[MNBoxster]

Originally Posted by xavier405

Excellent post, and thank you for your detail

Some questions:

- You mentioned that MTBE additive increases the octane rating, but also that octane rating is purely the ratio or percentace of octane in the 100% mixture? Are you saying that MTBE PLUS the octane percentage add up within the 100%?

- I read somewhere that higher octane also leads to slightly higher fuel mileage. From the previous essay, we can rule out higher energy content, so there may be something about the efficiency of combustion? Anybody have comments on that?

- I guess when I see a "110 Octane Racing Fuel" that it is a ploy, since apparently octane rating peaks at 100. Is that right?

Thanks very much
- X

Hi,

Thank you for your kind words.

To get to your questions, the intricacies of the subject matter are VERY complicated, so I'll simplify as best as I can and still pass on the basics. This post will still, by necessity, be somewhat lengthy, but here goes...

Gasoline is a very simple substance in it's components which are mainly Carbon and Hydrogen , but very complex in the way these two elements are first combined into over 500 different molecules, and then mixed in various quantities to form the end product - Gasoline.

That sounds a little confusing, but if you think about it, Gasoline must have many qualities at once, to meet with the dynamics of the environment and driving conditions. Just a short list includes it's ability to flow, it's ability to atomize, it's volatility (through a range of different altitudes and barometric pressures), it's ability to remain a liquid at Ambient Temperatures (or it would all evaporate from your tank, or not flow through the fuel rail into your injectors- vapor locking), and it must not readily freeze. Add to that it must be clean burning both in terms of efficiency in the Engine (so it doesn't muck-up the engine) but also with respect to post-combustion products (environmental pollution), non-corrosive, and maybe most important, it must readily burn and contain a lot of energy/vol. And, this is just a partial list of all the qualities it must possess.

Also, a gal. of gasoline weighs approximately 6.25lbs. at STP - Standard Temperature and Pressure (72°F and 1 ATM of pressure). But it is mostly made up of Carbon. In fact about 5lbs. of Carbon per Gal. ! Since elements do not readily decay, this means that every time you burn a gallon of gasoline, you are really throwing the equivalent of a 5lb. bag of Charcoal into the atmosphere as most of the carbon is released as CO²! Maybe those GreenHouse Gas people are on to something.


Gasoline comes from petroleum (Crude Oil), and contains more than 500 different Hydrocarbons. There are basically 3 categories of Hydrocarbons in Gasoline. They are:

Saturated Hydrocarbons ( aka paraffins, alkanes )

- stable, the major component of leaded gasolines.
- tend to burn in air with a clean flame.
- octane ratings depend on branching and number of carbon atoms.

Unsaturated Hydrocarbons

Their Characteristics include:
- Unstable, are the remaining component of gasoline.
- Tend to burn in air with a smoky flame.

Oxygenates

Oxygenates are just preused hydrocarbons :-). They contain oxygen, which can
not provide energy, but their structure provides a reasonable antiknock
value, thus they are good substitutes for aromatics, and they may also reduce
the smog-forming tendencies of the exhaust gases . Most oxygenates used
in gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain
1 to 6 carbon atoms. Alcohols have been used in gasolines since the 1930s. The relative advantages of aromatics and oxygenates are environmental-friendliness and low toxicity octane-enhancers. MTBE is one of these.

To get the right mix of qualities, one of which is the Octane Rating for higher compression Engines, the Octane component(s) are often a mix of different molecules from all 3 categories above. Remember not to confuse Octane Rating with Octane a Hydrocarbon molecule, one of which is 2,2,4-trimethylpentane. Once this Mix is determined, it is this mix which is combined with the Heptane to achieve a certain Octane Rating.

It is possible to have an octane rating greater than 100, but the mix making up the gasoline will never (cannot) exceed 100%.

Take Toluene for instance. It has an Octane Rating of 117. Twenty years ago, when Formula 1 Race Engines were forced (Turbocharged), the rules limited the Fuel to 105 Octane. The primary component of the Fuel was Toluene (Octane 117), but it was mixed with Heptane to lower the Octane Rating to the required 105. This mix allowed very high Boost Pressures to be run, producing lots of HP.

Could you run pure Toluene in your engine? The short answer is Yes (with some modifications), but you'd get lousy mileage, pollute like crazy and have a very inconsistent running and starting Engine depending upon environmental conditions (Temp, Humidity, Barometric Pressure), and your maintenance schedule(s) would be measured in the 100's of miles instead of the 10k+ miles. All-in-all, not a good all-around fuel.

With respect to your 2nd question about higher Octane fuels making more power, I think you may have misread whatever it was. With a higher Octane Fuel, you could increase the Engine's Compression Ratio allowing it to produce more power from the same displacement, but it is the changes to the Engine, not the Fuel which makes more power. You'd be feeding More Fuel to the Engine, hence more power (This is why the Corvette gets worse Gas mileage than say a Civic - it burns more Gas/mile) . And, combustion efficiency isn't primarily a factor of the Fuel at all in this case, but rather Engine Design.

As far as you last question, I think I've already answered this, but again, you can have Fuel with Octane Ratings over 100, but they don't usually contain any more energy and are purpose-designed Fuels, such as for Racing or Aviation and pretty much fail as all-around Fuels for a Street Car application. Hope this helps...

Happy Motoring!...Jim'99