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Air-Fuel Ratio (AFR)

Calculate air-fuel ratios, lambda (λ), equivalence ratio (φ), and analyze combustion stoichiometry for various fuels. Essential for engine tuning, burner optimization, and combustion analysis.

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Sample Examples

🚗 Gasoline Engine

Typical automotive gasoline engine at stoichiometric conditions

Internal Combustion Engine

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🚛 Diesel Engine

Diesel engine with lean burn for efficiency

Compression Ignition Engine

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🌾 E85 Ethanol Blend

E85 fuel (85% ethanol, 15% gasoline) in flex-fuel vehicle

Alternative Fuel Vehicle

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🔥 Natural Gas Burner

Industrial natural gas burner with 10% excess air

Industrial Burner

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🍖 Propane Grill

Propane grill with rich mixture for better flavor

Cooking Appliance

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⚡ Gas Turbine

Gas turbine with ultra-lean burn for low NOx emissions

Power Generation

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⚖️ Mass-Based Calculation

Calculate AFR from known air and fuel masses

Laboratory Analysis

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Calculation Mode

Mass-Based Calculation

Volume-Based Calculation (Alternative)

Standard: 1.225 kg/m³ at sea level, 15°C
Default for Gasoline: 750 kg/m³

Environmental Conditions

Used for air density calculation
Standard atmospheric pressure: 101.325 kPa

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For educational and informational purposes only. Verify with a qualified professional.

What is Air-Fuel Ratio (AFR)?

The Air-Fuel Ratio (AFR) is the mass ratio of air to fuel present in a combustion process. It is a critical parameter in combustion engineering, engine design, and emissions control.

Formula: AFR = Mass of Air / Mass of Fuel

The AFR determines whether a mixture is:

  • Rich: Too much fuel (AFR < stoichiometric) - incomplete combustion, higher emissions
  • Stoichiometric: Perfect balance (AFR = stoichiometric) - complete combustion
  • Lean: Too much air (AFR > stoichiometric) - may cause misfires, higher NOx

How to Calculate Air-Fuel Ratio

There are several methods to calculate AFR:

1. Mass-Based Calculation:

Measure the mass of air and fuel directly:
AFR = m_air / m_fuel

2. Volume-Based Calculation:

Convert volumes to masses using densities:
AFR = (V_air × ρ_air) / (V_fuel × ρ_fuel)

3. From Lambda:

If lambda is known:
AFR = λ × AFR_stoichiometric

When to Use AFR Calculations

AFR calculations are essential in:

  • Engine Tuning: Optimize performance and fuel economy
  • Emissions Control: Meet environmental regulations
  • Burner Design: Industrial and residential heating systems
  • Alternative Fuels: Analyze ethanol, biodiesel, natural gas blends
  • Research: Combustion studies and fuel development
  • Diagnostics: Troubleshoot engine performance issues

Key Formulas

Air-Fuel Ratio:

AFR = m_air / m_fuel

Lambda (λ):

λ = AFR_actual / AFR_stoichiometric

λ < 1: Rich mixture | λ = 1: Stoichiometric | λ > 1: Lean mixture

Equivalence Ratio (φ):

φ = 1 / λ = AFR_stoichiometric / AFR_actual

φ > 1: Rich mixture | φ = 1: Stoichiometric | φ < 1: Lean mixture

Excess Air:

Excess Air (%) = (λ - 1) × 100 (for lean mixtures)

Air Density (Ideal Gas Law):

ρ_air = P / (R × T)

Where R = 287 J/(kg·K) for dry air

Common Fuel Stoichiometric AFR Values

FuelFormulaStoichiometric AFR
GasolineC8H1814.7
DieselC12H2314.5
EthanolC2H5OH9.0
MethaneCH417.2
PropaneC3H815.7
MethanolCH3OH6.5
Natural GasCH417.2

📚 Official Data Sources

SAE International ↗
Automotive and combustion standards
Updated: 2025
NIST Chemistry WebBook ↗
Combustion and fuel data
Updated: 2025

⚠️ Disclaimer: AFR values are for reference. Actual combustion depends on conditions and equipment. Not a substitute for professional engine tuning.

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