ASTM D4175 Terminology Relating to Petroleum, Petroleum Products, and Lubricants
ignition delay, n - that period of time, expressed in degrees of crank angle rotation, between the start of fuel injection and the start of combustion.
immediate seizure region, n - that region of the scar-load curve characterized by seizure or welding at the startup or by large wear scars.

DISCUSSION - Under conditions of this test method, the immediate seizure region is shown by line CD. Also, initial deflection of indicating pen on the optional friction-measuring device is larger than with nonseizure loads.

IMO, n - International Maritime Organization
impervious carbon, n - the same as impervious graphite with the exception that the base stock has not been graphitized.
impervious graphite, n - manufactured graphite that has been impregnated with a resinous material to make the final article impervious to liquids in the recommended operating range.
impregnation, n - partial filling of the open pore structure with another material.
incipient seizure or initial seizure region, n - that region at which, with an applied load, there is a momentary breakdown of the lubricating film.

DISCUSSION - This breakdown is noted by a sudden increase in the measured scar diameter, shown in Fig. 1 as line BC, and a momentary deflection of the indicating pen of the optional friction-measuring device.

increment, n - a portion of a material to be combined with other portions of the same material to provide a larger sample which will represent the whole material.
index of refraction, n - see refractive index.
indicated mean effective pressure, n - for spark-ignition engines, the measure of engine power developed in the engine cylinder or combustion chamber.
inductively coupled plasma, n - a high temperature discharge generated by passing an ionizable gas through a magnetic field induced by a radio frequency coil surrounding the tubes that carry the gas.
inHg, n - inches of mercury
inhibition load XX (ILXX), n - a statistically or graphically estimated loading rate of test material that is expected to cause a XX % inhibition of a biological process (such as growth or reproduction) which has an analog as opposed to a digital measure.

DISCUSSION - An example of a digital measure would be alive or dead. This terminology should be used for hydraulic fluids instead of the standard inhibition concentration (ICXX) when the hydraulic fluid is not completely soluble under test conditions.

DISCUSSION - An example of a digital measure would be alive/dead. This terminology (ILXX) should be used instead of the standard ICXX when the test material is not completely soluble at the test treat rates.

inhibition load XX (ILXX), n - a statistically or graphically estimated loading rate of test material that is expected to cause a XX % inhibition of a biological process (such as growth or reproduction) of a representative subpopulation of organisms under specified conditions and is expressed as an analog as opposed to digital measure.

DISCUSSION - An example of a digital measure would be alive/dead. This terminology (ILXX) should be used instead of the standard ICXX when the test material is not completely soluble at the test treat rates.

initial boiling point (IBP), n - the point at which a cumulative volume count equal to 0.5 % of the total volume count under the chromatogram is obtained.
initial boiling point (IBP), n - the corrected thermometer reading that is observed at the instant the first drop of condensate falls from the lower end of the condenser tube.
initial boiling point (IBP), n - corrected temperature reading at the instant of the first detection of condensate in the receiver.
injection timing (injection advance), n - that time in the combustion cycle, measured in degrees of crank angle, at which fuel injection into the combustion chamber is initiated.
inoculum, n - spores, bacteria, single celled organisms or other live materials that are introduced into a test medium.
inoculum, n - living spores, bacteria, single celled organisms, or other live materials that are introduced into a test medium.
inorganic sulfate, n - sulfate species present as sulfuric acid, ionic salts of this acid, or mixtures of these.

DISCUSSION - Specifically in this test method, inorganic sulfate is present as sulfate in ethanol.

insolubles, n - in lubricating grease analysis, the material remaining after the acid hydrolysis, water extraction, and solvent extraction of soap-thickened greases.

DISCUSSION - Consisting of such products as graphite, molybdenum disulfide, insoluble polymers, and so forth.

in-statistical-control, adj - process, analytical measurement system, or function that exhibits variations that can only be attributable to common cause.
instrument response time, n - the time required for an indicating or detecting device to undergo a defined displacement following an abrupt change in the property being measured.
intake manifold pressure, n - for supercharged engines, the positive pressure in the intake manifold.
integrated tester, n - automated, or semi-automated stand alone instrument utilizing multiple technologies to provide diagnostic recommendations (on-site or in-line) for condition monitoring of in-service lubricants.
intermediate sample - a sample, representative of a gross sample, upon which no analysis is to be performed, yet required for generation of analysis samples after undergoing further division and reduction.
intermittent analyzer unit cycle time - the time interval between successive updates of the analyzer output.
internal reflux, n - the liquid normally running down inside a distillation column. (Synonym - reflux.)
internal reflux - the liquid normally running down inside the column.

DISCUSSION - In the case of an adiabatic column when distilling a pure compound, the internal reflux is constant from top to bottom and is equal to the reflux at the reflux divider. When distilling crude petroleum, the fractionation occurring in the dynamic holdup will cause a temperature gradient to be established with attendant greater amount of internal reflux at the bottom of the column.

IP - abbreviation for Institute of Petroleum.
isothermal secant bulk modulus - the product of original fluid volume and the slope of the secant drawn from the origin to any specified point on the plot of pressure versus volume change divided by volume at constant temperature.
isothermal tangent bulk modulus - the product of fluid volume at any specified pressure and the partial derivative of fluid pressure with respect to volume at constant temperature.
isotropic, adj - in carbon and graphite technology, having an isotropy ratio of 0.9 to 1.1 for a specific property of interest.
isotropic nuclear graphite, n - graphite in which the isotropy ratio based on the coefficient of thermal expansion (25–500°C) is 1.00–1.10.
isotropy ratio, n - in carbon and graphite technology, the ratio of a given property value in the against grain direction to its corresponding value in the with grain direction (for example, the ratio of coefficients of thermal expansion).

jet fuel, n - any liquid suitable for the generation of power by combustion in aircraft gas turbine engines.

DISCUSSION - Different grades are characterized primarily by volatility ranges and by freezing points.