Stoichiometric Air Fuel Ratio for Biofuels and Kerosene

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What is the Stoichometric Ratio?

The stoichiometric ratio is the chemically correct air and fuel ratio to ensure complete combustion of fuel. A pure fuel made up of only carbon and hydrogen like methane would burn into carbon dioxide and water when complete combustion takes place. However an incomplete combustion caused by lack of oxygen may result in formation of a mixture of carbon monoxide, carbon dioxide, and unburnt hydrocarbons. A Stoichiometric air fuel ratio is also called the ideal ratio. An air-fuel ratio containing more oxygen and less fuel is called a lean mixture. An air fuel ratio containing more fuel and less oxygen is called a rich mixture or ratio.

When the air fuel ratio is rich ,then complete combustion does not take place and loss of fuel in the form of unburnt hydrocarbons is there resulting in less efficiency to the engine. When the air fuel ratio is lean, then combustion is complete but cooling down of the engine occurs causing loss of efficiency. Thus it is the endeavor of all engine manufacturers to provide a Stoichiometric air to fuel ratio for better combustion. However practically it is not possible as extra air has to be supplied for the scavenging function.

Solving Ideal Stoichometric Equations

To find the amount of oxygen and hence the amount of air used to completely burn the fuel follow these steps to solve for the ideal stoichiometric equation.

  1. The ideal stoichiometric equation means no extra air is added for combustion.
  2. Write the equation for the combustion. For example CH4 + O2 = CO2 + H2O
  3. Substances always combine in definite proportions, and these proportions are determined by the molecular masses of the substances involved and the products formed.
  4. Balance the left hand side and the right hand side of the equation. That is CH4 + 2O2 = CO2 +2H2O
  5. Convert to molecular mass. For example, one molecule of CH4 weight is 1x12+4x1 = 16 and oxygen weighs 2x2x16 =64 for complete combustion.
  6. Now you know how much oxygen is required to completely burn the fuel. This means 64/16 = 4 kgs of oxygen is required for complete combustion of one Kg of methane.
  7. However remember some fuels have oxygen and it has to be subtracted accordingly.
  8. Remember oxygen in 21% by volume and 23% by weight in air and calculate accordingly. This means 4/0.23 = 17.39 or 17.4 Kgs of air is required for complete combustion of one kg of methane. So the stoichiometric air fuel ratio for methane is 17.4 : 1.

Stoichometric Ratio of different fuels

The Stoichiometric ratio of various fuels are as follows:

Gasoline has an air fuel ratio of 14.7 : 1

Kerosene has an air fuel ratio of 15.6 : 1

Natural Gas has an air fuel ratio of 17.2 : 1

Propane has an air fuel ratio of 15.5 : 1

Ethanol has an air fuel ratio of 9 : 1

Methanol has an air fuel ratio of 6.4 : 1

Hydrogen has an air fuel ratio of 34: 1

Diesel has an air fuel ratio of 14.6 : 1

Methane has an air fuel ratio of 17.4 : 1

Biogas has an air fuel ratio of 10: 1

Bio Ethanol (22 % mix) has an air fuel ratio of 12.7 : 1 as it is an oxygenated fuel as compared to Gasoline, and E85 has an air fuel ratio of 9.765.


Internal Combustion Engines by M.L. Mathur and R.P. Sharma

Thermodynamics and Heat Engines by R. Yadav

Biofuels for transport by Worldwatch Institute