Motor (Engine) Lubricating Oil

Motor (engine) oil is a type of liquid oil used for lubrication by various types of internal combustion engines.   The main function is to lubricate moving parts, but it cleans, inhibits corrosion and cools the engine by carrying heat away from the moving parts.   The majority of motor oils are derived from petroleum.   Synthetic motor oil, consisting of artificially synthesized compounds, currently has a majority share in the motor oil market place because, while more expensive, it offers enhanced performance.   Semi-synthetic motor oil was introduced in 1966. ^{Wiki n.p.}

In engines there are parts which move very closely against each other at high speeds, often for prolonged periods of time.   Such motion causes friction, absorbing otherwise useful power produced by the motor and converting the energy to useless heat.   Friction also wears away the contacting surfaces of those parts, which could lead to lower efficiency and degradation of the motor.   This increases fuel consumption and wears out the engine.   Lubricating oil makes a film between surfaces of parts moving next to each other so as to minimize direct contact between them decreasing friction, wear, and production of excessive heat, thus protecting the engine.   Motor oil also carries away heat from moving parts, which is important because materials tend to become softer and less abrasion-resistant at high temperatures.   Some engines have an additional oil cooler. ^{Wiki n.p.}

In gasoline engines, the top compression ring can expose the motor oil to temperatures of 320 F.   In diesel engines the top ring can expose the oil to temperatures over 600 F.   Motor oils with the higher viscosity tend to thin less at these higher temperatures.   Coating metal parts with oil also keeps them from being exposed to oxygen, which inhibits their oxidation at elevated operating temperatures to preventing them from rusting.   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 oil sludge buildup. ^{Wiki n.p.}

Rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces.   Sludge also accumulates in the engine.   Such particles could circulate in the oil and grind against the moving parts, causing erosion and wear.   Because particles inevitably build up in the oil, it is typically circulated through an oil filter to remove harmful particles.   An oil pump, a gear pump powered by the vehicle engine, pumps the oil through 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 journals, bearings, and rods connecting the pistons to the crankshaft.   The oil collects in an oil pan 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. ^{Wiki n.p.}

Automatic transmission fluid is a separate fluid.   It is typically colored red to distinguish it from the motor oil and other fluids in the vehicle.   Other kinds of motors, such as internal combustion engines in motorcycles, mopeds, outboard motors (for boats), snowmobile, ATV's, personal watercraft, scooters, and go-carts, etc., 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 lawn mowers and other engines, and special 2-cycle oil used in 2-stroke or 2-cycle internal combustion engines such as those used in various smaller engines like 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 than other motor oils in that it is pre-mixed with the gasoline or fuel, often in a gasoline : oil ratio of 50 : 1, and burned in use along with the gasoline.   In addition to the 2-cycle oil used if they have gasoline engines, chain saws also separately use "bar and chain oil" for lubricating the surfaces where the cutting chain moves around 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, turbines, and mechanisms using gears such as gear differentials for rear wheel-drive vehicles.   The oil properties will vary according to the needs of these devices. ^{Wiki n.p.}

Most motor oils are made from a heavier, thicker petroleum hydrocarbon base stock derived from crude oil, with additives added as needed to improve properties.   One of the most important properties of motor oil in maintaining a lubricating film between moving parts is its viscosity.   The viscosity of any fluid (gas or liquid) is a measure of its resistance to shear stress, which is also its resistance to flow, often described as its "thickness" or fluid internal friction.   Thus, water is "thin" and vegetable oil is "thick".   All fluids have a viscosity above zero and vary logarithmically with temperature.   The viscosity of any lubricating fluid, such as motor oil, 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 and not hinder movement of the working parts.   The viscosity index (VI) 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 (oil is "thick") than a lower viscosity index (oil is "thin").   For lubricating oils, such as motor oil, a high VI is desirable because its viscosity will not vary with temperautre, which is important when engines and hydraulic systems operate in large temperature ranges, such as in airplanes. ^{Wiki n.p.}

Oil film strength is its ability to keep moving aparts from abrading each other under load.   It includes the ability to hold lubricity under shock loads and vibration.   Film strength is a lubricant property that acts to prevent scuffing or scoring of metal parts.   Film strength is measured by the American Society for Testing Materials' (ASTM) Timken OK Load test, which measures the heaviest load that a lubricant will sustain without scoring the test block.^{Royal n.p.}   Chemical stability is another property of oils.   This is its ability to resist oxidation and deterioration over long periods.   Motor oil must be able to flow at cold winter temperatures to lubricate internal moving parts upon starting up the engine.   Therefore, 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 dangerous for the oil in a motor 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 from the motor oil fraction, thereby increasing the oil's flash point.   Another test done on oil is to determine the Total Base Number (TBN), which is a measurement of the reserve alkalinity of an oil to neutralize acids.   The resulting quantity is determined as mg KOH/(gram of lubricant).   Analogously, Total Acid Number (TAN) is the measure of a lubricant's acidity.   Other tests include amounts of zinc, phosphorus, or sulfur content, and testing for excessive foaming. ^{Wiki n.p.}

The Society of Automotive Engineers (SAE), has established a numerical code system for grading motor oils according to their kinematic viscosity.   For single-grade oils, the kinematic viscosity is measured at a reference temperature of 100 °C (212 °F) in units of mm²/s or the equivalent older non-SI units, 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, 60 or 70.   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. ^{Wiki n.p.}

The temperature range the oil is exposed to in most vehicles can be wide, ranging from cold ambient temperatures in the winter before the vehicle is started up to hot operating temperatures when the vehicle is fully warmed up in hot summer weather.   A specific oil will have high viscosity when cold and a low viscosity at the engines operating temperature.   The difference in viscosities for any single-grade oil is too large between the extremes of temperature.   To bring the difference in viscosities closer together, special polymer additives called viscosity index improvers are added to the oil.   These additives make the oil a multi-grade motor oil.   The idea is to cause the multi-grade oil to have the viscosity of the base number when cold and the viscosity of second number when hot.   The viscosity of a multi-grade oil still varies logarithmically with temperature, but the slope representing the change is lessened.   This slope representing the change with temperature depends on the nature and amount of the additives to the base oil.   The API/SAE designation for multi-grade oils includes two grade numbers; for example, 10W-30 designates a common multi-grade oil.   The first number associated with the W (again 'W' is for Winter, not Weight) is not rated at any single temperature.   The "10W" means that this oil can be pumped by your engine as well as a single-grade SAE 10 oil can be pumped.   "5W" can be pumped at a lower temperature than "10W".   "0W" can be pumped at a lower temperature than "5W", and thins less at temperatures above 99°C (210°F). The second number, 30, means that the viscosity of this multi-grade oil at 100°C (212°F) operating temperature corresponds to the viscosity of a single-grade 30 oil at same temperature.   The governing SAE standard is called SAE J300. The motor oil grade and viscosity to be used in a given vehicle is specified by the manufacturer of the vehicle. ^{Wiki n.p.}

Turbine motor oils are designed somewhat differently than reciprocating engine oils traditionally used in automobiles.   Deposit control and corrosion are not significant issues when formulating a turbine oil, and the shear stresses that turbine oils are exposed to are minimal in light of the fact that turbines are naturally balanced rotating machines unlike reciprocating engines.   In most aviation gas turbine applications, peak lubricant temperatures are not reached during engine operation, but after shutdown when heat has been able to migrate from the combustor cans and the compressors into the regions of the engine with lubricated bearings and gearboxes.   The gas flow associated with running the turbine provides significant convective cooling that disappears when the engine is shut down, leaving residual heat that causes temperatures within the turbine to rise dramatically. ^{Wiki n.p.}

Engine (motor) oil is used for the lubrication, cooling, and cleaning of internal combustion engines. Motor oil may be composed of a lubricant base stock only in the case of non-detergent oil, or a lubricant base stock plus additives to improve the oil's detergency, extreme pressure performance, and ability to inhibit corrosion of engine parts. Lubricant base stocks are categorized into five groups by the API (American Petroleum Institute). Group I base stocks are composed of fractionally distilled petroleum which is further refined with solvent extraction processes to improve certain properties such as oxidation resistance and to remove wax. Group II base stocks are composed of fractionally distilled petroleum that has been hydrocracked to further refine and purify it. Group III base stocks have similar characteristics to Group II base stocks, except that Group III base stocks have higher viscosity indexes. Group III base stocks are produced by further hydrocracking of Group II base stocks, or of hydroisomerized slack wax, (a byproduct of the dewaxing process). Group IV base stock are polyalphaolefins (PAOs). Group V is a catch all group for any other synthetic and mineral base stocks. Examples of group V base stocks include polyol esters, polyalkylene glycols (PAG oils), and perfluoropolyalkylethers (PFPAEs). Groups I, II, and III are sometimes referred to as mineral oils and groups IV and V as synthetic oils. However, most manufacturers have labeled their group III based oils as synthetic in the US for reasons of economy and marketing. ^{Wiki n.p.}

Motor oils are further categorized by their API service class. The API service classes have two general classifications: S for Service (typical passenger cars and light trucks using gasoline engines) and C for commercial applications (typical diesel equipment). Note that the API oil classification structure has eliminated specific support for wet-clutch motorcycle applications in their descriptors, and API SJ & newer oils are referred to be specific to automobile and light truck use. The latest API service standard designation is SM for gasoline automobile and light-truck engines. The SM standard refers to a group of laboratory and engine tests, including the latest series for control of high-temperature deposits. Current API service categories include SM, SL and SJ for gasoline engines. All previous service designations are obsolete, although motorcycle oils commonly still utilize the SF/SG standard. There are seven diesel engine service designations which are current: CJ-4, CI-4, CH-4, CG-4, CF-4, CF-2, and CF. All others are obsolete. It is possible for an oil to conform to both the gasoline and diesel standards. Engine oil which has been tested and meets the API standards has the API starburst symbol with the service designation on containers sold to oil users. ^{Wiki n.p.}

In engines there is inevitably some exposure of the oil to products of internal combustion, and microscopic coke particles from black soot accumulate in the oil during operation.   Also the 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 the part surfaces causing erosion and wear.   The oil filter removes many of the particles, but eventually the oil filter becomes clogged.   The motor oil and especially the additives also undergo thermal and mechanical degradation.   For these reasons, the oil and the oil filter need to be periodically replaced.   The vehicle manufacturer specifies which grade of oil should be used for the vehicles it produces.   The manufacturer also specifies how often the oil changes should be made.   Historically this has been 3,000 miles.   Many modern cars list higher intervals for changing of oil and filter.   There are many types (or sizes) of oil filters for vehicle engines.   Vendors of oil filters have information on which type of oil filter is compatible with a given vehicle. ^{Wiki n.p.}

In addition to the viscosity index improvers, motor oil manufacturers often include other additives such as detergents and dispersants to help keep the engine clean by minimizing sludge buildup, corrosion inhibitors, and alkaline additives to neutralize acidic oxidation products of the oil.   Most commercial oils have a minimal amount of zinc dialkyldithiophosphate as an anti-wear additive to protect contacting metal surfaces with zinc and other compounds in case of metal to metal contact.   The quantity of zinc dialkyldithiophosphate is limited to minimize adverse effect on catalytic converters. ^{Wiki n.p.}

Synthetic lubricants were invented initially for high-temperature gas turbine and jet engine applications where traditional mineral-derived lubricants provided inadequate performance.   In the mid-1970s, synthetic motor oils were formulated and commercially applied for the first time in automotive applications.   The same SAE system for designating motor oil viscosity applies to synthetic oils also.   Instead of making motor oil with the conventional petroleum base, "true" synthetic oil base stocks are artificially synthesized.   ("Commercial" synthetic oils are in fact Group III mineral base oils.)   True synthetics include classes of lubricants like synthetic esters and polyalpha-olefins.   Higher purity and therefore better property control means synthetic oil has good mechanical properties at extremes of high and low temperatures.   The molecules are made large and "soft" enough to retain good viscosity at higher temperatures, yet branched molecular structures interfere with solidification and therefore allow flow at lower temperatures.   Thus, although the viscosity still decreases as temperature increases, these synthetic motor oils have a much improved viscosity index (VI) over the traditional petroleum base.   Their specially designed properties allow a wider temperature range at higher and lower temperatures and often include a lower pour point.   With their improved VI, true synthetic oils need little or no viscosity index improvers, which are the oil components most vulnerable to thermal and mechanical degradation as the oil ages, and thus they do not degrade as quickly as traditional motor oils.   However, they still fill up with particulate matter at the same rate as the conventional oils do, and the oil filter still fills and clogs up over time.   So, periodic oil and filter changes should still be done with synthetic oil.   Some synthetic oil suppliers suggest that the intervals between oil changes can be longer, sometimes as long as 10,000 - 15,000 miles. ^{Wiki n.p.}

With improved efficiency, synthetic lubricants are designed to make wear and tear on gears far less than with petroleum-based lubricants, reduce the incidence of oil oxidation and sludge formation, and allow for extended drain intervals.   Synthetic lubricants are available for use in modern automobiles on nearly all lubricated components, potentially with superior performance and longevity as compared to non-synthetic alternatives.   Some tests have shown that fully synthetic oil is superior to conventional oil in many respects, providing better engine protection, performance, and better flow in cold starts than petroleum-based motor oil. However, a test by Consumer Reports showed that taxi-cabs exhibited the same amount of engine wear regardless of which oil was used. ^{Wiki n.p.}

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