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by mikestew
1580 days ago
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It's likely about as energetic as regular fuel...
Its Octane Number is higher than regular fuel I am not a petroleum chemist/engineer/whatever, but I'm pretty sure those two things cannot be true at the same time. Higher octane fuel contains less energy, which contributes to less engine knock/pre-detonation. https://en.wikipedia.org/wiki/Octane_rating#Effects "The other rarely-discussed reality with high-octane fuels associated with 'high performance' is that as octane increases, the specific gravity and energy content of the fuel per unit of weight are reduced. The net result is that to make a given amount of power, more high-octane fuel must be burned in the engine." |
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>I'm pretty sure those two things cannot be true at the same time.
I agree, I'd give better odds it's less likely myself. That's the type of petroleum chemist I am.
>as octane increases, the specific gravity and energy content of the fuel per unit of weight are reduced. The net result is that to make a given amount of power, more high-octane fuel must be burned in the engine.
But this isn't really true either.
The wiki snippet I've quoted here is just plain uncertain on its own, even though this can be an actual trend it is only true when the octane-boosting blend component(s) have a lighter specific gravity than the base stock hydrocarbons. Not nearly as commonly seen to support such a generalization. This happened more often with MTBE than ethanol.
As far as the amount of fuel that must be burned by comparison, ethanol's influence on the specific gravity of the gasoline does not have as big an effect as the oxygen content of the alcohol itself.
Ethanol is almost 35 percent oxygen by weight, the remainder of the molecule is carbon & hydrogen.
Plain hydrocarbons whether aromatic structures or not are just carbon & hydrogen arrangements with no other elements included.
The ethanol can be considered to have a big percentage of its carbon already "oxidized" or chemically combined with oxygen naturally in advance of its energy-releasing combustion, which then releases the remaining energy when it combusts in combination with fresh oxygen from the engine air intake.
So with oxygenate additives lots of space in your tank is occupied by the dead weight of its oxygen content, which doesn't contribute to energy release like the carbon content does. Unlike with conventional plain hydrocarbon fuels which pack more carbon per gallon.
This is where specific gravity can be easier to understand.
Even though it is a unitless number and for our purposes exists only in the mathematical realm 0.6 < SG < 0.9
Hydrocarbons pack the most punch per gallon, but some hydrocarbons are heavier than others depending on structure. To a very strong extent the more carbon molecules in the structure of a particular refined hydrocarbon, it will weigh more per gallon and have a resulting higher specific gravity.
And energy content is commonly measured in BTU per weight regardless of specific gravity.
So when it is plain hydrocarbons the higher the specific gravity, the more energy you can carry in the same tank, and with miles per gallon trending up naturally.
Which is one of the advantages of diesel since it doesn't contain any of the light flammable vapors that are essential to gasoline engines. So diesel is just plain heavier per gallon and that's a big reason you get more miles per gallon with diesel.
Now chemically aromatic ring structures have a higher carbon/hydrogen ratio compared to paraffinic chains, and physically turn out to be a big notch higher in specific gravity across the board compared to their paraffinic counterparts having the same number of carbon atoms per molecule.
Benzene itself is the simplest characteristic hexagonal aromatic ring structure, known as benzol in the first half of the 20th century, low freeze-point mixtures were sold as premium fuel for early gasoline motorcars. Since it basically runs 100 (R+M)/2 on its own people paid more.
Benzene has been widely reduced by regulations but along with a handful of other light aromatics they have a high octane contribution and a high specific gravity contribution at the same time.
Considering the fundamental basis of conventional hydrocarbon gasoline this is about the opposite of what this part of Wikipedia says.
Going further when you have for example 87 octane conventional unleaded gasoline this is a high aromatic fuel with high specific gravity.
If you were to boost the antiknock rating to 90 or more by adding alcohol or MTBE especially, that would be the time that the specific gravity would decrease as the octane increased.
But today the 87 octane fuel already has 10 percent alcohol so less aromatics are in there to begin with and the specific gravity is not that high. Adding more alcohol past that point has much less of an effect.
Always remember octane is not power it is just antiknock rating.
But when a gasoline engine doesn't knock, you can get more power out of it by consuming more fuel faster and more smoothly though. Higher octane can really let some seemingy weak engines perform quite a bit more responsively.
American cars of the 1960's were not usually designed to require premium gasoline, but they still really got the full performance from your V8 when you did pony up for the 100 octane high test available everywhere for pennies more.