Marine Diesel Lubricating Oils

The marine engine lubricating oil does not merely lubricate the moving parts, it also performs a number of specific functions.

1 It forms a film between the moving parts, preventing metal-to-metal contact. As a result, wear is kept to a minimum, power loss due to friction is minimized and engine noise is kept to a low level.

2 It acts as a cooling agent by carrying heat away from hot engine parts.

3 It forms a seal between the piston rings and the cylinder walls.

4 It acts as a cleaning agent.

5 It resists the corrosion of highly-polished engine surfaces by the acidic products of combustion that enter the sump past the piston rings.

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To efficiently fulfill these requirements the marine lubricating oil must posses a number of important properties. Of these, viscosity is one of the most important, and has already been defined as the reluctance of a fluid to flow. A fluid with high viscosity may be said to be thick or heavy, while a fluid with low viscosity is said to be thin, or light. Fluids tend to become less viscous as they are heated and, conversely, to become more viscous when cold. An engine oil must not become so thick in winter as to cause starting difficulties, but must not become so thin at operating temperatures as to fail in its requirements as a lubricant.

Because deposits and acids are formed through the combustion of fuel, and the marine engine oil should have the ability to wash the deposits from engine components and to neutralize the acids. The oil should not foam when agitated in the sump and should be able to withstand the extreme pressures encountered between certain engine components.

Mineral oil, by itself, cannot fulfill all the requirements of an engine lubricating oil, and certain chemicals are added it.

In the example at right, oil is pumped by a positive displacement pump which is driven by the engine to circulate the oil through the system.

Alternatively, some old engines are splash type where oil is thrown around the sump by the crankshaft scoops.

You can find a ton of information on marine diesel engines in the downloadable e-book from Amazon Marine Diesel Engine Basics. You can print out your own copy, insert the pages into plastic covers and take it with you when you work on your engine. It is a highly recommended book. Buy your copy now!

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