Motor oil , or engine oil , is an oil used for lubrication of various internal combustion engines. While the main function is to lubricate moving parts, motor oil also cleans, inhibits corrosion, improves sealing and cools the engine by carrying heat away from moving parts. Dieter Klamann's text provides extensive technical detail about motor oils.
Motor oils are derived from petroleum-based or non-petroleum synthesized chemical compounds. Motor oils are today mainly blended by using base oils composed of hydrocarbons (mineral, polyalphaolefins (PAO), polyinternal ofefins (PIO)), and are thus organic compounds consisting entirely of carbon and hydrogen. The base oils of some high-performance motor oils contain up to 20 wt.-% of esters.
Use
Motor oil is a lubricant used in internal combustion engines. These include motor or road vehicles such as cars and motorcycles, heavier vehicles such as buses and commercial vehicles, non-road vehicles such as go-karts, snowmobiles, boats (fixed engine installations and outboards), lawn mowers, large agricultural and construction equipment, locomotives and aircraft, and static engines such as electrical generators. In engines, there are parts which move against each other causing friction which wastes otherwise useful power by converting the energy to heat. Contact between moving surfaces also wears away those parts, which could lead to lower efficiency and degradation of the motor. This increases fuel consumption and decreases power output and can, in extreme cases, lead to engine failure.
Lubricating oil creates a separating film between surfaces of adjacent moving parts to minimize direct contact between them, decreasing heat caused by friction and reducing wear, thus protecting the engine. In use, motor oil transfers heat through convection as it flows through the engine by means of air flow over the surface of the oil pan, an oil cooler and through the build up of oil gases evacuated by the Positive Crankcase Ventilation (PCV) system.
In petrol (gasoline) engines, the top piston ring can expose the motor oil to temperatures of 320 °F (160 °C). In diesel engines the top ring can expose the oil to temperatures over 600 °F (315 °C). Motor oils with higher viscosity indices thin less at these higher temperatures.
Coating metal parts with oil also keeps them from being exposed to oxygen, inhibiting oxidation at elevated operating temperatures preventing rust or corrosion. Corrosion inhibitors may also be added to the motor oil. Many motor oils also have detergents and dispersants added to help keep the engine clean and minimize oil sludge build-up.
Rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces. Such particles could circulate in the oil and grind against moving parts, causing wear. Because particles accumulate in the oil, it is typically circulated through an oil filter to remove harmful particles. An oil pump, a vane or gear pump powered by the engine, pumps the oil throughout the engine, including the oil filter. Oil filters can be a full flow or bypass type.
In the crankcase of a vehicle engine, motor oil lubricates rotating or sliding surfaces between the crankshaft journal bearings (main bearings and big-end bearings), and rods connecting the pistons to the crankshaft. The oil collects in an oil pan, or sump, at the bottom of the crankcase. In some small engines such as lawn mower engines, dippers on the bottoms of connecting rods dip into the oil at the bottom and splash it around the crankcase as needed to lubricate parts inside. In modern vehicle engines, the oil pump takes oil from the oil pan and sends it through the oil filter into oil galleries, from which the oil lubricates the main bearings holding the crankshaft up at the main journals and camshaft bearings operating the valves. In typical modern vehicles, oil pressure-fed from the oil galleries to the main bearings enters holes in the main journals of the crankshaft. From these holes in the main journals, the oil moves through passageways inside the crankshaft to exit holes in the rod journals to lubricate the rod bearings and connecting rods. Some simpler designs relied on these rapidly moving parts to splash and lubricate the contacting surfaces between the piston rings and interior surfaces of the cylinders. However, in modern designs, there are also passageways through the rods which carry oil from the rod bearings to the rod-piston connections 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 chamber in the cylinder head from the crankcase. The oil then drips back down into the oil pan..
Other oils
While it may still be used in motor vehicles , ATF or Automatic Transmission Fluid is a separate type of specialized lubricating fluid. Varying specifications of ATF are used in automatic gearboxes and some power steering systems, and should not be used to lubricate the engine. It is typically colored dark red to distinguish it from the motor oil and other fluids in the vehicle. Other non-motor oils include gear or transmission, and differentials oils. These are used in manual gearboxes and driven axles. They could include specialty uses including EP (Extreme Pressure), hypoid, and limited slip functions. Again, they are not to be used for engine lubrication.
Non-vehicle oils
Other kinds of motors also use motor oil, as well as engines that are not in vehicles such as those for electrical generators. Examples include 4-stroke or 4-cycle internal combustion engines such as those used in many "walk behind" lawn mowers and other engines, and special 2-stroke oil used in 2-stroke or 2-cycle internal combustion engines such as those used in various smaller engines like mopeds, snow throwers (blowers), chain saws, toy engines like those in model airplanes, certain gardening equipment like weed/grass trimmers, leaf blowers, soil cultivators, etc. Often, the applications are not exposed to as wide a temperature range in use as vehicles, so these oils may be single grade or have less viscosity index improver. 2-cycle oil is used differently from other motor oils in that it is pre-mixed with the gasoline or fuel, often in a gasoline:oil ratio of 25:1, 40:1 or 50:1 up to 100:1, and burned in use along with the gasoline. Some 2-stroke engines used in cars, such as the Saab two-stroke engine, had an oil injection system rather than oil pre-mixed.
In addition to the 2-cycle oil used if they have gasoline engines, chain saws also separately use " bar and chain oil " for lubricating and cooling the surfaces where the cutting chain moves around the bar.
Other examples of mechanical equipment often using oil include oil-driven compressors, vacuum pumps, diffusion pumps, sewing machines and other devices with motors, oil-driven hydraulic equipment, and turbines.
The oil properties will vary according to the individual needs of these devices. Non-smoking 2-cycle oils are composed of esters or polyglycols. Environmental legislations for leisure marine applications, especially in Europe, enhanced the use of ester-based two cycle oils.
Properties
Most motor oils are made from a heavier, thicker petroleum hydrocarbon base stock derived from crude oil, with additives to improve certain properties. The bulk of a typical motor oil consists of hydrocarbons with between 18 and 34 carbon atoms per molecule. One of the most important properties of motor oil in maintaining a lubricating film between moving parts is its viscosity. 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.
Motor oil must be able to flow adequately at the lowest temperature it is expected to experience in order to minimize metal to metal contact between moving parts upon starting up the engine. The pour point defined first this property of motor oil, as defined by ASTM D97 as "...an index of the lowest temperature of its utility..." for a given application , but the "cold cranking similator" (CCS, see ASTM D5293-08) and "Mini-Rotary Viscometer" (MRV, see ASTM D3829-02(2007), ASTM D4684-08) are today the properties required in motor oil specs and define the SAE classifications.
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 dangerous for the oil in a motor to ignite and burn, so a high flash point is desirable. At a petroleum refinery, fractional distillation separates a motor oil fraction from other crude oil fractions, removing the more volatile components, and therefore increasing the oil's flash point.
Another manipulated property of motor oil is its total base number (TBN), which is a measurement of the reserve alkalinity of an oil, meaning its ability to neutralize acids. The resulting quantit
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