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Dictionary of Lubricant Terms


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DEMERIT RATING - definition of an engine part condition in numerical terms on an open-ended scale starting at 0 which represents a new part condition. In this system, the higher the rating number, the worse the part condition. Generally used for diesel engines.

DEMULSIBILITY - ability of an oil to separate from water, as determined by test method ASTM D 1401 or D 2711. Demulsibility is an important consideration in lubricant maintenance in many circulating lubrication systems.

DEMULSIFIER - additive that promotes oil-water separation in lubricants that are exposed to water or steam. See demulsibility.

DENSITY - See specific gravity.

DEPOSIT - material other than fuel or lubricant as such, so affixed to an engine surface as to have finite volume under operating conditions, and classifiable as sludge, lacquer, carbon, rust, etc.

DETERGENT - important component of engine oils that helps control varnish, ring zone deposits, and rust by keeping insoluble particles in colloidal suspension and in some cases, by neutralizing acids. A detergent is usually a metallic (commonly barium, calcium, or magnesium) compound, such as sulfonate, phosphonate, thiophosphonate, phenate, or salicylate. Because of its metallic composition, a detergent leaves a slight ash when the oil is burned. A detergent is normally used in conjunction with a dispersant. See ash content.

DETERGENT-DISPERSANT - engine oil additive that is a combination of a detergent and a dispersant, important in preventing the formation of a sludge and other engine deposits.

DETONATION - See knock.

DEWAXING - removal of paraffin wax from lubricating oils to improve low temperature properties, especially to lower the cloud points and pour point.

DIBASIC ACID ESTER (DIESTER) - synthetic lubricant base; an organic ester, formed by reacting a dicarboxylic acid and an alcohol; properties include a high viscosity index (VI) and low volatility. With the addition of specific additives, it may be used as a lubricant in compressors, hydraulic systems, and internal combustion engines.

DIESEL FUEL - that portion of crude oil that distills out within the temperature range of approximately 200C (392F) to 370C (698F), which is higher than the boiling range of gasoline. See distillation. Diesel fuel is ignited in an internal combustion engine cylinder by the heat of air under high compression--in contrast to motor gasoline, which is ignited by electrical spark. Because of the mode of ignition, a high cetane number is required in a good diesel fuel. Diesel fuel is close in boiling range and composition to the lighter heating oils. There are two grades of diesel fuel, established by the American Society for Testing and Materials (ASTM): Diesel 1 and Diesel 2. Diesel 1 is a kerosene-type fuel, lighter, more volatile, and cleaner than Diesel 2, and is used in engine applications where there are more frequent changes in speed and load. Diesel 2 is used in industrial and heavy mobile service.

DIESELING - See afterrunning.

DILUENT - a usually inert (unreactive) liquid or solvent, used to dilute, carry, or increase the bulk of some other substance. Petroleum oils and solvents are commonly used as dilutents in such products as paints, pesticides, and additives.

DISPERSANT - engine oil additive that helps prevent sludge, varnish, and other engine deposits by keeping particles suspended in a colloidal state. Dispersants are normally used in conjunction with detergents. A dispersant is commonly distinguished from a detergent in that the former is non-metallic and, thus, does not leave an ash when the oil is burned; hence, the term ashless dispersant. Also, a dispersant can keep appreciable larger quantities of contaminants in suspension than a detergent.

DISTILLATE - any of a wide range of petroleum products produced by distillation, as distinct from bottoms, cracked stock (see cracking), and natural gas liquids. In fuels, a term referring specifically to those products in the mid-boiling range, which include kerosene, turbo fuel, and heating oil--also called middle distillates and distillate fuels. In lubricating oils a term applied to the various fractions separated under vacuum in a distillation tower for further processing (lube distillate).

DISTILLATION (FRACTIONATION) - the primary refining step, in which crude is separated into fractions, or components, in a distillation tower, or pipe still. Heat, usually applied at the bottom of the tower, causes the oil vapors to rise through progressively cooler levels of the tower, where they condense into plates and are drawn off in order of their respective condensation temperatures, or boiling points--the lighter-weight, lower-boiling-point fractions, exiting higher in the tower. The primary fractions, from low to high boiling point are: hydrocarbon gases (e.g. ethane, propane); naphtha (e.g. gasoline); kerosene, diesel fuels (heating oil); and heavy gas for cracking. Heavy materials remaining at the bottom are called the bottoms, or residuum, and include such components as heavy fuel oil and asphaltic substances. Those fractions taken in liquid form from any level other than the very top or bottom are called overhead product. Distillation may take place in two stages: first, the lighter fractions--gases, naphtha, and kerosene--are recovered at essentially atmospheric pressure; next, the remaining crude is distilled at reduced pressure in a vacuum tower, causing the heavy lube fractions to distill a much lower temperatures than possible at atmospheric pressure, thus permitting more lube to be distilled without molecular cracking that can occur at extensively high temperatures.

DISTILLATION TEST - method for determining the full range of volatility characteristics of a hydrocarbon liquid by progressively boiling off (evaporating) a sample under controlled heating. Initial boiling point (IBP) is the fluid temperature at which the first drop falls into a graduated cylinder after being condensed in a condenser connected to a distillation flask. Mid-boiling point (MBP) is the temperature at which 50% of the fluid has collected in the cylinder. Dry point is the temperature at which the last drop of fluid disappears from the bottom of the distillation flask. Final boiling point (FBP) is the highest temperature observed. Front-end volatility and tail-end volatility are the amounts of test sample that evaporate, respectively, at the low and high temperature ranges. If the boiling range is small, the fluid is said to be narrow cut, that is, having components with similar volatilities; if the boiling range is wide, the fluid is termed wide cut. Distillation may be carried out by several ASTM test methods, including ASTM D 86, D 850, D 1078, and D 1160.

DUMBBELL BLEND - mixture of hydrocarbons, usually two components, that have markedly different volatilities, viscosities, or other properties. See volatility, viscosity.

DYNAMOMETER - device for measuring the mechanical power output or driving torque of a rotating machine.

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