• Octane Number

    • Octane number is a standard measure of the anti-knock properties (i.e. the performance) of a motor or aviation fuel (Petrol or Gasoline). “Knock” refers to the sound caused by fuel prematurely igniting during compression.

      Octane number is a measure of the knocking tendency of gasoline fuels in spark ignition engines. The ability of a fuel to resist auto-ignition during compression and prior to the spark ignition gives it a high octane number.

    • Pure iso-octane (2,2,4-trimethylpentane), burns smoothly in a high compression engine, and is assigned an octane number of 100. n-Heptane that has a strong tendency to knock, is assigned an octane number zero. Hence, an 80 vol% iso-octane mixture (of iso-octane + n-heptane) has an octane number of 80.

    • The higher the octane number, the less likely is the fuel to ignite prematurely in the engine’s cycle and cause the engine damage.

    • In general, the octane number of a straight chain hydrocarbon is inversely proportional to molecular weight and directly proportional to the degree of branching or unsaturation.

    • If the octane number of the blend is not high enough, it may be raised by increasing the proportion of high octane alkylate, catalytically reformed product, or aromatics in the blend. Or an antiknock compound may be added to accomplish the same thing inexpensively. Tetraethyl, or tetramethyl lead were used in the early days (till 1990s) as antiknock compounds, have been phased out due to the formation of lead oxide (litharge).

    • With the advancement in technology octane number of petrol was increased without using lead compounds leading to an era of unleaded petrol

    • By 1990s MTBE (methyl tertiary butyl ether) became the dominant replacement additive used to raise the octane rating.

    • Many oxygen-containing compounds other than MTBE, such as t-butanol, t-butyl acetate, and ethanol also have antiknock capabilities.

    • Aromatics presence in gasoline increases the octane number.

    • Butane has a high blending octane number and is a very desirable component of gasoline; refiners put as much in their gasolines as vapor pressure limitations permit.

    • Octane number increases in the order: \[\text{Paraffins (low octane number)} \rightarrow \text{Naphthenes} \rightarrow \text{Aromatics (high octane number)}\]

    • Regular gasoline—has an octane rating of 85–88.