I Know Nothing About Car Systems

[cRiPpLe_rOoStEr]

Originally Posted by Occasionally6

Propane would work (is used sometimes - not anywhere near where I am though )

Some backyard-mechanics recharge air-conditioners with Propane, but it's extremely dangerous. I wouldn't advice not even my enemies to do so.

[mechman600]

My brother in law charged his house A/C with propane. He says it cools better than ever.
I think he's crazy.

[cRiPpLe_rOoStEr]

Originally Posted by mechman600

My brother in law charged his house A/C with propane. He says it cools better than ever.
I think he's crazy.

I'd rather get a swamp-cooler instead of charging an A/C with Propane, specially considering the most usual building methods used in American houses. Even if it won't explode, I wouldn't want to take the risk. You might also remember Propane can be used as a poison, so if he's sleeping and the A/C starts to leak he may eventually not wake up anymore...

[jeff88]

What changes the octane of gas? I've searched and searched, but the only thing I can come up with is how much ethanol is added. But according to the CARB website, the amount of ethanol is typically 10%. So is there something else that changes the octane? Is there more hydrogen chains in 91 vs. 87? Or is ethanol the major factor at this point?

[wdb]

It's an octane rating. It's changed by adding and removing octanes, a family of hydrocarbons. Octanes affect the tendency of fuel to self-ignite under pressure, or to burn too quickly when ignited. High octane fuels do not necessarily contain any more or less energy than low octane fuels. High octane fuels do, however, allow engines to run at higher compression ratios without unwanted self-ignition or too-fast combustion of the fuel.

The higher compression ratio is where the performance improvement is found; higher fuel octane ratings are just a means to that end.

[jeff88]

I know about the compression and burning capabilities, I'm just trying to understand what is actually in the fuel that makes the difference. Sorry if I wasn't clear in my original question.

So if I want to make regular gas a premium, I just add more of the octane family of hydrocarbons, but what does it replace? According to HowItWorks, there are different families like the octane family, so does regular just have more of the other families like hexane, heptane etc.?

[some_other_dave]

There are a few different ways to look at it.

One is that the octane rating is an experimentally-determined number that represents the auto-ignition resistance of a particular batch of fuel. They use a one-cylinder test engine that I think they can change the compression ratio of dynamically, and when they start detecting knocking, they see what the numbers are and they do some math and there is the "octane rating".

One is the origin of the term "octane rating". Originally, at least, the octane scale was how the auto-ignition resistance compared to a mixture of iso-octane and n-heptane fuel. A 100-octane fuel would have the same resistance to auto-ignition as 100% iso-octane, a 50-octane fuel would have the same resistance as a 50/50 mixture of iso-octane and n-heptane, etc.

As for the exact mechanism that makes them resist auto-ignition differently, I'm not sure. It probably has at least as much to do with the shape the molecule is, especially since other isomers of octane (same atoms, arranged differently) have different "octane" values.

For details, see:
Octane rating - Wikipedia, the free encyclopedia
Octane - Wikipedia, the free encyclopedia

I would bet that the exact mixture of chemicals that go into a particular gasoline are a fairly closely-held secret by the manufacturer(s).

-soD