AFR Calculator - Air Fuel Ratio
Free Afrcalculator air fuel ratio Calculator for stoichiometry. Enter variables to compute results with formulas and detailed steps.
Reviewed by Manoj Kumar, Mathematics Educator
Formula
AFR = mass of air / mass of fuel | Lambda = AFR / stoichiometric AFR
The air-fuel ratio is the mass of air divided by the mass of fuel. Lambda normalizes this by dividing by the stoichiometric AFR for the specific fuel. Lambda = 1 is stoichiometric, less than 1 is rich, greater than 1 is lean.
Worked Examples
Example 1: Gasoline Engine AFR
Problem:An engine consumes 500 g of air and 35 g of gasoline per minute. Find the AFR and lambda.
Solution:AFR = 500 / 35 = 14.286:1\nStoichiometric AFR (gasoline) = 14.7:1\nLambda = 14.286 / 14.7 = 0.972\nMixture is slightly rich
Result:AFR = 14.29:1, Lambda = 0.972 (slightly rich)
Example 2: E85 Target AFR
Problem:Find the target AFR for E85 fuel at lambda = 0.88 (max power).
Solution:Stoichiometric AFR (E85) = 9.8:1\nTarget AFR = lambda * stoich = 0.88 * 9.8\nTarget AFR = 8.624:1
Result:Target AFR = 8.62:1 for E85 at max power
Frequently Asked Questions
What is the air-fuel ratio (AFR)?
The air-fuel ratio is the mass ratio of air to fuel in a combustion mixture. For gasoline engines, the stoichiometric AFR is 14.7:1, meaning 14.7 grams of air are needed to completely burn 1 gram of fuel. Running richer (lower AFR, more fuel) increases power but wastes fuel and increases emissions. Running leaner (higher AFR, less fuel) improves efficiency but can cause higher combustion temperatures and NOx emissions. Modern engines use oxygen sensors to maintain the AFR near stoichiometric for optimal catalytic converter performance.
What is lambda and the equivalence ratio?
Lambda is the ratio of the actual AFR to the stoichiometric AFR. A lambda of 1.0 means the mixture is stoichiometric. Lambda greater than 1.0 is lean (excess air), and lambda less than 1.0 is rich (excess fuel). The equivalence ratio (phi) is the inverse of lambda: phi = 1/lambda. Engine tuners use lambda because it normalizes across different fuels. For example, lambda = 0.85 means the same degree of richness whether you are burning gasoline, ethanol, or any other fuel, even though the actual AFR numbers differ.
What AFR gives maximum power vs maximum efficiency?
Maximum power in gasoline engines typically occurs at a rich mixture around lambda 0.85-0.90 (AFR of about 12.5-13.2:1). The extra fuel provides cooling and ensures all available oxygen is used. Maximum fuel efficiency occurs at a lean mixture around lambda 1.1-1.3 (AFR of 16-19:1), where all fuel is burned with excess air. However, very lean mixtures risk misfires and high NOx emissions. Diesel engines typically run lean (lambda 1.3-5.0) because they control power by fuel quantity, not throttle position.
How do I calculate fuel costs for a road trip?
Divide total trip distance by your car's MPG to get gallons needed, then multiply by the price per gallon. Example: 500 miles at 25 MPG = 20 gallons x $3.50 = $70 in fuel. Use GasBuddy to find prices along your route and add 10% for city driving and detours.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy