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.
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.