Air–Fuel Ratio & Lambda¶
Lambda (λ) is the actual air–fuel ratio divided by the chemically ideal one. λ = 1.0 is stoichiometric; below 1 is rich, above 1 is lean.
What it is¶
Every fuel has a stoichiometric ratio — the air-to-fuel mix that burns with nothing left over. For petrol that's about 14.7:1 by mass. Rather than quote raw ratios that change with fuel, tuners use lambda:
- λ = 1.0 — stoichiometric (≈ 14.7:1 for petrol).
- λ < 1.0 — rich (extra fuel).
- λ > 1.0 — lean (extra air).
Why it matters¶
Mixture is a balance of power, temperature, efficiency, and safety, and the best value changes with load:
- Lean mixtures burn hotter and can stop making power; too lean risks misfire and high combustion temperatures.
- Rich mixtures cool the charge and resist knock, but waste fuel and can wash oil from the bores and make black exhaust smoke.
In Engone the optimal target shifts with operating point: around λ ≈ 0.98 at idle, λ ≈ 0.88 at wide-open throttle, and slightly richer again at high RPM (about 0.02 lower at the top).
In practice (approximate): lean misfire begins around λ 1.05–1.15 (earlier under load and at high RPM), and the engine can't run beyond roughly λ 1.3–1.4. Rich misfire starts near λ 0.75 and combustion stops near λ 0.5. At low RPM, λ below about 0.65 wets the plugs and the engine floods (see Cranking Enrichment).
How to tune it¶
You don't set lambda directly while running — you set targets in the lambda table and let fuelling (plus closed-loop trim) chase them. Use this page's intuition to decide what those targets should be.
Rule of thumb
Lean where you want economy (light cruise), stoich where you want clean and steady (mid-load), rich where you want power and knock safety (full load and high RPM).
Watch out
Chasing economy with very lean full-load mixtures is how you melt pistons. Keep WOT rich.
On a boosted engine
Engone currently simulates naturally aspirated engines only. For background: a boosted engine packs far more air into the cylinder, so the tune changes — target lambda moves richer (to cool the charge and protect against knock), ignition advance comes down in the boosted cells, compression ratio is usually lowered, and higher octane matters more. See Boosted Engines (Background).
In the game¶
The λ DEVIATION panel in the left readouts shows λ target, λ actual, Δ, and a centre-zero bar
(full scale ±0.30). The reading is the delayed sensor value — see
O2 / Lambda. Compare it to the target for the current cell while tuning.
OPTIONS → GAMEPLAY → UNITS → MIXTURE switches every λ display and the lambda table between λ and
AFR (stoich 14.7).