Isooctane, reference fuel

A motor fuel has an octane number X if it behaves under tightly defined experimental conditions the same as a mixture of X volume % of isooctane and (100 - X)% of n-heptane. The isooctane-heptane binary mixtures are called primary reference fuels. Octane numbers higher than 100 can also be defined the reference material is isooctane with small quantities of tetraethyl lead added the way in which this additive acts will be discussed later. 

Octane number is a measure of a fuel s abiUty to avoid knocking. The octane number of a gasoline is deterrnined in a special single-cylinder engine where various combustion conditions can be controlled. The test engine is adjusted to give trace knock from the fuel to be rated. Various mixtures of isooctane (2,2,4-trimethyl pentane) and normal heptane are then used to find the ratio of the two reference fuels that produce the same intensity of knock as that by the unknown fuel. 

By defining isooctane as having an octane number of 100 and / -heptane as having an octane number of 0, the volumetric percentage of isooctane in heptane that matches the knock from the unknown fuel can be calculated as the octane number of the fuel. For example, 90 vol % isooctane and 10 vol % normal heptane produce a 90-octane-number reference fuel. 

Isooctane and heptane are utilized as primary reference fuels for octane number determinations. Isooctane has an octane rating of 100, and n-heptane has an octane... 

ON Octane number. A numerical measure of the antiknock properties of motor fuel, based on percentage by volume of isooctane in a standard reference fuel. 

Fig. 7.10. Comparison of ignition delay-times measured in a rapid compression machine (points) with Shell model predictions (lines) [71]. Fuels are all RON 90 with different sensitivities PRF, primary reference fuel, 10% n-heptane, 90% isooctane, MON = 90 TRF toluene reference fuel, 30% heptane, 70% toluene MON = 77.9 2-methyl-2-hexene, MON = 78.9. Compression ratio 9.6, 0.9 stoichiometric mixtures, wall temperatures 373 K. (a) Effect of temperature at end of compression charge density 3.20 x 10" mol cm . (b) Effect of charge density end of compression temperature 690 K. (Note all end of compression temperatures are averages over whole charge.) From [71].

S.K. Prabhu, H. Li, D.L. Miller and N.P. Cernansky, The Effect of Nitric Oxide on Autoignition of a Primary Reference Fuel Blend [(87 RON -Heptane/Isooctane)] in a Motored Engine, SAE Technical Paper 932757 (1993). 

Liquid paraffinic hydrocarbon containing seven carbon atoms in the molecule, which may be straight-chain (normal) or branched-chain (iso). Heptane can be used in place of hexane where a less volatile solvent is desired, as in the manufacture of certain adhesives and lacquers, and in extraction of edible and commercial oils. Heptane is blended with isooctane to create a standard reference fuel in laboratory determinations of octane number. 

ASTM Reference Fuel C modified Fuel C 85% isooctane, 15% toluene 23 60 8 Data from SPE ANTEC 1991 Conference Proceedings 92.7 Strained in flexure Thermocomp WF-1006 Filler 30% Glass fiber Specimen Type V tensile specimen... 

ASTM Reference Fuel C (cont) 50% isooctane, 50% toluene 121 90 9 0 0 Vectra A130 Hoechst Celanese Filler 30% Glass fiber Specimen 127X12.7X3.2 mm (5X0.5X0.125 in) flex bars... 

Isooctane—ASTM Knock Test Reference Fuel. ... 

The motor fuel under test is compared to two pure hydrocarbons chosen as references. The first is 2, 2, 4-trimethylpentane or isooctane which is very resistant to auto-ignition and to which is arbitrarily given the number 100 /... 

Octane Number—The octane rating of a motor fuel is defined in terms of its knocking characteristics relative to those of blends of isooctane (2.3,4-trimethylpentane) and n-heptane. and a rating of 100 to isooctane, The octane number of an unknown fuel is numerically equal to the volume percent ot isooctane in a blend with -heptane which has the same knocking tendency as the unknown fuel when both the unknown and the reference blend are run in a standard single-cylinder engine operated at specified conditions. Motor Method octane numbe-is are measured at moie- severe engine conditions and are numerically lower than those determined by the milder Research Method. The difference between the two numbers is termed sensitivity. 

Isooctane, although technically an incorrect systematic name, is the name used in the fuel industry to refer to 2,2,4-trimethylpentane. 

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