Talk:Octane

I propose a split. One article for octane(the chemical) and one for octane rating. GPTHURSTON

Yes, but what does octane do? How does it prevent against engine knocking? Should I bother filling up my 2002 Camry with 91 octane or will 87 suffice? Why do television announcers talk about "high-octane racing?"

I propose that the article absolutely not be split. Octane is a thing, not a rating, though it is treated in the media as a rating. The article needs to explain (by someone qualified to do so) how, when, and where octane is "added" to gasoline, or does it just "appear" during the refining process. Or is octane not actually in the gasoline ever, but just used as some reference measure?

Higher octane number, more octane present, more energy released from combustion

(Higher octane has nothing to do with energy being released for combustion. It's all about resisting pre-ignition)

Request - please give the reaction formula for combustion.

Engine knocking is autoignition AFTER the spark

In the early days of automotive engineering, it was assumed that engine knock was due to autoignition before the spark, but research has shown that to be incorrect. It is autoignition after the spark but before complete combustion of the fuel.

See http://www.faqs.org/faqs/autos/gasoline-faq/part3/ 74.70.32.60 02:56, 18 December 2006 (UTC)

If this is true, then it would appear that the fuel is not completely mixed - some components are igniting more readily than others. The pre-ignition theory was straight forward..

From C Ed Wright: Ignition is initiated at the spark plug gap as the spark jumps it, but autoignition occurs as the flame front traverses the compressed fuel-air mix, the burning mix expands and further compresses remaining unburned mix ahead of the flame front, causing it to ignite from the heat of compression exactly as diesel fuel ignites. This "knocking" is sometimes called "dieseling" because it is the same phenomenon but in a non-diesel engine, particularly when it causes a gasoline engine to continue to run after the ignition is shut off and no spark occurs.

Octane rating is not a media fiction, it refers to the theoretical amount of octane in a given blend that would cause resistance to autoignition to the degree that an actual blend does, even if no actual octane is in the blend. For example, if a gasoline was 100% octane, it would also have an octane rating of 100; if only 90% octane, its rating would be 90; but any mix of other components and octane rating boosters, called "anti-knock agents", that behave the same as those percentages of octane, get the same rating as actual octane percentages. And the higher the compression ratio, and to a lesser extent the further the spark advance, actually the combination of both; the higher the octane rating must be to prevent autoignition.

Octane is merely one compound of many that can be found in gasoline. It has the highest resisance to autoignition of all, and ideally, all gasoline would be pure octane, and all gas engines could then have something like 12:1 compression, for maximum power or greatest economy or a compromise of both. Unfortunately, only a tiny amount of octane per gallon is produced during "cracking" & refining gasoline, and difficult to purify as well, altogether making pure octane extremely expensive hence absolutely not feasible for general use. —Preceding unsigned comment added by 69.141.125.243 (talk) 16:29, 1 October 2008 (UTC)

Aviation fuel -- so-called Av-Gas -- and certain auto racing fuels actually have octane ratings higher than 100, despite the fact that fuel cannot be more than 100% octane, or more than 100% ANYTHING. This is simply accomplished by adding even more antiknock compound(s) and the rating number extrapolated according to the resistance to autoignition.

The octane rating has absolutely nothing to do with the amount of energy in the blend or the amount of power or speed that can be had, in any given engine rated for a given octane rating. In other words, an engine designed to run on "Regular", typically 87 Octane, actually runs best on that; using "Premium" or "Super" at 93 Octane in the same engine actually loses a little power because the higher octane-rated blend actually burns slightly slower, but costs a lot more -- you DON'T get what you think you paid for. Higher octane gas is not "better" (higher quality in any sense), it just burns slower to resist autoignition in higher-compression engines. Some turbocharged or otherwise supercharged engines require higher octane fuel because the supercharging raises the effective compression ratio, depending on actual compression ratio, relief settings, etc.

"High-octane racing" on the other hand is just excited mediababble suggesting that the racing action is more fast & furious than at some other times. Virtually all auto racing involves high-compression engines, lots of spark advance, and supercharging depending on Racing Class, hence all requiring high-octane fuel. —Preceding unsigned comment added by 69.141.125.243 (talk) 15:37, 1 October 2008 (UTC)

The formula for both gasoline & diesel fuel combustion would be a really messy mix something like:

(v)C(x)H(2x+2) + (w)O(2) + (y)N(2) = (a)CO(2) + (b)CO + (c)C + (d)H(2)O + (e)NO + (f)NO(2) + (z)C(x)H(x+2)

But this does not include all the various additives including-but-not-limited-to detergents, antiknock agents such as MTBE, ethanol, etc., that differ from one place to another, and all their unpredictable miscellaneous post-reaction compounds.

On the other hand:

(w)Ethanol + (x)O(2) = (y)CO(2) + (z)H(2)O —Preceding unsigned comment added by 69.141.125.243 (talk) 16:06, 1 October 2008 (UTC)

Octane is not octene

Common isomer for Octane is Octene, with the same C₈H₁₈.

This is false as octene is C₈H₁₆

~AQ 20:57, 2 November 2008 (UTC)