kw
Idle User
Posts: 65
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Post by kw on Dec 10, 2005 3:38:36 GMT -5
Thanks Brian. So, if you go back up to the original post, isn't it true he also would have gotten better fuel mileage while in the mountains (as he claimed). I can get 39 MPG down here at near sea level, out on the highway, so it would seem that 45 MPG in the mountains might not be such an exaggeration.
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Post by Brian5475E on Dec 10, 2005 16:15:38 GMT -5
You get worse milage at higher elevations due to pressure and less oxygen in the air
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kw
Idle User
Posts: 65
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Post by kw on Dec 10, 2005 23:56:39 GMT -5
Now you are confusing me, Brian. As I posted, I got the best mileage I'd ever gotten, while driving my van in the Rocky Mts. They sold gas at a slightly lower octane. I thought that was unusual, but you responded: "Because lower octane gas will ignite easier under pressure than a higher octane. so it was less work for your engine to ignite the fuel. In turn freeing power from the engine, uses less gas. Then when I suggest applying that rationale to the original post on this thread, you switch 180 degrees in your last post. Please explain further.
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exodus
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Posts: 886
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Post by exodus on Dec 24, 2005 10:47:29 GMT -5
Octane rating basically the amount of hydrocarbons in the gasoline. The higher the amount of these hydrocarbons the more resistant to combustion due to compression and heat. The spark will always ignite the gasoline.
Engines produce power by compressing the air/fuel mixture and igniting it. The aveo 1.6l engine has a 9.5:1(if I recall correctly) compression ratio. Meaning at BDC(bottom dead center) of the intake stroke there is 9.5ml^3(I think it's ml...) of air/fuel. At TDC(top) of the combustion stroke, that 9.5ml^3 is compressed to 1ml^3 (the mass stays the same, but the volume of space the mixture takes up is reduced). This compression allows the engine to harness the maximum amount of energy from the mixture.
Compression brings up a problem, though. As that mixture is compressed, it generates heat (the atoms are closer together, creating friction heat as the bounce off one another). If the gas heats up too much, it'll ignite before the piston reaches the end of the combustion stroke. This pushes down on the piston before the rod has "looped" over the crankshaft... so the rod is pushed in the opposite direction the crankshaft is turning... you can see how this presents a problem.
A modern ECU will adjust for this (to an extent). It'll put less fuel into the combustion chamber, thus limiting your power. However a higher octane fuel is more resistant to this heat, thus allowing a greater amount of fuel to be injected... and generating more power.
It should also be noted that a car that is designed with 87 octane in mind really wouldn't benefit from a higher grade fuel. The ECU isn't designed for it, and won't put more fuel in. Unless of course it is reprogrammed. Though without the computer being upgraded, you would just suffer a minor decrease in gas milage (probably not even noticable).
Now that we've all had petrol 101, on to the topic at hand. At higher elevations the air is "thinner" meaning there is less oxygen per volume of air. Less oxygen means less power. Meaning you'll have to work your engine harder to maintain a regular pace. I don't think it's really that you get worse gas milage, rather you're adjusting your driving habits to compensate for the difference in performance. At least that's my take on it.
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Post by TheAmazingJ on Jan 3, 2006 2:26:21 GMT -5
i have seen fuel estimates vary wildly before on ppls cars, it could just be that his math is off?? i must admit that is a bit optimistic of an estimate lol
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exodus
Nitrous User
Posts: 886
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Post by exodus on Jan 3, 2006 8:21:37 GMT -5
i have seen fuel estimates vary wildly before on ppls cars, it could just be that his math is off?? i must admit that is a bit optimistic of an estimate lol They always do. If you look at the EPA estimates and review actual MPG they are usually always vary by +/-5. The biggest determining factors are always driving habits, air cleaner, and tire pressure. Your estimates will always be off, also... because you're estimating.
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