ICE-26 — Lambda

ASSUMPTION — Stoichiometric AFR varies by fuel; ~14.7:1 for gasoline is a typical illustrative value.

In this lesson you will learn:

• Lambda and AFR
• Rich/lean/stoichiometric differences
• The importance of lambda control

1. AFR and Lambda

Air-fuel ratio (AFR) is the mass of air divided by the mass of fuel. Lambda (λ) is the actual AFR divided by the stoichiometric AFR.

FACT — λ = AFR_actual / AFR_stoich. λ = 1 is stoichiometric, λ < 1 rich (excess fuel), λ > 1 lean (excess air).

2. Mixture Types

• Rich (λ < 1) — more power but more CO/HC and fuel use.
• Stoichiometric (λ = 1) — optimum for the TWC.
• Lean (λ > 1) — better efficiency, less CO/HC, but NOx control is harder.

FACT — Gasoline runs near λ = 1 for the TWC; diesel always runs lean.

3. Lambda Sensor and Control

FACT — The lambda (O2) sensor measures exhaust oxygen; the ECU keeps injection near stoichiometric via closed-loop control. Open-loop is used at full load and cold start.

4. Summary

• λ = actual AFR / stoichiometric AFR.
• λ<1 rich, λ=1 stoichiometric, λ>1 lean.
• The lambda sensor enables closed-loop fuel control.
• TWC efficiency depends on the λ ≈ 1 window.

5. Sources

• Bosch, Gasoline-Engine Management
• Heywood, Internal Combustion Engine Fundamentals

Next Lesson

• ICE-27 — Knock: abnormal combustion.