulated contamination from unburned fuel and combustion residues, and in the worst case, metal to metal contact, known as “scuffing”.
■ Over-lubrication
If too much cylinder oil is supplied, the loss of fresh, unused oil in the scavenge ports
will be high, and the piston rings might be prevented from moving (rotating) in their grooves by the so called
“hydraulic lock”. Furthermore, the cylinder liner running surface structure might over time become closed and smooth like a mirror, and will no longer be able to retain the lubricating oil. This is sometimes called “chemical bore polish”, and when alkaline deposit build-up on the piston top land from excessive cylinder oil is in contact with the cylinder liner running surface, it can cause what is sometimes called “mechanical bore polish”. All of these phenomena might eventually result in scuffing.
Acid Condensation in the combustion chamber
The cooling system must be operated so that the piston and cylinder liner temperature is not dropped below the temperature at which the Sulphuric acid may condense on the cylinder liner.
Acid condensation depends on:
• the engine combustion pressure
• the liner temperature
• the concentration of the sulphur oxides
• the humidity of the intake air.
So, to help the lubricant in neutralising the acid, the engineer must ensure that the temperature of the scavenge air should be maintained in accordance with the manufacturers’ recommendation. Too low a scavenge air temperature will result in condensation with the risk of moisture entering the cylinders; too high a scavenge air temperature will adversely affect the combustion characteristics of the engine.
Engine Runing-in
Critical to this lubrication area is the way the engine has been run in at commissioning. A good run in procedure will create a good wear in of the cylinder liner and piston ring. A good gas seal is obtained between them whereby a thin oil film provides reliable and effective lubrication.
The period and method of running in should be decided upon in accordance with the engine manufacturer’s recommendation. Even if only new rings have been fitted the running in procedures should be as near as possible to that recommended for new engines.
The running in recommendation may specify the use of a particular type of lubricant and the feed rate should be high. After running in, the normal cylinder oil will be used and the feed rate gradually adjusted until the recommended feed rate is reached.
So, the cylinder lubricating oil must create a lubricating film between the piston ring and the liner, and must maintain effective lubrication. It must also combat corrosive wear. The use of the correct lubricant and the correct feed rate for the engine load will help to achieve the best result from the lubricant.
Lubrication Of Medium Speed Trunk Piston Engine
In medium speed diesel the cylinder is open to the crank case. This means that contamination of the crank case oil by combustion products requires the oil to be different in character to that which may be used in a slow speed engine. Generally, the lubricant must:
* create and maintain effective lubrication between moving components under high mechanical and thermal loads;
* transport solid contaminants from the cylinder to the cleaning devices, such as filters and centrifuges;
* withstand heat; fight contamination, corrosion and wear; resist oxidation and thermal breakdown; keep the engine clean.
References:
1. www.marinediesels.info
2. The Running and Maintenance of Marine Machinery – Cowley
3. Reeds Marine Engineering Series, Vol. 12 – Motor Engineering Knowledge for Marine Engineers
4. Lamb’s Question and Answers on Marine Diesel Engines – S. Christensen
5. Principles and Practice of Marine Diesel Engines – Sanyal
6. www.wartsila.com