This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
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 .
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
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
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 :-
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)
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
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