Gasoline and diesel oil are both mixtures of dozens of different hydrocarbons. However, the hydrocarbon molecules in gasoline have less carbon atoms and boil at lower temperatures than do the hydrocarbons in diesel oil.

The hydrocarbons in gasoline have from 5 to 11 carbon atoms, and have a boiling point temperature range from 100 to 400 degrees Fahrenheit.

The hydrocarbons in diesel oil have from 10 to 16 carbon atoms, and have a boiling point temperature range from 350 to 600 degrees Fahrenheit.

Gasoline also has a lighter specific gravity (about 0.72) than diesel oil (about 0.85).

OCTANE Numbers:

The octane number is a measure of the antiknock quality of gasoline ... in other words, the ability of the gasoline to resist knocking when it is burned in an automobile engine. Pure iso-octane has been designated as having an octane number of 100.

Samples of gasoline are tested by running them in a single-cylinder engine mounted in a laboratory. For example, if a gasoline's antiknock performance during the test is 85 percent as good as the performance of pure iso-octane in the same test, then that gasoline is said to have an octane number of 85.

There are two different sets of engine operating conditions used during the tests to measure the octane number of a gasoline. One set of conditions is used to determine the so-called Research octane number (RON), and another set of conditions is used to determine the so-called Motor octane number (MON). The average of the RON and MON is abbreviated to (R + M)/2 and that is the octane rating usually posted at service stations in the US.

CETANE Numbers:

Similarly, the cetane number of a diesel fuel is a measure of how well that fuel performs (in a laboratory-mounted, one cylinder, 4 stroke cycle, continuously variable compression ratio diesel engine) relative to the performance of pure cetane in that same engine. Diesel fuel cetane numbers typically range from about 45 to 53.

The octane number of a gasoline and the cetane number of a diesel fuel can not be calculated. They must be measured as described above.

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FRIENDLY DISCUSSIONS:

All of the hydrocarbons in gasoline are mutually fully soluble in each other (i.e., the mixture is completely homogeneous) and there should be no separation out of a clear liquid.

If you live in an area where "gasohol" is marketed, that is a blend of gasoline and ethyl alcohol... and the alcohol will settle out under certain conditions. Perhaps, that is what is happening.

======================= Another sort of related question... I was wondering what happens when gasoline separates. I've noticed that when I buy gas for the lawn mower and let it sit too long, it separates. I was wondering how and what separates. What is the clear liquid?

Very interesting. I have never heard of that. However, since gas is a mixture of different HC's, each having a different molecular weight ( i.e. it's a heterogeneous solution), then I guess it's natural for it to separate a bit. No problem with water or rubbing alcohol because they are homogeneous solutions.

Over period of time, gasoline can oxidize and polymerize and then can form 2 layers.  These reactions are catalyzed by copper and also by exposure to sunlight.  I've seen gasoline (not gasohol) in a glass jar exposed to sunlight form 2 layers surprisingly quickly  (a few days at most I think) whereas other gasoline in a jar exposed to the atmosphere in the dark did not form 2 layers after 8 months.