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INDUSTRIAL LUBRICANT - any petroleum or synthetic base fluid (see synthetic lubricant) or grease commonly used in lubricating industrial equipment, such as gears, turbines, compressors.

INHIBITOR - additive that improves the performance of a petroleum product through the control of undesirable chemical reactions. See corrosion inhibitor, oxidation inhibitor, rust inhibitor.

INSOLUBLES TESTING - using ASTM D 893, material not soluble in pentane or toluene, with or without the use of a coagulant.

INTERCOOLING - cooling of a gas at a constant pressure between stages in a compressor. It permits reduced work in the compression phase because cooler gas is more easily compressed. Aftercooling is the final cooling following the last compression stage.

INTERFACIAL TENSION (IFT) - the force required to rupture the interface between two liquid phases. The interfacial tension between water and a petroleum oil can be determined by measuring the force required to move a platinum ring upward through the interface, under conditions specified by test method ASTM D 971. Since the interface can be weakened by oxidation products in the oil, this measurement may be evidence of oil deterioration. The lower the surface tension below the original value, the greater the extent of oxidation. ASTM D 971 is not widely used with additive-containing oils, since additives may affect surface tension, thus reducing the reliability of the test as an indicator of oxidation.

INTERNAL COMBUSTION ENGINE - heat engine driven directly by the expansion of combustion gases, rather than by an externally produced medium, such as steam. Basic versions of the internal combustion engine are: gasoline engine and gas engine (spark ignition), diesel engine (compression ignition), and gas turbine (continuous combustion). Diesel compression-ignition engines are more fuel-efficient than gasoline engines because compression ratios are higher, and because the absence of air throttling improves volumetric efficiency. Gasoline, gas (natural gas, propane), and diesel engines operate either on a four-stroke cycle (Otto cycle) or a two-stroke cycle. Most gasoline engines are of the four-stroke type, with operation as follows: (1) intake--piston moves down the cylinder, drawing in a fuel-air mixture through the intake valve; (2) compression--all valves closed, piston moves up, compressing the fuel-air mixture, and spark ignites mixture near top of stroke; (3) power--rapid expansion of heat combustion drives piston down, all valves remain closed; (4) exhaust--exhaust valves open and piston returns, forcing out spent gases. The diesel four-stroke cycle differs in that only air is admitted on the intake stroke, fuel is injected at the top of the compression stroke, and the fuel-air mixture is ignited by the heat of compression rather than by an electric spark. The four-stroke cycle engine has certain advantages over a two-stroke, which include: higher piston speeds, wider variation in speed and load, cooler pistons, no fuel lost through exhaust, and lower fuel consumption. The two-stroke cycle eliminates the intake and exhaust strokes of the four-stroke cycle. As the piston ascends, it compresses the charge in the cylinder, while simultaneously drawing a new fuel-air charge into the crankcase, which is air-tight. (In the diesel two-stroke cycle, only air is drawn in; the fuel is injected at the top of the compression stroke.) After ignition, the piston descends on the power stroke, simultaneously compressing the fresh charge in the crankcase. Toward the end of the power stroke, intake ports in the piston skirt admit a new fuel-air charge that sweeps exhaust products from the cylinder through exhaust ports; this means of flushing out exhaust gases is called “scavenging”. Because the crankcase is needed to contain the intake charge, it cannot double as an oil reservoir. Therefore, lubrication is generally supplied by oil that is pre-mixed with the fuel. An important advantage of the two-stroke cycle engine is that it offers twice as many power strokes per cycle and, thus, greater output for the same displacement and speed. Because two-stroke engines are light in relation to their output, they are frequently used where small engines are desirable, as in chain saws, outboard motors, and lawnmowers. Many commercial, industrial, and railroad diesel engines are also of two-stroke type. Gas turbines differ from conventional internal combustion engines in that a continuous stream of hot gases is directed at the blades of a rotor. A compressor section supplies air to a combustion chamber into which fuel is sprayed, maintaining continuous combustion. The resulting hot gases expand through the turbine unit, turning the rotor and driveshaft.

INTERNATIONAL SYSTEM OF UNITS - See SI.

INVERSE EMULSION - See emulsion.

ION - electrically charged atom, or group of atoms, that has lost or gained electrons. Electron loss makes the resulting particle positive, while electron gain makes the particle negative.

ISOMER - molecule having the same molecular formula as another molecule, but having a different structure and, therefore, different properties. As the carbon atoms in a molecule increase, the number of possible combinations, or isomers, increases sharply. For example, octane, an 8-carbon-atom molecule, has 18 isomers; decane, a 10-carbon-atom molecule, has 75 isomers.

ISOOCTANE - an isomer of octane (C8H18) having very good anti-knock properties. With a designated octane number of 100, isooctane is used as a standard for determining the octane number of gasolines.

ISOPRENE RUBBER - See polyisoprene rubber.

ISOTHERMAL - pertaining to the conduct of a process or operation of equipment under conditions of constant temperature.

ISO VISCOSITY CLASSIFICATION SYSTEM - international system, approved by the International Standards Organization (ISO), for classifying industrial lubricants according to viscosity. Each ISO viscosity grade number designation corresponds to the mid-point of a viscosity range expressed in centistokes (cSt) at 40°C. For example, a lubricant with an ISO grade of 32 has a viscosity within the range of 28.8 - 35.2 cSt, the mid-point of which is 32.