Automotive Lubricants Part I - Oil
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Types Of Oils

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.

What Are The Properties Of A Good Lubricating Oil?

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.