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 FAQ: Octane Number and what it means 
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Post FAQ: Octane Number and what it means
Taken from the Maxima.org forums.

Octane Number and what it means

There is a lot in this thread about octane. To help you find what you are looking for, I've listed the different discussions by Post Number:

Post #1: Technical Explanation of Octane Number.
Post #4: What Determines an Engine's Octane Requirements?
Post #6: Octane at Altitude (above sea level).
Post #7: Reaearch Octane Number (RON) in the Rest of the World. Also in #1.
Post #9: Should I Try Regular Gasoline in My Max?
Post #29: Octane Requirement Increase from Deposits
Post #30: Detergent Additives to Combat Deposits

An Australian posted a thread on the 6th Gen Maxima site and talked about using 98 octane gasoline in his Maxima in Australia. I made a number of comments to him about this gasoline octane not being the same as the octane posted in the USA. I pointed out that he was using a Research Octane Number (RON) while the octane you see posted at all US gas stations is a blend of Research and Motor Octane.

Now I’ve done a little research and found this more detailed discussion from the February 15, 1988 edition of the “Octane Week” Newsletter. To better help you understand what is being discussed, I've placed clarifying information inside of these { } brackets in the body of this short paper. I’ve also placed the one section of this discussion that requires some knowledge of organic chemistry in these [ ] brackets. You can skip that section and still get quite a bit out of this paper.
SilverMax_04

~~~~~~~~~~~~~~~~~~~
A Review of the Term “Octane Number” by George H Unzelman of Octane Week’s advisory board.

Recently I was asked to explain the difference between Research {R} and Motor {M} octane numbers. A second part of the question was: Why does the industry use (R+M)/2 octane?

In general terms the octane number of a gasoline is a measure of its antiknock quality or ability to resist detonation during combustion. A little history is helpful in understanding the different methods of measuring octane quality.

Not too many years after the discovery of the antiknock characteristics of tetreathyl lead, {the lead used in leaded gasoline -- see the next post to get the full spelling of this word} it became obvious that some yardstick was needed to define the antiknock quality of motor fuel. The octane scale was developed in 1926 by Dr Graham Edgar. Iso-octane was the hydrocarbon selected as 100 octane because it would not knock in the highest compression engine in existence at the time. Normal heptane, on the other hand, was designated as zero octane because it would cause intense knock in a very low compression engine. Mixtures of the two hydrocarbons established the linear scale between zero and 100. For example 20% normal heptane and 80% iso-octane has an octane number of 80. Later, methods were established to extend the octane scale above 100.

The Cooperative Fuel Research (CFR) knock-test engine was developed to determine {gasoline} octane number in the laboratory. It is a single-cylinder engine in which the compression ration can be adjusted during operation. The knock intensity of the fuel {gasoline} under test is bracketed between standard mixtures of iso-octane and normal heptane or other standard fuels of known octane number.

Two standard ASTM {American Society of Testing Materials} test methods define knock characteristics of motor fuels, by the Research (D 2699) and Motor (D 2700) methods. The Research method correlates with engine antiknock performance at low {engine} speed, while the Motor method correlates with high-speed performance. Both methods employ the same basic laboratory engine under different speed, spark advance, mixture temperature, and intake air temperature conditions.

The Motor method was developed first and was adopted and used extensively by the petroleum refining industry in setting finished gasoline specifications. However, during the 1950’s new refinery processes materially changed the hydrocarbon characteristics of gasoline. The road performance of gasoline no longer matched Motor octane number. Another measure of performance was needed and the Research method became the designating octane quality at the gasoline pump.

Because the Research method is less severe than the Motor method, most gasolines have a higher Research octane. The difference between the two numbers is called sensitivity. [For example a cat-cracked gasoline {component} with a Motor octane number (MON) of 81 and a Research octane number (RON) of 93 has a sensitivity of 12. The sensitivity of gasoline depends on hydrocarbon type and cat-cracked gasoline is “sensitive” because it contains a high percentage of olefins and aromatics. By contrast alkylate and isomerate {other gasoline blend streams} are paraffinic in nature and have little or no sensitivity.]

Sensitivity also relates directly to road octane quality. {Road octane is not defined by a specific lab test, but is the observed performance of gasoline on the road – in actual use in a vehicle on the road.} Following the switch to RON for octane specifications in the 1950’s, oil industry research laboratories worked extensively with instrumented vehicles to determine road octane numbers for the various makes and models. There was almost a frenzy of activity during the so-called octane-race period of the late 1950’s. Detroit auto makers steadily increased compression ratios to achieve superior performance and the oil industry followed with higher octane quality of fuels to match vehicle needs.

Because of the tremendous expense of equipping and operating road test facilities, the oil industry continued to use RON for specifying the octane quality of gasoline at the refinery and at the station pump until 1981. On Sept 4, 1981 the Environmental Protection Agency {EPA} published a notice in the Federal Register to amend unleaded gasoline regulations by substituting (R+M)/2 for RON as a measure of unleaded gasoline octane. Since that time the term “octane number” {at least in the USA} has been accepted as (R+M)/2 unless otherwise designated.


Last edited by johnnyb on May 09 2005, 4:16 PM, edited 1 time in total.



May 09 2005, 4:04 PM
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What Determines Engine Octane Requirements?


Thought I would expand a little more on what I posted above on gasoline octane while trying to answer this question and also cover some misconceptions about octane. [All octanes are (R+M)/2].

By using high octane gasoline, we are attempting to prevent engine knock. Engine knock is the premature and spontaneous ignition of gasoline. In effect the fuel EXPLODES rather than BURNS, and this results in incomplete combustion, a loss of power and (over time) engine damage. When this happens, you hear an audible "knock" or "ping", sometimes referred to as detonation. Detonation may vary from a faint noise on light acceleration to a constant, deep hammering noise while driving at speed.

Octane requirements are dictated by the following three engine operating factors:
- Cylinder Pressure,
- Spark Advance, and
- Engine Temperature.

Also, engine deposits can affect all of these factors, and produce engine knock where it otherwise would not exist.

Modern engines (like the VQ35) do a good job of controlling the spark advance and to some extent engine temperature. Cylinder pressure is less well controlled and is more dependent on the environment (the engine’s compression ratio, the altitude where you’re operating and the throttle position). But things get real exciting when you go to maximum power output at WOT (Wide Open Throttle). That's when bad things can happen fast. That is typically one of two times when you need maximum gasoline octane to prevent engine knock. The other is lugging your engine when engine temperatures are high.

Anything you do to increase any or all of these three factors will increase your engine’s octane requirement. So, if you put a turbo-charger on you car, you will increase cylinder pressure. If you’re driving a loaded car up a steep grade at high speeds, you will increase the engine’s temperature (but also slowly decrease the cylinder pressure due to ever higher elevation).

Because many OEMs (Original Equipment Manufacturers) are conservative, their stated octane requirement for their vehicles are frequently based on a series of very strenuous tests on the engine dynamometer that are in all likelihood never seen in the real world. In effect they have you driving at WOT (maximum power) for several hours at a time. The last I checked, all Highway Patrols kind of frown on that.

All of this being true, it is also possibly true that the current VQ35 engine (with a 10.3 to 1 compression ratio and advanced engine controls) is operating on the edge of what can be safely done burning regular gasoline. And this even though the owners’ manual says it can be operated on 87 octane regular gasoline – although they recommend premium. The 3.5L VQ engine in the Maxima is among the most advanced in engine technology today (see Wards Automotive 10 Best Engines for more details). The Honda Accord V-6 advertises use of 87 octane regular, but it only has a 10.0 to 1 compression ratio. Because of this, perhaps the VQ is not so close to the edge with 87 octane gasolines. If you do run 87 octane, I recommend that you work to keep these factors above as low as possible (but there's not much you can do about spark advance).

Now I would like to quote from the 2004 Maxima owners’ manual:
"Use unleaded gasoline with an octane rating of at least 87 AKI (Anti-Knock Index). For improved performance . . . use unleaded premium gasoline with an octane rating of at least 91. . . However, you may use unleaded gasoline with an octane rating as low as 85 AKI in high altitude areas [over 4,000 ft] such as Colorado . . ." {goes on to list all or parts of 10 other high-altitude western states}. I will discuss octane and altitude in more detail later on this thread. Then the owners' manual has a comment {with my interpretation}:

"However, now and then you may notice light spark knock for a short time while accelerating or driving up hills. This is not a cause for concern; because you get the greatest fuel benefit {read 'efficiency'} when there is light spark knock for a short time under heavy engine load." Higher efficiency means better gasoline mileage and when you are very close to getting light spark knock, the engine is apparently at its most efficient. {This is engine lab 101 for Mechanical Engineers.}

You’ve probably heard it said that improper engine timing or excessively lean air/fuel ratios will cause engine knock. Well the engine timing impacts the spark advance factor cited above. And the excessively lean air/fuel ratio will increase engine operating temperatures. So both of these statements are true because of their impact on the important three factors.

Some slight misconceptions about octane and gasoline blending;
1) The higher the octane rating on a gasoline the less volatile it is (evaporative qualities) and the slower the fuel burns.
Comment: While this is correct about the slower burning property of premium gasoline, the volatility of premium gasoline (evaporative qualities) is not directly correlated with its octane rating. The volatility of each grade of gasoline is determined in the blending process (and is usually set as high as the applicable government regulations allow for that time of year). This is because the higher volatility components used to blend gasoline are relatively cheap and have reasonable octane characteristics.

2) Most fuel refiners blend fuels for geographic areas and adjust their blends seasonally.
Comment: All refiners blend their gasolines according to strict EPA regulations for their marketing area and the season of the year.

3) These blending techniques compensate for the decrease in oxygen content with an increase in altitude and compensate for volatility during the warmer or cooler seasons.
Comment: It's not the decrease in oxygen content with altitude that's important, but the lower air pressure at higher altitudes. Because of the lower ambient air pressure, the cylinder pressure is reduced versus the pressure at sea level. Thus, it's possible to use a lower octane fuel at higher elevations without engine knock. Generally above 4,000 feet, 85 octane performs better than 87 octane at sea level. Thus, if you transported (via carrier) a vehicle with 85 octane in the fuel tank from Denver to Los Angeles, you would likely have noticeable knock (unless the engine technology compensated, which it might.)

Comment: Volatility is the ability of the fuel to vaporize. In the 1950s "vapor lock" was a common occurrence because the fuel vaporized in the fuel line rather than in the carburetor. With EPA mandated lower volatility gasolines in use today, you no longer see "vapor lock." What you do see is sometimes difficult starting situations in cold weather (particularly when a cold snap is premature).


Last edited by johnnyb on May 09 2005, 4:15 PM, edited 1 time in total.



May 09 2005, 4:06 PM
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Additive packages in the US vary from company to company, expect the same is true in Canada, but I'm not familiar with Canadian law on the subject. The US EPA has a mandated minimum additive treatment rate (so many parts per million), but I can't remember the exact rate. Suffice it to say, the rate is not high enough to keep modern fuel injected engines (any vehicle bought in the 90's or later) clean. Some oil companies in the States do provide a better treatment rate, particularly in their premium grade, but I can not specifically name them. When I worked for Amoco in the 80's and early 90's they put 50% more Techron (purchased from Chevron) in their premium than in their regular; and they advertised this fact. Recently, I've not seen any oil company advertising their additive treatment rate in any of their gasolines.

During the time when oil and gasoline prices fell, most oil companies cut back on the treatment rate in their fuel (to the EPA minimum) to save money. Gasoline was considered a commodity by many, and so many companies tended to give up on trying to convince people that gasoline was not a commodity.

Every refinery in the world produces slightly different fuels (both gasolines and distillates). I suspect that the Canadian refinery that supplies the stations you frequent makes good quality. Some refineries even end up "giving away octane" (the fuel is higher than specified) in order to meet other fuel specifications. That may be happening to you in Canada. Obviously, the US refinery that supplied the gasoline you bought in the States does not make their gasoline as well as the one in Canada. Chevron does not have a refinery in Washington state, so the gasoline may have been obtained on exchange from one of the 5 larger refineries in the state. (Or Chevron could have barged the product up from their Richmond CA refinery.) Bottom line: you can't generalize about gasoline in the two countries, you need to know the suppling refinery in each case to determine what is really happening.


May 09 2005, 4:07 PM
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What About 110 RON Racing Fuel?


Racing fuel at 110 Research Octane rating will likely have a higher sensitivity, and so will likely have an (R+M)/2 rating of from 102 to 104.

Once you have a gasoline with an octane that takes full advantage of an engine's capabilities, further increases in octane do not yield any benefits (but do increase your operating cost). The engine (like a racing engine) has to be designed for such a high-octane fuel to see any benefit. The Maxima engine was designed to operate on 91 to 93 octane fuel (96 to 98 RON) and should not show any improvement with a higher octane gasoline.

If you turbo-charged your Maxima, (remember the 3 critical knock factors from Post #3 above) you will increase the cylinder pressure and could then benefit from a higher octane fuel like racing gasoline.

I also remember seeing a post on this site that Sunoco has a higher octane gasoline available at some of their stations in the east. When I last bought Sunoco in S.C., their premium was 93. So I'm not certain where this higher octane fuel is marketed by them.


Last edited by johnnyb on May 09 2005, 4:14 PM, edited 1 time in total.



May 09 2005, 4:10 PM
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Octane Requirement Increase from Deposits


I have an SAE (Society of Automotive Engineers) paper done by researchers from Shell back in 1997 on “The Octane Requirement Increase (ORI) from Deposits.” It is quite technical, but I will quote key points this paper makes summarizing earlier research on this topic {with my comments in these brackets}:

“Engine knock arises from pressure pulses generated by the auto-ignition {or pre-ignition} of fuel in the end-gas region of the combustion chamber {above the piston and furthest from the spark plug} before the arrival of the flame front originating from spark ignition” {the spark plug firing}.

“Typical values for octane requirement increase (ORI) range between 5 and 10 {octane} numbers, with the most rapid increase in octane requirement occurring during early stages of deposit formation. . . Deposits cause ORI in these different ways:”

- “The volume occupied by deposits increases the compression ratio and thus the octane requirement of the engine. This contributes about 10% to 20% of observed ORI.”
- “Deposits are thermally insulating which leads to an increase in the end-gas temperature inside of each engine cylinder through reduced heat loss during compression, making knock more likely.”
- “Deposits also influence octane requirements through the transfer of heat from one engine cycle to the next, thus raising the temperature of each fresh fuel-air charge during induction into the combustion chamber. . . Modeling of this effect accounts for up to 50% of the total ORI observed”
- “Inlet system deposits have been shown to affect engine octane requirements, but the causes for this are not clear. . . Theories include . . . changing the extent to which fuel is vaporized in the inlet port. Less evaporation would result in less charge cooling and increase knock tendency. Alternately, deposits could provide a source of heat which raises the temperature of the fuel-air charge as it enters the combustion chamber. . . Such effects are less significant in gasolines containing detergent additives which reduce the level of deposits in inlet systems.”

There are other reasons why deposits may cause engine knock; they have not been fully studied. The ones listed above account for most of the causes.

Be certain to read my next post on “Detergent Additives to Combat Deposits.


Last edited by johnnyb on May 09 2005, 4:12 PM, edited 1 time in total.



May 09 2005, 4:11 PM
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Detergent Additives to Combat Deposits


I've said it on other threads, but it’s worth repeating again here, in somewhat more detail. Given the minimal level of fuel injector detergent mandated in US gasoline, you probably need to put a bottle of top-quality fuel injector cleaner through your fuel tank from time to time. The best cleaner are PEA based and include Chevron’s Techron, BG 44-K, and other high-end additives that are PEA based. A “keep clean” dose of these additives runs from 200 to 400 parts per million (ppm) – which few branded gasolines and no unbranded gasolines meet – thus, the reason for a “clean-up” dose of additive. This dose generally results in concentrations of about 2,000 ppm. This treatment rate works well for clean-up but will generally result in oil viscosity changes if used frequently. One dose just before each oil change is usually sufficient, but this rate is also dependent on how frequently you change oil and the deposit propensity of the fuel-engine combination you are running. You probably need to experiment with how frequently you do this to your Max. I change synthetic motor oil every 7,500 miles and find this frequency of clean-up injector detergent works for my Max. Although these PEA additives are only sold to clean deposits from the inlet system and fuel injectors, there are good indications that they also reduce the level of deposits on valves and inside of cylinders.[/b]


May 09 2005, 4:11 PM
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What About Aviation Gasoline?


When I last worked in the oil industry (1998) there were two grages of AvGas sold in the USA. 80-87 (this sometimes was called low lead) and 100-130. I believe that the 80-87 grade was being discontinued at that time. Given the high numbers on the 100-130, I'm certain that these are RON. In 1998 they were using lead in both versions. If the 80-87 is gone, the specs may now require low lead content for the 100-130. The octane boost effect of lead is very high for small amounts of lead, and then falls off when more is added. So low lead content will provide the most "octane kick" for very little lead used in the fuel.

The 100-130 AvGas was a single fuel with both ratings. I'm not certain exactly what these ratings mean, but suspect they represented the RON for different airplane operating conditions. My memory was that there were very few planes in the 90's that required 80-87 so it was dropped, requiring the remaining planes to buy 100-130 AvGas. My memory was that there are also problems using automotive gasoline in airplanes. This was never an area of petroleum products that I knew a lot about. Amoco only sold AvGas in a few restricted locations. You need someone who worked for Phillips to address these questions. Phillips sold AvGas in many locations.


May 09 2005, 4:14 PM
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Thanks goes to SilverMax_04 on the maxima.org forums for his wealth of knowledge


May 09 2005, 4:17 PM
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Thanks for that info. Have seen some of it before but not all, and it's great to see it in one place.

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May 09 2005, 5:40 PM
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neutral wrote:
Thanks for that info. Have seen some of it before but not all, and it's great to see it in one place.


For those reasons this info should be permanently retained/archived for future referance.
JohnnyB :2thumbsup: :2thumbsup: :2thumbsup: :2thumbsup: can't say it enough, many thanks for the extra effort.

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May 09 2005, 9:16 PM
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Another vote to have it Stickied or moved to the FAQs. Very good informative read, man. :2thumbsup:


May 11 2005, 4:42 PM
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johnnyb wrote:
What About Aviation Gasoline?


When I last worked in the oil industry (1998) there were two grages of AvGas sold in the USA. 80-87 (this sometimes was called low lead) and 100-130. I believe that the 80-87 grade was being discontinued at that time. Given the high numbers on the 100-130, I'm certain that these are RON. In 1998 they were using lead in both versions. If the 80-87 is gone, the specs may now require low lead content for the 100-130. The octane boost effect of lead is very high for small amounts of lead, and then falls off when more is added. So low lead content will provide the most "octane kick" for very little lead used in the fuel.

The 100-130 AvGas was a single fuel with both ratings. I'm not certain exactly what these ratings mean, but suspect they represented the RON for different airplane operating conditions. My memory was that there were very few planes in the 90's that required 80-87 so it was dropped, requiring the remaining planes to buy 100-130 AvGas. My memory was that there are also problems using automotive gasoline in airplanes. This was never an area of petroleum products that I knew a lot about. Amoco only sold AvGas in a few restricted locations. You need someone who worked for Phillips to address these questions. Phillips sold AvGas in many locations.


yeah its been proven avgas doesnt do anything for performance in cars, even though it has high octane, airplane gas has other chemicals which stop it from igniting/combusting/explodeing etc..etc... And is meant for high altitude. Drag cars tryed this and it didnt do anything, it infact made their car run poorly.

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May 11 2005, 6:17 PM
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