104 Octane too much?

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The reason higher-octane fuels generally have lower energy content (it's not a huge difference, but still present) is that it takes more of the anti-knock additives to raise the octane, and those additives have less energy value than the fuel they replace.

You end up with less high-energy fuel per gallon, so the overall energy content per volume of the fuel is lower. However, since you can burn it more efficiently in the correct engine, you end up with more energy out, just because you're making better use of the energy present in the fuel.

Consider for a moment:
Ethanol heat content: 76,000 BTU
Gasoline heat content: 120,000 BTU

Everyone here knows that a common octane-booster is to add ethanol to gasoline to get your 93 or 95 octane fuel. You should be able to see that adding any ethanol to gasoline is going to reduce the mixture's heat content.

That's pump gas, of course, but ethanol-free high octane fuel has similarly lower-energy-content octane boosters added to gasoline and the end result is that the heat content of a gallon of high-octane fuel has lower energy content -- though that energy is more available in the appropriate high-compression engine (but not, in an engine which doesn't have high enough compression to cause detonation with the low octane fuel).

 

[bwalker]

The reason higher-octane fuels generally have lower energy content (it's not a huge difference, but still present) is that it takes more of the anti-knock additives to raise the octane, and those additives have less energy value than the fuel they replace.

You end up with less high-energy fuel per gallon, so the overall energy content per volume of the fuel is lower. However, since you can burn it more efficiently in the correct engine, you end up with more energy out, just because you're making better use of the energy present in the fuel.

Consider for a moment:
Ethanol heat content: 76,000 BTU
Gasoline heat content: 120,000 BTU

Everyone here knows that a common octane-booster is to add ethanol to gasoline to get your 93 or 95 octane fuel. You should be able to see that adding any ethanol to gasoline is going to reduce the mixture's heat content.

That's pump gas, of course, but ethanol-free high octane fuel has similarly lower-energy-content octane boosters added to gasoline and the end result is that the heat content of a gallon of high-octane fuel has lower energy content -- though that energy is more available in the appropriate high-compression engine (but not, in an engine which doesn't have high enough compression to cause detonation with the low octane fuel).

None of that is really true accross the board. Some of the t highest energy fuels are also some of the highest octane ones.
When it comes to oxygenated like ethanol, MTBE, etc. They might have a lower energy value per gallon but they make more power because you can burn more of it. The limiting factor of a motor isn't how much fuel you can force in, but how much air. Anything that adds more o2 will thus make more power. This of course requires adding more fuel.

 

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None of that is really true accross the board. Some of the t highest energy fuels are also some of the highest octane ones.
When it comes to oxygenated like ethanol, MTBE, etc. They might have a lower energy value per gallon but they make more power because you can burn more of it. The limiting factor of a motor isn't how much fuel you can force in, but how much air. Anything that adds more o2 will thus make more power. This of course requires adding more fuel.

I mean, you basically restated my point.
The raw energy content of the fuel is lower per unit volume, but it's more usable by the right engine.
We appear to agree.

 

[bwalker]

I mean, you basically restated my point.
The raw energy content of the fuel is lower per unit volume, but it's more usable by the right engine.
We appear to agree.

My issue is with the first sentence. Some of the highest octane streams are also some of the most energy dense.

 

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My issue is with the first sentence. Some of the highest octane streams are also some of the most energy dense.

We're talking about gasoline, right?
Do you have a reference for this high-octane most energy dense stuff?

 

[bwalker]

Yes, I am talking about gasoline. Toluene, a component often used in racing gasoline has a R+M/2 rating of 114 also has a high BTU rating compared to other streams.

We're talking about gasoline, right?
Do you have a reference for this high-octane most energy dense stuff?

 

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Yes, I am talking about gasoline. Toluene, a component often used in racing gasoline has a R+M/2 rating of 114 also has a high BTU rating compared to other streams.

Good point then. Considering Toluene is apparently about 132K btu/gal, it would raise the general energy level.
So all you powerheads, add toluene to get more power! Careful with the rubber parts... and cold starting could be a problem, if you go too high a mixture.

 

[bwalker]

Good point then. Considering Toluene is apparently about 132K btu/gal, it would raise the general energy level.
So all you powerheads, add toluene to get more power! Careful with the rubber parts... and cold starting could be a problem, if you go too high a mixture.

The other thing as it pertains to two strokes. A fuel only does work that's vaporized before the plug fires. High boiling point fuels like Toluene and to a lesser extent ethanol often don't vaporize and either pass through the engine unburnt, or partially burnt or leeches heat from the combustion process to vaporize them which hurts HP.