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    Determining the Octane Rating of Gasoline

    Introduction
    Most internal combustion (IC) engines obtain their energy from the combustion of a hydrocarbon fuel with air, which converts the chemical energy of the fuel to internal energy in the gases produced in the cylinder. The mechanical linkages in the engine then convert the internal energy to the rotating crankshaft output. There are many thousands of different hydrocarbon fuel compounds which consist mainly of hydrogen and carbon, but may also contain oxygen as in the case of alcohols, as well as nitrogen and sulfur in some other formulations. The maximum possible amount of chemical energy is released from the fuel, in the form of heat, when its combustion occurs in a stochiometric amount of oxygen. Stoichiometric oxygen (also called theoretical oxygen) is just enough to convert all carbon in the fuel to CO2, and all hydrogen to H2O with no oxygen left over. Combustion can occur, within limits, when more than stoichiometric air is present (lean) or when less than stoichiometric air is available (rich), for a given amount of fuel.

    Atmospheric air is made up of about 78% nitrogen, 21% oxygen and 1% argon with traces of CO2, Ne, CH4 (methane), He, H2O, etc. Nitrogen and argon are essentially chemically neutral and do not react in the combustion process, but their presence does affect the temperature and pressure in the combustion chamber. To simplify calculations without introducing appreciable error, the neutral argon is assumed to be combined with the neutral nitrogen in the air, and atmospheric air can then be modeled as being composed of 21% oxygen and 79% nitrogen.

    Principle References:
    Engineering Fundamentals of the Internal Combustion Engine, Second Edition by Willard W. Pulkrabek; Chapter 4, Thermochemistry and Fuels, pages 139-189.

    Conversations with my brother-in-law, a chemical engineer (petroleum).

    I recognize that most who read this won't have access to the above book but many websites are available that cover this material - so many that specific recommendations need not be given. Google reveals all.

    Hydrocarbon Fuels: General
    The main fuel in use for spark ignition (SI) engines is gasoline which is a mixture of many hydrocarbon components and is produced from crude petroleum. Crude oil was first discovered by Drake in Pennsylvania in 1859 and the fuels generated from it developed along with the development of the IC engine. Crude oil is made up almost entirely of carbon and hydrogen with traces of other species; it varies from 83-87% carbon and 11-14% hydrogen by weight. The carbon and hydrogen can combine in an extraordinarily large number of ways to form many different molecular compounds. One test identified over 25,000 different hydrocarbon compounds.

    Crude oil is separated into its component products, at the refinery, by catalytic cracking and/or fractional distillation. Cracking is the process of breaking large molecular components into more useful components of smaller molecular weight. Distillation is used to separate the components by boiling them off at different temperatures. Components with smaller molecular weights, such as solvents and fuels (gasoline), boil off at lower temperatures while those with larger molecular weights, such as tar and asphalt, boil off at higher temperatures thus separating out discreet substances from the original mix. Examples of some other components resulting from the refining process include jet fuel, diesel fuel, heating oils, lubrication oils, natural gas, alcohol, rubber, paint, plastics and explosives. Crude oil is not a uniform substance and contains different amounts and combinations of hydrocabon species depending on what part of the world from which it derives. In the US there are two general classifications: Pennsylvania crude has a large concentration of paraffins with little or no asphalt; western crude has an asphalt base with little paraffin. Crude oil from some parts of the Mideast is made up of component mixtures that could be used, as is, for IC engine fuel with little or no refining. Refineries are generally optimized for particular types of crude oil inputs but newer facilities are more adaptable to a variety of inputs. On average, a 42-gallon barrel of crude can yield about 19.5 gallons of gasoline, about 46%.

    General Characteristics of Gasoline
    A number of different families of hydrocarbons have been identified and some of the more important ones that are used in formulating gasoline will be described here. Gasoline is not a monolithic product but instead is a blend of many components chosen to maximize its performance in a given set of circumstances. A specific hydrocarbon is defined not only by the number of carbon and hydrogen atoms it contains, but also by the type of structure that binds them together. In some hydrocarbons the carbon atoms are bound together in a straight line known as an unbranched or straight chain molecule. In others with the same chemical formula, the chains that bind the carbon atoms are branched leading to a different molecular structure with somewhat different chemical properties. The branched molecule is known as an isomer. There are many ways in which chemical chains can be branched giving a very large number of chemical species. Thus, for example, the straight chain molecule n-butane (C4H10) has the same formula, but different molecular structure, than the branched chain molecule isobutane, also (C4H10). For a pictorial representation of these two types of molecular structures, see the photos at the end of this post.

    The prefix for a hydrocarbon fuel component is associated with the number of carbon atoms in its main chain or ring: one carbon atom = meth; 2 = eth; 3 = prop; 4 = but; 5 = pent; 6 = hex; 7 = hept; 8 = oct; 9 = non; 10 = dec, 11 undec, 12 = dodec. The suffix for a given hydrocarbon name is determined by the family it inhabits: for example, paraffins end in ane; aromatics in ene.

    Paraffins
    Although many hydrocarbon families make up the constituents of the various gasolines, with some of the more important ones being Paraffins, Olefins, Cycloparaffins and Aromatics, for the purpose of our discussion about octane ratings we need concentrate only on Paraffins. The components of this family, also referred to as alkanes, have a chemical formulation of CnH2n+2, where n = any number. The simplest member of this family and the simplest of all stable hydrocarbon molecules is methane (CH4), which is the main component of natural gas. Two components of this family that have major importance in reference to octane rating, to be discussed at length in the next post, are n-heptane (C7H16) where n = the normal or straight chain form of the molecule, and isooctane (C8H18) an isomer of n-octane. Isooctane is also referred to as 2, 2, 4-trimethylpentane; pentane because it has 5 carbon atoms in the main chain, trimethyl because it has three methyl radicals (CH3), and 2, 2, 4 because the three radicals are on the second, second, and fourth carbon atoms in the chain. Isn't organic chemistry just the greatest thing ever?

    If gasoline were to be approximated as a single-component hydrocarbon fuel (theoretical), it would have a molecular formula of C8H15 and a corresponding molecular weight of 111. Many textbooks use these values for purposes of discussion. On the other hand, sometimes gasoline is approximated by the real hydrocarbon component isooctane (C8H18), that most closely matches its component structure and thermodynamic properties.

    In the next post, since we now have the necessary background, we'll discuss how octane ratings are measured.

    Happy Motoring!

    Photos 1 and 2: Straight chain and branched chain representations of the n-octane and isooctane molecular structures. The black spheres represent the carbon atoms and the light-colored ones the hydrogen atoms. The isomeric form of the molecule has a stronger chemical bond and more energy than the n-form. Note that only two of the carbon atoms are attached to three hydrogen atoms in the straight chain form while five carbon atoms are attached to three hydrogen atoms in the branched chain version.
    Attached Files
    Last edited by goldstar; 03-11-2012, 08:30 AM.
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    MP3 Shifter, Knob and Aluminum Pedal Set
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    #2
    Octane Rating and its Measurement

    The Combustion Process
    To better understand the meaning of the octane rating of a fuel, it would be a good idea to review the combustion process in an SI engine. If the temperature of an A/F mixture is raised high enough, the mixture will self-ignite without the need of a spark plug. The temperature above which this occurs is called the self-ignition temperature (SIT). This is the basic principle of ignition in a diesel, or compression ignition, engine. Self-ignition (or preignition, or autoignition) is not desirable, or acceptable, in an SI engine, where a spark plug is used to ignite the A/F mixture at the proper time in the cycle. The compression ratios of gasoline-fueled SI engines are limited to about 11:1 to avoid self-ignition. When self-ignition does occur in an SI engine, higher than desirable pressure pulses are generated that can cause damage to the engine and quite often can be audibly heard as knock or ping.

    Pulkrabek explains the processes going on during the combustion cycle as follows. If the A/F mixture is heated to a temperature above SIT, self-ignition will occur after a short time delay called the ignition delay (ID). The higher the initial temperature rise above SIT, the shorter will be the ID. The values for SIT and ID for a given A/F mixture are difficult to specify as they depend on many variables including temperature, pressure, density, turbulence, swirl, A/F ratio and the presence of inert gases. However, ID is generally on the order of thousandths of a second and, during this time, preignition reactions occur including oxidation of some fuel components and even cracking of some large hydrocarbon components into smaller molecules. Additionally, these preignition reactions raise the temperature at local spots, which then promotes additional reactions leading to the combustion reaction.

    Combustion starts at the spark plug and the flame front begins to travel through the, as yet, unburned part of the mixture. As combustion occurs, the temperature of the burned gases is increased to a high value which in turn raises its pressure and expands its volume. The unburned gases in front of the flame front are compressed by this higher pressure and this compressive heating raises the temperature of the gas. The temperature of the unburned gas is further raised by radiation heating from the flame and this action raises the pressure even higher. Consequently, the flame front moving through the remaining unburned gas does so at an accelerated rate because of the higher temperature and pressure, which further increases the reaction rate. In addition, the energy release in the combustion process further raises the temperature of burned gases behind the flame front. Thus, the flame front continues its travel through an unburned mixture that is progressively higher in temperature and pressure. By the time the flame reaches the last portion of unburned gas, this gas is at a very high temperature and pressure because of the foregoing events. In this end gas near the end of the combustion process is where self-ignition and knock can occur. To avoid this, it is necessary for the flame to pass through and consume all unburned gases which have risen above SIT before the ID time elapses. This is accomplished through a combination of fuel property control, including its octane rating, and the design of the combustion chamber geometry.

    By limiting the compression ratio in an SI engine, the temperature at the end of the compression stroke where combustion starts is limited. The reduced temperature at the start of combustion then reduces the temperature throughout the entire combustion process and self-ignition is avoided. Alternatively, a high compression ratio will result in a higher temperature at the start of combustion causing all temperatures for the rest of the cycle to be higher. The higher temperature of the end gas will shorten the ID time making self-ignition more likely.

    Measuring Octane
    The fuel characteristic that best describes how likely it is that a fuel will, or will not, self-ignite is called the octane number, or just octane. This is a numerical scale generated by comparing the self-ignition characteristics of the fuel to that of standard fuels in a specific test engine under specific operating conditions. The two standard reference fuels used are isooctane (C8H18) which is defined as having an octane number (ON) of 100 and n-heptane (C7H16) which is given the ON of 0. The higher the octane number of a fuel, the less likely that it will self-ignite.

    There are several different tests used for rating ONs, each of which yields a slightly different ON value. The two most common methods of rating gasoline and other automotive SI engine fuels are the Motor Method and the Research Method. These produce the Motor Octane Number (MON) and the Research Octane Number (RON), respectively. There is also the Aviation Method which is used for piston-engine aircraft and provides the Aviation Octane Number (AON). The test engine used to measure MON and RON was developed in the 1930s and is a single cylinder, OHV, four-stroke cycle type. It has a variable compression ratio that can be adjusted from 3 to 30.

    To take a RON measurement, the engine is run at 600 rpm, inlet air temperature of 125 deg F (52 deg C), and ignition timing of 13 deg BTDC.

    For a MON measurement, the engine is run at 900 rpm, inlet air temperature of 300 deg F (149 deg C), and ignition timing of from 19-26 deg BTDC.

    All other engine parameters are the same for both tests.

    To find the ON of a fuel, the following test proceedure is utilized. The test engine is run under the specified conditions using the fuel being tested. Compression ratio is adjusted until a standard level of knock (I don't know what this means) is experienced. The test fuel is then replaced with a mixture of the two standard fuels. The intake system is designed so that the blend of the two standard fuels can be varied to any percent from all isooctane to all n-heptane. The blend of the fuels is varied until the same knock characteristics are obseved as with the test fuel. The percent of isooctane in the fuel blend is the ON given to the test fuel. For example, a fuel that has the same knock characteristics as a blend of 87% by volume isooctane and 13% by volume n-heptane would have an ON of 87. This doesn't mean that the test fuel actually contains those proportions of the two substances, only that it has the same self-ignition resistance.

    Waukesha Engine Division of Dresser Industries manufactures the fuel research engine and has ever since its design was laid down in 1931.
    http://www.waukeshaengine.com/index....ctane-category

    Since operating conditions used to measure MON are more severe than those used to measure RON, many fuels will have a RON greater than MON. Depending on the fuel composition, MON generally will be about 8-10 points lower than RON. MON is a better measure of how the fuel performs under load. In most countries (including all of Europe and Australia), the octane rating shown on the pump is the RON. In the US, Canada and some other countries, the number displayed on the pump, although often referred to as the ON of the fuel, is actually the anti-knock index (AKI). It represents the mean value of the sum of the RON and the MON.
    AKI = (RON + MON)/2
    Because of the 10-point difference noted above, the octane number in the US and Canada will be about 4 to 5 points lower than the same fuel elsewhere. Thus, 87 octane (regular) fuel in the US and Canada would be equivalent to 91-95 octane (regular) in Europe.

    Common octane numbers (AKI) for automotive gasoline fuels range from 87 to 95, with higher values available for high performance and racing engines. Aviation gasoline, in which low levels of tetraethyl lead [(C2H5)4Pb] additives are still permitted, have octane numbers in the 85 to 100 range, and although close, are not directly comparable with automotive gasoline numbers. A few milliliters of tetraethyl lead (TEL) in several liters of gasoline can raise the ON several points in a very predictable manner. TEL works because its easily decomposed to its component radicals, which react with the radicals from the fuel and with the oxygen from the air that would start the combustion, thereby delaying ignition. It's possible for a fuel to have a RON greater than 100. Isooctane is not the highest octane substance available. Toluene, benzene (both derivatives of the aromatics family) and ethanol are some of the additives available to raise the ON of the fuel above 100.

    An example of a pure racing gasoline is 76 Competition 110 Racing Gasoline made by Conoco-Philips. It is said to be the official fuel of Nascar. Since it contains TEL, it can only be used off-highway at sanctioned racing events. It has a RON of 115, a MON of 106 and an AKI of 110. It has been tested in engines with compression ratios up to 16:1 operating at both 6,000 and 9,000 rpm. It is claimed to be highly resistant to detonation under high speed, high output conditions. No ingredients are listed, of course, as the fuel is undoubtedly proprietary, but I assume it contains a high proportion of toluene and perhaps xylene. It contains no alcohols or oxygenates.
    http://www.leesracing.com/fuelspec/g3.html

    As Pulkrabek notes, since the test engine has a combustion chamber designed in the 1930s and the tests are carried out at low speeds, the obtained octane number will not always indicate how the fuel will respond in a modern, high speed engine. Therefore, octane numbers should not be seen as absolute in predicting knock characteristics for a given engine. Given two engines with the same compression ratio but with different combustion chamber geometries, one may exhibit knock and the other not, with the same fuel used in both. In connection with this, the difference between RON and MON is referred to as the fuel sensitivity (FS).
    FS = RON - MON

    Fuel sensitivity is a good measure of how sensitive knock characteristics of a fuel will be to engine geometry. A low FS number will usually mean that knock characteristics are insensitive to engine geometry. FS numbers generally range from 0 to 10.

    Factors Determining the Octane Number of a Fuel
    The higher the flame speed in an A/F mixture, the higher the octane number. This is because, with a higher flame speed, the A/F mixture that is heated above SIT will more likely be consumed during the ID time thereby preventing knock. The high flame speed of alcohols is partially responsible for their high octane numbers.

    Gasolines containing relatively large amounts of of straight chain molecules, such as n-heptane, have an increased tendency to knock, whereas those containing branched chain forms, such as isooctane, have a reduced tendency to knock while burning more smoothly.

    For a compound with a given number of carbon and hydrogen atoms, the more these atoms are combined in side chains and not in a few long, straight chains, the higher will be the octane number. For example, an isomer compared with the n-form of the molecule.

    Fuel components with ring molecules have higher octane numbers. For example, cycloparaffins such as cyclohexane and aromatics such as benzene and toluene.

    As already stated, combustion chamber geometry can also affect octane rating.


    Comparative Octane Ratings
    Two cautions must be kept in mind when attempting to precisely specify octane numbers of gasoline and its component hydrocarbons. First, its necessary to distinguish between RON, MON, and AKI. Second, within those categories, individual variation seems to exist depending on whose listing is being consulted. For example, Pulkrabek's table differs in part from some online listings, which in turn differ from each other. That being said, here is a partial list of octane ratings, and stoichiometric A/F ratios, taken from Pulkrabek that represent at least reasonable accuracy. A few values are missing. We already know that n-heptane and isooctane are defined as being 0 octane and 100 octane, respectively.

    Fuel ______________ MON ______RON ______AKI___Stoich A/F

    gasoline (theoretical)
    (C8H15) _____________80-91 ______92-99 _____86-96_____14.6

    isooctane (C8H18) _____100 ________100_______100_______15.1

    n-heptane (C7H16) _____0 __________0__________________15.2

    n-pentane (C5H12) _____62 ________62 ________62

    butene-1 (C4H8) _______80 ________99 ________90________14.8

    cyclohexane (C6H12) _________________________97

    benzene (C6H6) _____________________________101

    isodecane (C10H22) ____92 ________113 _______103________15.1

    triptane (C7H16) ______101 ________112 _______107________15.2

    toluene (C7H8) _______109 _________120 _______115_______13.5

    I welcome your comments, additions, corrections, and questions.

    Happy Motoring!
    Last edited by goldstar; 11-09-2011, 07:28 AM.
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    Comment


      #3
      we just went over this in school, i want to know how the variable compression engine works.
      sigpic

      Comment


        #4
        Originally posted by 1992tracerlts View Post
        we just went over this in school, i want to know how the variable compression engine works.
        Basically, the entire cylinder is moved up and down with respect to the piston. This method prevents any change in combustion chamber shape as would occur if, for example, adjustable or removable plugs were used to vary the combustion chamber volume.

        For more information about the Waukesha engines, go to:

        Runyard's Octane Engine Page. Pictures and history of the Research & Motor Method Octane engines. CFR F-1 ASTM D 2699 & F-2 ASTM D 2700


        Happy Motoring!
        Last edited by goldstar; 01-06-2011, 08:48 AM.
        02 DX Millenium Red - The Penultimate Driving Machine
        MP3 Strut Tower Bar kit; Cusco Front Lower Arm Tie Bar
        MSP Springs, Struts, Stabilizer Bars, Trailing Links, #3 Engine Mount
        Kartboy Stabilizer Bar Bushings; Nyloil Shifter Bushings; Red Line MT-90 Gear Oil
        MP3 Shifter, Knob and Aluminum Pedal Set
        Suvlights HD Wiring Harness; Osram Night Breaker H4 Bulbs; Exide Edge AGM Battery
        Summer: 5Zigen FN01R-C 16 x 7" Wheels; Yoko S.drive 205/45-16s
        Winter: Enkei OR52 16 x 7" Wheels; Falken Ziex ZE-912 205/45-16s
        Modified OEM Air Intake; Racing Beat Exhaust System; Techna-Fit SS Clutch Line
        Denso SKJ16CR-L11 Extended Tip Spark Plugs; Magnecor Wires
        Power Slot Front Brake Rotors; Techna-Fit SS Brake Lines; Hawk HPS Pads
        Red Line Synthetic Engine Oil; C/S Aluminum Oil Cap
        Cyberdyne Digital Gauges: Tach; Ambient Air Temp; Voltmeter

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