MadSci Network: Chemistry

Re: What is the chemical equation for unleaded gasoline?

Date: Sun Apr 4 22:37:47 1999
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Chemistry
ID: 922918196.Ch


Important references for your questions are the American Petrolium Institute at URL:

The Shell Oil Company at URL:

I highly recommend that you first read the following Frequently Asked Questions (FAQ) web pages on gasoline etc.for an historic perspective of what is put into gasoline and why it is there and how it works. Also; the cumbustion products of gasoline are discussed in detail. I have abstracted the following discussion from the FAQ web site:


The combustion of gasoline is a very complex chemical process for there are over 500 different hydrocarbons in gasoline plus many other elements and additives.

Hydrocarbons ( HCs ) are any molecules that just contain hydrogen and carbon, both of which are fuel molecules that can be burned to form water ( H2O ) or carbon dioxide ( CO2 ). If the combustion is not complete, carbon monoxide ( CO ) may be formed. As CO is burned to produce CO2, it also becomes a fuel.The way the hydrogen and carbons bond determines which hydrocarbon family they belong to. If they have only one bond they are called "saturated hydrocarbons" because they can not absorb additional hydrogen. If the carbons have two bonds they are called "unsaturated hydrocarbons" because they can be converted into "saturated hydrocarbons" by the addition of hydrogen to the double bond.

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. In past years the boiling range has been 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. (Reference given on web pages)

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 burned, 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 . (Reference on web pages). 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 For normal heptane C7H16 with a molecular weight = 100.204 C7H16 + 11O2 = 7CO2 + 8H2Othus 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 American Petrolium Institute Project 45 using 60 octane base fuel, and the numbers in brackets are Blending Octane Numbers currently used for modern fuels.

Typical values for commercial fuels in megajoules/kilogram are:

    - Gross Net 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 

I'm sure you will find most of the information that you require on the FAQ pages. The AIP and Shell pages may fill in some gaps.

Best regards, Your Mad Scientist

Adrian Popa

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