General principles of engine lubrication
Updated 11 Oct 2019
All machines with moving parts where friction can be created require lubrication but in marine engines – particularly two-stroke engines – the subject is more complex than merely ensuring that a level of lubricant is maintained.
Tribology – the science of friction, wear and lubrication – has particular application in marine engines where conditions and parameters vary probably more than in any other internal combustion engine.
Marine four-stroke engines require just a single system lubricant, but lubrication of two-stroke marine diesel crosshead engines is achieved using two quite different lubricants. In these engines, the combustion chamber and scavenging air spaces is separated from the crankcase by the piston rod stuffing box and a different lubricant is used for each area. Furthermore, the cylinder lubricant must be matched to the fuel being used so as to neutralise the acidity of the fuel that is determined by the sulphur content and to the load conditions under which the engine is operating. This means that the type of cylinder lubricant for such an engine may need to be changed several times in the course of a day.
In any engine operating on fuels containing sulphur, cylinder lubricants have three main purposes: to provide a barrier to metal to metal contact between piston rings and the cylinder liner, neutralising any sulphuric acid to control corrosion and to clean the cylinder liner and piston rings preventing damage from combustion and neutralisation residues.
In four-stroke trunk piston engines the same oil is used for cylinder lubrication and cooling but some of the cylinder 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. In general, four-stroke engines are less complicated as regards lubrication than two-strokes.
The two-stroke crosshead engine is different from the four-stroke in that it has no connection between the piston underside space and the oil pan. The cylinder lubricant is not recycled and will be consumed in the combustion process making precise dosing imperative and allowing for different oil types to be used in the two zones of the engine – cylinder lubricant in the combustion chamber and system oil in other parts of the engine including the turbocharger and valves.
Two-stroke cylinder lubricating oil has a number of important parameters including viscosity, base number (BN) and detergent additives. The viscosity is typically SAE 50 across most suppliers although other grades might be specified under certain operating conditions. The BN can vary and is normally in the range of 40 to 70 if running on a typical HFO with 3.5% sulphur content. This will change after 2020 when only ships with scrubbers will be burning 3.5% sulphur fuel.
The BN indicates the content of alkaline additives used to neutralise the sulphuric acid produced during combustion. Higher sulphur content fuels require a lube with a higher BN and low-sulphur fuels a lube with a low BN.
Before the advent of SECAs and controls on SOx emissions, most operators would choose a lube with a BN that could deal with the 4.5% global sulphur cap. This would normally require a lube of BN70. As low-sulphur fuels became more common there was a great deal of debate over what strategy to adopt.
In order to avoid problems with deposit build-up on piston crowns, it was accepted wisdom to reduce the BN of lubes when operating on low-sulphur fuels. Short periods of operating on low-sulphur fuels normally present no problem and would not require a switch away from the high BN lube normally used.
In order to avoid problems connected with the use of lubes with the wrong composition, any lube suppliers experimented with producing universal lubricants which were typically of BN40 or 50 and these are now readily available and favoured by some operators. The use of these universal products is not without a degree of controversy and some engine makers have retracted previous approval of their use under some conditions and in specific engines.
Clearly it is unwise to ignore OEM recommendations but the benefits of using a single lubricant are attractive and relieve crew of having to cope with the problems of changing lubes as well as fuel oils during approaches to ECAs. Owners should therefore communicate with their engine and lube suppliers to determine the best and safest operational strategy.