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Old 03-14-2006, 11:33 PM   #1
DerrickRH
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Question Fuel Octane rating and its relation to engine knock

hey guys, this is more of just a chemistry of fuel related question. ive been studying for a chem test (university chem I) and had some questions about octane rating and engine knock that can occur from using too low of an octane rating of gasoline in certain cars. to my understanding so far, engine knock occurs when the fuel and air mixture prematurely fires in the cylinder before the spark plug fires. ive read that this kind of engine knock occurs when you use too low of an octane rating of gas for an engine designed to run a higher octane (i.e. a WRX trying to run 87 octane). ive learned that the octane rating of gasoline is determined by the amount of isooctane vs. the amount of heptane present (so 87 octane gas is composed of 87 parts isooctane and 13 parts heptane, is this right?) why is it then that a fuel of lower octane rating is more prone to cause engine knock than that of higher octane rating?

any replies would be appreciated. and mods, if this thread is not in the right forum, please move it to a more appropriate one.

thanks,

Derrick
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Old 03-14-2006, 11:36 PM   #2
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Quote:
Originally Posted by DerrickRH
hey guys, this is more of just a chemistry of fuel related question. ive been studying for a chem test (university chem I) and had some questions about octane rating and engine knock that can occur from using too low of an octane rating of gasoline in certain cars. to my understanding so far, engine knock occurs when the fuel and air mixture prematurely fires in the cylinder before the spark plug fires. ive read that this kind of engine knock occurs when you use too low of an octane rating of gas for an engine designed to run a higher octane (i.e. a WRX trying to run 87 octane). ive learned that the octane rating of gasoline is determined by the amount of isooctane vs. the amount of heptane present (so 87 octane gas is composed of 87 parts isooctane and 13 parts heptane, is this right?) why is it then that a fuel of lower octane rating is more prone to cause engine knock than that of higher octane rating?

any replies would be appreciated. and mods, if this thread is not in the right forum, please move it to a more appropriate one.

thanks,

Derrick
don't have an answer for you, but will ask another question. if 87 is 87 parts iso-octane and 13 parts heptane, then what composes fuel that's rated over 100 octane? what's c16?
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Old 03-15-2006, 01:36 AM   #3
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I'm dumb but low lower octane isn't prone to causing knock, it's less resistant to premature ignition.

Sorry, probably stating the obvious.
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Old 03-15-2006, 09:53 AM   #4
Silverpike
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Quote:
Originally Posted by DerrickRH
hey guys, this is more of just a chemistry of fuel related question. ive been studying for a chem test (university chem I) and had some questions about octane rating and engine knock that can occur from using too low of an octane rating of gasoline in certain cars. to my understanding so far, engine knock occurs when the fuel and air mixture prematurely fires in the cylinder before the spark plug fires. ive read that this kind of engine knock occurs when you use too low of an octane rating of gas for an engine designed to run a higher octane (i.e. a WRX trying to run 87 octane). ive learned that the octane rating of gasoline is determined by the amount of isooctane vs. the amount of heptane present (so 87 octane gas is composed of 87 parts isooctane and 13 parts heptane, is this right?) why is it then that a fuel of lower octane rating is more prone to cause engine knock than that of higher octane rating?

any replies would be appreciated. and mods, if this thread is not in the right forum, please move it to a more appropriate one.

thanks,

Derrick
I can answer this one.

Octane rating does not indicate the types of hydrocarbons in the fuel. It is a made-up number that acts as a reference value. Modern pump gas is composed of literally hundreds of different hydrocarbons, all blended uniquely by each gasoline vendor. To read all the specifics about Octane (including its history), read the Gasoline FAQ.

The short answer is that the "reference" iso-octane and heptane blend has an octane rating of 100. Any other fuel composition is compared to this and assigned an number based on relative knock properties. A value of less than 100 is worse, a value higher than 100 is better.
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Old 03-15-2006, 10:41 AM   #5
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just read the gas faq... everything will be revealed to you.
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Old 03-15-2006, 11:43 AM   #6
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Knock and preignition are different problems, but both can be caused by low octane gas.

Knock is caused by the spark and happens after the spark event. The extra pressure and heat of the combustion initially started by the spark event causes a *different* location in the combustion chamber to spontaneously start its own combustion. So you have two flame fronts, the entire mixture burns too quickly, and knocks.

Preignition is not caused by the spark. The pressure and heat of the piston compressing the gas and fuel mixture causes the mixture to combust without spark. I think this is typically caused by a hot spot in the combustion chamber, or absolutely horrible gas.

As silverpike pointed out, octane is an industrial standard and measure that isn't directly tied to a specific chemical composition. It is outside the box, so to speak. I do believe the specification was originally validated as heptane and iso-octane only, but it is largely irrelevent now. Just like it is irrelevent if we decided how much 1 liter is or 1 kilogram is. Did we say "this is 1 liter of water" then say that 1 liter of water is a kilogram? Or did we say "this is 1 kg of water" then decide that amount was considered a liter? It's nothing but trivia and a history lesson now.
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Old 03-15-2006, 12:27 PM   #7
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Quote:
Originally Posted by DerrickRH
why is it then that a fuel of lower octane rating is more prone to cause engine knock, or pre-ignition than that of higher octane rating?

Derrick
alright, there's the question i asked hopefully a bit clearer.

Derrick
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Old 03-15-2006, 12:31 PM   #8
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Quote:
Originally Posted by ride5000
just read the gas faq... everything will be revealed to you.
What gas faq?
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Old 03-15-2006, 12:41 PM   #9
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Old 03-15-2006, 12:56 PM   #10
DerrickRH
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finally found the answer in the gasoline faq. thanks for showing the way you guys.

the chemical structure of the heptane chain and the bonds in it are weaker than those in a branched iso-octane chain.

thanks again,

Derrick
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Old 03-16-2006, 09:56 AM   #11
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Quote:
Originally Posted by jlagace
don't have an answer for you, but will ask another question. if 87 is 87 parts iso-octane and 13 parts heptane, then what composes fuel that's rated over 100 octane? what's c16?
By definition, in the standard test for octane, isooctane is 100 and n-heptane is 0. However aromatics, alcohols, mesitylene, etc, have higher motor and research octane numbers than isoooctane. Also, additives like tetraethyl lead and MMT can boost pure isooctane itself to over 100 octane. C16 is over 100 octane through one or more of those effects.
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Old 03-16-2006, 11:40 AM   #12
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When a test fuel resists knock better than pure iso-octane (100), then the standards call for adding tetraethyl lead to the iso-octane, up to a maximum concentration of 6 ml per US gallon, which corresponds to an octane number of 120.34. A fuel with an octane number of 116 resists knock as well as iso-octane with a TEL concentration of 3.22 ml/gal. If the fuel under test is still not knocking while adding 6 ml/gal TEL to the iso-octane does knock, then all you can say is that it's octane number is greater than 120. Examples of such fuels are mesitylene and methane.
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