Many hundreds of years ago someone figured out that fluids are not compressible. Therefore, fluid can be used to form a film between two moving metal parts, preventing them from touching together. That's pretty handy, since automobile engines have moving parts and we don't want them touching.
Keeping parts from touching doesn't mean we won't get heat from friction, but we can remove the heat as long as we keep the fluid (oil, of course) moving. A lubricating oil with the necessary properties and characteristics will serve six functions, among which is the removal of heat. Here they are:
1. Controls friction between load-bearing surfaces
2. Reduces wear by preventing metal-to-metal contact between moving parts
3. Limits the temperature by carrying away heat from fluid friction and combustion of fuel
4. Reduces corrosion by coating metal parts and by flushing debris from between moving parts
5. Dampens mechanical shock in gears
6. Forms a seal on the walls of the cylinders
That, in a nutshell, is what oil and other lubricants do in engines. This is how they do it...
It was Irving Langmuir (1881-1957), American chemist, who figured out why oil lubricates. In his research on surface tension and surface chemistry he developed a new technique (employing monolayers, i.e., layers of molecules one molecule thick) for the study of molecules. According to his theoretical work, at least three layers of oil exist between two lubricated bearing surfaces. Two of the oil films are boundary films, a condition in which the oil film is neither so thin as to cause seizure nor so thick as to create a full film of oil between the shaft and the bearing. One of the boundary films clings to the surface of the rotating journal. The other boundary film clings to the stationary lining of the bearing. However, boundary film lubrication alone is not sufficient to protect metal surfaces from friction, so between the two boundary films are one or more fluid films that slide layer upon layer. This combination lubricates the parts and prevents seizure.
For his contributions in surface chemistry he received the 1932 Nobel Prize in Chemistry. (It was Langmuir, by the way, who discovered that the introduction of particles of dry ice and iodide into a cloud of low temperature containing sufficient moisture in tiny droplets triggered a chain reaction producing rain or snow, depending on the condition of the weather.)
Anyway, back to the lubrication process. In a bearing journal, for instance, as the journal begins to turn, the oil adhering to the surface of the journal is carried into a space between it and the bearing. The oil film increases in thickness and tends to lift the journal away from the bearing. (Remember, a liquid is incompressible.) As the shaft speed increases, the journal takes a position somewhere between the bearing surfaces and rides on the sliding layers of oil. This means that, in properly-spaced components where oil of the right viscosity is pumped at correct pressure, no mechanical wear takes place during operaton.
A number of factors determine the effectiveness of oil film lubrication. They include load, temperature, viscosity, flow rate, speed, alignment, condition of the bearing surfaces, running clearances, and purity of the lubricant. Many of these factors, of course, are interrelated and interdependent. All are taken into account by the engine designer before specifying a particular grade or type of oil, but there are fundamental properties that lubricants must possess, such as adhesiveness and cohesiveness.
A lubricant must stick (adhesiveness) to the bearing surfaces and support the load at operating speeds. More adhesiveness is required to make a lubricant stick to bearing surfaces at high speeds than at low speeds.
At low speeds, resistance from being squeezed out from between the bearing surfaces (cohesiveness) is required on the part of the lubricant. Generally, the greater the viscosity, the greater the cohesiveness. Large clearances between bearing surfaces require the use of a lubricant with a high viscosity and cohesiveness that will provide an adequate lubricating oil film. The larger the clearances, the greater resistance the lubricant must have so it will not be pounded out.
If the lubricant is pounded out, the lubricating oil film will be destroyed. High unit loading of a bearing will also require the use of a lubricant with a high viscosity. A lubricant that is subjected to high loading must be sufficiently cohesive to hold together and maintain the oil film.
Conventional oils come from crude oil that is pumped from the ground. Crude oil is made up of a twisted and jumbled mass of carbon atoms that form chains and rings of different sizes and shapes. Long chains of carbon atoms produce a thick viscous fluid that flows slowly. Shorter chains produce fluid that flows more readily. In an oil refinery, crude oil is separated into various stocks. These become the basis for lubricating oils and fuels. While petroleum refining is an advanced science, small amounts of contaminants, such as sulfur, reactive hydrocarbons and other materials cannot be completely removed from petroleum, and may end up in motor oil base stocks. All motor oils are made up of base oils and additives.
Base oils can contain hydrocarbon molecules that are cycloparaffinic (sometimes called cycloalkane or paraffinic) or napthenic depending upon where the oil was found.
Paraffinic oils are relatively non-reactive and have excellent oxidation stability. Such oils have a high wax content (paraffin) and therefore a higher pour point and viscosity index, described below.
Napthenic oils have low pour points because of their lower wax content. They tend to have lower viscosity index properties and are high in solvency properties.
Newer oils have come on the market, known as Synthetic oils. These are essentially "built" from conventional oils in combination with fluids such as polyalphaolefins and additives. Such oils can be tailored to specific applications because they don't suffer from as many impurities as conventional oils. Synthetic motor oils are designed to perform even under severe conditions, such as very cold starting temperatures, extreme high-temperature operations and high-load conditions.
It is a myth that synthetic oils can't be mixed with traditional oils. They cannot be mixed with other synthetics, however.
The properties of lube oil are briefly explained in the following paragraphs.
VISCOSITY - Perhaps the most misunderstood property of oils, viscosity is its tendency to resist flow. The viscosity must always be high enough to keep a good oil film between the moving parts. Otherwise, friction will increase, resulting in power loss and rapid wear on the parts. Oils are graded by their viscosities at a certain temperature, since most oils tend to be thicker when cold and thinner when hot. (Grading is set up by noting the number of seconds required for a given quantity (60 ml) of the oil at the given temperature to flow through a standard orifice.)
Every oil has a viscosity index based on the slope of the industry's standard temperature-viscosity curve. The viscosity index depends on the rate of change in viscosity of a given oil with a change in temperature. A low index figure means a steep slope of the curve, or a great variation of viscosity with a change in temperature; a high index figure means a flatter slope, or lesser variation of viscosity with the same changes in temperatures. If you are using an oil with a high viscosity index, its viscosity or body will change less when the temperature of the engine increases.
For decades automobiles have used multi-grade (viscosity) oils rather than single viscosity, as detailed above, to avoid seasonal changes to different oils. Multi-grade oils have two viscosity grade numbers indicating their lowest and highest classification, e.g., SAE 10W-40. The lower grade number indicates the relative fluidity of the oil in cold weather for easy starting and immediate oil flow. The higher grade number indicates the relative viscosity of the oil at high operating temperatures for adequate wear protection. The "W" means "winter" grade, now used universally. Multi-grade oils generally contain viscosity index improvers (additives) that reduce the tendency of an oil to lose viscosity, or thin out, at high temperatures. Modern oils, especially synthetics, have viscosity grades as low as zero (e.g., SAE 0W-30), meaning simply that the oil flows more freely at low temperature than, for instance, a 5W-30 oil.
POUR POINT — The pour point of an oil is the lowest temperature at which the oil will [barely] flow from a container. At a temperature below the pour point, oil congeals or solidifies. Lube oils used in cold weather operations must have a low pour point. (NOTE: The pour point is closely related to the viscosity of the oil. In general, an oil of high viscosity will have a higher pour point than an oil of low viscosity.)
FLASH POINT — The flash point of an oil is the temperature at which enough vapor is given off to ignite when a flame or spark is present. The minimum flash points allowed for most lube oils are all above 300°F. However, the actual temperatures of the oils are always far below 300°F under normal operating conditions.
FIRE POINT — The fire point of an oil is the temperature at which the oil will continue to burn when it is ignited.
AUTOIGNITION POINT — The auto-ignition point of an oil is the temperature at which the flammable vapors given off from the oil will burn. This kind of burning will occur without the application of a spark or flame. For most lubricating oils, this temperature is in the range of 465° to 815°F.
DEMULSIBILITY — The demulsibility, or emulsion characteristic, of an oil is its ability to separate cleanly from any water present- an important factor in forced-feed systems.
NEUTRALIZATION NUMBER — The neutralization number of an oil indicates its acid content and is defined as the number of milligrams of potassium hydroxide (KOH) required to neutralize 1 gram of the oil. All petroleum products deteriorate (oxidize) in the presence of air and heat. Oxidation produces organic acids which, if present in sufficient concentrations, will cause deterioration of alloy bearings at elevated temperatures as well as galvanized surfaces.
PRECIPITATION NUMBER — The precipitation number of an oil is a measure of the amount of solids classified as asphalts or carbon residue contained in the oil. The number is reached when a known amount of oil is diluted with naphtha and the precipitate is separated by centrifuging-the volume of separated solids equals the precipitation number. This test detects the presence of foreign materials in used oils. An oil with a high precipitation number may cause trouble in an engine. It could leave deposits or plug up valves and pumps.
The American Petroleum Institute (API) has established a classification system for the designation of gasoline and diesel engine oils that reflects the quality, performance and suitability of the oils for various engines. These classifications, or categories, have no bearing on oil viscosity, whose limits are set by the Society of Automotive Engineers (SAE). Contrary to popular belief, the SAE grade only defines oil viscosity and has absolutely nothing to do with oil quality. Under federal law both the API service designation and the SAE viscosity grade are required.
In the United States, API also administers the licensing and certification of engine oils through a classification system that reflects the warranty, maintenance and lubrication requirements of the automotive industry. Through this system, API has standardized the labeling of engine oils by adopting the "donut" logo, which tells the user the oil's viscosity grade, engine service classification and any energy conserving capabilities.
Note: Engine oil performance requirements, test methods and limits for the various classifications are established by the engine and vehicle manufacturers and technical societies.
These classifications are arranged into two different groups, one for automotive gasoline engine service and the other for commercial diesel engine service. All gasoline engine oils use the classification "S," or "Service," presently SA through SL, as follows. Note that categories SA through SG are now obsolete, meaning any oil now available will more than meet the requirements of older engines.
SA - SB: SA and SB denotes service typical of older engines operated under such mild conditions that the protection afforded by compounded oils is not required. Oils designed for this service have been used since the 1930s and provide only mild anti-scuff capability and resistance to oxidation and bearing corrosion. They should not be used in any engines unless specifically recommended by the engine manufacturer.
SC — Category SC denotes service typical of gasoline engines in 1964 through 1967 models of passenger cars and some trucks, operating under engine manufacturers' warranties in effect during those model years. Oils designed for this service provide control of high and low temperature deposits, wear, rust and corrosion in gasoline engines.
SD — Category SD denotes service typical of gasoline engines in 1968 through 1970 models of passenger cars and some trucks, operating under engine manufacturers' warranties in effect during those model years. This category may also apply to certain 1971 or later models as specified or recommended in the owners' manuals. Oils designed for this service provide more protection against high and low-temperature deposits, wear, rust and corrosion in gasoline engines than oils that are satisfactory for API Service Category SC and may be used when API Engine Service Category SC is recommended.
SE — Category SE denotes service typical of gasoline engines in passenger cars and some trucks beginning 1972 and certain 1971 through 1979 models operating under engine manufacturers' warranties. Oils designed for this service provide more protection against oil oxidation, high-temperature engine deposits, rust and corrosion in gasoline engines than oils that are satisfactory for API Engine Categories SD or SC and may be used when either of these categories is recommended.
SF — Category SF denotes service typical of gasoline engines in passenger cars and some trucks beginning with 1980 through 1989 models operating under engine manufacturers' recommended maintenance procedures. Oils developed for this service provide increased oxidation stability and improved anti-wear performance relative to oils that meet the minimum requirements of API Service Category SE. These oils also provide protection against engine deposits, rust and corrosion. Oils meeting API Service Category SF may be used when API Engine Service Categories SE, SD, or SC are recommended.
SG - Category SG denotes service typical of gasoline engines in passenger cars, vans and light trucks beginning with 1989 through 1993 models operating under manufacturers' recommended maintenance procedures. Category SG oils include the performance properties of API Service Category CC as certain manufacturers of gasoline engines require oils that also meet the diesel engine oil category CD. Oils developed for this service provide improved control of engine deposits, oil oxidation, and engine wear relative to oils developed for previous categories. These oils also provide protection against rust and corrosion. Oils meeting API Service Category SG may be used when API Engine Service Categories SF, SE, SF/CC, or SE/CC are recommended.
SH — Category SH, first introduced in 1993, is for use in service typical of gasoline engines in present and earlier passenger cars, vans and light trucks operating under vehicle manufacturers' recommended maintenance procedures. Engine oils developed for this category provide performance exceeding the minimum requirements of API Service Category SG, which it is intended to replace, in the areas of deposit control, oil oxidation, wear, rust, and corrosion. Oils meeting API SH requirements have been tested according to the Chemical Manufacturers Association (CMA) Product Approval Code of Practice and may utilize the API Base Oil Interchange and Viscosity Grade Engine Testing Guidelines. They may be used where API Service Category SG and earlier categories are recommended. This service category was valid until August 1, 1997.
SJ — Category SJ was adopted for use in describing engine oils available in 1996. These oils are for use in service typical of gasoline engines in current and earlier passenger car, sport utility vehicle, van, and light truck operations under vehicle manufacturers' recommended maintenance procedures. Engine oils that meet the API Service Category SJ may be used where API Service Category SH and earlier Categories have been recommended.
SL — Category SL was adopted for use in describing engine oils available after July 1, 2001. Engine oils that meet the API Service Category SL may be used where API Category SJ and earlier categories have been recommended.
Don't make the mistake of "second-guessing" the engineers who designed your engine by using higher or lower viscosity oils than specified, since doing so makes it difficult for the pump to provide a sufficient film between bearings. In the former case there will be insufficient oil distribution and in the latter case the parts will pound out the thinner film and damage themselves.
The most important thing we can say about lubrication is this. Trust the engineers who designed your engine to have chosen the correct type and viscosity of oil for its operation.