Two-stroke main engine cylinder lubrication explained…

Cylinder lubrication is load dependent and the system is designed to deliver a specified quantity of lube-oil in the cylinder liner, of which main purpose is to reduce corrosion and friction and to increase the sealing effect between piston ring and the cylinder liner. Because today’s cylinder oils have complex chemistry, the particular feed rate must be determined for each oil brand, viscosity class, and BN level. A cylinder oil is blended to obtain the requisite detergency and dispersancy to keep the piston rings and crown clean, as well as the required base number (BN) to neutralize the acids created during combustion. Systems which are reliable and which can consistently sustain heavy duty loads are required for cylinder lubrication of 2-stroke large diesel engines for use as drive units for ships or generators.

Cylinder lubricating oil is supplied to each cylinder liner at points around the liner circumference, there being a single row or, on very large 2-stroke diesel engines, an upper row and lower row of lubricator quills. Lubricating oil from the cylinder lubricator pump is distributed to the row or rows of quills and on some of the engine models the cylinder lubricator unit also acts to lubricate the exhaust valve spindle of the associated cylinder.

The cylinder lubricator can be a kind of reciprocating specific volumetric pump with plunger and barrel (Vogel type) or a pulse jet type (Alpha lubrication, Hans Jensen). The latter is more common nowadays, on modern vessels as the system ensures improved distribution of cylinder oil on the running surface of the cylinder liner, with precise feed timing and dosage of lubricating oil.

Cylinder lubrication on 2-stroke engine. Source and credit: Supreme_Engineer
Alpha lubrication. Source and credit: The Engineer
Hans Jensen lubrication. Source and credit: Hans Jensen Lubricators

The quills are fitted with accumulators which maintain oil pressure and control the injection of lubricating oil into the cylinder. Should the accumulator fail, the oil delivery will no longer be controlled by the pressure within the associated engine cylinder but is controlled by the delivery stroke of the lubricator pump.

A single electric motor drives all cylinder lubrication pump modules, there being two pump elements or plungers for each cylinder, one for the upper quills and one for the lower quills. Oil is supplied by the pump plunger to a progressive distributor block and the distributor block supplies the oil to the associated lubricator quills. The oil supply from pump/distributor recharges the lubricator quill`s accumulator. The lubricator quill releases a quantity of oil into the cylinder depending upon the cylinder pressure. When the pressure at the quill injection point falls below the accumulator pressure oil flows into the cylinder through the quill. When the cylinder pressure rises above the accumulator pressure the non-return valve in the quill closes and stops the flow of oil to the cylinder. For each cylinder a piston distributor, with two outlets, is fitted after the lower level distribution block and this allows part of the oil for one quill to be directed to lubricate the exhaust valve spindle.

The Pulse Jet lubricating system involves the spraying of cylinder oil above, onto and below the ring pack from a single or two row of quills arranged around the liner, each quill having a number of nozzle holes. The quills are simple non-return valves.

Source and credit: WinGD – Winterthur Gas & Diesel Ltd

The oil jets are individually directed to separate points on the ring pack as it moves past the quills. There is no atomisation and negligible loss of lubricating oil to the scavenge air. Cylinder lubricating oil is delivered to the quills by a lubricator pump which is connected to the engine chain drive shaft via coupling or, on modern common rail engines, powered by the engine’s pressurized servo oil system. The feed rate can be adjusted by increasing or decreasing the stroke of the plungers (which can be changed by the adjusting bolts) or, on modern engines, is electronically controlled through a solenoid valve at the lubricator pump. The timing is adjusted by means of timing gauge, where a special mark is stamped for the adjustment or, on modern engines, is electronically controlled in which case there is full flexibility in the setting of the lubrication timing point, and the volumetric metering ensures constant spray pattern across the engine’s load range. In this last case, the dosage is precisely regulated even for low feed rates. Normal cylinder lubrication is load dependent and this is controlled by the engine control system. The amount of cylinder lubricating oil going to each cylinder can be individually controlled.

Cylinder lubricating oil differs from the oil used for crankcase lubrication as it needs to neutralize the acid products of combustion, maintain an oil film at conditions of high temperature and pressure, and keep the liner surface clean at all times. It is essential that the correct cylinder lubricating oil is used for the fuel being burned. Nowadays, when very low sulphur fuel is used, attention must be paid with regard to cylinder oil TBN (total basic number) as running the engine with high TBN oil while using very low sulphur fuel can create high alkaline deposits between piston rings and their grooves, leading to piston rings being stuck and ultimately their failure. Moreover high deposits will be carried over between piston rings and cylinder liner and will damage them both due scuffing. In order to simplify the lubrication process onboard the ships, as well the logistics of supply, the oil companies started the process of developing a cylinder lube oil that can lubricate the cylinders regardless of the sulphur content in the fuel. Such oils have BN levels that are lower than the traditional BN 70 cylinder lube oils and can be used on engines that are not affected by cold corrosion, but are not applicable on newer engine designs with higher levels of cold corrosion.

Cylinder lubrication is normally automatically controlled via the engine control system, but a manual operation is also possible in case the control system is shut down or during engine maintenance.

Emergency cylinder lubrication is available and this is started automatically when normal cylinder lubrication control fails, provided that the emergency lubrication switch at the cylinder lubrication starter panel is set to the AUTO mode. Under emergency lubrication load control of the cylinder lubrication is no longer available and the feed rate is set to 100% of the normal lubrication output.

When the engine has been idle for a long period of time or if any part of the pump or system has been replaced with a new part, it may happen that the remaining oil may not be pumped at all or only small amount of oil may be discharged due to accumulated air in the pump. In such cases, it is necessary to bleed the pump manually.

In conclusion, it is essential to remember that correct cylinder lubrication is essential to efficient engine operation, minimum lubricating costs and optimum maintenance costs. It is essential that the cylinder lubricators are correctly set and the correct cylinder lubricating oil is used for the fuel being burned and to prevent rapid wear of cylinder liner.

Used oil taken from the engine through the scavenge bottom drain can be used for cylinder condition evaluation and drain oil analysis is also a strong tool for judging the engine wear condition. In general, the oil feed rate must be kept and optimized while keeping the BN above 10-25 mg KOH/g and the iron (Fe) content below 200 mg/kg in the drain oil, but it is important to note that high iron values may be experienced during piston ring running-in when coating gradually wears off.

When the vessel is slow steaming, the engine is operated at low load, and the liner surface will become cooler and, therefore, increase the risk of corrosion. The feed rate should be re-assessed and adjusted accordingly.

2-stroke engines require extra attention and lubrication during their first running hours. The first 500 running hours are the most demanding, as this is the period where the liners are run in, which is also referred to as the breaking-in period. The purpose of the breaking-in period is to flush away wear particles and facilitate running-in of the liner surface and rings. Cylinder liner and piston ring breaking-in takes 500 running hours maximum and during this period, the running-in coating on the piston rings will gradually wear off, and the cylinder liner surface will smoothen. During this process, extra lubrication oil is required to flush away wear particles and assure a satisfactory oil film between the relatively rough sliding surfaces and is recommend to check piston rings and cylinder liners through scavenge air port inspections for every 100 hours. Do not proceed to the next lubrication step if the scavenge air port inspection reveals seizures or
other irregularities. The feed rate during breaking-in must not be set lower than the fuel sulphur content depending feed rate.

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