Octane Question

[SilverSurfer]

Hello all. First post on BITOG!

Anyway, I am curious how gasoline is blended with ethanol. If I buy E5 or E10 pump gas that is rated at 91...is that 91 octane that was mixed with ethanol (114 octane equivalent), to produce something higher than 91? Or is it a lower octane gas blended with ethanol to produce a final product of 91 octane?

Please, no need to argue about the merits of ethanol in gasoline. It has been beaten to death and I am not interested in that thread of debate.

 

[Benzadmiral]

Originally Posted By: SilverSurfer
Hello all. First post on BITOG!

Anyway, I am curious how gasoline is blended with ethanol. If I buy E5 or E10 pump gas that is rated at 91...is that 91 octane that was mixed with ethanol (114 octane equivalent), to produce something higher than 91? Or is it a lower octane gas blended with ethanol to produce a final product of 91 octane?

Please, no need to argue about the merits of ethanol in gasoline. It has been beaten to death and I am not interested in that thread of debate.

Hello, Silver,

From reading here I think it's the latter. No way would the fuel companies give you more than you're paying for!

 

[SilverSurfer]

Originally Posted By: Benzadmiral
Originally Posted By: SilverSurfer
Hello all. First post on BITOG!

Anyway, I am curious how gasoline is blended with ethanol. If I buy E5 or E10 pump gas that is rated at 91...is that 91 octane that was mixed with ethanol (114 octane equivalent), to produce something higher than 91? Or is it a lower octane gas blended with ethanol to produce a final product of 91 octane?

Please, no need to argue about the merits of ethanol in gasoline. It has been beaten to death and I am not interested in that thread of debate.

Hello, Silver,

From reading here I think it's the latter. No way would the fuel companies give you more than you're paying for!


Thank you for the reply. This what I figured, but then that begs the question, what is the initial octane of 87 E10? Did they have to intentionally retool their refineries to produce a lower octane gasoline to start with?

 

[Eddie]

Less pure gas and ethanol to bring octane up to 87. Ed

 

[old1]

I think I read somewhere that they now make 84 octane gas, and add the 10% ethanol to get it to the 87 octane. I think 87 octane E0 is made with the 84 and a percentage of the E0 premium.

 

[bigt61]

Remember, that octane is an explosion inhibitor - lower octane is more volatile and has higher energy content (BTU). High octane is needed in high performance engines so they can squeeze the air-fuel tighter(high compression) for a bigger bang which equals more horsepower. Low octane fuel will explode too easily causing pre-ignition (it ignites all by itself from being squeezed - like a diesel) This pre-ignition causes power loss and engine damage in a gasoline engine. Octane additives are expensive - therefor the price difference.

 

[zach1900]

Originally Posted By: bigt61
Remember, that octane is an explosion inhibitor - lower octane is more volatile and has higher energy content (BTU). High octane is needed in high performance engines so they can squeeze the air-fuel tighter(high compression) for a bigger bang which equals more horsepower. Low octane fuel will explode too easily causing pre-ignition (it ignites all by itself from being squeezed - like a diesel) This pre-ignition causes power loss and engine damage in a gasoline engine. Octane additives are expensive - therefor the price difference.

Exactly, anybody I've spoken to off BITOG was under the impression that high octane = faster combustion. when the opposite is true.

 

[HerrStig]

Originally Posted By: bigt61
Remember, that octane is an explosion inhibitor - lower octane is more volatile and has higher energy content (BTU). High octane is needed in high performance engines so they can squeeze the air-fuel tighter(high compression) for a bigger bang which equals more horsepower. Low octane fuel will explode too easily causing pre-ignition (it ignites all by itself from being squeezed - like a diesel) This pre-ignition causes power loss and engine damage in a gasoline engine. Octane additives are expensive - therefor the price difference.
So what's the per gallon cost of octane additives?

 

[Donald]

So 91 octane gas sold around Albany is E0.

 

[SwampSurvivor]

Originally Posted By: Donald
So 91 octane gas sold around Albany is E0.


We have both around here. Fastrack seems to be the biggest distributor of E0 91 octane. I usually run it in the Cherokee in the summer.

 

[SilverSurfer]

Originally Posted By: bigt61
Remember, that octane is an explosion inhibitor - lower octane is more volatile and has higher energy content (BTU). High octane is needed in high performance engines so they can squeeze the air-fuel tighter(high compression) for a bigger bang which equals more horsepower. Low octane fuel will explode too easily causing pre-ignition (it ignites all by itself from being squeezed - like a diesel) This pre-ignition causes power loss and engine damage in a gasoline engine. Octane additives are expensive - therefor the price difference.


Not exactly, but you have the basic idea. Octane is the name of a molecule. The more of these molecules, the greater the gasoline's ability to resist detonation caused by high compression ratios and too much ignition advance. That is, it raises the temperature at which the fuel begins to combust. Pre-ignition is usually caused by hot spots, like carbon build up on the exhaust valve and dirty spark plugs. Either way, its bad news for engines.

Ethanol has no octane molecules, but it has an octane equivalency based on what temperature it will combust — and it is higher than gasoline.

But yes, you have identified why I am asking the question! I have E0 91 octane and E10 91 octane available locally to me (no reliable 93 octane sources). So what do I pick? Where is the power? Pure gasoline has 3% more energy (BTU's), but ethanol can withstand more compression and spark advance for more power. If E10 91 starts life as E0 91 and ethanol is added to it, then it is higher than 91 octane, and I can use more spark advance to make more power. On the other hand, if it is E0 87 and ethanol is added to it, then I am still at 91 octane, but with less BTU's, so even worse.

Yes, I have two vehicles with high compression engines (9.9:1 and 10.7:1) and I do tune them for power, so this is not just a factory motor and factory tune.

I have heard that higher octane = faster burn by uninformed people, and I have heard it is slower burn from more educated people. Personally, I am not positive there is a correlation between octane and flame speed. I could be wrong though.

 

[901Memphis]

You can always get race gas fuel additive if you want more octane.

http://race-gas.com/

 

[MNgopher]

87 octane E10 is a blend of 84/85 octane gas plus ethanol resulting in 87 octane. Not a lot of changes needed to make a lower octane gas at the refining level.

 

[SilverSurfer]

Originally Posted By: 901Memphis
You can always get race gas fuel additive if you want more octane.

http://race-gas.com/


Yes I have seen that, and it seems legit. One of the vehicles is my daily driver, so I don't want to get some radical tune that requires me to bring race-gas additive with me everywhere I go. If I go on a roadtrip or something I just want to be able to use what is available. I just want to get the most bang for my buck!

 

[kschachn]

No, not all octane molecules are alike. It depends on how highly branched the 8C molecule is. n-octane has a very low octane number, whereas iso-octane is high.

And the octane rating is an aggregate of the gasoline's composition including lower and higher carbon chains.

Originally Posted By: SilverSurfer
Not exactly, but you have the basic idea. Octane is the name of a molecule. The more of these molecules, the greater the gasoline's ability to resist detonation caused by high compression ratios and too much ignition advance. That is, it raises the temperature at which the fuel begins to combust. Pre-ignition is usually caused by hot spots, like carbon build up on the exhaust valve and dirty spark plugs. Either way, its bad news for engines.

 

[SilverSurfer]

Originally Posted By: MNgopher
87 octane E10 is a blend of 84/85 octane gas plus ethanol resulting in 87 octane. Not a lot of changes needed to make a lower octane gas at the refining level.


Thank you. Any idea what E10 91 octane starts as?

 

[kschachn]

In terms of "bang for your buck" as you put it, there will be no difference between any gasoline with identical octane ratings. As you probably already know, the test (which I have done) is what determines the rating, so it is agnostic to the composition (bang meaning resistance to detonation). The energy content of the gasoline will be different however.

 

[SilverSurfer]

Originally Posted By: kschachn
In terms of "bang for your buck" as you put it, there will be no difference between any gasoline with identical octane ratings. As you probably already know, the test (which I have done) is what determines the rating, so it is agnostic to the composition (bang meaning resistance to detonation). The energy content of the gasoline will be different however.


I am new around here, what test are you speaking of? Can you send a link?
Any validity to a correlation between octane rating of gasoline and flame speed?
I guess while I am asking, what about comparing flame speed of pure gasoline to ethanol?

 

[kschachn]

It's a standard ASTM test (D2700?) which at the time used a standardized single-cylinder engine. I did it as part of a fuels & lubricants class I took in college. If you Google around enough you will probably find descriptions of the ASTM test.

As far as flame front propagation speeds between hydrocarbons and alcohols go.... That's going to have to be from someone with a pay grade higher than mine. I'm sure pressure makes a difference but that's about all I can guess.

Octane rating is the relative resistance to detonation, detonation (I assume) meaning the transition to rapid combustion caused by a shockwave.

Originally Posted By: SilverSurfer
I am new around here, what test are you speaking of? Can you send a link?
Any validity to a correlation between octane rating of gasoline and flame speed?
I guess while I am asking, what about comparing flame speed of pure gasoline to ethanol?

 

[Pontual]

In average engine compression, Ethanol has a speed flame of roughly 30 meters per sencond and pure gas has a 25 meters per sencond speed flame, but thats dependant of static cc compression, but the ratio is 20% in favor to ethanol. And the alcohol richier mixture (9:1 air to fuel ratio, instead of 14:1) also contributes to makes a higher speed flame. Enrichment and compression makes closer O2 + fuel molecules and the power comes by in form of resultant 3rd law from pressure. That's why an E85 engine runs rough compared to the same engine at pure gas. Speedier flames bring the combustion close to a detonation situation and that translates to roughness.