Equivalence Ratio & Excess Air Analysis
| Parameter | Formula |
|---|---|
| Actual A/F Ratio | Air Flow / Fuel Flow (by volume) |
| Equivalence Ratio | φ = Stoich A/F / Actual A/F |
| Lambda (λ) | λ = 1 / φ = Actual A/F / Stoich A/F |
| Excess Air | (λ - 1) × 100% |
Understand air-fuel ratio fundamentals, combustion stoichiometry, and lean burn engine operation
The stoichiometric air-fuel ratio for natural gas is approximately 9.7:1 by volume (17.2:1 by mass). This is the exact ratio needed to completely combust the fuel with no excess air remaining.
Equivalence ratio (phi) is the stoichiometric A/F ratio divided by the actual A/F ratio. Lambda is the inverse (actual/stoichiometric). Phi greater than 1 indicates a rich mixture, while phi less than 1 indicates lean. Lambda is commonly used in automotive applications while phi is standard in industrial gas engine analysis.
Lean burn engines operate with 50-100% excess air (phi = 0.5-0.67) to reduce NOx emissions, improve fuel efficiency, and lower exhaust temperatures. Most modern integral and separable compressor engines in midstream service use lean burn combustion for emissions compliance.