Does Premium Gasoline Have More Energy Than Regular?

[Rmay635703]

I don't remember which forum I read it on but some guy said that he used Premium in his mowers because " It started quicker , ran cooler , and made more power " .

My one underpowered 1999 era brigs 3.5 that’s bogs down on even the slightest wet or thick grass actually revs a million mph when I have fresh e15 in the tank and makes it faster to cut grass in the continuous 100% humidity tropic rainforest conditions that Wisconsin seems to have months at a time

 

[TiGeo]

Depends on if the premium 91/93 is thick or thin.

 

[goingplacesanddoingstuff]

Does a higher compression engine convert chemical energy to kinetic energy more efficiently?

 

[Rygrego]

Yeah, that's why they call it premium

Premium used to be called Ethyl when tetraethyl lead was used as an octane booster until the very early 80s.

 

[kschachn]

Does a higher compression engine convert chemical energy to kinetic energy more efficiently?

It’s a higher thermal efficiency, so yes.

 

[goingplacesanddoingstuff]

It’s a higher thermal efficiency, so yes.

Then does premium effectively have more energy under the right conditions?

 

[TiGeo]

Then does premium effectively have more energy under the right conditions?

No. It can be used to produce more energy in the combustion cycle b/c of the engine's more advanced timing which is only allowed due to the premium fuel's increase in detonation resistance.

 

[kschachn]

No. It can be used to produce more energy in the combustion cycle b/c of the engine's more advanced timing which is only allowed due to the premium fuel's increase in detonation resistance.

Or if the engine is designed with a higher compression ratio.

It’s also why automakers are lobbying for a higher minimum octane fuel. All the low-hanging efficiency fruit was picked long ago, and these days the high fruit is gone as well. This is one of the remaining ways to increase efficiency.

 

[goingplacesanddoingstuff]

No. It can be used to produce more energy in the combustion cycle b/c of the engine's more advanced timing which is only allowed due to the premium fuel's increase in detonation resistance.

I think a “yes” in front of that is equally valid…

 

[kschachn]

I think a “yes” in front of that is equally valid…

Sort of. It doesn't intrinsically have more energy, but if an engine is designed to take advantage of the higher octane fuel it can extract more of the available energy. But if the engine cannot do this then it will not.

Of course all this hinges on the way higher octane fuels are formulated. Two samples of different isomers of octane burned in a calorimeter will not have a strikingly different result but they will have a differing resistance to compression ignition. However, two formulated gasoline samples that have adjuncts other than octane may have a lower or higher BTU content quite separate from the octane rating.

 

[CarlB]

Then does premium effectively have more energy under the right conditions?

No.

 

[TiGeo]

Or if the engine is designed with a higher compression ratio.

It’s also why automakers are lobbying for a higher minimum octane fuel. All the low-hanging efficiency fruit was picked long ago, and these days the high fruit is gone as well. This is one of the remaining ways to increase efficiency.

I've read some rumblings about this, just to make 91/93 "regular" and nix 87. Tough for Americans to swallow that extra $$ I guess. We have 5/6 vheicles that take higher octane fuel so I'm just used to the cost at this point.

 

[TiGeo]

I think a “yes” in front of that is equally valid…

I'm not seeing it like you then - the gas itself is just gas, it has no additional contained energy vs. 87. The engine is what is creating more power by optimizing it's combusion cycle which is only allowed if you have more octane/resistance to detonation in a higher compression ratio engine/forced induction. It could create the same power from 87 if it didn't det early b/c of the lower octane rating. It's not more energy dense.

 

[RDY4WAR]

I think a “yes” in front of that is equally valid…

It's not. Premium pump gas does not have more thermal energy (BTU/gal or BTU/lb) than regular pump gas. End of story.

The higher octane allows more compression and timing (if applicable) to improve efficiency and thus make more power, but you'd get the same effect adding 0.5 g/l of TEL to regular 87.

 

[goingplacesanddoingstuff]

No.

I'm not seeing it like you then - the gas itself is just gas, it has no additional contained energy vs. 87. The engine is what is creating more power by optimizing its combusion cycle which is only allowed if you have more octane/resistance to detonation in a higher compression ratio engine/forced induction. It could create the same power from 87 if it didn't det early b/c of the lower octane rating. It's not more energy dense.

I think we may be missing the point. The key word was “effective.” If a higher octane fuel allows certain engines to run higher compression and convert a higher proportion of chemical energy into kinetic energy, then in those circumstances that premium gas effectively contains more usable net energy than regular gas, even though the gross chemical energy is the same.

It’s just a different way of thinking about it.

 

[CarlB]

I think we may be missing the point. The key word was “effective.” If a higher octane fuel allows certain engines to run higher compression and convert a higher proportion of chemical energy into kinetic energy, then in those circumstances that premium gas effectively contains more usable net energy than regular gas, even though the gross chemical energy is the same.

It’s just a different way of thinking about it.

You are describing something else. It's not that energy was somehow not "usable" it was just used at possibly different point in the crankshaft rotation. The BTU of the fuel cannot tell you the octane. The octane cannot tell you the BTU.

 

[goingplacesanddoingstuff]

You are describing something else. It's not that energy was somehow not "usable" it was just used at possibly different point in the crankshaft rotation. The BTU of the fuel cannot tell you the octane. The octane cannot tell you the BTU.

If you eat 100 calories of fat, is it the same effect in your body as eating 100 calories of sugar? Your body is more efficient at processing the sugar, and therefore sugar has a higher effective net energy density with respect to your metabolism.

Similarly, if a higher compression, higher thermal efficiency engine can turn a higher octane fuel of a given chemical energy into a higher proportion of kinetic energy (rather than heat), then the higher octane fuel has a higher effective net energy density with respect to that engine.

 

[CarlB]

If you eat 100 calories of fat, is it the same effect in your body as eating 100 calories of sugar? Your body is more efficient at processing the sugar, and therefore sugar has a higher effective net energy density with respect to your metabolism.

Similarly, if a higher compression, higher thermal efficiency engine can turn a higher octane fuel of a given chemical energy into a higher proportion of kinetic energy (rather than heat), then the higher octane fuel has a higher effective net energy density with respect to that engine.

No, if the timing of an engine is fixed, it makes the same power (cylinder pressure) independent of the octane you put in.

The only variance would be how the fuel was blended, since you have have higher or lower BTU at different octane ratings. It isn't because of octane, it is because of the chemicals that aren't gasoline.

Again, you are trying to make octane into something it isn't.

 

[nomas]

Some of ya'll need to go back and read the original post . This as usual has gotten way off track .

 

[RDY4WAR]

The octane rating is nothing more than a measure of the auto-ignition point of the end gases. That's it. It's actually not even the sole determining factor of pre-ignition. Factors like air/fuel homogenization, intake swirl/tumble, turbulent squish velocity, and heat of vaporization all affect how prone a cylinder will be to pre-ignition. This is aside from piston/chamber shape.

E10 pump gasoline, regardless if regular or premium, varies between 1,270-1,276 BTU/lb of stoich air with 1,274 being the average. This varies seasonally and also from one batch to another depending on the feedstock used. There is no consistency in one being more than the other.

To give an example of knock resistance, let's compare Sunoco 260 GT with E85 as they both have an anti-knock index of ~100.

Sunoco 260 GT
- BTU/lb = 17,900
- Stoich AFR = 14.1
- HoV = 12.6

17,900 / 14.1 = 1,270 BTU/lb of stoich air
12.6 / 14.1 = 0.89 BTU/lb of stoich air

Sunoco E85-R
- BTU/lb = 12,500
- Stoich AFR = 9.8
- HoV = 35.4

12,500 / 9.8 = 1,276 BTU/lb of stoich air
35.4 / 9.8 = 3.61 BTU/lb of stoich air

The difference in BTU is just 0.4% and both are ~100 octane so they should have the same power potential and same knock resistance, right? No. The E85 has +280% more cooling per lb (+406% per lb of stoich air) resulting in a cooler combustion chamber prior to ignition. Since the auto-ignition point is a matter of heat, having a cooler chamber greatly skews the knock resistance higher. This is why E85 has equivalent knock resistance to a ~116 octane race fuel.

That's just an example of how octane value can give a false perception of knock resistance. It's only 1 part of an equation for 1 variable in fuel properties.