Induction, carb sizes, fuel types and their effects.

[RednGold86Z]

There are NO olefins that have high octane values.

In the Automotive Fuels Reference Book (second edition, appendix 10), Alkenes (Olefins) in general have a very high RON and Blending Octane numbers above 100, except a few in the octenes and heptenes - however, they're not used much in fuel (a few %). Olefins aren't stable, smell, and form gums, though. The Alkanes are in general all pretty poor Octane. Aromatics looks the best, except Benzene.

But, there's one clear phrase in the book that supports Mr. Redszus's previous statement of light front ends having lower octane (in practice). So, I retract my claim/idea that the lighter front end stuff is higher octane (with some exceptions here and there of course).

But, as most realize, fuel is a blend, and there's many exceptions - it's difficult to generalize things to always and nevers.

A snippet from: http://chemistry.about.com/gi/dynamic/o ... oline.html

The octane rating of hydrocarbons is determined by the structure of the molecule, with long, straight hydrocarbon chains producing large amounts of easily-autoignitable pre-flame decomposition species, while branched and aromatic hydrocarbons are more resistant. This also explains why the octane ratings of paraffins consistently decrease with carbon number.
....
6.12 How do other fuel properties affect octane?
Several other properties affect knock. The most significant determinant of octane is the chemical structure of the hydrocarbons and their response to the addition of octane enhancing additives. Other factors include:-
• Front End Volatility - Paraffins are the major component in gasoline, and the octane number decreases with increasing chain length or ring size, but increases with chain branching. Overall, the effect is a significant reduction in octane if front end volatility is lost, as can happen with improper or long term storage. Fuel economy on short trips can be improved by using a more volatile fuel, at the risk of carburettor icing and increased evaporative emissions.
• Final Boiling Point.- Decreases in the final boiling point increase fuel octane. Aviation gasolines have much lower final boiling points than automotive gasolines. Note that final boiling points are being reduced because the higher boiling fractions are responsible for disproportionate quantities of pollutants and toxins.

[David Redszus]

A snippet from: http://chemistry.about.com/gi/dynamic/offsite.htm?

I have a great deal of respect for Bruce Hamilton. He gets most things right. Anyone who is serious about learning fuel chemistry would do well to study Bruces's papers, even if they are a bit old.

The octane rating of hydrocarbons is determined by the structure of the molecule, with long, straight hydrocarbon chains producing large amounts of easily-autoignitable pre-flame decomposition species,

Long chain alkanes do not have high octane, no matter how many carbons they may have.

while branched and aromatic hydrocarbons are more resistant. This also explains why the octane ratings of paraffins consistently decrease with carbon number.

Branched chain alkanes have much better octane but also have higher boiling points. Aromatics all have about the same octane values but have rather higher boiling points.

Several other properties affect knock. The most significant determinant of octane is the chemical structure of the hydrocarbons and their response to the addition of octane enhancing additives.

That is quite true. The most important is the addition of TEL or (rarely) TML. Even a small amount of TEL will raise the octane number of most hydrocarbons. The degree to which octane is improved is a function of the specific hydrocarbon. Some hydrocarbons have a negative reaction to the addition of TEL; some respond very positively.

• Front End Volatility - Paraffins are the major component in gasoline, and the octane number decreases with increasing chain length or ring size, but increases with chain branching.

As mentioned earlier, straight chain hydrocarbons, regardless of carbon number, have lower octane values.

Aromatic compounds are based on the benzene ring which contains C6H6. Aromatics can be modified by the substitution of a radical group in place of one of the hydrogen atoms with a CH3 radical. Isomers of aromatics are formed when radicals are attached to different carbon atoms in the hexagonal ring.

Overall, the effect is a significant reduction in octane if front end volatility is lost, as can happen with improper or long term storage.

Not true at all. A loss of front ends will raise octane but will reduce the ability to ignite easily.

Fuel economy on short trips can be improved by using a more volatile fuel, at the risk of carburettor icing and increased evaporative emissions.

Partially true. In cold weather, a more volatile fuel is beneficial to assist ignition and avoid misfires. In warm weather, a fuel with a high stoich value and high specific gravity will produce the best fuel economy.

Final Boiling Point.- Decreases in the final boiling point increase fuel octane. Aviation gasolines have much lower final boiling points than automotive gasolines.

Actually, the opposite is true. Lower boiling points do not provide higher octane values. Aviation gasolines depend on lead additives to increase octane and do not have lower boiling points.

Note that final boiling points are being reduced because the higher boiling fractions are responsible for disproportionate quantities of pollutants and toxins.

Sort of true. But some higher fractions are useful in pump gas for street cars in order to maintain the proper temperature of the catalytic converter. Generally speaking, the lighter fractions do reduce hydrocarbon pollutants but they increase NOx emissions.

We are faced with two possibilities when it comes to light fuel fractions.
Supposed we have a fuel that does a poor job of fuel mixture preparation. The light fractions will evaporate (with lower octane) while the heavier fraction (with higher octane) remain as a liquid and produce carbon buid-up. The combustion chamber concludes that the mixture is too lean and must be enriched.

One the other hand, we have a fuel that has lost its light fractions due to evaporaton during transport or storage. Even if the fuel metering system does an excellent job of mixture preparation, there are no light fractions available to ignite easily. Now it becomes necessary to advance the timing in order to produce the proper combustion pressure angle. But throttle response is sluggish and we run the risk of misfires.