AFR Calculator (Air-Fuel Ratio)

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, rockets, 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.

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:

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

or...

where:

• $\rm AFR$ — Air-fuel ratio on mass basis;
• $\overline{\rm AFR}$ — Air-fuel ratio on molar basis; and
• $M_{\rm air}$ and $M_{\rm fuel}$ — Respective molar masses.

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 are 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:

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:

$\text C_\alpha \text H_\beta + a(\text O_2 + 3.76 \text N_2) \longrightarrow b\text C\text O_2 + c\text H_2\text O + d\text N_2$

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 $\text C_\alpha \text H_\beta$.

• Where $\alpha$ and $\beta$ subscripts indicate the respective number of atoms of carbon and hydrogen.

• The combustion air $a(\text O_2 + 3.76\text N_2)$, assuming that air is composed of 21% oxygen and 79% nitrogen.

• Here, the coefficient $a$ 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 $b$, $c$ and $d$ 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 ($\alpha$) and hydrogen ($\beta$) of a particular fuel, assigning the following values:

• $b=\alpha$

• $c=\dfrac{{\beta}}{2}$

• $a=\alpha+\dfrac{{\beta}}{4}$

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

You may check our combustion reaction calculator and learn more on how to balance a combustion recation ⚗

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 shown 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.

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.