Air-Fuel Ratio Calculator

The air-fuel ratio (AFR) is a value that specifies the mass of air divided by the mass of fuel in a combustion mixture. It indicates whether an engine or burner has enough air to completely burn the fuel. This calculator allows you to calculate AFR from the masses of air and fuel, and if AFR is known one of the two masses can be recalculated. For the selected fuel it also shows whether it's a lambda ratio, whether it's stoichiometric and whether the mixture is lean, rich or optimal.

The importance of air-fuel ratio:

For each fuel there is a certain amount of air required for complete combustion. For petrol the ratio of mass of air to mass of fuel is about 14.7 which is expressed as 14.7:1. This reaction using only this amount of air is called stoichiometric combustion since both oxygen and fuel are completely consumed. If less air is supplied to the engine then the mixture is rich, if more air is supplied it is lean.

Since this ratio is a mass ratio and not a volume ratio, the same units must be used for both air and fuel. If the air is measured in grams, then the fuel must also be measured in grams. As it is a ratio of like units, the AFR itself has no unit. The value 12.5 always means the mass of air 12.5 divided by the mass of fuel 1.

Lambda and stoichiometric ratio:

Experts rarely speak of the actual AFR directly as the ideal value varies with fuel. Instead Lambda is used to compare the actual AFR to the stoichiometric AFR for the fuel in the tank. Regardless of what fuel is being used, a lambda of 1.0 indicates an ideal mixture. A value below 1.0 means a rich mixture while a value above 1.0 indicates a lean mixture. Thus Lambda is a way to represent a mixture independent of the fuel and this is why wide band oxygen sensors directly measure this value.

The stoichiometric ratio is represented by the Greek letter φ and is the inverse of λ. If φ exceeds a value of 1.0, then the mixture is rich; if it falls below 1.0, then it is lean. The values of both quantities therefore give the same information but in opposite directions. Air excess is calculated from λ. A value of λ equal to 1.10 means that there is about a 10 percent excess of air over what is actually required for the fuel mixture.

Here's how to use this calculator:

First select the fuel and let the tool set the stoichiometric reference values. Then enter the required value for air mass, fuel mass or AFR, and the tool will calculate the remaining values. If you only want to check the datasheets of the bench or the readings from your instruments, simply enter the value for AFR to retrieve λ and the mixture rating. By activating the "target mixture" option, you can also calculate the values backwards. Enter the desired value for λ and the tool will return the corresponding AFR value and the corresponding fuel mass.

Formula:

The basic relationship is very simple: AFR is the quotient of air mass and fuel mass.

λ is the quotient of the actual ratio and the stoichiometric ratio of the fuel, where φ is the inverse.

Suppose a petrol engine is running at an actual air-fuel ratio (AFR) of 13.5:1. As the stoichiometric ratio for petrol is 14.7:1, lambda would be approximately equal to 13.5 divided by 14.7, which equals about 0.92. This value is less than 1.0, meaning that the mixture is around 8% too rich. This is a setting often used when an engine is being run at its maximum power output. The deviation from stoichiometric is expressed as a percentage and based on the difference of lambda.

The calculation results in the following AFR for the respective mass of Air and Fuel.

Stoichiometric ratios for various fuels

Since each fuel has a different hydrogen to carbon ratio, there is an ideal value for each one. Hydrogen requires significantly more air per gram than gasoline, while alcohols such as methanol and ethanol require considerably less air. The following table shows the values used in this tool.

Why Air-Fuel Ratio is Important

In a petrol engine the mixture affects power output, fuel consumption and emissions. When running at near stoichiometric ratio the catalytic converter can clean up exhaust gases and fuel consumption is low. At high load the management systems will increase the amount of fuel being injected and lower lambda to about 0.85 to 0.90. This is because excess fuel mixture cools down combustion and prevents knock. If the mixture becomes too lean then the engine runs hotter and there is a risk of component damage. Therefore, lean mixture fault codes should be taken seriously.

The same principle applies to diesel engines, gas turbines, furnaces and laboratory burners. Diesel engines always run with a lean mixture, where lambda is often significantly greater than 1, and both soot and nitrogen oxide emissions are closely related to this value. Regardless of where the fuel and air meet, this ratio determines the completeness and cleanliness of combustion.

Tips for meaningful measurements:

Measure air and fuel in the same units to ensure that the ratio is correct. When reading a wideband sensor, note that it displays the lambda value. Multiply the lambda value by the stoichiometric AFR of the fuel being used to get the actual air excess. Choose a fuel that matches the actual fuel in your tank. Even at the same lambda value, the target AFR values can be significantly different when tuning E85 and gasoline.