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Motor oil is used for lubrication by various kinds of motors, especially internal combustion engines in vehicles, boats, lawn mowers, trains, some airplanes, etc. In motors, there are contacting parts which move against each other at high speeds, often for prolonged periods of time. Such rubbing motion causes friction, absorbing otherwise useful power produced by the motor and converting the energy to relatively useless heat. Friction also wears away the contacting surfaces of those parts, which could lead to lower efficiency and wearing out of the motor.
Lubricating oil makes a film between surfaces of parts moving against each other so as to minimize direct contact between them. This lubricating film decreases friction, wearing, and production of excessive heat between the moving parts. Also as a moving fluid, motor oil carries away heat building up at the surfaces of parts, because some heat is inevitably produced anyway due to friction from parts moving against each other or the oil film. Materials tend to become softer and less abrasion-resistant at high temperatures.
In the crankcase of a vehicle engine, the crankshaft, bearings, and bottoms of rods connecting the pistons to the crankshaft are submerged in oil for lubrication of these rapidly moving parts. Also, the rapid motion of these parts causes churning of the oil to splash and lubricate the contacting surfaces between the piston rings and interior surfaces of the cylinders. This oil film also serves as a seal between the piston rings and cylinder walls to separate the combustion volume in the cylinders from the space in the crankcase.
Coating metal parts with oil also keeps them from being exposed to oxygen, which inhibits their oxidation at elevated operating temperatures. Corrosion inhibitors may also be added to the motor oil. Many motor oils also have detergent additives to help keep the engine clean and minimize sludge buildup.
Rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces. Sludge accumulates in the engine also. Such particles could circulate in the oil and grind against the moving parts, causing erosion and wear. Because undesired particles inevitably build up in the oil, the motor oil is circulated through an oil filter to remove harmful particles. An oil pump powered by the engine pumps the oil around and through the oil filter.
Automatic transmission fluid is a separate fluid used for vehicle's transmissions. It is typically colored red to distinguish it from the motor oil and other fluids in the vehicle.
Most motor oils are made from a heavier, thicker petroleum hydrocarbon base stock derived from crude oil, with additives added as needed to improve the properties. One of the most important properties of motor oil in maintaining a lubricating film between moving parts is its viscosity. In layman's terms, the viscosity of a liquid can be thought of as its "thickness" or a quantity of resistance to flow. The viscosity must be high enough to maintain a satisfactory lubricating film, but low enough that the oil can flow around the engine parts satisfactorily to keep them well coated under all conditions. The viscosity index is a measure of how much the oil's viscosity changes as temperature changes. A higher viscosity index indicates the viscosity changes less with temperature than a lower viscosity index.
Another important property of motor oil is its pour point, which is indicative of the lowest temperature at which the oil could still be poured satisfactorily. The lower the pour point temperature of the oil, the more desirable the oil is when starting up at cold temperature.
Oil is largely composed of hydrocarbons which can burn if ignited. Still another important property of motor oil is its flash point, the lowest temperature at which the oil gives off vapors which can ignite. It is, of course, dangerous for the oil to ignite and burn, so a high flash point is desirable. At a petroleum refinery, fractional distillation to separate crude oil fractions removes the volatile components, which more easily ignite, from the motor oil fraction; therefore, increasing the oil's flash point.
Another test done on oil is Total Base Number (TBN), which is a measurement of reserve alkalinity of an oil for neutralizing acids. The resulting quantity is determined as mg KOH/(gram of lubricant). Analogously, Total Acid Number (TAN) is the measurement of a lubricant’s acidity content. Other tests include zinc, phosphorus, or sulfur content, and testing for excessive foaming.
Gasoline engines and Diesel fuel engines use slightly different kinds of motor oil.
The Society of Automotive Engineers, usually abbreviated as SAE, has established a numerical code system for grading motor oils according to their kinematic viscosity. See the article on viscosity for details on kinematic viscosity. For single-grade oils, the kinematic viscosity is measured at a reference temperature of 100°C (212°F) in units of centistokes (abbreviated cSt). Based on the range of viscosity the oil falls in at that temperature, the oil is graded as an SAE number 0, 5, 10, 20, 30, 40, 50, or 60. The higher the viscosity, the higher the SAE grade number is. These numbers are often referred to as the weight of a motor oil. The reference temperature is meant to approximate the operating temperature to which motor oil is exposed in an engine.
The viscosity of single-grade oil derived from petroleum unimproved with additives changes considerably with temperature. As the temperature increases, the viscosity of the oil decreases logarithmically in a relatively predictable manner. On single-grade oils, viscosity testing can be done at cold winter ( W ) temperature (as well as checking minimum viscosity at 100°C or 212°F) to grade an oil as SAE number 0W, 5W, 10W, 15W, 20W, or 25W. A single-grade oil graded at the hot temperature is expected to test into the corresponding grade at the winter temperature; i. e. a 10 grade oil should correspond to a 10W oil. For some applications, such as when the temperature ranges in use are not very wide, single-grade motor oil is satisfactory; for example, lawn mower engines.
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