The octane rating doesn't imply how much potential energy it has, that's all down to the actual chemical composition of the particular fuel. Higher octane gas can contain either more or less energy than lower octane gas, depending on the chemical constituents of both, you can't make a blanket generalization about that. You can easily make a high octane fuel have more energy than low octane fuel, or have less energy. It's all chemistry and the number of carbon bonds in that particular fuel formulation, etc. Sure, you may have a problem actually using the energy of high octane fuel in an engine optimized for low octane, because you won't have the compression ratio to take advantage of the heat needed (activation energy actually) to release all of the energy within a useful time frame. Having the energy within the fuel and being able to release/use it are two different things obviously.

Saying that diesel has higher energy content because of it's octane rating is misleading at best, as the octane rating and energy aren't necessarily dependent on each other. Diesel fuel, having a completely different chemical makeup than gasoline, can't really be compared to it. Diesel fuel is formulated both for high energy and low octane (cetane) but it is just that..formulated. You can make a diesel fuel that has high octane and high energy (you wouldn't want to), or low octane and low energy instead (wouldn't want to do that either).

 

It's not necessarily true, but given the extremely close composition of the products from various manufacturers, it's a pretty good guideline.... Things tend to be less stable when they have more potential....

And although it may be possible, storing that diesel, or finding a vehicle that could use it even..... would be very difficult at best.

Aircraft fuels with octane ratings of 130+ tend not to run good in cars.... they have that high octane because it's needed at the low temps, and thin air at thousands of feet elevation. Can you blend it more powerful? Sure... but only to a point, then it's not stable enough, or loses the octane needed for its purpose....

Again, octane is a measure of stability, and usually, less stable indicates more power.... all else being equal.

87 pump has more potential than 93 pump.... (partially because of additives in the 93) But, it does no good if it detonates prematurely.

 

This is all about what assumptions can be made, and which can't.

You're still relating octane rating to energy. You can't draw that conclusion (you're comparing two independent things), and it's a very reckless generalization to make. Octane and energy are both related to the chemical composition, and each can be adjusted while staying largely independent from the other.

Saying a formulation is 93 or 87 doesn't say anything about it's energy, and therefore 93 and 87 can't be compared in those terms. All you know is their octane.

It's quite common for higher octane gas to be formulated to have more energy and higher octane (than lower octane gas), which is the opposite of what you are saying. This isn't always the case though, because as I said, octane and energy are largely independent. So yes, high octane could have lower energy. You just can't make assumptions for all types of gas, all 93 and 87 aren't the same in all markets during all seasons and from all companies based solely on the rated octane.

 

Aircraft fuels have very different requirements than automotive fuels, but not for the reasons you stated. Stability for aircraft fuels is for safety (high flashpoint temp) and for high altitude use (vapor pressure becomes very important) but that stability doesn't have anything to do with the octane rating. The different grades of aircraft fuel have different amounts and types of octane additives in them for use in different types of engines at different altitude and average load operating points, often with forced induction. Again, the chemical makeup can be tailored to suit whatever purpose is needed, and the octane can be adjusted largely independently of the other properties. If I remember, there are lead or MTBE additives used to increase octane, which don't really alter many of the other properties of the fuel at all. The flashpoint requirements make the high octane necessary, because higher compression is needed (for the higher activation energy) because of the high flashpoint.

So I guess aircraft fuel is probably the perfect example of being able to adjust octane independently of energy or stability.

Aircraft fuel also has a lot of additives for icing, corrosion, flashpoint, etc, which could lower energy. It's a very specialized fuel, and isn't designed for the lower operating requirements and lower purity standards of automotive fuel.

 

I really love these discussions by the way.

You guys can't see it but we're still debating this topic behind the scenes.

 

As do I my friend. As do I!