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AFR Calculator

Chemistry

Calculate Air-Fuel Ratio (AFR), stoichiometric AFR, lambda (λ), and equivalence ratio (φ) for petrol, diesel, CNG, LPG, and other fuels instantly.

Reviewed by the thecalcu.com team · Last updated March 1, 2026

14.7

Stoichiometric AFR

14.7
Lambda (λ)
1
Equivalence Ratio (φ)
1
Mixture Condition
Stoichiometric (λ ≈ 1, φ ≈ 1)

This calculator computes your Stoichiometric AFR, Lambda (λ), Equivalence Ratio (φ), Mixture Condition from the values you enter.

Inputs
Fuel TypeActual Air-Fuel Ratio
Outputs
Stoichiometric AFRLambda (λ)Equivalence Ratio (φ)Mixture Condition

What is a Air-Fuel Ratio?

The AFR Calculator computes Lambda (λ = actual AFR / stoichiometric AFR), Equivalence Ratio (φ = 1/λ), and Mixture Condition (rich/lean/stoichiometric) for petrol, diesel, CNG, LPG, ethanol, and hydrogen fuels. Select the fuel type and enter the actual air-fuel ratio.

Air-Fuel Ratio is the mass ratio of air to fuel in a combustion mixture. Stoichiometric AFR is the theoretically perfect ratio for complete combustion (14.7 for petrol, 17.2 for CNG, 9.0 for ethanol). Lambda and equivalence ratio express how far the actual mixture deviates from stoichiometric, controlling emissions, power output, and fuel economy. Modern BS6 engines maintain λ = 1 ± 0.01 via closed-loop oxygen sensor feedback for optimal three-way catalyst performance.

For the underlying combustion chemistry producing CO₂ and H₂O, the Combustion Reaction Calculator balances the stoichiometric equation for hydrocarbon fuels. For green chemistry efficiency metrics, the Atom Economy Calculator provides the analogous concept for synthetic reactions.

Why Use an AFR Calculator?

The stoichiometric AFR must be calculated from the fuel composition (CₓHᵧOᵤ + O₂ → CO₂ + H₂O), not memorised. Lambda requires knowing the correct stoichiometric AFR for each fuel and dividing. Equivalence ratio inverts lambda. This calculator implements the chemistry for all common fuels and computes all three outputs instantly.

Who Should Use This Calculator?

Automotive engineers at Maruti Suzuki, Tata Motors, Mahindra, Hero MotoCorp, and Bajaj Auto developing BS6-compliant engine calibration.

CNG/LPG retrofit technicians setting fuel injection maps for alternative fuel conversions.

Environmental engineers at ARAI (Automotive Research Association of India) and iCAT (International Centre for Automotive Technology) conducting emission certification tests.

Combustion researchers at IIT Bombay, IIT Delhi, and IIT Madras studying alternative fuels (biogas, hydrogen) for transportation applications.

What Insights Does the AFR Calculator Give You?

Stoichiometric AFR, the theoretically correct ratio for complete combustion of the selected fuel.

Lambda (λ), dimensionless excess air ratio: λ = 1 (stoich), < 1 (rich), > 1 (lean).

Equivalence Ratio (φ), fuel-referenced ratio: φ = 1 (stoich), > 1 (rich), < 1 (lean).

Mixture Condition, plain-English classification of the combustion mixture state.

How to use this Air-Fuel Ratio calculator

  1. Select the Fuel Type, petrol for spark ignition engines; diesel for compression ignition; CNG for natural gas vehicles; ethanol for E85 or flex-fuel; hydrogen for fuel cell vehicles.
  2. Note the Stoichiometric AFR shown, this is the reference.
  3. Enter the Actual AFR, from ECU data, exhaust analysis, or design specification.
  4. Read Lambda, values 0.85–1.15 are the operating range for most engines; TWC requires 0.99–1.01.
  5. Read Mixture Condition, use to diagnose rich (CO emissions) vs lean (NOx emissions) issues.
Show formula & methodology ↓Show less ↑

Formula & Methodology

AFR, lambda, and equivalence ratio:

AFR_stoich depends on fuel CₓHᵧOᵤ:   O₂ required = (x + y/4 − z/2) mol per mol fuel   AFR_stoich = [(x + y/4 − z/2) × 32 / 0.232] / M_fuel    Petrol (C₈H₁₈): AFR = 14.7   Diesel (C₁₂H₂₃): AFR = 14.5   CNG / CH₄:       AFR = 17.2   LPG / C₃H₈:      AFR = 15.7   Ethanol C₂H₅OH:  AFR = 9.0   Hydrogen H₂:      AFR = 34.3  λ = AFR_actual / AFR_stoich φ = 1/λ = AFR_stoich / AFR_actual  λ < 1 → rich (φ > 1, excess fuel) λ = 1 → stoichiometric (φ = 1) λ > 1 → lean (φ < 1, excess air)

Worked example, E20 (20% ethanol blend) combustion:

E20 fuel: 80% petrol + 20% ethanol by volume (approximately 80% by mass after density correction; petrol: 0.72 kg/L, ethanol: 0.79 kg/L → mass fractions: 77.5% petrol, 22.5% ethanol).

AFR_stoich(E20) = 0.775 × 14.7 + 0.225 × 9.0 = 11.39 + 2.025 = 13.42  If engine calibrated for petrol (AFR = 14.7) but running E20: Actual AFR delivered = 14.7 (same fuel injection volume for air) But stoichiometric AFR for E20 = 13.42 λ = 14.7 / 13.42 = 1.095 → lean mixture (φ = 0.91)

Result: lean combustion → higher NOx, potential knocking. This is why E20 requires ECU recalibration, India's NITI Aayog E20 mandate (target: nationwide E20 by 2025) requires all new vehicles sold from 2023 to be E20-compatible. Maruti Suzuki, Hyundai, and Tata Motors have already launched E20-compatible model variants recalibrated for the lower stoichiometric AFR.

Frequently Asked Questions

What is Air-Fuel Ratio (AFR)?
Air-Fuel Ratio (AFR) is the mass ratio of air to fuel in a combustion mixture: AFR = mass of air / mass of fuel. It is the most fundamental parameter in engine and combustion engineering. A higher AFR means more air relative to fuel (lean mixture); a lower AFR means more fuel relative to air (rich mixture). The stoichiometric AFR is the theoretically perfect ratio for complete combustion, exactly enough air to burn all the fuel without excess of either. For petrol (isooctane, C₈H₁₈), stoichiometric AFR = 14.7 (14.7 kg air per 1 kg fuel).
How do I use the AFR Calculator?
Select the Fuel Type from the dropdown (petrol, diesel, CNG, LPG, ethanol, hydrogen, or custom). The stoichiometric AFR for that fuel is shown automatically. Enter the Actual Air-Fuel Ratio for your engine or combustion system. The calculator returns Stoichiometric AFR, Lambda (λ = actual AFR / stoichiometric AFR), Equivalence Ratio (φ = 1/λ), and Mixture Condition (rich/lean/stoichiometric). Default: petrol at 14.7 = stoichiometric (λ = 1, perfect combustion).
What is lambda (λ) and what values indicate rich or lean mixture?
Lambda (λ) = actual AFR / stoichiometric AFR. λ = 1: stoichiometric (perfect ratio, complete combustion, all O₂ consumed, all fuel burned). λ < 1: rich mixture (excess fuel, unburned fuel, CO and HC emissions high, but maximum power). λ > 1: lean mixture (excess air, more complete combustion, lower CO/HC, higher NOx, fuel economy improved, but risk of misfire at λ > 1.4). Modern petrol engines use a three-way catalytic converter (TWC) that works efficiently only in the narrow window λ = 1.0 ± 0.01, controlled by the engine ECU using an O₂ (lambda) sensor in the exhaust. This window is called the 'stoichiometric window' or 'lambda window'.
What are the stoichiometric AFR values for different Indian fuels?
Stoichiometric AFR by fuel (mass of air / mass of fuel): Petrol/Gasoline (C₈H₁₈): 14.7. Diesel (C₁₂H₂₃ approximation): 14.5. CNG / Methane (CH₄): 17.2. LPG / Propane (C₃H₈): 15.7. Ethanol (C₂H₅OH): 9.0. Hydrogen (H₂): 34.3. Natural gas blends (PNG, Piped Natural Gas): 16.5–17.5 depending on composition. Biogas (55–65% CH₄, balance CO₂): ~5.5–7.0. India's Bharat Stage VI (BS6, equivalent to Euro 6, implemented April 2020) fuel specifications for petrol and diesel define compositions that give stoichiometric AFR values in these ranges.
How does the equivalence ratio (φ) differ from lambda?
φ (phi, equivalence ratio) = 1/λ = stoichiometric AFR / actual AFR. φ = 1: stoichiometric. φ > 1: rich (more fuel than stoichiometric, excess fuel, equivalently deficient air). φ < 1: lean (less fuel than stoichiometric, excess air). The equivalence ratio φ is preferred by combustion researchers because it scales naturally: φ = 2 means twice the stoichiometric fuel. Lambda λ is preferred by engine calibration engineers because λ = 1 is universal across all fuels. Both describe the same mixture condition from opposite perspectives. Under Bharat Stage VI norms, all passenger vehicle ECUs must maintain λ = 0.99–1.01 during steady-state operation for optimal TWC performance.
Why does ethanol have a lower stoichiometric AFR than petrol?
Ethanol (C₂H₅OH) contains oxygen in its molecular structure, the combustion reaction requires less atmospheric O₂ because the fuel already provides some oxygen: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Stoichiometric O₂ needed per gram of ethanol = 3×32/46 = 2.087 g O₂/g ethanol. Air (23.2% O₂ by mass): stoich AFR = 2.087/0.232 = 9.0. Petrol C₈H₁₈ + 12.5O₂ → 8CO₂ + 9H₂O: stoich O₂ = 12.5×32/114 = 3.508 g/g → AFR = 3.508/0.232 = 15.1 (literature: 14.7 for practical gasoline). E10 (10% ethanol, 90% petrol) blend: stoich AFR = 0.9×14.7 + 0.1×9.0 = 14.13. India's E10 rollout (2022–23) and E20 mandate (2025) require engine recalibration to account for this AFR shift.
How does AFR affect engine emissions and why does India have BS6 norms?
AFR controls combustion emissions: Rich (λ < 1): high CO (carbon monoxide, toxic), high HC (unburned hydrocarbons), low NOx. Stoichiometric (λ = 1): minimum combined CO + HC + NOx with TWC. Lean (λ > 1): low CO and HC, HIGH NOx (high temperature, excess O₂ promotes N₂ + O₂ → 2NO). Bharat Stage VI (April 2020) limits for petrol cars: CO 1.0 g/km, HC + NOx 0.10 g/km. Delhi's air quality crisis (AQI > 400 in November) is driven by diesel vehicle NOx + particulate matter, winter inversion, and crop stubble burning. The NGT (National Green Tribunal) has imposed BS6 enforcement to reduce vehicular emissions, AFR calibration is central to achieving BS6 compliance.
What is the role of oxygen sensors (lambda sensors) in modern Indian vehicles?
Wideband lambda sensors (also called universal exhaust gas oxygen / UEGO sensors) measure exhaust O₂ to compute λ precisely (range 0.7–1.3). Narrowband sensors (NTK, Bosch, Delphi) used in older vehicles only detect whether λ < 1 (rich, low O₂ voltage ~0.9V) or λ > 1 (lean, high O₂ voltage ~0.1V). Modern BS6 engines use wideband sensors for precise λ control. BS6 OBD-II regulations (implemented April 2020 by ARAI, Automotive Research Association of India) require: two lambda sensors (pre-catalyst and post-catalyst), DTC (Diagnostic Trouble Codes) for sensor faults, MIL (Malfunction Indicator Light) activation if λ control fails. Maruti Suzuki, Hyundai India, Tata Motors use Bosch and Delphi lambda sensor systems in their BS6 vehicles.
How is stoichiometric AFR calculated from fuel composition?
For a fuel CₓHᵧOᵤ: Complete combustion: CₓHᵧOᵤ + (x + y/4 − z/2)O₂ → xCO₂ + (y/2)H₂O. O₂ required per mole fuel = (x + y/4 − z/2) moles O₂. Mass of O₂ = (x + y/4 − z/2) × 32 g. Air (23.2% O₂ by mass): mass of air = O₂ mass / 0.232. Stoichiometric AFR = air mass / fuel mass = [(x + y/4 − z/2) × 32 / 0.232] / (12x + y + 16z). Example: Methane CH₄ (x=1, y=4, z=0): stoich AFR = [(1+1) × 32 / 0.232] / (12+4) = 275.9/16 = 17.2 ✓. The [Combustion Reaction Calculator](/combustion-reaction-calculator/) computes the full balanced equation for CₓHᵧOᵤ fuels including these stoichiometric coefficients.
What AFR does a CNG vehicle operate at compared to petrol?
CNG (Compressed Natural Gas, predominantly CH₄) stoichiometric AFR = 17.2 vs petrol 14.7. CNG engines require recalibration: the ECU must target a different AFR (17.2 instead of 14.7). CNG-fitted vehicles in India (taxi fleets in Delhi, Mumbai, Pune, mandated by NGT for commercial vehicles; dedicated TATA and Maruti CNG models) use bi-fuel ECUs that switch maps between CNG and petrol modes. India has 5.5 million CNG vehicles (2023), the second largest fleet in the world after China, with 7,000+ CNG stations along highways. Delhi's auto-rickshaw and taxi fleet (500,000 vehicles) was converted to CNG from 1998–2003 following a Supreme Court order, reducing PM and NOx dramatically.