Why Lambda (λ) — Not Raw Air-Fuel Ratio (AFR) — Governs Modern ECU Fueling
Gasoline Direct Injection (GDI) controllers such as Bosch MED17, MG1CS, Continental Simos 18/19, and Marelli 8GMF/10GM calculate fuel mass entirely in equivalence ratio Lambda (λ = AFR_actual / AFR_stoich) rather than gasoline AFR. Why? Because wideband planar oxygen sensors (such as the 6-wire Bosch LSU 4.9 controlled by an internal Bosch CJ125 or CJ135 ASIC) measure oxygen excess or deficiency relative to complete stoichiometric oxidation regardless of hydrocarbon blend:
- 100% Pump Gasoline (E0):
AFR_stoich ≈ 14.70 : 1atλ = 1.00 - European E10 Pump Fuel:
AFR_stoich ≈ 14.13 : 1atλ = 1.00 - E85 Bioethanol (85% Ethanol / 15% Gasoline):
AFR_stoich ≈ 9.76 : 1atλ = 1.00(~33% higher volumetric fuel mass required for identical oxygen consumption, accompanied by ~2.5× higher latent heat of vaporization!).
Evaluating Short-Term (STFT) and Long-Term Fuel Trims (LTFT) Before Any Calibration
Before performing dyno Stage 1 optimization or E85 FlexFuel calibration on any turbocharged petrol engine, log Short-Term Fuel Trim (STFT) and Long-Term Multiplicative/Additive Fuel Trim (LTFT) across idle and 2,500 RPM steady cruise. Combined fuel trims exceeding ±7% reveal underlying mechanical defects—boost charge-pipe leaks, stuck PCV diaphragms, weak high-pressure fuel pump (HPFP) cam followers, or leaking piezo/solenoid injectors—that must be repaired mechanically before touching software maps.