11th
Octane Number
“When such an explosion occurs in the reaction chamber of an internal combustion engine, the piston is driven forward with a violent, rather than a gentle, stroke. Such premature explosions cause the phenomenon known as knocking. The tendency of a fuel to knock depends markedly on the nature of the hydrocarbons used. In general, branching of an alkane chain tends to inhibit knocking. The knocking characteristic of a fuel is expressed quantitatively by an “octane number.” On this arbitrary scale, n-heptane is given a value of 0 and 2,2,4-trimethylpentane (“iso-octane”) is assigned the value of 100. An octane number of 90, typical of a medium grade “standard” or “regular” gasoline has a knocking characteristic that is equivalent to that of a mixture of 90% 2,2,4-trimethylpentane and 10% n-heptane. The octane rating may be upgraded by the addition of small amounts of tetraethyllead (C2H5)4Pb, which is called an “antiknock” agent. Its function is to control the concentration of free radicals and prevent the premature explosions which are characteristic of knocking.”
