Best octane?

Makes sense.

Remember that octane rating isn't somehow magical power. The higher the octane (RM/2) rating the *lower* the ignitability. For high compression motors you need that to prevent pre-ignition, "spark knock"

Or if working the motor it prevents pinging. Like if towing a heavy load with a pickup, most newer trucks will allow the timing to advance a bit more with premium.

At the very least, using Premium gas in a Prius is wasting money. The lowest grade unleaded I can get from Mohawk here is 90 which is an ethanol blend. My Prius seems to like it so far.

 

I think I heard somewhere that automotive fuel is a mixture of two kinds of hydrocarbons: hexane and octane. That's probably a simplistic view of things, but that's all I remember from that discussion.

So apparently higher-octane fuels become that way because they have a higher proportion of octane in them than lower-octane fuels do. So both hexane and octane burn, but if you change the proportion then they burn in a slightly different way, appropriate for sporty cars and whatnot.

What do we know about the energy density of these different components? How much energy is created (output - input) by burning each molecule of hexane or octane? How much space is taken up by a mol of each?

Also, kinda offtopic, what's chemically different about ethanol? I've heard people say they get slightly worse mileage on ethanol-blended fuel, but how much worse, compared with how many barrels of *political statement censored* oil saved?

--Michael Spencer

 

actually gas has a mixture of several hydrocarbon chains. hexane, pentane, and heptane and octane whose ratio represented the octane rating.

heptane is a 7 carbon chain while octane is an 8 carbon chain. because of enhancements to the gasoline formula, there are other factors involved in determining octane rating. because of the longer carbon chain of octane, in a perfect world, it would provide more power. that is one reason why solutions like CNG is not more widely used.

CNG uses smaller carbon chains like propane (3) and butane (4). less energy and much lower boiling point makes it a gas (with its appropriate storage issues) at temperatures we live at.

ethanol is a 2 carbon chain with an OH added to it which chemically makes it organic solvant. ethanol is desired because when any carbon chain is burned, the best solution we can hope for is exhaust consisting of carbon dioxide and water. however cars dont burn gasoline efficiently so we end up with carbon monoxide (a poison) carbon dioxide and water. the longer the chain, the more monoxides we get.

now additives have helped a lot. but what helps the most is burning shorter carbon chains. sooo ethanol with its 2 carbons convert to carbon dioxide (a good thing in moderation since all plant life uses CO2 like we use air. now if we could just stop killing off the plant life but that is another thread) this also explains the claim of clean burning for CNG.

 

Started with 93 octane, did research switched to 87 octane, and have noticed no changes between the two.

 

I had _TERRIBLE_ mileage with 93. I was getting low 40ish MPG. Switched back to 87 and was back up to the middle 50ish MPG.

 

Dave, I've never paid much attention.. will it be obvious if the pump your at has E10?.. are they required to say?