Octane number of gasoline indicates the content of carbon chain of C8 (octane) contained therein, similar to the Cetane number for diesel fuel as a measure of fuel quality despite having a different understanding. In the case of combustion, octane rating will determine the fuel ability to not ignite spontaneously in the combustion chamber. Spontaneous ignition means with the compression process without any sparks from spark plugs air and fuel mixture burn by itself. The occurrence of spontaneous combustion is characterized by the occurrence of knocking in the engine and can cause engine damage. Fuel does not burn completely but explode.
The octane number is determined by comparing the characteristics of a gasoline to isooctane (2,2,4-trimethylpentane) and heptane. Isooctane is assigned an octane number of 100. It is a highly branched compound that burns smoothly, with little knock. On the other hand, heptane, chain a straight, unbranched molecule is given an octane rating of zero because of its bad knocking properties.
Straight-run gasoline (directly from the refinery distillation column) has an octane number of about 70. In other words, straight-run gasoline has the Same knocking properties as a mixture of 70% isooctane and 30% heptane, although actually it is a complex mixture of over 500 hydrocarbons. Many of these compounds are straight-chain alkanes. Cracking, isomerization, and other refining processes can be used to Increase the octane rating of gasoline to about 90. Anti-knock agents may be added to further increase the octane rating. Fuel octane requirements for gasoline engines vary with the compression ratio of the engine.
Some of the octane number for fuel:
87 → Gasoline standards in the United States
88 → Gasoline Premium unleaded
92 → Gasoline standards in Europe, Pertamax (Pertamina product)
94 → Premix-TT (Pertamina product)
98 → PertamaxPlus (Pertamina product)