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Dictionary of Lubricant Terms

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VACUUM TOWER - see distillation.

VALVE BEAT-IN - wear on the valve face or valve seat in internal combustion engines resulting from the pounding of the valve on the seat. Also called valve sink or valve recession.

VALVE LIFTER - mechanical or hydraulic device for opening and closing valves by transmitting cam rotation to vertical valve movement.

VAPOR LOCK - disruption of fuel movement to a gasoline engine carburetor caused by excessive vaporization of gasoline. Vapor lock occurs when the fuel pump, which is designed to pump liquid, loses suction as it tries to pump fuel vapor. The engine will usually stall, but in less severe cases may accelerate sluggishly or knock due to an excessively lean fuel mixture. Automotive engines are more likely to experience vapor lock during and acceleration that follows a short shutdown period. Vapor lock problems are most likely to occur in the late spring on unseasonably warm days, before the more volatile winter grades of gasoline have been replaced by the less volatile spring and summer grades (see volatility). Vapor lock can also occur in other types of pumping systems where volatile liquids are being handled.

VAPOR PRESSURE - pressure of confined vapor in equilibrium with its liquid at a specified temperature; thus, a measure of a liquids volatility. Vapor pressure of gasoline and other volatile petroleum products is commonly measured in accordance with test method ASTM D 323 (Reid vapor pressure). The apparatus is essentially a double-chambered bomb. One chamber, fitted with a pressure gauge, contains air at atmospheric pressure; the other chamber is filled with the liquid sample. The bomb is immersed in a 37.8C (100F) bath, and the resulting vapor pressure of the sample is recorded in pounds per square inch (psi). Reid vapor pressure is useful in predicting seasonal gasoline performance (e.g., higher volatility is needed in cold weather, and lower volatility in hot weather), as well as the tendencies of gasolines, solvents, and other volatile petroleum products toward evaporative loss and fire hazard.

VARNISH - hard, dry, generally lustrous oil insoluble deposit which cannot be removed by wiping with a soft cloth. Generally associated with gasoline engines. VI - see viscosity index (VI).

VISCOSIMETER - see viscometer.

VISCOSITY - measurement of a fluids resistance to flow. The common metric unit of absolute viscosity is the poise, which is defined as the force in dynes required to move a surface one square centimeter in area past a parallel surface at a speed of one centimeter per second, with the surfaces separated by a fluid film one centimeter thick. For convenience, the centipoise (cp) - one one-hundredth of a poise - is the unit customarily used. Laboratory measurements of viscosity normally use the force of gravity to produce flow through a capillary tube (viscometer) at a controlled temperature. The measurement is called kinematic viscosity. The unit of kinematic viscosity is the stoke, expressed in square centimeters per second. The more customary unit is the centistoke (cSt) - one one-hundredth of a stoke. Kinematic viscosity can be related to absolute viscosity by the equation:

cSt = cp - fluid density

In addition to kinematic viscosity, there are other methods for determining viscosity, including Saybolt Universal viscosity, Saybolt Furol viscosity, Engler viscosity, and Redwood viscosity. Since viscosity varies inversely with temperature, its value is meaningless unless the temperature at which it is determined is reported.

VISCOSITY INDEX (VI) - empirical, unitless number indicating the effect of temperature change on the kinematic viscosity of an oil. Liquids change viscosity with temperature, becoming less viscous when heated; the higher the VI of an oil, the lower its tendency to change viscosity with temperature. The VI of an oil - with known viscosity at 40C and at 100C - is determined by comparing the oil with two standard oils having and arbitrary VI of 0 and 100, respectively, and both having the same viscosity at 100C as the test oil. The following formula is used , in accordance with test method ASTM D 2270:

vi.gif (1030 bytes)

where L is the viscosity at 40C of the 0-VI oil, H is the viscosity at 40C of the 100-VI oil, and U is the viscosity at 40C of the test oil. There is an alternative calculation, also in ASTM D 2270, for oils with VIs above 100. The VI of paraffinic oils (see paraffin) is inherently high, but is low in naphthenic oils (see naphthene), and even lower in aromatic oils (often below 0). The VI of any petroleum oil can be increased by adding a viscosity index improver. High-VI lubricants are needed wherever relatively constant viscosity is required at widely varying temperatures. In an automobile, for example, an engine oil must flow freely enough to permit cold starting, but must be viscous enough after warm-up to provide full lubrication. Similarly, in an aircraft hydraulic system, which may be exposed to temperatures above 38C at ground level and temperatures below -54C at high altitudes, consistent hydraulic fluid performance requires a high viscosity index.

VISCOSITY INDEX (VI) IMPROVER - lubricant additive, usually a high-molecular-weight polymer, that reduces the tendency of an oil to change viscosity with temperature. Multi-grade oils, which provide effective lubrication over a broad temperature range, usually contain VI improvers. See viscosity index.

VISCOSITY-TEMPERATURE RELATIONSHIP - the manner in which the viscosity of a given fluid varies inversely with temperature. Because of the mathematical relationship that exists between these two variables, it is possible to predict graphically the viscosity of a petroleum fluid at any temperature within a limited range if the viscosities at two other temperatures are known. The charts used for this purpose are the ASTM Standard Viscosity-Temperature Charts for Liquid Petroleum Products, available in 6 ranges. If two known viscosity-temperature points of a fluid are located on the chart and a straight line drawn through them, other viscosity-temperature values of the fluid will fall on this line; however, values near or below the cloud point of the oil may deviate from the straight-line relationship.

VOLATILITY - expression of evaporation tendency. The more volatile a petroleum liquid, the lower its boiling point and the greater its flammability. The volatility of a petroleum product can be precisely determined by tests for evaporation rate; also, it can be estimated by tests for flash point and vapor pressure, and by distillation tests.

VOLUMETRIC EFFICIENCY - ratio of the weight of air drawn into the cylinder of an operating internal combustion engine to the weight of air the cylinder could hold at rest when the piston is at the bottom of the stroke and the valves are fully closed. Any restriction of air flow into the cylinder reduces volumetric efficiency, which, in turn, reduces power output. The volumetric efficiency of an automotive engine is usually slightly more than 80% at about half the rated speed of the engine, then decreases considerably at higher speed, thus limiting the power output of the engine. The air charge to the cylinder can be increased at high speeds by means of supercharging. See supercharger.

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