Talk:Octane rating

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wtfspeak[edit]

This article is written in what I call 'wtfspeak'; to get anything out of it, you need to have some preexisting techincal knowledge. I think it needs to be dumbified for those of us who lack that knowledge. As is, I have no idea what the author is talking about or how it applies to consumers. —Preceding unsigned comment added by 65.113.71.3 (talkcontribs) 18:25, 30 July 2008

You have a point to the extent that the introduction didn't explain why a normal person would care about octane rating. I have added a sentence which says that high-performance engines need high octane fuels, and that it's got to do with their compression ratio. I have also removed a sentence which effectively duplicates other information and uses some jargon, and I have added the comment (which is buried lower in the article) that higher-octane fuel does not generally help for low-performance engines, as well as some info on how it is defined and tested. For most people, this is all they really need to know. If you have some time and energy to help, read WP:MTAA, and make suggestions or even WP:Be bold and edit the article yourself (but then also be willing to accept that not all your edits will stick!) Especially be careful of "dumbing down" the article, as that is explicitly not recommended by the "MTAA" guideline.
I hope this helps, let us know what you don't understand; maybe we can make some more improvements. --Slashme (talk) 07:02, 31 July 2008 (UTC)

says that methamphetamynes are good for the environment"

what !?? "methamphetamynes" I don't think so ? and who is

Thank you for your suggestion! When you feel an article needs improvement, please feel free to make those changes. Wikipedia is a wiki, so anyone can edit almost any article by simply following the Edit this page link at the top. You don't even need to log in (although there are many reasons why you might want to). The Wikipedia community encourages you to be bold in updating pages. Don't worry too much about making honest mistakes — they're likely to be found and corrected quickly. If you're not sure how editing works, check out how to edit a page, or use the sandbox to try out your editing skills. New contributors are always welcome.

Octane[edit]

I have removed the following sentance from the end of the paragraph.

"A higher octane will burn faster and allow the auto to run more clean."

This is simply not true. It depends on the chemical composition of the fuel. Flame front propagation speeds are not goverened by octane.

I'm taking this out! This is so wrong. "However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane." This is what people tell themselves to rationalize getting premium fuel when they don't really need it. High octane fuel actually has fewer BTU compared to low octane fuel; thus LESS ENERGY. —Preceding unsigned comment added by 67.175.221.197 (talk) 03:13, 27 September 2007 (UTC)

Octane refers to Gasoline ONLY. So the statement that Octane refers to a fuel is in error. Krontach (talk) 08:14, 22 May 2011 (UTC)

Gasoline energy content varies from about 110k BTU to 125k BTU. Adding ethanol, which is mandated, does lower this. Ethanol at 10% lowers mileage by about 2 to 2.5 mpg. Krontach (talk) 08:49, 22 May 2011 (UTC)

Recommended Octane[edit]

Is the assertion that recommended octane making "a huge difference" adequately supported by the Dinan article? The article is about one test with one model of BMW, one time. It wasn't peer-reviewed or stablished for other cars, other models, or even shown repeatable. -- Mikeblas 16:49, 4 January 2006 (UTC)

11 HP over 280 is about 4%. This is "NOT" huge. This is minor.
Furthermore - the only way you can see this is that the engine can produce slightly more torque at a given RPM. The velocity of the car at a given engine RPM is fixed. This is a function of the gear ratio of the transmission and the rear end. Since this is the case, at any given RPM and consequently velocity of the vehical - there is nothing that suggests that the torque the engine _can_ produce is even needed. IE - the additional "power" the additional octane may confer may not be required.
This can be seen by looking at the lower RPM portions of the curve. In order to conduct this test the amount of power the engine is asked to produce at a given RPM can be scaled back from the maximum then the fuel consumption at this lower power setting can be compared between the various fuels. The cited white paper does not do this.
The differences in octane between fuels makes little difference in an engine which is performing at below its maximum power for the following simple reasons. In order for the engine at a given RPM to produce less than its maximum horsepower, the engine must be choked. The choking is accomplished by restricting the air flow to the engine (usually by a manifold restriction such as the butterfly value in the intake manifold). Given the amount of air restriction and resultant intake manifold vacuum, the fuel mixture is then optimized for these conditions. Choking the engine by reducing the air flow is exactly the same as reducing the final compression ratio. Another way of looking at this is that the average gasoline engine at idle is experiencing air pressures akin to the top of Mount Everest.
To measure an engine's performance at maxium compression ratio when it is normally performing at far less than this limit - and possibly at only 1/3 to 1/2 in fact - leads to a totally incorrect conclusion.
Terrell Larson 6 January 2006
A modest suggestion: Seems to me that what this page needs is a definitive statement on whether there are any real benefits - in an engine that accepts both - to using higher-octane fuel. As I understand it, there isn't.
But UK fuel companies are apparently very keen to foster the belief that 98 octane petrol is more powerful, cleaner, and in every way a superior product to 95, and the higher cost is just the price one pays for all this extra wonderfulness.
As far as I'm aware, as long as your engine compression ratio is below the autoignition point of 95 octane fuel, you should receive no advantage at all from 98. Is this the case? If so, this information really deserves to be propagated, because at present many people fork out the extra money - which may be considerable over time - in the firm belief that their engines and their driving will benefit. Those who pocket the price difference, all the way up the line, will be in no hurry to disillusion them.
Indeed, the choice of names for 98-octane fuel, which usually contain superlatives like Ultra- or Super-, reinforce the idea that a fuel that is basically just harder to light is better in every way: after all, 98 is more than 95, isn't it?
Is anyone qualified to confirm this one way or the other? Cdavis999 21:04, 1 March 2007 (UTC)
This page is a good summary of the above: http://www.petrolprices.com/about-fuel.html Paraphrased (talk) 22:59, 19 May 2010 (UTC)

In an engine that has active knock control (ignition is advanced or retarded depending on knock sensor feedback) and is knock-limited at full load (the ignition timing cannot be advanced due to knock), an engine can produce more torque with the same amount of fuel due to a increase in the combustion efficiency. Once the ignition timing is advanced to it's optimal point, there are no more thermodynamic benefits to advancing ignition timing. But almost all production engines run very retarded ignition timings at full load due to knock limitations, so an advance is almost always possible. Thus, in an engine with active knock control, a higher octane fuel will lead to increased torque at a given engine speed than if it had lower octane fuel.198.202.236.129 (talk) 21:41, 23 August 2010 (UTC)Dan


"The differences in octane between fuels makes little difference in an engine which is performing at below its maximum power for the following simple reasons. In order for the engine at a given RPM to produce less than its maximum horsepower, the engine must be choked."

This is misleading because of the terminology. The term "choked" does not apply here. Choking implies restricting all air. You mean to "throttle" an engine with a "throttle valve". This implies that you add a restriction upstream such that the air pressure after the restriction is lower than ambient.

"Choking the engine by reducing the air flow is exactly the same as reducing the final compression ratio."

No. The geometric compression ratio is calculated by comparing the chamber volume at BDC to the chamber volume at TDC. This doesn't change. You still compress the air (even at a vacuum) at this ratio. You do not change the effective compression ratio this way. This is completely incorrect. One can change the effective expansion ratio or compression ratio with various valve timing techniques, but not with throttling.

"Another way of looking at this is that the average gasoline engine at idle is experiencing air pressures akin to the top of Mount Everest."

But that volume of air (with whatever density you assume) is still being compressed at the compression ratio. That's why it's presented as a ratio, so it can be normalized with whatever pressure you have to start the cycle with.

"To measure an engine's performance at maxium compression ratio when it is normally performing at far less than this limit - and possibly at only 1/3 to 1/2 in fact - leads to a totally incorrect conclusion."

I can't understand what this means.198.202.236.129 (talk) 21:51, 23 August 2010 (UTC)Dan

Change powerful from link to ordinary word[edit]

The article contains this passage:

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful.

The word powerful was a link to the disambiguation page Power, which lists many possible meanings of 'power'. Since this sentence is describing the way the public misunderstand the meaning of 'powerful', I believe a precise definiton of power would only be a distraction in this context.

I came across this issue while I was disambiguating the Power page. Gerry Ashton 04:57, 26 May 2006 (UTC)

Disambiguation link repair - You can help!
In the absence of any objection, I have de-linked as described above. Gerry Ashton 13:58, 31 May 2006 (UTC)

The idea that higher octane is more powerful is supported by all race engines using very high octanes (typically 110).Dac8 (talk) 13:12, 12 December 2011 (UTC)

Ethanol under Knock resistance[edit]

This seems to be a pro-ethanol plug that somebody just decided to place there. It just seems really alone...but that may just be bias talking. — Lunarbunny 20:10, 6 August 2006 (UTC) The idea that higher octane is more powerful is supported by all race cars and boats using around 110 octane. — Preceding unsigned comment added by Dac8 (talkcontribs) 13:03, 12 December 2011 (UTC)

Maximum power vs. maximum torque[edit]

"On a typical high-rev'ving motorcycle engine, for example, the maximum power occurs at a point where the movements of the intake and exhaust valves are timed in such a way to maximize the compression loading of the cylinder"

I believe this is where the maximum torque occurs. The maximum torque always occurs at the rpm where the maximum engine efficiency occurs. And it sounds like this is the point they are talking about. Does anyone have any ideas, or a reference to where the maximum power information came from?

Not necessarily. Motorcycle engines tend to be very primitive. Maximum power occurs at a specific rpm and is determined by the torque multiplied by the rpm. But torque is determined by a number of factors which change. Chief among these is volumetric efficiency which is measured as BMEP (Brake Mean Effective Pressure).
The efficiency of cylinder filling (BMEP), engine friction loss, and the ability of the engine to revolve at a certain speed determines instantaneous power, which is nearly irrelevant to driving performance. Variable valve timing is now being used to maximize engine performance over a wider range of rpms by varying the torque producing characteristics of the engine to better suit the rpm.
Aside from issues of volumetric efficiency, torque is almost exactly a measurement of engine size. In engine design though, a larger piston is harder to move and will disintegrate at a lower rpm than a smaller piston. So engine cylinder numbers, rpm, bmep, and friction all provide limits to power output. In modern engines, power output is almost directly related to the amount of fuel burned. Note that engines typically have a specific rpm range where specific fuel consumption is the lowest for a given output. This is usually not in the same rpm range as the highest power output. So engines setup for high power are inefficient. Variable valve timing helps to compensate somewhat for this.
Higher loading of the piston, by definition, also means more friction loss through the rings. The Miller cycle design of the toyota prius engine is designed to lower friction to a minimum, but is very inefficient. The Prius is no more efficient than conventional engines that are simply sized correctly for the load.
Motorcycle engines, due to their emphasis on high performance, also won't tolerate lower octane fuels unless they have modern fuel injection. They are also very high polluting. The worst polluting engines in use today are the Day and Clerk cycle engines used in motorcycles. But Harley-Davidson V-Twins are massive polluters due to their primitive hot running designs. Krontach (talk) 09:11, 22 May 2011 (UTC)

--Alex 130.83.244.129 15:55, 14 October 2006 (UTC)

In the article the Btu value of gasoline is compared to other fuels. A question is asked regarding the volume assumed in the comparison; the text states a value of 19,000 Btu for gasoline, but does not specify a volume (in other words, is this per gallon, per liter, or what?)

Various websites indicate gasoline is rated somewhere between 125,000 and 114,000 Btu / gallon

So the article's mention of 19,000 Btu for gasoline does not refer to a gallon volume. I wonder what they were referring to?

66.123.2.35 21:28, 27 March 2007 (UTC) Rich Flynn

The article is very misleading here and as someone else has hinted could be read as a shameless plug for ethanol motor fuel. The 19,000 BTU figure is approximately correct for a pound of gasoline and is how the specific heat of combustion of fuels is measured in the industry, i.e. in BTU/lb. The figure for the ethanol is obviously not for a pound of ethanol but for it's optimal fuel ratio, given the same size of air charge as used for the gasoline, so I figure it's for 12.5/6.5lbs, i.e. ~1.9lbs of ethanol. Macdonaldinho 08:17, 11 April 2007 (UTC)

Examples of octane ratings[edit]

I have removed the statement "Note: The octane rating of cyclohexane significantly varies form source to source. See for example [3]" since it is wrong and the reference given has no relevant info to confirm the observation. Cyclohexane is a pure chemical and has the same octane no matter its source; what can vary, like many other blending agents but dramatically for cyclohexane, is its actual octane contribution... depending on the mix of chemicals to which it's being added.

Also note that the "table" which is supposed to appear in this section is misplaced and actually appears later in the article, currently globbed together with the "specific energy" table. It appears to have moved in the past few days so maybe the author responsible can take care of that. Scunnerous 04:45, 14 April 2007 (UTC)

Octane question[edit]

Removed the question: [Can someone please determine the correct unit for this measurement? Does the gasoline release 19,000 BTU per cycle, per gallon, or what. It is tough to determine whether ethanol is actually being proven to be a better fuel in this article]. It should be in the discussion, not in the article.

Gasoline, developed first in 1908 by the United States Navy, has undergone decades of improvement. Originally you had to go to your local apothecary with your hygrometer and measure the specific density of your fuel to make sure it would run in your engine. The first filling station was built in Detroit in 1910 after the Navy created the specification for gasoline.
By 1912 all the known reserves of crude oil from which gasoline could be made were known to be running out. Charles F. Kettering of Dayton Engineering Laboratories (Delco) was given the task of finding a way to boost the compression ratio of Otto engines. At that time 6 to 1 was about all that an engine could run at prior to destruction. Kettering put two of his assistants to work studying Kerosene (which had NEVER been able to be used as a motor fuel). His belief was that the reason engines blew up using kerosene was that the fuel did not burn totally and was continuing to burn in the exhaust causing a melt down, broken rods, pistons, valves, guides, etc. etc.
What was found instead was Pre-Ignition. Ethanol is much less likely to pre-ignite. But it has less heat energy. Ethanol is bio-degradable, carbon neutral. From the standpoint of whether humans will poison themselves to death with soot, CO, CO2, NOS, H2S etc. etc. Ethanol is preferable to preserve life on the planet. Otherwise gasoline is a better fuel for high power and to preserve the petroleum monopolies that keep us all permanently indebted to heat our uninsulated homes and to drive our inefficient vehicles.
Gasoline is good for the Oil Monopoly. —Preceding unsigned comment added by Krontach (talkcontribs) 09:24, 22 May 2011 (UTC)

"Energy released per air fuel ratio"?[edit]

Energy cannot be released per air fuel ratio!

  • energy per kg of fuel (values mentioned by factor 10 too low!)
  • energy per kg of air (most propable case for these values)
I agree, this doesn't make much sense to me...The energy_density article shows ethanol having a much lower energy density than gasoline (30 and 46, respectively), so I'd like to know how these figures are calculated. While I agree that due to running a richer mixture "the net energy released per cycle is higher", this doesn't translate to an increased net energy of ethanol. Mattjm 22:16, 15 November 2007 (UTC)

Fuel octane's effect on pollutants emitted[edit]

I was wondering whether higher or lower octane fuels emit more pollants as a result of combustion (and why). If you have any idea please let me know. 199.159.108.208 17:23, 5 July 2007 (UTC)

I can't see why it would make a difference with the possible exception of when the engine is actually knocking, but even then... —Ben FrantzDale (talk) 00:36, 18 April 2008 (UTC)

Detonation vs. autoignition[edit]

I just raised this complaint on gasoline and figured I'd check if it came from here. This article repeated uses the terms "detonation" and "autoignition" interchangeably, which is incorrect. Detonation is not the correct term. A detonation is a premixed flame wave structure which is self-sustaining through the coupling of a shock front and subsequent heat release. There is no wave structure in knock. I think the correct term is autoignition, referring to the phenomenon of a fuel/air premixture which self-ignites due to high ambient temperatures and pressures, and should be made to be consistent throughout the article. Unless someone else wants to argue that point, I'll make the change in a week or so. Thermodude 16:52, 10 July 2007 (UTC)

detonation is actually the correct term. it is easy to prove as well, an engine running on fuel such as methanol can be made to auto ignite very easily without knocking. "knock" is quite audible and can be detected with a microphone type pick up while pre ignition or auto ignition makes nearly no sound at all. it is mealy a dull "thud" if it can be heard. the knocking sound is the result of the fuel exploding rather than burning. this happens well after combustion has started, during the period where the pressures and temperatures are highest. its a bit like comparing gunpowder to high explosives, normal burning vs knocking. indeed there are many fuels with high octane ratings that have extremely poor resistance to auto ignition, methyl ethyl ketone is one fine example. one reason i think the two are grouped together is that they often accompany each other. an engine that starts to knock can over heat the spark plug and start to preignite. this has killed countless methanol engines. i intend to edit this article to be accurate but will not until i have better data. —Preceding unsigned comment added by 4.227.193.229 (talk) 02:07, 14 December 2009 (UTC)
Haven't found the shift key on the keyboard yet, have we, archyl?—QuicksilverT @ 03:55, 24 September 2010 (UTC)

Definition of knock[edit]

I was reading this article as a n00b and the term 'knock' doesn't mean anything to me. Can anyone create a stub and link it? Jddriessen (talk) 13:34, 29 January 2008 (UTC)

The article already exists, and is rather more than a stub. :-) Indeed, it's already linked to in the introduction. Angus Lepper(T, C, D) 14:56, 29 January 2008 (UTC)

Effects of octane rating[edit]

The sentence "It might seem odd that fuels with higher octane ratings explode less easily and are therefore more powerful." was simply incorrect, so I changed it to read "It might seem odd that fuels with higher octane ratings explode less easily and can therefore be used in more powerful engines."

The whole section needs a re-write, however. It's disjointed and confusing. --Slashme (talk) 07:29, 31 July 2008 (UTC)

Octane rating of hydrogen[edit]

Maybe this article will have a good reference to the octane rating of pure hydrogen as fuel. Does someone here have access to it? --Slashme (talk) 07:34, 31 July 2008 (UTC)

I don't have the specifics on Hydrogen but I can tell you it requires that ignition timing be AFTER TDC due to it's burning so fast. I don't believe Hydrogen will ever be a viable fuel due to the necessity to compress it to about 10,000 psi to get a decent range out of it. Krontach (talk) 09:28, 22 May 2011 (UTC)

Effects of octane rating[edit]

The entire premise of this article is just wrong wrong WRONG... Octane is the measurement of combustion speed at a given pressure and temperature, yet the article repeatedly speaks of the fuel's resistance to pre-ignition, which is just nonsense. This article needs a complete rewrite with verifiable sources. Shreditor (talk) 05:19, 17 August 2008 (UTC)

This book disagrees with you: They say that "... knocking tendency depends on the autoignition temperature, ignition lag and flame speed of the air-fuel mixture..." --Slashme (talk) 13:36, 4 September 2008 (UTC)
I don't think the book disagrees with me. I think the terms used in engineering parlance in that book are being improperly used in the article. For example, auto-ignition is different from pre-ignition. Auto-ignition temperature is the temperature needed to ignite the fuel at a given pressure. Pre-ignition is a mechanic's term, defined as the fuel igniting before the spark plug fires. Auto-ignition temperature is definitely a factor in detonation, and it is a factor in pre-ignition. However, pre-ignition does not necessarily cause detonation, or vice versa. Whoever wrote this article got their words mixed up and it turned out very confusing. I'm going to start making some changes. See what you think. Shreditor (talk) 04:13, 5 September 2008 (UTC)
Sorry, I didn't express myself clearly enough. When you say "Octane is the measurement of combustion speed at a given pressure and temperature" (I assume you mean "the octane rating of a fuel is ...") that disagrees with what the book says. The octane rating is defined by the knocking tendency of the fuel, which is dependent on more factors than just the flame speed. --Slashme (talk) 05:42, 5 September 2008 (UTC)
I've taken a look at your edit, and it's very good. I've just done some copy-editing on the prose here and there. One thing that you took out was the comparison with the cetane number. I presume it's because it was just wrong, but can you possibly write a better version? I think it's a natural comparison to make in this article. --Slashme (talk) 06:08, 5 September 2008 (UTC)
I am not an engineer, nor a chemist. I am a mechanic and I see it from that perspective. As I have understood it from the textbooks I have read, the speed of combustion is the main factor in detonation. If the fuel starts burning too fast, the pressure rises rapidly and the remaining fuel explodes due to shock compression. I could be oversimplifying here. These matters are rarely as simple as one factor causing one effect. If you or someone else can make better sense of the engineering speak and explain it in layman's terms, it would help out the article a lot.
Now, as I understand it, octane rating and cetane rating are not really related. Octane rating measures the fuel's tendency to combust supersonically (explode; detonate). Cetane rating has to do with the fuel's ignition temperature. That is why I removed that section from this article. Shreditor (talk) 18:56, 6 September 2008 (UTC)

its hard to generalize anything about this subject as its just so complex. i think the whole article should be rewritten myself. its my understanding that rate of burning has an influence on tendency to knock but there is no real correlation that can be measured. a good example is that adding TEL to some fuels dose not change the speed at which it burns but greatly inhibits knock. alcohols tend to resist knock and burn slower but many compounds that burn fast are quite knock free. cetane and octane ratings are quite different too, many fuels have a high octane # but also a high cetane # while others don't. —Preceding unsigned comment added by 4.227.193.229 (talk) 02:19, 14 December 2009 (UTC)

Usage in Russia[edit]

"In Russia and CIS countries 80 RON (76 MON) is the minimum available, the standard is 92 RON, however, the most used type is 95 RON."

I dare say this is incorrect. Most people actually use 92, particularly so outside Moscow; 95 is generally considered above average. 98 is also available here and there, but is very rarely used. -- int19h (talk) 20:24, 30 November 2008 (UTC)

Where was that gas station picture taken?[edit]

I'm kind of an anthro-dork, and I found it really interesting. I've been all over the US and have never seen anything like that.

Sit booboo sit (talk) 11:06, 6 December 2008 (UTC)

Hmmm... There has been a recent discussion about this photograph on the Wikimedia Commons. One people thinks this photograph is photoshopped. I completely agree with him: the labels "Plus" are roughly made and added, so is also at least one of labels at the bottom of the image. -- Basilus (talk) 15:16, 2 March 2010 (UTC)


I've seen int the USA at a full service home heating oil comapny's gas station have gasoline in 5 different octanes. Then a diferent fullservice station, a Sunoco, have 4 different octanes, 87, 89, 91, and 93(or 92?). 71.58.198.190 (talk) 00:21, 7 November 2011 (UTC)

Star system[edit]

Prior to the switch to unleaded there was afaict a star system used to rate the knock resistance of petrol in the UK. Afaict all grades except four star gradually dissapeared (and four star was eventually replaced by LRP) as unleaded (which wasn't rated on the star system) came in. Anyone got any further information on this system. Plugwash (talk) 22:15, 22 December 2008 (UTC)

Under this system one star was the lowest grade, 2 star was 92 octane, 3 star 95 octane, 4 star 98 octane and 5 star 101 octane. — Preceding unsigned comment added by 78.144.90.52 (talk) 01:41, 4 August 2011 (UTC)

Actually as I remember it, it was: 5-Star = 100 octane, 4-Star = 98 octane, 3-Star = 93 octane, 2-Star = 87 octane, but your figures may be correct. The 5-Star rating was the motor equivalent of avgas and was only used in high-performance cars with high compression-ratio engines, such as Jaguars, etc. It was also used for racing engines.
During the Second World War the country's petrol stocks for motor vehicles were pooled (combined and amalgamated into one grade) and was of a fairly low octane and was known as 'pool petrol' ('motor spirit'). The best was reserved for avgas ('aviation spirit') of 100 octane, although a higher rating blend of 150 octane was used for fighters later in the war. All the fuel used had to be brought in by ship. — Preceding unsigned comment added by 80.4.57.101 (talk) 19:48, 26 September 2011 (UTC)
The "Star System" was detailed in BS4040 "Specification for leaded petrol (gasoline) for motor vehicles", introduced in the 1960s as a replacement for informal bandings of "Best" or "Super" (99-101 RON); "Premium" (96-98 RON); "Mixture" (95 RON, from mixing Premium and Regular); "Standard" or "Regular" (89-91 RON). I believe the octane ratings were a (seldom encountered) 1-Star (89 RON); 2-Star (92 RON); 3-Star (95 RON); 4-Star (98 RON); 5-Star (101 RON) Somersetlevels (talk) 14:00, 28 April 2013 (UTC)

Has anyone considered the effect of increased ignition advance?[edit]

Most modern engines use a knock sensor to control ignition advance. Using a higher-than-necessary octane fuel will allow the engine to use a more aggressive ignition advance than if the minimum octane fuel were used. Therefore, using a higher octane fuel can increase power, especially at high engine speeds at high throttle. Is this correct?

On the other hand, use of an increased ignition advance will probably increase total wear on engine parts - since the combustion will be started earlier, the average amount of downward force on the piston will be greater during end of the compression stroke. Does this increased friction overwhelm the benefit of increased ignition advance (assuming a normal compression ratio which does not require high octane)? —Preceding unsigned comment added by 66.235.43.99 (talk) 13:08, 20 March 2009 (UTC)

I don't think your logic is correct. There is a sweet spot for ignition timing, and advancing it more than the engine was designed for isn't going to produce more power. The idea behind ignition advance is to give the fuel time to burn so the cylinder pressure goes as high as it's going to go right as the piston passes top-dead-center. This produces maximum power. For cars that feature a knock sensor which causes the computer to retard the timing, this is only a safety feature designed to keep detonation from damaging the engine. The only way to produce more power with a given engine displacement is to increase the compression ratio or add forced induction, or increase the maximum RPM. Any of these modifications may require a higher octane fuel to prevent detonation, but using high octane fuel without any modifications is not going to increase power. Shreditor (talk) 06:04, 30 March 2009 (UTC)

This is incorrect on several points. Yes, there is a sweet spot for ignition timing called "maximum timing for best torque" (MBT). But the idea behind ignition advance is not to place peak cylinder pressure at TDC. The optimal location of peak pressure is somewhere around 12 deg aTDC for most engines...roughly. It is absolutely true that most engines have active knock control which retards or advances timing as necessary dependant on knock sensor feedback. Most engines at full load (WOT) have a heavily retarded ignition timing, much later than optimal. So, with active knock control an engine will run more advanced relative to the very retarded WOT timing with higher octane fuel. This increases combustion efficiency and thus increases torque at any specific engine speed that the ignition is knock-limited. So, effectively speaking, yes, a higher octane fuel can increase the specific power output of an engine (or prevents it from being derated with a lower octane fuel). This is especially true for our European brethren, who can often be found driving turbocharged vehicles, which are often knock-limited at full load conditions. The action of retarding timing by the knock control is indeed a "safety feature", but that safety is at the expense of combustion efficiency. So preventing that loss of efficiency effectively allows one to run higher output. If you run more advanced timing closer to optimal (MBT) timing the engine produces higher temperatures in-cylinder during combustion. Higher in-cylinder combustion temperatures lead to increased thermodynamic efficiency. This is a thermodynamic principle.198.202.236.129 (talk) 21:26, 23 August 2010 (UTC)Dan

RON vs. AKI[edit]

This article explains the difference between the "Researched octane number" and the "Anti-knock index" at least three times at different places. IMHO one time is sufficient for one aritcle! axpdeHello! 17:07, 31 May 2009 (UTC)

P.S.: I just redid the definiton section and added an own section just for explaining the difference. All other occurances should be redundant now and can be safely deleted! axpdeHello! 17:28, 31 May 2009 (UTC)

Does the table contain RON or AKI values?[edit]

I just merged the text from Difference between RON and AKI into the table (in section Examples of octane ratings). It seemed to me the values referred to MON, but then I noticed that the text before the table says "the following table ... gives the 'AKI' ratings." I'm not sure if we can trust that sentence. It has been changed in February 2008 without any justification [1]. I am now deleting this sentence, which has become obsolete since the new table contains a column for each. I assume that all existing values were in fact MON values, and I am keeping them in the MON column. But because the change has been around for one and a half years, I am not sure if my assumption is correct. In particular, there is a risk that values that may have been added after the sentence changed are in fact AKI values. — Sebastian 18:45, 13 July 2009 (UTC)

I should add that the statement "Most of these ratings are given as 'AKI' ratings", which has been there prior to the edit I cite above, was itself added without any justification[2]. — Sebastian 21:24, 13 July 2009 (UTC)

How about higher?[edit]

The article said that lower octane that engine require will lower performance and efficiencies. How about higher? —Preceding unsigned comment added by Kelvin1704 (talkcontribs) 09:48, 5 June 2010 (UTC)

It's like penicillin. If you reach a certain amount it does the job. Beyond that there is no further effect. Krontach (talk) 09:32, 22 May 2011 (UTC)


'Octane/Altitude/Combustion Coefficient ''''' I am trying to find an octane/altitude/ combustion coefficient relative to petrol powered electrical generators in order to establish what percentage mixture of European '95 & '98 RON fuels would compensate for reduction in air density. If a petrol generator designed to run on '95 runs on pure '98 there is a risk of knocking but would the appropriate percentage mixture give better performance without risk of dammage to the motor?—Preceding unsigned comment added by 193.252.53.93 (talk) 13:39, 28 October 2010 (UTC)

Why would altitude have any impact on knock behaviour of an engine. James 80.176.88.36 (talk) 22:40, 4 July 2011 (UTC)

Octane Index and Relevance to RON/MON tests[edit]

I think it's very important to outline octane index (OI) here. It's a key parameter in describing knock in S.I. engines. A higher RON doesn't automatically mean better resistance to knock at all. It's quite possible to have a RON 100 fuel knock well before a RON 95 fuel.

The actual performance of a fuel in on-the-road terms of knocking performance is the OI which is determined as follows. OI = RON - KS, where S is the fuel sensitivity (RON - MON) and K is a weighting factor dependant on each engine and operating condition. When the RON MON tests were conceived in the 30s carburettor engines required far higher intake temperatures than we see today. As compression ratios and intake pressures have increased and intake temperatures have decreased, the value of K has gone from 1.0 to values < 0 in the most modern engines. With negative K values for modern engines it is clear to see that a higher RON can be worse for modern engines with GDI and FI technologies.

Please see the SAE paper "The Shift in Relevance of Fuel RON and MON to Knock Onset in Modern SI Engines Over the Last 70 Years" for more details. — Preceding unsigned comment added by Gtconway (talkcontribs) 13:42, 29 June 2011 (UTC)

Since when was octane a gas??[edit]

With a stated boiling point of 125 C then surely octane is a liquid and NOT a gas as the opening sentence says...

Could I also suggest that editors refer to "Internal Combustion Engine Fundamentals" by Heywood. There is a whole chapter on knock and this page could certainly use some input from there. James 80.176.88.36 (talk) 22:38, 4 July 2011 (UTC) Why is there only speculation and amateur short term test results?Dac8 (talk) 13:17, 12 December 2011 (UTC)

Octane numbers greater than 100[edit]

According to the definition, the octane number is the percentage of iso-octane in a mixture with n-heptane. How are octane numbers greater than 100 defined? — Preceding unsigned comment added by Thalb2000 (talkcontribs) 10:39, 21 December 2011 (UTC)

Well I am not expert, but here goes. 100% iso-octane has an octane number of 100, but the scale is not really about the iso-octane content but performance that is like isooctane. Some other fuels are even better than iso-octane in terms of resisting pre-ignition. Apparently toluene is one such species. So if you load up with toluene, you get better performance. --Smokefoot (talk) 11:35, 21 December 2011 (UTC)

Hydrogen MON[edit]

Why this section was removed?

  • Hydrogen does not fit well into the normal definitions of octane number. It has a very high RON and a low MON,[16] so that it has low knock resistance in practice,[17] due to its low ignition energy (primarily due to its low dissociation energy) and extremely high flame speed. These traits are highly desirable in rocket engines, but undesirable in Otto-cycle engines. However, as a minor blending component (e.g. in a bi-fuel vehicle), hydrogen raises overall knock resistance. Flame speed is limited by the rest of the component species; hydrogen may reduce knock because of its high thermal conductivity — Preceding unsigned comment added by 87.110.181.186 (talk) 16:13, 18 June 2012 (UTC)

Petrol V. Gasoline[edit]

This is not the UK version of Wiki is it? Take out all the "petrol" jargon and save it for the .uk site. — Preceding unsigned comment added by 173.218.135.200 (talk) 01:55, 21 October 2014 (UTC)

Nor is it the US version. It is the English-language version. GrindtXX (talk) 07:53, 21 October 2014 (UTC)