What you need to know about oil
Issue 55 : Jul/Aug 2007
The motor oil in any sailboat’s auxiliary engine is second in importance only to the supply of fuel and air. If neglected, certain — and often sudden — engine failure will occur. Here’s a look at the complex role motor oil plays.
Motor oil has four primary functions: to lubricate, seal, cool, and clean. In order to be a good lubricant, oil must maintain a slippery surface between moving parts. The oil film must tenaciously adhere to the engine’s metal surfaces and resist being pushed off, regardless of temperature or pressure.
During combustion, gases are formed under high pressure. This pressure, acting against the piston head, is what is converted into working horse-power. Therefore, it is necessary to prevent these gases from leaking past the pistons. Oil acts as the sealant.
The cooling system (water) removes the bulk of unwanted heat from the engine. However, oil assists the cooling system by removing heat from some critical areas, such as the bearings. In doing so, it frequently becomes much hotter than the cooling water.
Waste materials are formed as the byproduct of combustion. The type of fuel, gasoline, or diesel influences the nature and quantity of these waste materials. A few of the most common and troublesome include water, soot, carbon, and acids. The motor oil maintains these contaminants in a state of dispersion so they can be removed with routine oil changes rather than forming harmful deposits.
Major challenges
Some of the major chemical and physical factors that make the job of engine lubrication challenging include temperature extremes, oxidation, corrosion, contaminants, and foaming.
Motor oils encounter both high and low temperatures. When cool, motor oil becomes viscous and resists flowing. In cold weather, oil may become so thick that it will not circulate freely throughout the engine. This can result in excessive engine wear. In extreme cases, the oil can become so thick that the engine cannot be started. On the other hand, when it is hot, oil thins out. If it becomes too thin, high oil consumption and engine wear will occur.
When oxygen in the air comes in contact with the oil, a chemical reaction called oxidation begins to take place. As long as the oil is cold, oxidation occurs very slowly. However, heat speeds up the reaction. In addition to the products of oxidation, such as deposits and acids, motor oil becomes more viscous as it oxidizes.
Two types of corrosion commonly occur in a sailboat’s auxiliary engine: atmospheric corrosion and acid corrosion. While atmospheric corrosion is caused by water or moisture in the air, acids are a byproduct of the combustion process. Both water and acids are routinely formed in quantities significant enough to cause engine damage in a short time.
Most sailors are under the false impression that their motor oil requires changing because it has broken down or worn out. Not so. The oil needs to be removed because it has accumulated contaminants in the course of performing its job. These contaminants include water, soot, carbon, acids, metallic particles, and dirt. If allowed to remain in the engine, they can combine and form sludge.
Small air bubbles
Foaming can take place when oil and air are beaten together in the crankcase. It is the result of small air bubbles being trapped in an oil film. Foaming reduces lubrication effectiveness and can result in the malfunctioning of such engine components as the oil pump and hydraulic valve lifters.
Viscosity is the physical property that determines the oil’s ability to flow. Oils that are thick offer great resistance to flow and possess a high viscosity. Those that flow easily possess a low viscosity and are often described as non-viscous.
Oil viscosity controls the ease of engine starting in cold weather. To put it another way, it determines the temperature below which an engine cannot be started.
Viscosity determines the thickness of an oil film in an engine. A viscous oil leaves a thicker film, under any set of conditions, than a non-viscous oil. This fact influences engine wear.
Oil viscosity also explains the reason for most oil consumption, in as much as consumption increases as oil viscosity decreases. Oil can be too heavy. If so, it can fail to uniformly lubricate or remove excess heat.
For many years, the naturally occurring chemical and physical characteristics of motor oil were adequate. However, there have been major developments since World War II. Varying compression ratios, higher temperatures, greater power-to-weight ratios, and emission controls have made it necessary to incorporate a number of additional agents in the oil. These additives, including detergents, anti-oxidants, corrosion inhibitors, and anti-foaming agents, are designed to enhance the oil’s performance.
Routine changes needed
For the most part, these additives are expendable. They can be depleted if the oil is used for too long a time. Routine oil changes not only insure that these additives will be replenished, along with fresh oil, but also that destructive contaminants will be removed.
Synthetic motor oils have been commercially available since the early 1930s and were used primarily for military applications. During World War II, they were used to prevent the oil from freezing in army tanks during the winter months. Synthetic oils are often made from a mineral-oil base stock, which may be partially disassembled and reconstructed to meet a set of specific performance requirements. Synthetic motor oils are generally more expensive than conventional motor oils.
The combination of letters and numbers on each container of oil was created by the combined efforts of the American Petroleum Institute (API) in conjunction with the American Society for Testing and Materials (ASTM) and the Society of Automotive Engineers (SAE). It is the motor oil’s service classification and identifies in which type of engine and under what conditions the oil can be used.
The API Engine Service Category S indicates that the oil is designed to provide service in a gasoline (spark) engine. The letter immediately following the S suggests the type of additive package the oil contains. For example, SD indicates use in gasoline engines 1968 through 1970, while SG is for gasoline engines beginning with the 1989 model year.
The API Engine Service Category C indicates that the oil is designed for use in diesel (compression) engines. As is the case with category S oils, the letter immediately following the C also defines the oil’s use. CA oils were widely used in diesel engines in the late 1940s and early 1950s. Diesel engines manufactured since 1983 require oils with an API Engine Service Category of CE.
“Backward compatible”
The second letter in the API Engine Service Category is issued in alphabetical order. The oldest and least complex oils are SA and CA. The most current are SL and CF. In most instances, higher API Engine Service Category oils are “backward compatible.” In other words, they may be used in place of earlier recommendations.
The SAE viscosity numbers constitute a classification of crankcase lubricating oils in terms of viscosity. These SAE grade recommendations are linked directly to temperature. The lower the number, such as SAE 5, the thinner or less viscous the oil. The higher the number, such as SAE 40, the thicker or more viscous the oil. Thin oils meet SAE low-temperature requirements, while more viscous oils meet high-temperature requirements. A multi-SAE or viscosity-graded oil, such as SAE 10W-30, is one that, at low temperature, meets the SAE requirements of a lower-grade number and also meets the high-temperature requirements of the heavier-grade oil.
While today’s modern motor oils are amazingly effective, they are not magic. They have an operating life and need to be replaced periodically. Follow the engine manufacturer’s recommendations regarding the proper API Engine Service Category, the SAE viscosity, and change frequency.
Before changing the oil, bring your engine up to operating temperature. This ensures that any damaging contaminants are in suspension and can be pumped out with the old oil. In addition to periodic oil changes based on hours and conditions of usage, always change the oil before seasonal layup. And be sure to change the filter with every oil change.
Thank you to Sailrite Enterprises, Inc., for providing free access to back issues of Good Old Boat through intellectual property rights. Sailrite.com
