My initial thoughts

[Weldangrind]

Interesting findings. I presumed the mileage figure would drop with a higher octane level, but I hadn't considered that mileage would improve with an identical octane number.

Do you recall what the final octane level was in those tests? I presume there would be a sweet spot for the average car or truck.

[oldqwerty]

Originally Posted by Weldangrind

Interesting findings. I presumed the mileage figure would drop with a higher octane level, but I hadn't considered that mileage would improve with an identical octane number.

Do you recall what the final octane level was in those tests? I presume there would be a sweet spot for the average car or truck.

We tested gasolines from 82.3 to 106 octane (R+M)/2, diesel fuels with cetane numbers from 38 to 62, and other fuels with octanes up to 117. The happy sweet spot for all vehicles not specifically designed to run on pure biofuels is 0%.

It is impossible to recalibrate an engine to run as efficiently per unit volume on 100% ethanol or in any % mixture with gasoline because ethanol only has 5/8 the potential energy per unit of volume as gasoline.

To make matters worse, engines operated on nonimiscible fuel mixtures cannot be tuned to maximum efficiency for BOTH fuels, so efficiency of one fuel will be sorely compromised. In the case of gasohol, the ethanol does not contribute to forcing the piston down until well after the beginning of the power stroke, and typically is still burning well into the exhaust stroke, which is kind of like two riders on a tandem bicycle, and the one in the back has the brake on to keep the bike from rolling backwards. Therefore, poor combustion efficiency compromises fuel efficiency.

Also, the additives that allow immiscibility occupy fuel volume and not only do not burn, they inhibit burning. Sort of like replacing a little of each intake charge with a tiny burst from a fire extinguisher.

These are the reasons why theoretically E10 has about 97% of the potential energy of gasoline, but in the real world only provides 73 to 89% of the power and efficiency per unit volume.

[Weldangrind]

Originally Posted by oldqwerty

We tested gasolines from 82.3 to 106 octane (R+M)/2, diesel fuels with cetane numbers from 38 to 62, and other fuels with octanes up to 117. The happy sweet spot for all vehicles not specifically designed to run on pure biofuels is 0%.

It is impossible to recalibrate an engine to run as efficiently per unit volume on 100% ethanol or in any % mixture with gasoline because ethanol only has 5/8 the potential energy per unit of volume as gasoline.

To make matters worse, engines operated on nonimiscible fuel mixtures cannot be tuned to maximum efficiency for BOTH fuels, so efficiency of one fuel will be sorely compromised. In the case of gasohol, the ethanol does not contribute to forcing the piston down until well after the beginning of the power stroke, and typically is still burning well into the exhaust stroke, which is kind of like two riders on a tandem bicycle, and the one in the back has the brake on to keep the bike from rolling backwards. Therefore, poor combustion efficiency compromises fuel efficiency.

Also, the additives that allow immiscibility occupy fuel volume and not only do not burn, they inhibit burning. Sort of like replacing a little of each intake charge with a tiny burst from a fire extinguisher.

These are the reasons why theoretically E10 has about 97% of the potential energy of gasoline, but in the real world only provides 73 to 89% of the power and efficiency per unit volume.

That was an excellent explanation.

I realize that I failed to ask my question properly. I knew that Ethanol only has a fraction of the potential energy of gasoline, but I thought the number was closer to propane, which I think is around 80%. In Canada, we went through these issues in the '80's, when our gov't offered rebates to those who would have propane conversions performed. The only reason that propane seemed to be more efficient was that is was far less expensive than gasoline in the beginning. Then the price started creeping up...

The question I meant to ask was, is there an octane sweet spot for straight gasoline for modern cars and trucks? I ask that knowing that some vehicles require premium, but I'm not interested in that info. I'd like to know your thoughts about everyday passenger vehicles that run on regular.

[oldqwerty]

Originally Posted by Weldangrind

That was an excellent explanation.

I realize that I failed to ask my question properly. I knew that Ethanol only has a fraction of the potential energy of gasoline, but I thought the number was closer to propane, which I think is around 80%. In Canada, we went through these issues in the '80's, when our gov't offered rebates to those who would have propane conversions performed. The only reason that propane seemed to be more efficient was that is was far less expensive than gasoline in the beginning. Then the price started creeping up...

The question I meant to ask was, is there an octane sweet spot for straight gasoline for modern cars and trucks? I ask that knowing that some vehicles require premium, but I'm not interested in that info. I'd like to know your thoughts about everyday passenger vehicles that run on regular.

87, unless at high altitudes. 85 at high altitudes. Make sure the octane rating is (R+M)/2. Some places use different definitions of "octane rating."

Many engines loose power and efficiency as octane ratings go up because the slower flame front propagation does not allow peak cylinder pressure early enough in the combustion stroke.

Tdub (stock engine with rejet) lost power slightly over 88 octane, exponentially.

Tdub2 (267cc EFI, 11.6:1 compression, timing advanced 4* with ping sensor and ignition retard to run 87 when necessary) gains power with up to 97 octane, then levels off.

Pirate (276cc carbed, 12.7:1 compression) requires 93, and really likes 103. My son put 87 in Pirate, rode 1/4 mile, shut the bike off and pushed it home. It pinged so bad he thought he had blown the engine.

To vary octane we started with 87 E0 and added toluene, a common octane booster and paint thinner.