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.
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.
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