Excerpted from FAQ: Automotive Gasoline, Bruce Hamilton, B.Hamilton@irl.cri.nz
4.4 What are the hydrocarbons in gasoline?
Hydrocarbons ( HCs ) are any molecules that just contain hydrogen and
carbon, both of which are fuel molecules that can be burnt ( oxidised )
to form water ( H2O ) or carbon dioxide ( CO2 ). If the combustion is
not complete, carbon monoxide ( CO ) may be formed. As CO can be burnt
to produce CO2, it is also a fuel.
The way the hydrogen and carbons hold hands determines which hydrocarbon
family they belong to. If they only hold one hand they are called
"saturated hydrocarbons" because they can not absorb additional hydrogen.
If the carbons hold two hands they are called "unsaturated hydrocarbons"
because they can be converted into "saturated hydrocarbons" by the
addition of hydrogen to the double bond. Hydrogens are omitted from the
following, but if you remember C = 4 hands, H = 1 hand, and O = 2 hands,
you can draw the full structures of most HCs.
Gasoline contains over 500 hydrocarbons that may have between 3 to 12
carbons, and gasoline used to have a boiling range from 30C to 220C at
atmospheric pressure. The boiling range is narrowing as the initial boiling
point is increasing, and the final boiling point is decreasing, both
changes are for environmental reasons. Detailed descriptions of structures
can be found in any chemical or petroleum text discussing gasolines [14].
4.4.1 Saturated hydrocarbons ( aka paraffins, alkanes )
- stable, the major component of leaded gasolines.
- tend to burn in air with a clean flame.
- octane ratings depend on branching and number of carbon atoms.
alkanes
normal = continuous chain of carbons ( Cn H2n+2 )
- low octane ratings, decreasing with carbon chain length.
normal heptane C-C-C-C-C-C-C C7H16
iso = branched chain of carbons ( Cn H2n+2 )
- higher octane ratings, increasing with carbon chain branching.
iso octane = C C
( aka 2,2,4-trimethylpentane ) | |
C-C-C-C-C C8H18
|
C
cyclic = circle of carbons ( Cn H2n )
( aka Naphthenes )
- high octane ratings.
cyclohexane = C
/ \
C C
| | C6H12
C C
\ /
C
4.4.2 Unsaturated Hydrocarbons
- Unstable, are the remaining component of gasoline.
- Tend to burn in air with a smoky flame.
Alkenes ( aka olefins, have carbon=carbon double bonds )
- These are unstable, and are usually limited to a few %.
- tend to be reactive and toxic, but have desirable octane ratings.
C
| C5H10
2-methyl-2-butene C-C=C-C
Alkynes ( aka acetylenes, have carbon-carbon triple bonds )
- These are even more unstable, are only present in
trace amounts, and only in some poorly-refined gasolines.
_
Acetylene C=C C2H2
Arenes ( aka aromatics )
- Used to be up to 40%, gradually being reduced to <20% in the US.
- tend to be more toxic, but have desirable octane ratings.
- Some countries are increasing the aromatic content ( up to 50% in some
super unleaded fuels ) to replace the alkyl lead octane enhancers.
C C
// \ // \
C C C-C C
Benzene | || Toluene | ||
C C C C
\\ / \\ /
C C
C6H6 C7H8
Polynuclear Aromatics ( aka PNAs or PAHs )
- These are high boiling, and are only present in small amounts in gasoline.
They contain benzene rings joined together. The simplest, and least toxic,
is Naphthalene, which is only present in trace amounts in traditional
gasolines, and even lower levels are found in reformulated gasolines.
The larger multi-ringed PNAs are highly toxic, and are not present in
gasoline.
C C
// \ / \\
C C C
Naphthalene | || | C10H8
C C C
\\ / \ //
C C
4.9 What energy is released when gasoline is burned?
It is important to note that 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 oxidised (burnt),
to form water and carbon dioxide. Octane rating is not fundamentally
related to the energy content, and the actual hydrocarbon and oxygenate
components used in the gasoline will determine both the energy release and
the antiknock rating.
Two important reactions are:-
C + O2 = CO2
H + O2 = H2O
The mass or volume of air required to provide sufficient oxygen to achieve
this complete combustion is the "stoichiometric" mass or volume of air.
Insufficient air = "rich", and excess air = "lean", and the stoichiometric
mass of air is related to the carbon:hydrogen ratio of the fuel. The
procedures for calculation of stoichiometric air-fuel ratios are fully
documented in an SAE standard [35].
Atomic masses used are:- Hydrogen = 1.00794, Carbon = 12.011,
Oxygen = 15.994, Nitrogen = 14.0067, and Sulfur = 32.066.
The composition of sea level air ( 1976 data, hence low CO2 value ) is
Gas Fractional Molecular Weight Relative
Species Volume kg/mole Mass
N2 0.78084 28.0134 21.873983
O2 0.209476 31.9988 6.702981
Ar 0.00934 39.948 0.373114
CO2 0.000314 44.0098 0.013919
Ne 0.00001818 20.179 0.000365
He 0.00000524 4.002602 0.000021
Kr 0.00000114 83.80 0.000092
Xe 0.000000087 131.29 0.000011
CH4 0.000002 16.04276 0.000032
H2 0.0000005 2.01588 0.000001
---------
Air 28.964419
For normal heptane C7H16 with a molecular weight = 100.204
C7H16 + 11O2 = 7CO2 + 8H2O
thus 1.000 kg of C7H16 requires 3.513 kg of O2 = 15.179 kg of air.
The chemical stoichiometric combustion of hydrocarbons with oxygen can be
written as:-
CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O
Often, for simplicity, the remainder of air is assumed to be nitrogen,
which can be added to the equation when exhaust compositions are required.
As a general rule, maximum power is achieved at slightly rich, whereas
maximum fuel economy is achieved at slightly lean.
The energy content of the gasoline is measured by burning all the fuel
inside a bomb calorimeter and measuring the temperature increase.
The energy available depends on what happens to the water produced from the
combustion of the hydrogen. If the water remains as a gas, then it cannot
release the heat of vaporisation, thus producing the Nett Calorific Value.
If the water were condensed back to the original fuel temperature, then
Gross Calorific Value of the fuel, which will be larger, is obtained.
The calorific values are fairly constant for families of HCs, which is not
surprising, given their fairly consistent carbon:hydrogen ratios. For liquid
( l ) or gaseous ( g ) fuel converted to gaseous products - except for the
2-methylbutene-2, where only gaseous is reported. * = Blending Octane Number
as reported by API Project 45 using 60 octane base fuel, and the numbers
in brackets are Blending Octane Numbers currently used for modern fuels.
Typical Heats of Combustion are [36]:-
Fuel State Heat of Combustion Research Motor
MJ/kg Octane Octane
n-heptane l 44.592 0 0
g 44.955
i-octane l 44.374 100 100
g 44.682
toluene l 40.554 124* (111) 112* (94)
g 40.967
2-methylbutene-2 44.720 176* (113) 141* (81)
Because all the data is available, the calorific value of fuels can be
estimated quite accurately from hydrocarbon fuel properties such as the
density, sulfur content, and aniline point ( which indicates the aromatics
content ).
It should be noted that because oxygenates contain oxygen that can
not provide energy, they will have significantly lower energy contents.
They are added to provide octane, not energy. For an engine that can be
optimised for oxygenates, more fuel is required to obtain the same power,
but they can burn slightly more efficiently, thus the power ratio is not
identical to the energy content ratio. They also require more energy to
vaporise.
Energy Content Heat of Vaporisation Oxygen Content
Nett MJ/kg MJ/kg wt%
Methanol 19.95 1.154 49.9
Ethanol 26.68 0.913 34.7
MTBE 35.18 0.322 18.2
ETBE 36.29 0.310 15.7
TAME 36.28 0.323 15.7
Gasoline 42 - 44 0.297 0.0
Typical values for commercial fuels in megajoules/kilogram are [37]:-
Gross Nett
Hydrogen 141.9 120.0
Carbon to Carbon monoxide 10.2 -
Carbon to Carbon dioxide 32.8 -
Sulfur to sulfur dioxide 9.16 -
Natural Gas 53.1 48.0
Liquified petroleum gas 49.8 46.1
Aviation gasoline 46.0 44.0
Automotive gasoline 45.8 43.8
Kerosine 46.3 43.3
Diesel 45.3 42.5
Obviously, for automobiles, the nett calorific value is appropriate, as the
water is emitted as vapour. The engine can not utilise the additional energy
available when the steam is condensed back to water. The calorific value is
the maximum energy that can be obtained from the fuel by combustion, but the
reality of modern SI engines is that thermal efficiencies of only 20-40% may
be obtained, this limit being due to engineering and material constraints
that prevent optimum thermal conditions being used. CI engines can achieve
higher thermal efficiencies, usually over a wider operating range as well.
Note that combustion efficiencies are high, it is the thermal efficiency of
the engine is low due to losses. For a water-cooled SI engine with 25%
useful work at the crankshaft, the losses may consist of 35% (coolant),
33% (exhaust), and 12% (surroundings).