This calculator determines the minimum octane rating, also know as the knock index, by the formula (R+M)/2.
Octane rating number is defined as a value used to indicate the resistance of a motor fuel to knock. Octane numbers
are based on a scale on which isooctane is 100 (minimal knock) and heptane is 0 (bad knock). A gasoline with an octane
number of 92 has the same knock as a mixture of 92% isooctane and 8% heptane. To use our calculator, simply enter the Research Octane Number and the Motor Octane Number; click on Calculate and the minimum octane number, what you see at
the pump, is returned. We have inserted potential defaults that are correct for the most popular octane rating of 87.
You may however, change the values appropriately. This method of calculation is good for all internal combustion engines,
including automotive or general aviation. Consult the appropriate engine manual or vehicle owner's manual for the suggested
minimum octane rating. You may wish to experiment to see if you can use a grade lower fuel without creating engine ping,
in order to be optimally cost efficient.
Gasoline consists of a complex mixture of hydrocarbons. Most of these are alkanes with 4-10 carbon atoms per molecule.
Smaller amounts of aromatic compounds are present. Virtually no alkenes or alkynes are present in gasoline.
Gasoline is most often produced by the fractional distillation of petroleum, also known as crude oil (it is also
produced from coal and oil shale). The crude oil is separated according to different boiling points into fractions.
This fractional distillation process yields approximately 250 mL of straight-run gasoline for each liter of crude oil.
The yield of gasoline may be doubled by converting higher or lower boiling point fractions into hydrocarbons in the
gasoline range. Two of the main processes used to perform this conversion are cracking and isomerization.
In cracking, high molecular weight fractions and catalysts are heated to the point where the carbon-carbon bonds break. Products of the reaction include alkenes and alkanes of lower molecular weight than were present in the original fraction. The alkanes from the cracking reaction are added to the straight-run gasoline to increase the gasoline yield from the crude oil. An example of a cracking reaction is:
In the isomerization process, straight chain alkanes are converted into branched chain isomers, which burn more
efficiently. For example, pentane and a catalyst may react to yield 2-methylbutane and 2,2-dimethylpropane.
Also, some isomerization occurs during the cracking process, which increases the gasoline quality.
In internal combustion engines, the compressed gasoline-air mixtures have a tendency to ignite prematurely rather than burning smoothly. This creates engine knock, a characteristic rattling or pinging sound in one or more cylinders. The octane number of gasoline is a measure of its resistance to knock. 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 is given an octane rating of zero. It is an unbranched compound and knocks badly.
Straight-run gasoline 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. Cracking, isomerization, and other 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. Tetraethyl lead, Pb(C2H5)4, was one such agent, which was added to gas
at the rate of up to 2.4 grams per gallon of gasoline. The switch to unleaded gasoline has required the addition of
more expensive compounds, such as aromatics and highly branched alkanes, to maintain high octane numbers.
Gasoline pumps typically post octane numbers as an average of two different values. Often you may see the octane
rating quoted as (R+M)/2. One value is the research octane number (RON), which is determined with a test engine running
at a low speed of 600 rpm. The other value is the motor octane number (MON), which is determined with a test engine
running at a higher speed of 900 rpm. If, for example, a gasoline has an RON of 98 and a MON of 90, then the posted
octane number would be the average of the two values or 94.
High octane gasoline does not outperform regular octane gasoline in preventing engine deposits from forming, in
removing them, or in cleaning the engine. Consumers should select the lowest octane grade at which the car's engine
runs without knocking. Occasional light knocking or pinging won't harm the engine, and doesn't indicate a need for
higher octane. On the other hand, a heavy or persistent knock may result in engine damage.