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#1610 - 09/11/01 11:56 AM Gas makeup  
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( check this forum section for other parts)

Gasoline is blend of many different hydrocarbons, all derived from crude oil. Gasoline has been around since approximately the late 1800's. Since that time, gasoline has gone through many changes in formulations and intended applications. For example, early 1900 cars used 40-60 octane fuel. 1960's cars needed octane values over 100 by comparison, as some Chevy, Ford, Mopar and Pontiac factory race cars were using up to 13:1 compression ratios! High compression was nice in the 60's as it made prodigious amounts of horsepower on the same cubic inch engine compared to one with less compression. In addition, high octane gas was abundant and their was little concern about pollution and the atmosphere compared with today.

Aside from powering factory race cars, most gasoline has to work in everyday cars going to and from work, start and stop, starting in sub zero weather, driving at 100+ temperatures and extended periods of storage. This is why gasoline is made from many different chemicals, as it must work over a broad range of situations. In addition, additives are used to provide icing protection, anti-wear properties, corrosion inhibitors, detergents and sometimes dyes. All of these are needed to protect your engine so it will last hundreds of thousands of miles under various conditions.


Gasoline is mixed with the air from your carburetor and distributed through the intake system to the chamber. It ignited with a spark and orchestrated with precise timing of intake and exhaust valves in relation to the piston stroke. All engines need a specific air/fuel ratios or amount of gas per volume of air your engine needs to work properly. For general purposes, it has been agreed that a ratio of around 14.0:1 (14.0 parts air to 1 part gas) is typical for carburated engines, using a non-oxygenated gasoline. The oxygen in air is what provides the explosive force when mixed with gas and set off with a spark. The interesting thing about the air we breathe is, it is only approximately 21% by volume Oxygen (remember this for later). The majority is Nitrogen, thank god, because it keeps everything from catching fire, oxidizing and rusting. Non-oxygenated gasoline contains 0% by volume of Oxygen in the chemical formula and therefore your engine must get all of its Oxygen from the intake system.

Deviating or going up and down on the air/fuel ratio will have an effect on power output and will also affect knocking. Leaning the mixture will require higher octane values. Each 1.0 increase in leaning (15.0) will require an increase of 2.0 (R+M/2) octane points. Mid to late 70's cars with EGR and lean calibrated Q-jets fall into this category. Timing will also have an effect as the timing is increased, the tendency to knock is increased. Typically, late 70's cars have lazy advance curves and limited total timing. However, higher cylinder temperatures and lean jetting will cause problems on lower octane gas with lean air/fuel ratios. Computer controlled newer cars will automatically adjust this ratio along with the timing for optimum performance for given conditions and octane used.

* To help with knocking on high performance cars with higher compression ratios, run air/fuel ratios richer (14.0:1) this will reduce chance of knocking.


The topic of octane produces more controversy than any other pertaining to fuels.

It has been explained in various ways many accurate and some not so accurate. In a nutshell, octane is the resistance to knock during combustion. Knocking is when the fuel will auto-ignite under heat and pressure after the spark and causes extreme forces by having multiple flames fronts within the chamber. Many factors effect knocking in addition to octane, air inlet temperature, cylinder head temperature, air/fuel ratio, engine load, plug heat range and compression ratio. There are others I am sure but these are the primary.

The octane value of a fuel is measured under very specific and controlled conditions. A company (very close to my home in Wisconsin) manufacturers a single cylinder octane test engine with a variable compression ratio (4:1-18:1) that is used for the ASTM test. Ironically, the standard single engine design uses a longer stroke than bore similar to a 455 Pontiac! This device burns a test fuel while increasing the compression by moving the head and measuring ultra sonic vibrations in the chamber on a test apparatus that measures sound waves. Predictable knocking patterns at specific compression ratios on various fuels are assigned an octane value.

The two most common values are, Research Octane RON and Motor Octane MON. The antiknock index is calculated by the average of the two, RON+MON/2. Both RON and MON use specific but different test parameters. The RON uses lower RPM, variable intake temperature, variable barometric pressure, and fixed timing. The MON uses higher RPM, fixed intake temperature and variable timing. An average of the two provides a good cross section of how a fuel will work under all conditions. Other octane values are tested as well as other tests to determine how the fuel will actually behave in the engine but the RON and MON are what most applies to our engines.

Increasing the compression ratio is the most common reason for using a higher octane fuel. Increasing compression will make more horsepower up to a point. The maximum effective compression ratio before you hit a point of diminishing returns is 16:1. This is about the compression that NHRA Pro Stock engines use for this reason. One important point is, as compression is increased from lower compression ratios (7:1 to 10:1) there is more brake thermal efficiency increase from lower than higher ratios. The point is, there is a bigger difference going from 7:1 to 10:1 than 10:1 to 13:1 compression. Turbos and blowers also have specific requirements as they increase cylinder pressure by artificial means.

As mentioned earlier, as compression ratios increased, the need for higher octane became important to reduce engine failures. Airplanes used in World War 1 really were the first to need higher octane. Shortly after, automotive engines were getting more powerful, more cubic inches and higher compression dictated the need for higher octane values.

During these years one of the most effective and economical means to increase octane was to use Tetra Ethyl Lead. Problem was, lead compounds are toxic by absorption, inhalation and even when burned. By the late 70's the EPA mandated reduction and elimination of TEL in gas. Similar replacements were used with limited success. This meant with lower octane fuel manufactured and the elimination of lead, auto manufacturers had to manufacture cars with lower compression ratios, something they were not used to doing! Compression ratios were dropped and timing curves were trimmed to function on 87 octane fuel and thus 70's cars got a bad rap by the press.


The conditions of the MON rated octane value represent severe, sustained high speed, high load driving, like racing. For most fuels, including those with either lead or oxygenates, the motor octane number (MON) will be more applicable for racing applications. R+M/2 is more applicable for street driving. The MON will also be lower than the research octane number (RON). The following is a table showing the RON increase with the addition of TEL.

92 R+M/2 octane pump gas:

Lead g/l Research Octane Number

0 96
0.1 98
0.2 99
0.3 100
0.4 101
0.5 101.5
0.6 102
0.7 102.5
0.8 102.75

Keep in mind that the MON and the R+M/2 will be lower. This illustrates that TEL does not add a significant amount of octane. Other compounds are used as well.

As previously mentioned, there are many factors that dictate how much octane an engine needs. This value is effected by more than mechanical compression ratio. It is primarily affected by cylinder pressure and valve opening and closing in relation to mechanical compression.

Most G.M engines calculate to less compression than the factory literature or repair manuals state. If you think you have a 10.75:1 G.M engine, you don't! That does not mean you don't need good fuel, but rather your requirement might be actually for a 10.0:1 engine. Because of the aforementioned reasons, there is no easy way to determine the octane value for a given engine. The standard ASTM test engine changes barometric pressure, intake temperature, variable timing and variable compression ratio to determine octane values!


Today, pump gas is of very high quality and has some inherent advantages over race fuels for street cars. Back in the early 1980's, the EPA began to mandate in certain regions of the country that gas manufacturers or terminals use oxygenated fuels. The single and primary reason, as far as the EPA is concerned is, reduced emissions. A secondary benefit is, in the case of Ethanol, it is a renewable resource as it is fermented from corn, distilled and dehydrated. This oxygenate is very popular here in the mid west.

Oxygenates, Ethanol, MTBE, ETBE and Methanol have the benefit of reducing HC emissions, raising octane, adding Oxygen to the fuel, and provide more anti-icing properties to the fuel. Ethanol is the most predominant, followed by MTBE, ETBE and Methanol. Current fuel formulations use approximately 3-4% by volume of Oxygen in the formula in the form of one of the above oxygenates. Remember what I said previously about air only containing 21% Oxygen. By using an oxygenated fuel, you are introducing more Oxygen to your engine without changing any engine components. An increase of 3-4% Oxygen does not seem like a lot, until you remember that air only contains 21% by volume. The ratio is 14:1 but you are still adding more Oxygen without changing any external or internal engine parts.


The theoretical energy content of gasoline when burned in air is only related to the hydrogen and carbon contents. The energy is released when the hydrogen and carbon are oxidized to form water and carbon dioxide. Important to note: The octane rating is not related to the energy content. The actual hydrocarbon and oxygenate compounds used in the gasoline will determine both the energy release and octane rating.

BTU's of a given fuel do not have much effect on the power produced by an internal combustion engine when compared at the correct stochoimetric A/F ratio. Example: Gasoline has a BTU/lb value of roughly 20,000 BTU's. Methanol has a BTU value of 8,600. Methanol can produce much more power than gasoline because of this increased Oxygen content, thus, changing its stochiometric ratio and creating a denser charge. However, much more fuel per pound must be consumed. Any time you can introduce more Oxygen into the engine, it's a benefit.

For years, racers have used Methanol as a race fuel because half the weight of a molecule of Methanol is, Oxygen, at 49%. Because of this, the air/fuel ratio is now 6:1 (6 parts air to 1 part fuel). You must burn roughly twice the amount of Methanol as gasoline because of this. Methanol lost acceptance by the EPA as it produces Formaldehyde emissions and it attacks epoxy which is sometimes used to secure rubber fuel hose to gas tanks and filler necks.

Ethanol, which is the primary EPA oxygenate, has an air fuel ratio of approximately 9:1. When 10% Ethanol is blended in gas, it can cause a loss of gas mileage and power because of this stochoimetric air/fuel ratio. Computer controller cars will notice this also. However, if compensated and properly jetted, Ethanol and MTBE can produce more power because of the increased Oxygen content even over race gas if you have a lower compression ratio.


The primary objective from the EPA's standpoint was, Oxygenates caused an engine to run lean, reducing CO and HC emissions. However, they also have high a blending octane and can be used to replace high levels of aromatics in fuel. Oxygenates (contain oxygen), really do not provide much BTU/lb of energy.

For an engine that runs on RGF, more fuel is required to obtain the same power, thus, the power ratio is not identical to the energy content ratio. They also require more energy to vaporize.

Oxygen Content wt %

Methanol 49.9%
Ethanol 34.7%
Gasoline 0.0%

Older higher compression cars in stock tune, are going to have the most problem with pump gas. With 10.0:1 actual compression ratios, stock carbs and short duration camshafts they will be the most prone to problems on lower octane gas. This means the resto crowd will have most of the problems. Modified cars with lower compression (<10:1) longer duration cams, ram air systems and richer jetting, will be OK with pump gas. Ethanol blended gas compared to non-oxygenated gas, can provide additional problems as it will lean out the air/fuel mixture as discussed before. This is why I recommended jetting carbs richer when using Ethanol/MTBE blended pump gas.

The most important thing to remember is, higher octane on an engine that does not require higher octane, will absolutely be a waste of time and money and you might slow down as well! Race engines with higher compression's (>10.5:1) will require a product with higher octane than pump gas can offer. And this is just as important. If the octane requirement is not met, the engine will be subjected to very severe cylinder pressures which will be transmitted to the rest of the reciprocating parts. Most street cars are not going to need race gas or octane boosters. Timing and carb adjustments will make it possible to function with pump gas.


Most octane boosters are hydrocarbon solvents already present in gas. Some claim to be real lead. Others are products have been banned by the EPA.

Most of us have a pretty big gas tank in our Corvette's. It would take a lot of little 8 oz. bottles to actually provide any measurable increase. Claims that, "adding a 8 oz. bottle of our octane booster will raise octane 4 points". Actually, they mean .4 points.
Sunoco did a test a couple of years ago and tested the major products on the market. None of them raised octane when used be the manufactures recommendation. In fact a couple actually lowered it. Only when dosing 4X the recommended volume, one product raised it a few points. The problem was they were only now up to 96-97 octane and had surpassed the cost of a tank of race gas.

Some of the products claim they will minimize valve seat recession. They do this by adding a heavy weight oil that gets introduced to the chamber via the gas and lubes the valves. Valve seat recession is primarily a problem in sustained high speed operation (racing, autobahn, etc.) Test data shows a 1970 engine operated at 70 MPH averaged 1.5 mm of seat recession in 7,500 miles per year. However, this is 7,500 miles at 70 MPH.
Gas line antifreeze products are typically used by us northern and mid western people. These products are typically, Ethyl, Methyl or Isopropyl Alcohol. If you are using Ethanol blended gas, using these products would be a total waste of money, as there is more alcohol already in your tank than you are adding from an 8 oz. bottle.

I am not stating that none of the products work. The better products under the right conditions might provide some advantage to a car owner that has problems with knocking. What I am saying is, the additional octane provided is less than you think, meaning your car does not require as much octane as you think. Additionally, there are thousands of people out there that are adding fuel additive's that do not need to.


Products are usually formulated for a specific application. The automotive engineer or aircraft engineer usually has a pretty good idea what will work best for the engine design. This is why you have auto gas, AV gas and race gas.

The density of AV gas is much less than automobile gas or race gas. Thus, for every gallon of AV gas passed through your carb jets, you have less fuel per pound compared to pump gas/race gas. This will lean out the air/fuel ratio and produce less power if jetted for pump gas.

AV gas has a different vapor pressure than pump gas. It contains a lighter fraction of solvents that can cause or lead to vapor lock in an automotive engine that uses a heated intake plenum. This might not be problem for a race car but a street car running in hot weather, it will cause hesitation or vapor lock.

A mild race car (11:1) might be an OK application for true high octane AV gas. If the owner is going to take the time to specifically jet the carb to run on the AV gas, he might be OK. However, all out race motors running 12:1 actual compression ratios are going to need more than the typical 100 LL octane. The common available AV gas (100 LL) will not be enough octane for a race motor.

If you need to raise your octane, as you fall into one of the above mentioned categories, do the following. Current high octane, unleaded fuel and many race gas formulations increase octane by cranking up the T&X content. Most race gas formulations use Toluene or Xylene to achieve the octane boost.

Typical T&X content of 92 octane pump gas.
Typical T&X content of race gas.

Toluene 9% by vol.
Toluene 27% by vol.

Xylene 8% by vol.
Xylene 1% by vol.

Total T&X 17%
Total T&X 28%

Adding 10% Toluene by volume to your gas tank will achieve the same thing. Toluene has a RON+MON/2 of 118 so it is very effective. Caution, do not spill on you paint or you will be sorry. Toluene is a major constituent in lacquer thinner and enamel reducers. *Toluene is available from most hardware stores and will boost octane a lot more than a little 8oz. bottle of a flash in the pan product.

one should look at the amount of energy (heat) released in the burning of a particular fuel. This is described by the specific energy of the fuel. This quantity describes the amount of power one can obtain from the fuel much more accurately. The specific energy of the fuel is the product of the lower heating value (LHV) of the fuel and molecular weight of air (MW) divided by the air-fuel ratio (AF):

Specific Energy = LHV*MW/AF

For example, for gasoline LHV= 43 MJ/kg and AF=14.6, while for methanol LHV= 21.1MJ/kg (less "heat" than gasoline) and AF=6.46 (much richer jetting than gasoline). Using the above formula we see that methanol only has a 10% higher specific energy than g asoline! This means that the power increase obtained by running methanol, with no other changes except jetting, is only 10%. Comparing the specific energy of racing and premium pump gas you can see that there is not much, if any, difference. Only alcohol s (such as methanol or ethanol) have a slightly higher specific energy than racing or pump gas.

Other oxygen-bearing fuels, besides the alcohols and nitromethanes, such as the new ELF fuel, will also produce slightly more power once the bike is rejetted. However, at $15.00 to $20.00 at gallon for the fuel the reportedly minor (1% - 2%) improvement is hardly worth the cost for the average racer.

The real advantage of racing gasolines comes from the fact that they will tolerate higher compression ratios (due to their higher octane rating) and thus indirectly will produce more power since you can now build an engine with a higher compression rati o. Also, alcohols burn cooler than gasoline, meaning even higher compression ratios are possible with them, for even more power.

The bottom line here is that, in a given engine, a fuel that doesn't knock will produce the same power as most expensive racing gasolines.

However, it sometimes happens that when you use another fuel, the engine suddenly seems to run better. The reasons for this are indirect: First, the jetting may be more closely matched to the new fuel. Secondly, the new fuel may improve the volumetric e fficiency (that is, the "breathing") of the motor. This happens as follows: Basically a fuel that quickly evaporates upon contact with the hot cylinder wall and piston crown will create additional pressure inside the cylinder, which will reduce the amount of fresh air/fuel mix taken in. This important--but often overlooked--factor is described by the amount of heat required to vaporize the fuel, described by the 'enthalpy of vaporization' (H), or 'heat of vaporization' of the fuel.

A high value of H will improve engine breathing, but the catch is that it leads to a different operating temperature within the engine. This is most important with two-strokes, which rely on the incoming fuel/air mix to do much of the cooling--even mode rn water-cooled two-strokes rely on incoming charge to cool the piston. For two-strokes a fuel that vaporizes, drawing a maximum amount of heat from the engine, is essential--the small variations in horsepower produced by different fuels is only of second ary concern.

Also important is the flame speed: Power is maximized the faster the fuel burns because the combustion pressure rises more quickly and can do more useful work on the piston. Flame speed is typically between 35 and 50 cm/sec. This is rather low compared to the speed of sound, at which pressure waves travel, or even the average piston speed. It is important to note that the flame propagation is greatly enhanced by turbulence (as in a motor with a squish band combustion chamber).

The most amazing thing about all this is that you can get the relevant information from most racing gasoline manufacturers. Then, just look at the specification sheet to see what fuel suits you best: Hot running motors and 2-strokes should use fuels wit h a value of "H" that improves their cooling, while more power (and more heat) is obtained from fuels with a high specific energy.

By the way, pump gas has specific energies which are no better or worse than most racing gasolines. The power obtained from pump gas is therefore often identical to that of racing fuels, and the only reason to run racing fuels would be detonation probl ems, or, since racing fuels are often more consistent than pump gas--which racers call "chemical soup"--a consistent reading of the spark plugs and exhaust pipe.

Nitrous Oxide

Nitrous oxide ( N2O ) contains 33 vol% of oxygen, consequently the combustion chamber is filled with less useless nitrogen. It is also metered in as a liquid, with can cool the incoming charge further, thus effectively increasing the charge density. With all that oxygen, a lot more fuel can be squashed into the combustion chamber. The advantage of nitrous oxide is that it has a flame speed, when burned with hydrocarbon and alcohol fuels, that can be handled by current IC engines, consequently the power is delivered in an orderly fashion, but rapidly. The same is not true for pure oxygen combustion with hydrocarbons, so leave that oxygen cylinder on the gas axe alone

* I will stress this again, if you don't need it, higher octane fuel won't make a difference. In fact, the slower burn rate of higher octane solvents actually might slow the car down if you have a low compression motor.

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#1611 - 09/11/01 12:43 PM Re: Gas makeup  
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teamzr1 Offline
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teamzr1  Offline
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Gasoline FAQ Contents:

Chapter 6) What do Fuel Octane ratings really indicate?
6.1) Who invented Octane Ratings?
6.2) Why do we need Octane Ratings?
6.3) What fuel property does the Octane Rating measure?
6.4) Why are two ratings used to obtain the pump rating?
6.5) What does the Motor Octane rating measure?
6.6) What does the Research Octane rating measure?
6.7) Why is the difference called "sensitivity"?
6.8) What sort of engine is used to rate fuels?
6.9) How is the Octane rating determined?
6.10) What is the Octane Distribution of the fuel?
6.11) What is a "delta Research Octane number"?
6.12) How do other fuel properties affect octane?
6.13) Can higher octane fuels give me more power?
6.14) Does low octane fuel increase engine wear?
6.15) Can I mix different octane fuel grades?


Chapter 6) What do Fuel Octane ratings really indicate?


6.1) Who invented Octane Ratings?
Since 1912 the spark ignition internal combustion engine's compression ratio had been constrained by the unwanted "knock" that could rapidly destroy engines. "Knocking" is a very good description of the sound heard from an engine using fuel of too low octane. The engineers had blamed the "knock" on the battery ignition system that was added to cars along with the electric self-starter. The engine developers knew that they could improve power and efficiency if knock could be overcome.
Kettering assigned Thomas Midgley, Jr. to the task of finding the exact cause of knock [24]. They used a Dobbie-McInnes manograph to demonstrate that the knock did not arise from preignition, as was commonly supposed, but arose from a violent pressure rise _after_ ignition. The manograph was not suitable for further research, so Midgley and Boyd developed a high-speed camera to see what was happening. They also developed a "bouncing pin" indicator that measured the amount of knock [9]. Ricardo had developed an alternative concept of HUCF ( Highest Useful Compression Ratio ) using a variable-compression engine. His numbers were not absolute, as there were many variables, such as ignition timing, cleanliness, spark plug position, engine temperature. etc.

In 1927 Graham Edgar suggested using two hydrocarbons that could be produced in sufficient purity and quantity [11]. These were "normal heptane", that was already obtainable in sufficient purity from the distillation of Jeffrey pine oil, and " an octane, named 2,4,4-trimethyl pentane " that he first synthesized. Today we call it " iso-octane " or 2,2,4-trimethyl pentane. The octane had a high antiknock value, and he suggested using the ratio of the two as a reference fuel number. He demonstrated that all the commercially- available gasolines could be bracketed between 60:40 and 40:60 parts by volume heptane:iso-octane.

The reason for using normal heptane and iso-octane was because they both have similar volatility properties, specifically boiling point, thus the varying ratios 0:100 to 100:0 should not exhibit large differences in volatility that could affect the rating test.

Heat of
Melting Point Boiling Point Density Vaporisation
C C g/ml MJ/kg
normal heptane -90.7 98.4 0.684 0.365 @ 25C
iso octane -107.45 99.3 0.6919 0.308 @ 25C

Having decided on standard reference fuels, a whole range of engines and test conditions appeared, but today the most common are the Research Octane Number ( RON ), and the Motor Octane Number ( MON ).


6.2) Why do we need Octane Ratings?
To obtain the maximum energy from the gasoline, the compressed fuel-air mixture inside the combustion chamber needs to burn evenly, propagating out from the spark plug until all the fuel is consumed. This would deliver an optimum power stroke. In real life, a series of pre-flame reactions will occur in the unburnt "end gases" in the combustion chamber before the flame front arrives. If these reactions form molecules or species that can autoignite before the flame front arrives, knock will occur [21,22].
Simply put, the octane rating of the fuel reflects the ability of the unburnt end gases to resist spontaneous autoignition under the engine test conditions used. If autoignition occurs, it results in an extremely rapid pressure rise, as both the desired spark-initiated flame front, and the undesired autoignited end gas flames are expanding. The combined pressure peak arrives slightly ahead of the normal operating pressure peak, leading to a loss of power and eventual overheating. The end gas pressure waves are superimposed on the main pressure wave, leading to a sawtooth pattern of pressure oscillations that create the "knocking" sound.

The combination of intense pressure waves and overheating can induce piston failure in a few minutes. Knock and preignition are both favoured by high temperatures, so one may lead to the other. Under high-speed conditions knock can lead to preignition, which then accelerates engine destruction [27,28].


6.3) What fuel property does the Octane Rating measure?
The fuel property the octane ratings measure is the ability of the unburnt end gases to spontaneously ignite under the specified test conditions. Within the chemical structure of the fuel is the ability to withstand pre-flame conditions without decomposing into species that will autoignite before the flame-front arrives. Different reaction mechanisms, occurring at various stages of the pre-flame compression stroke, are responsible for the undesirable, easily-autoignitable, end gases.
During the oxidation of a hydrocarbon fuel, the hydrogen atoms are removed one at a time from the molecule by reactions with small radical species (such as OH and HO2), and O and H atoms. The strength of carbon-hydrogen bonds depends on what the carbon is connected to. Straight chain HCs such as normal heptane have secondary C-H bonds that are significantly weaker than the primary C-H bonds present in branched chain HCs like iso-octane [21,22].

The octane rating of hydrocarbons is determined by the structure of the molecule, with long, straight hydrocarbon chains producing large amounts of easily-autoignitable pre-flame decomposition species, while branched and aromatic hydrocarbons are more resistant. This also explains why the octane ratings of paraffins consistently decrease with carbon number. In real life, the unburnt "end gases" ahead of the flame front encounter temperatures up to about 700C due to piston motion and radiant and conductive heating, and commence a series of pre-flame reactions. These reactions occur at different thermal stages, with the initial stage ( below 400C ) commencing with the addition of molecular oxygen to alkyl radicals, followed by the internal transfer of hydrogen atoms within the new radical to form an unsaturated, oxygen-containing species. These new species are susceptible to chain branching involving the HO2 radical during the intermediate temperature stage (400-600C), mainly through the production of OH radicals. Above 600C, the most important reaction that produces chain branching is the reaction of one hydrogen atom radical with molecular oxygen to form O and OH radicals.

The addition of additives such as alkyl lead and oxygenates can significantly affect the pre-flame reaction pathways. Antiknock additives work by interfering at different points in the pre-flame reactions, with the oxygenates retarding undesirable low temperature reactions, and the alkyl lead compounds react in the intermediate temperature region to deactivate the major undesirable chain branching sequence [21,22].

The antiknock ability is related to the "autoignition temperature" of the hydrocarbons. Antiknock ability is _not_ substantially related to:

The energy content of fuel, this should be obvious, as oxygenates have lower energy contents, but high octanes.
The flame speed of the conventionally ignited mixture, this should be evident from the similarities of the two reference hydrocarbons. Although flame speed does play a minor part, there are many other factors that are far more important. ( such as compression ratio, stoichiometry, combustion chamber shape, chemical structure of the fuel, presence of antiknock additives, number and position of spark plugs, turbulence etc.) Flame speed does not correlate with octane.


6.4) Why are two ratings used to obtain the pump rating?
The correct name for the (RON+MON)/2 formula is the "antiknock index", and it remains the most important quality criteria for motorists [39].
The initial knock measurement methods developed in the 1920s resulted in a diverse range of engine test methods and conditions, many of which have been summarised by Campbell and Boyd [103]. In 1928 the Co-operative Fuel Research Committee formed a sub-committee to develop a uniform knock-testing apparatus and procedure. They settled on a single-cylinder, valve-in-head, water-cooled, variable compression engine of 3.5"bore and 4.5" stroke. The knock indicator was the bouncing-pin type. They selected operating conditions for evaluation that most closely match the current Research Method, however correlation trials with road octanes in the early 1930s exhibited such large discrepancies that conditions were changed ( higher engine speed, hot mixture temperature, and defined spark advance profiles ), and a new tentative ASTM Octane rating method was produced. This method is similar to the operating conditions of the current Motor Octane procedure [12,103]. Over several decades, a large number of alternative octane test methods appeared. These were variations to either the engine design, or the specified operating conditions [103]. During the 1950-1960s attempts were made to internationally standardise and reduce the number of Octane Rating test procedures.

During the late 1940s - mid 1960s, the Research method became the important rating because it more closely represented the octane requirements of the motorist using the fuels/vehicles/roads then available. In the late 1960s German automakers discovered their engines were destroying themselves on long Autobahn runs, even though the Research Octane was within specification. They discovered that either the MON or the Sensitivity ( the numerical difference between the RON and MON numbers ) also had to be specified. Today it is accepted that no one octane rating covers all use. In fact, during 1994, there have been increasing concerns in Europe about the high Sensitivity of some commercially-available unleaded fuels.

The design of the engine and vehicle significantly affect the fuel octane requirement for both RON and MON. In the 1930s, most vehicles would have been sensitive to the Research Octane of the fuel, almost regardless of the Motor Octane, whereas most 1990s engines have a 'severity" of one, which means the engine is unlikely to knock if a changes of one RON is matched by an equal and opposite change of MON [32]. I should note that the Research method was only formally approved in 1947, but used unofficially from 1942 ),


6.5) What does the Motor Octane rating measure?
The conditions of the Motor method represent severe, sustained high speed, high load driving. For most hydrocarbon fuels, including those with either lead or oxygenates, the motor octane number (MON) will be lower than the research octane number (RON).

Test Engine conditions Motor Octane
====================== ==================================
Test Method ASTM D2700-92 [104]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 900 RPM
Intake air temperature 38 C
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature 149 C
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - variable Varies with compression ratio
( eg 14 - 26 degrees BTDC )
Carburettor Venturi 14.3 mm<>


6.6) What does the Research Octane rating measure?
The Research method settings represent typical mild driving, without consistent heavy loads on the engine.

Test Engine conditions Research Octane
====================== ==================================
Test Method ASTM D2699-92 [105]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 600 RPM
Intake air temperature Varies with barometric pressure
( eg 88kPa = 19.4C, 101.6kPa = 52.2C )
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature Not specified
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - fixed 13 degrees BTDC
Carburettor Venturi Set according to engine altitude
( eg 0-500m=14.3mm, 500-1000m=15.1mm ) <>


6.7) Why is the difference called "sensitivity"?
RON - MON = Sensitivity. Because the two test methods use different test conditions, especially the intake mixture temperatures and engine speeds, then a fuel that is sensitive to changes in operating conditions will have a larger difference between the two rating methods. Modern fuels typically have sensitivities around 10. The US 87 (RON+MON)/2 unleaded gasoline is recommended to have a 82+ MON, thus preventing very high sensitivity fuels [39]. Recent changes in European gasolines has caused concern, as high sensitivity unleaded fuels have been found that fail to meet the 85 MON requirement of the EN228 European gasoline specification [106].


6.8) What sort of engine is used to rate fuels?
Automotive octane ratings are determined in a special single-cylinder engine with a variable compression ratio ( CR 4:1 to 18:1 ) known as a Cooperative Fuels Research ( CFR ) engine. The cylinder bore is 82.5mm, the stroke is 114.3mm, giving a displacement of 612 cm3. The piston has four compression rings, and one oil control ring. The intake valve is shrouded. The head and cylinder are one piece, and can be moved up and down to obtain the desired compression ratio. The engines have a special four-bowl carburettor that can adjust individual bowl air-fuel ratios. This facilitates rapid switching between reference fuels and samples. A magnetorestrictive detonation sensor in the combustion chamber measures the rapid changes in combustion chamber pressure caused by knock, and the amplified signal is measured on a "knockmeter" with a 0-100 scale [104,105]. A complete Octane Rating engine system costs about $200,000 with all the services installed. Only one company manufactures these engines, the Waukesha Engine Division of Dresser Industries, Waukesha. WI 53186.


6.9) How is the Octane rating determined?
To rate a fuel, the engine is set to an appropriate compression ratio that will produce a knock of about 50 on the knockmeter for the sample when the air-fuel ratio is adjusted on the carburettor bowl to obtain maximum knock. Normal heptane and iso-octane are known as primary reference fuels. Two blends of these are made, one that is one octane number above the expected rating, and another that is one octane number below the expected rating. These are placed in different bowls, and are also rated with each air-fuel ratio being adjusted for maximum knock. The higher octane reference fuel should produce a reading around 30-40, and the lower reference fuel should produce a reading of 60-70. The sample is again tested, and if it does not fit between the reference fuels, further reference fuels are prepared, and the engine readjusted to obtain the required knock. The actual fuel rating is interpolated from the knockmeter readings [104,105].


6.10) What is the Octane Distribution of the fuel?
The combination of vehicle and engine can result in specific requirements for octane that depend on the fuel. If the octane is distributed differently throughout the boiling range of a fuel, then engines can knock on one brand of 87 (RON+MON)/2, but not on another brand. This "octane distribution" is especially important when sudden changes in load occur, such as high load, full throttle, acceleration. The fuel can segregate in the manifold, with the very volatile fraction reaching the combustion chamber first and, if that fraction is deficient in octane, then knock will occur until the less volatile, higher octane fractions arrive [27,28].
Some fuel specifications include delta RONs, to ensure octane distribution throughout the fuel boiling range was consistent. Octane distribution was seldom a problem with the alkyl lead compounds, as the tetra methyl lead and tetra ethyl lead octane volatility profiles were well characterised, but it can be a major problem for the new, reformulated, low aromatic gasolines, as MTBE boils at 55C, whereas ethanol boils at 78C. Drivers have discovered that an 87 (RON+MON)/2 from one brand has to be substituted with an 89 (RON+MON)/2 of another, and that is because of the combination of their driving style, engine design, vehicle mass, fuel octane distribution, fuel volatility, and the octane-enhancers used.


6.11) What is a "delta Research Octane number"?
To obtain an indication of behaviour of a gasoline during any manifold segregation, an octane rating procedure called the Distribution Octane Number was used. The rating engine had a special manifold that allowed the heavier fractions to be separated before they reached the combustion chamber [27]. That method has been replaced by the "delta" RON procedure.
The fuel is carefully distilled to obtain a distillate fraction that boils to the specified temperature, which is usually 100C. Both the parent fuel and the distillate fraction are rated on the octane engine using the Research Octane method [107]. The difference between these is the delta RON(100C), usually just called the delta RON. The delta RON ratings are not particularly relevant to engines with injectors, and are not used in the US.


6.12) How do other fuel properties affect octane?
Several other properties affect knock. The most significant determinant of octane is the chemical structure of the hydrocarbons and their response to the addition of octane enhancing additives. Other factors include:

Front End Volatility
Paraffins are the major component in gasoline, and the octane number decreases with increasing chain length or ring size, but increases with chain branching. Overall, the effect is a significant reduction in octane if front end volatility is lost, as can happen with improper or long term storage. Fuel economy on short trips can be improved by using a more volatile fuel, at the risk of carburettor icing and increased evaporative emissions.

Final Boiling Point
Decreases in the final boiling point increase fuel octane. Aviation gasolines have much lower final boiling points than automotive gasolines. Note that final boiling points are being reduced because the higher boiling fractions are responsible for disproportionate quantities of pollutants and toxins.

Preignition tendency
Both knock and preignition can induce each other.


6.13) Can higher octane fuels give me more power?
On modern engines with sophisticated engine management systems, the engine can operate efficiently on fuels of a wider range of octane rating, but there remains an optimum octane for the engine under specific driving conditions. Older cars without such systems are more restricted in their choice of fuel, as the engine can not automatically adjust to accommodate lower octane fuel. Because knock is so destructive, owners of older cars must use fuel that will not knock under the most demanding conditions they encounter, and must continue to use that fuel, even if they only occasionally require the octane.
If you are already using the proper octane fuel, you will not obtain more power from higher octane fuels. The engine will be already operating at optimum settings, and a higher octane should have no effect on the management system. Your driveability and fuel economy will remain the same. The higher octane fuel costs more, so you are just throwing money away. If you are already using a fuel with an octane rating slightly below the optimum, then using a higher octane fuel will cause the engine management system to move to the optimum settings, possibly resulting in both increased power and improved fuel economy. You may be able to change octanes between seasons ( reduce octane in winter ) to obtain the most cost-effective fuel without loss of driveability.

Once you have identified the fuel that keeps the engine at optimum settings, there is no advantage in moving to an even higher octane fuel. The manufacturer's recommendation is conservative, so you may be able to carefully reduce the fuel octane. The penalty for getting it badly wrong, and not realising that you have, could be expensive engine damage.


6.14) Does low octane fuel increase engine wear?
Not if you are meeting the octane requirement of the engine. If you are not meeting the octane requirement, the engine will rapidly suffer major damage due to knock. You must not use fuels that produce sustained audible knock, as engine damage will occur. If the octane is just sufficient, the engine management system will move settings to a less optimal position, and the only major penalty will be increased costs due to poor fuel economy. Whenever possible, engines should be operated at the optimum position for long-term reliability. Engine wear is mainly related to design, manufacturing, maintenance and lubrication factors. Once the octane and run-on requirements of the engine are satisfied, increased octane will have no beneficial effect on the engine. Run-on is the tendency of an engine to continue running after the ignition has been switched off, and is discussed in more detail in Section 8.2. The quality of gasoline, and the additive package used, would be more likely to affect the rate of engine wear, rather than the octane rating.


6.15) Can I mix different octane fuel grades?
Yes, however attempts to blend in your fuel tank should be carefully planned. You should not allow the tank to become empty, and then add 50% of lower octane, followed by 50% of higher octane. The fuels may not completely mix immediately, especially if there is a density difference. You may get a slug of low octane that causes severe knock. You should refill when your tank is half full. In general the octane response will be linear for most hydrocarbon and oxygenated fuels eg 50:50 of 87 and 91 will give 89.
Attempts to mix leaded high octane to unleaded high octane to obtain higher octane are useless for most commercial gasolines. The lead response of the unleaded fuel does not overcome the dilution effect, thus 50:50 of 96 leaded and 91 unleaded will give 94. Some blends of oxygenated fuels with ordinary gasoline can result in undesirable increases in volatility due to volatile azeotropes, and some oxygenates can have negative lead responses. The octane requirement of some engines is determined by the need to avoid run-on, not to avoid knock.

Team ZR-1
True Custom Performance Tuning
#1612 - 09/11/01 02:32 PM Re: Gas makeup  
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Posts: 5,232
Part 3

6.16 What happens if I use the wrong octane fuel?

If you use a fuel with an octane rating below the requirement of the engine,
the management system may move the engine settings into an area of less
efficient combustion, resulting in reduced power and reduced fuel economy.
You will be losing both money and driveability. If you use a fuel with an
octane rating higher than what the engine can use, you are just wasting
money by paying for octane that you can not utilise. The additive packages
are matched to the engines using the fuel, for example intake valve deposit
control additive concentrations may be increased in the premium octane grade.
If your vehicle does not have a knock sensor, then using a fuel with an
octane rating significantly below the octane requirement of the engine means
that the little men with hammers will gleefully pummel your engine to pieces.

You should initially be guided by the vehicle manufacturer's recommendations,
however you can experiment, as the variations in vehicle tolerances can
mean that Octane Number Requirement for a given vehicle model can range
over 6 Octane Numbers. Caution should be used, and remember to compensate
if the conditions change, such as carrying more people or driving in
different ambient conditions. You can often reduce the octane of the fuel
you use in winter because the temperature decrease and possible humidity
changes may significantly reduce the octane requirement of the engine.

Use the octane that provides cost-effective driveability and performance,
using anything more is waste of money, and anything less could result in
an unscheduled, expensive visit to your mechanic.

6.17 Can I tune the engine to use another octane fuel?

In general, modern engine management systems will compensate for fuel octane,
and once you have satisfied the optimum octane requirement, you are at the
optimum overall performance area of the engine map. Tuning changes to obtain
more power will probably adversely affect both fuel economy and emissions.
Unless you have access to good diagnostic equipment that can ensure
regulatory limits are complied with, it is likely that adjustments may be
regarded as illegal tampering by your local regulation enforcers. If you are
skilled, you will be able to legally wring slightly more performance from
your engine by using a dynamometer in conjunction with engine and exhaust gas
analyzers and a well-designed, retrofitted, performance engine management

6.18 How can I increase the fuel octane?

Not simply, you can purchase additives, however these are not cost-effective
and a survey in 1989 showed the cost of increasing the octane rating of one
US gallon by one unit ranged from 10 cents ( methanol ), 50 cents (MMT),
$1.00 ( TEL ), to $3.25 ( xylenes ) [108]. Refer to section 6.20 for a
discussion on naphthalene ( mothballs ). It is preferable to purchase a
higher octane fuel such as racing fuel, aviation gasolines, or methanol.
Sadly, the price of chemical grade methanol has almost doubled during 1994.
If you plan to use alcohol blends, ensure your fuel handling system is
compatible, and that you only use dry gasoline by filling up early in the
morning when the storage tanks are cool. Also ensure that the service station
storage tank has not been refilled recently. Retailers are supposed to wait
several hours before bringing a refilled tank online, to allow suspended
undissolved water to settle out, but they do not always wait the full period.

6.19 Are aviation gasoline octane numbers comparable?

Aviation gasolines were all highly leaded and graded using two numbers, with
common grades being 80/87, 100/130, and 115/145 [109,110]. The first number is
the Aviation rating ( aka Lean Mixture rating ), and the second number is the
Supercharge rating ( aka Rich Mixture rating ). In the 1970s a new grade,
100LL ( low lead = 0.53mlTEL/L instead of 1.06mlTEL/L) was introduced to
replace the 80/87 and 100/130. Soon after the introduction, there was a
spate of plug fouling, and high cylinder head temperatures resulting in
cracked cylinder heads [110]. The old 80/87 grade was reintroduced on a
limited scale. The Aviation Rating is determined using the automotive Motor
Octane test procedure, and then converted to an Aviation Number using a
table in the method. Aviation Numbers below 100 are Octane numbers, while
numbers above 100 are Performance numbers. There is usually only 1 - 2
Octane units different to the Motor value up to 100, but Performance numbers
varies significantly above that eg 110 MON = 128 Performance number.

The second Avgas number is the Rich Mixture method Performance Number ( PN
- they are not commonly called octane numbers when they are above 100 ), and
is determined on a supercharged version of the CFR engine which has a fixed
compression ratio. The method determines the dependence of the highest
permissible power ( in terms of indicated mean effective pressure ) on
mixture strength and boost for a specific light knocking setting. The
Performance Number indicates the maximum knock-free power obtainable from a
fuel compared to iso-octane = 100. Thus, a PN = 150 indicates that an engine
designed to utilise the fuel can obtain 150% of the knock-limited power of
iso-octane at the same mixture ratio. This is an arbitrary scale based on
iso-octane + varying amounts of TEL, derived from a survey of engines
performed decades ago. Aviation gasoline PNs are rated using variations of
mixture strength to obtain the maximum knock-limited power in a supercharged
engine. This can be extended to provide mixture response curves which define
the maximum boost ( rich - about 11:1 stoichiometry ) and minimum boost
( weak about 16:1 stoichiometry ) before knock [110].

The 115/145 grade is being phased out, but even the 100LL has more octane
than any automotive gasoline.

6.20 Can mothballs increase octane?

The legend of mothballs as an octane enhancer arose well before WWII when
naphthalene was used as the active ingredient. Today, the majority of
mothballs use para-dichlorobenzene in place of naphthalene, so choose
carefully if you wish to experiment :-). There have been some concerns about
the toxicity of para-dichlorobenzene, and naphthalene mothballs have again
become popular. In the 1920s, typical gasoline octane ratings were 40-60
[11], and during the 1930s and 40s, the ratings increased by approximately 20
units as alkyl leads and improved refining processes became widespread [12].

Naphthalene has a blending motor octane number of 90 [52], so the addition of
a significant amount of mothballs could increase the octane, and they were
soluble in gasoline. The amount usually required to appreciably increase the
octane also had some adverse effects. The most obvious was due to the high
melting point ( 80C ), when the fuel evaporated the naphthalene would
precipitate out, blocking jets and filters. With modern gasolines,
naphthalene is more likely to reduce the octane rating, and the amount
required for low octane fuels will also create operational and emissions

Team ZR-1
True Custom Performance Tuning

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