Combustion Analysis for Gas Compressor Engines
Stoichiometric combustion, equivalence ratio, lean-burn operation, emissions control strategies, and fuel composition effects.
Most pipeline lean-burn engines operate at AFR 25:1 to 35:1 (equivalence ratio φ = 0.55–0.70). This range minimizes NOx without crossing the misfire limit, which typically begins below φ = 0.50 (AFR ≳ 33:1).
The fuel-mass-per-cycle formula assumes a 2-stroke integral compressor engine — Cooper-Bessemer GMV, Ingersoll-Rand HBA, Clark TLA — where each cylinder fires once per revolution. For a 4-stroke engine, divide RPM by 2 in the fuel-mass denominator.
Lower atmospheric pressure (altitude) and higher air manifold temperature both reduce trapped air density. This raises actual AFR (leaner) and lowers φ — typically about 3% per 1,000 ft elevation and roughly 1% per 10°F AMT increase. Engines must be derated or turbo boost adjusted to compensate.
Equivalence ratio (φ = AFRstoich / AFRactual) normalizes for fuel composition. When fuel gas shifts from pipeline-quality (16.7:1 stoich) to heavier field gas (16.2:1) or propane (15.7:1), the same actual AFR represents different combustion conditions. Tuning to a φ target keeps NOx and misfire margin consistent regardless of fuel.