Gasoline Octane Rating 1

[lownox]

Is it a fair description of octane rating to say it is relative to the flash point of the fuel?

I have read the Wiki(octane rating) on the subject already.

I have been having a discussion with a collegue about this and his understanding is that it is related to the flame speed... which I think is not correct.

Thanks.

 

[ivymike]

no. perhaps you should also read the wikipedia article on flash point. "The flash point of a flammable liquid is the lowest temperature at which it can form an ignitable mixture in air. "

"Octane number is the number which gives the percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptane, that would have the same anti-knocking capacity as the fuel which is under consideration."

That sounds like a pretty clear description to me. You take a fuel, you run it in a test engine where you can vary compression ratio, you note the compression ratio where you observe the onset of knock, you look at a table showing the test results from the same test run w/mixtures of iso-octane and heptane, and you pick the corresponding octane number.

Higher flame speeds tend to be associated with lower propensity to knock, so your colleague is probably "more right" than you are.

 

[vc66]

Octane rating is relative to resistance to detonation. Flash point is the point at which a fuel mixture can be ignited in air.

Therefore, the higher the flash point, the longer the fuel resists detonation, the higher the octane. I'm sure there are other factors, but just by definition it seems you're correct.

Anyone, please let me know if I'm incorrect.

V

 

[dgallup]

"Therefore, the higher the flash point, the longer the fuel resists detonation, the higher the octane. I'm sure there are other factors, but just by definition it seems you're correct."

Not at all. Propane has a flash point of -156 F but has a higher octane rating than iso-octane with a flash point of 10 F

 

[dcasto]

you are correct. This has been discussed before and on other fourms especially relation to the use of hydrogen to improve flame speeds.

 

[lownox]

Interesting. So, the three typical octane rating that we see 87, 91, & 93... the flash points don't increase respectively for these three??? I assumed the flame speed would be about the same.

This discussion assumes the engine design is constant... maybe and engine designed for 87 octane being used with 93.

Thanks for all the replies so far.

 

[lownox]

My mistake... I was saying flash point when I should have said autoignition temperature. Sorry.

 

[dcasto]

since all three grades must meet the same exact physical properties of flash, end point, IP, EP and the like to be called gasoline, the only difference is in how the mixture of differnt feedstocks can be combined to meet the octane test requirements. The BTU/lb of each octane is within measurement errors too.

 

[patprimmer]

Still wrong, but not so much.

THE TEST IS TO RUN THE FUEL IN A TEST ENGINE. END OF STORY.

Higher auto ignition point helps increase octane, but there are other factors like a:f ratio and latent heat of evaporation of the fuel.

Do a google site search and save us the problem of doing this yet one more time.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[black2003cobra]

lownox - Read section 6.3 of the web document below. I believe it will answer your questions.

http://chemistry.about.com/gi/dynam...tp://www.turborick.com/gsxr1127/gasoline.html

 

[dicer]

Octane, is resistance to burn. So it is as far as you can get from flash point.

Cetane, is the very oppposite of Octane.

And when it comes to the fuel types each designation pertains to, they are both very oppposing.

High octane is hard to self ignite, so it has a very low cetane rating. High cetane is very easy to self ignite, thus it has a very low octane rating.

And if these fuels are used in the opposite engine in which they were intended. You would get knocking for reasons that are counter intuitive.
High octane gasoline will knock in a diesel because it will not ignite fast enough. And diesel would knock in an SI engine because of the low octane rating, it wants to burn too soon.

 

[Lou Scannon]

Seems intuitive to me, after all these years...

 

[lownox]

black2003cobra, You nailed it. Thanks for the doc. I was thinking about it correctly after all... except I had somehow confused flash point with autoignition temperature. These things happen I suppose. Thanks again for all the replies and sorry to put you through a nub thread.

Steve