AFR Calculator (Air-Fuel Ratio)

Created by Gabriela Diaz
Reviewed by Hanna Pamuła, PhD candidate and Steven Wooding
Last updated: Mar 24, 2022

The AFR calculator (air-fuel ratio) will give you the rate of air to fuel and the mass of air needed for its complete combustion.

Combustion is a process found in different technologies such as heating devices, internal combustion engines, gas turbines, rocket engines, etc., where AFR is an important parameter.

Keep reading to learn about what is the air-fuel ratio, the AFR of some fuels and how to calculate the stoichiometric air-fuel ratio for fossil fuels.

What is the air-fuel ratio (AFR)?

As Lavoisier found out, oxygen is a key substance in any combustion process, and it turns out that for different types of fuels, the amount of oxygen required is also different.

This specific oxygen requirement is expressed by the parameter known as the air-fuel ratio (AFR), indicating the amount of air needed to achieve the complete combustion of a given quantity of a fuel 🔥.

The AFR is often expressed on a mass basis, as mass of air divided by mass of fuel:

AFR=massairmassfuel\quad AFR= \dfrac{mass_{air}}{mass_{fuel}}

This air-fuel ratio calculation formula can also be expressed in terms of the molar ratio and the molar mass of each substance as:

AFR=molesair×Mairmolesfuel×Mfuel\quad AFR= \dfrac{moles_{air}\times M_{air}}{moles_{fuel}\times M_{fuel}}

or...

AFR=AFR×MairMfuel\quad AFR= \overline{AFR}\times \dfrac{M_{air}}{M_{fuel}}

where:

  • AFRAFR – Air-fuel ratio on mass basis;
  • AFR\overline{AFR} – Air-fuel ratio on molar basis; and
  • MairM_{air} and MfuelM_{fuel} – Respective molar masses.

Air-fuel ratio of common fuels

From the stove in our homes to rockets leaving Earth, we’re surrounded by different appliances and equipment that include the combustion process as part of their functioning system, all of which use a specific fuel. For example, natural gas used on stoves, heating systems and power generation plants 🏭,; gasoline and diesel, mainly used for cars, buses and other means of transportation 🚗,; aviation turbine fuel (ATF) for airplanes ✈ or liquid hydrogen as rocket fuel 🚀.

Here are some air-fuel ratios for most common hydrocarbon fuels:

Fuel

Formula

AFR (mass)

AFR (molar)

Molar Mass (g/mol)

Methane

CH4

17.19

9.52

16.04

Ethane

C2H6

16.95

16.66

30.07

Propane

C3H8

15.64

23.80

44.09

Butane

C4H10

15.42

30.94

58.12

Pentane

C5H12

15.29

38.08

72.15

Octane

C8H18

15.09

59.50

114.23

Diesel

C12H23

14.6

84.49

167.31

Hydrogen

H2

34.21

2.38

2.02

💡 Did you know that natural gas is mainly composed of methane (CH4), around 80-90%?

How to calculate the stoichiometric air-fuel ratio

The minimum amount of air needed for complete combustion is known as theoretical or stoichiometric air. That is the quantity of air used when calculating the stoichiometric air-fuel ratio.

The general formula for the complete combustion of a hydrocarbon fuel with theoretical air is:

CαHβ+a(O2+3.76N2)bCO2+cH2O+dN2\text C_\alpha \text H_\beta + a(\text O_2 + 3.76 \text N_2) \longrightarrow b\text C\text O_2 + c\text H_2\text O + d\text N_2

On the left side of the reaction we have:

  • The reactants of the combustion, which are a hydrocarbon fuel and oxygen.
  • The generic formula of a hydrocarbon fuel is represented as CαHβ\text C_\alpha \text H_\beta.
  • Where α\alpha and β\beta subscripts indicate the respective number of atoms of carbon and hydrogen.
  • The combustion air a(O2+3.76N2)a(\text O_2 + 3.76\text N_2), assuming that air is composed of 21% oxygen and 79% nitrogen.
  • Here, the coefficient aa represents the required moles of air to balance the combustion reaction.

And on the right side of the reaction:

  • Here we find the products of a complete combustion: carbon dioxide (CO2), water (H2O) and nitrogen (N2). Note that for theoretical air there's non free oxygen (O2).
  • The coefficients bb, cc and dd of combustion's products balance the equation.

The coefficients that balance the chemical reaction can be obtained in terms of the number of atoms of carbon (α\alpha) and hydrogen (β\beta) of a particular fuel, assigning the following values:

  • b=αb=\alpha

  • c=β2c=\dfrac{{\beta}}{2}

  • a=α+β4a=\alpha+\dfrac{{\beta}}{4}

  • d=3.76×(α+β4)d=3.76 \times \bigg(\alpha+\dfrac{{\beta}}{4}\bigg)

Once these are known, we can calculate the number of moles of combustion air and the moles of fuel and relate them using the AFR equation showed before.

On the given occasions when the molecular formula of the fuel is unknown, this one can be obtained through combustion analysis calculation. Once the molecular formula is known, the procedure to balance the combustion reaction is the same as mentioned above.

How to use the AFR calculator?

To use the AFR calculator, follow these steps:

  1. Select one of the substances from the list of fuels.

  2. Once selected, the calculator will show the Air-fuel ratio (AFR) for that substance.

    As an example, if choosing methane (CH4), the calculator will show 17.19:1, indicating that for the combustion of every unit mass of methane (i.e., 1 kg), 17.19 unit mass of air (i.e., 17.19 kg) will be required for its complete combustion.

  3. On the Mass of air and fuel section, enter either the mass of fuel or air, and the calculator will give the mass of the other substance.

💡 The air-fuel ratio calculator also works the other way around. You can enter both the mass of air and fuel, and the calculator will show you the Air-fuel ratio (AFR) as a result.

Gabriela Diaz
Select a fuel
Fuel type
Methane (CH4)
Air-fuel ratio (AFR)
:1
Mass of air and fuel
Mass of fuel ⛽
lb
Mass of air 🌬
lb
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