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Chuck Husick: Techno-Talk, March 2006 from BoatUS Magazine -

Engine Oil

The “lube” oil in any internal combustion engine is all that stands between normal operation and certain, sudden, and gruesome mechanical death. However the oil you use in your gasoline fueled car engine may not be the right oil for the gas engine in your boat.

The most significant difference between the engine in your car and the one in your boat is how hard it must work, usually expressed as how much of its maximum power it is required to deliver for sustained periods of time. In many boats the engine is constantly delivering 60-80% of its maximum power rating, compared with the 20% of maximum power demanded from a car engine. The more highly stressed marine engine needs superior lubricating oil if it is to operate reliably. Marine diesel engines also require special oils since most diesel engine applications involve high continuous loads. Just about any of the “C” diesel oils will work well. (The first “C” in the specification identifies the oil as being suitable for a compression ignition engine).

The lubricating oil identification you see on a container of oil is created under the auspices of the American Petroleum Institute, the “API”. The specification that defines the “weight” of the oil, for example 10W-50, is set by the Society of Automotive Engineers, the “SAE”. However, in recognition of the special and often severe stress placed on oil by gasoline fueled marine engines, the National Marine Manufacturer’s Association (NMMA), has created a series of specifications for two- and four-stroke marine engine oils. NMMA Specification FC–W™ defines the oil requirements for four-stroke engines, NMMA TC–W3® specification applies to two-stroke engines. The oils that meet these specifications are specially formulated to provide the special protection required by our hard working marine engines.

All engine oils serve multiple purposes. Controlling friction between opposing surfaces is only one of those jobs. One of the oil’s most important tasks is to transfer heat from parts of the engine that are best cooled by being sprayed with oil, for example, undersides of the pistons. Oil provides the critical seal between the inner surfaces of the cylinder bores and the piston rings, keeping the high pressure gases created by combustion of fuel from leaking past the piston rings. This ensures that the expanding combustion gases are used to provide useful energy while reducing contamination of the oil in the crankcase.

Additionally, oil provides a vital cushion between surfaces that impact on one another, for example the tops of the valve stems and the tappets and some types of gear teeth. Its lubricating qualities prevent adjacent moving surfaces from contacting one another and welding themselves into a solid mass from the heat of friction. Oil also fulfills a particularly critical need in marine engines, corrosion protection. Recreational vessel engines spend most of their lives just sitting in what is often a high humidity, salt air environment. The saying that our engines usually rust out before they wear out is unfortunately true.

NMMA specification oils are usually identified as synthetic or part synthetic. The use of the word synthetic does not necessarily mean that it was created directly from hydrogen and carbon atoms in a laboratory. Synthetic oils are often made from a mineral oil base stock which may be partially disassembled and reconstructed to yield a set of specific performance requirements. Oil constructed entirely from the chemists’ stockroom tends to be more costly and is used primarily for special applications such as aircraft jet engines.

All of today’s engine oils contain chemical additives, each designed to fulfill a specific purpose. These include anti-foaming agents, corrosion inhibitors, acid neutralizers, viscosity and pour-point stabilizers and detergents that help keep the inside of the engine clean and hold the contaminants in suspension so that they can be removed when the oil is changed.

Although claims that the “oil” in engine oil does not wear out are true to a degree, the chemical additives do have a finite service life. Properly conducted oil changes are primary in ensuring long and trouble free engine life. Change oil only after the engine has reached full operating temperature. Use the correct oil, one that matches the engine manufacturer’s specifications. In most cases, change the filter when you change the oil.

By Chuck Husick

Chuck Husick is a pilot, engineer, sailor and former president of Chris Craft Boats.

© Copyright BoatUS Magazine 2006





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