Lambda (λ) represents the air-fuel ratio in combustion engines relative to the stoichiometric ideal. Calculated as \( \lambda = \frac{\text{Actual Air-Fuel Ratio}}{\text{Stoichiometric Ratio}} \), this dimensionless value is critical for engine management and emissions control.

The optimal λ value is 1.0 (14.7:1 air-fuel mass ratio for gasoline), where all fuel completely burns with available oxygen. This balance maximizes catalytic converter (CAT) efficiency while minimizing emissions.

Fundamental differences in combustion create distinct lambda requirements:

Petrol Engines:

• Require precise λ=1.0 for three-way catalysts to simultaneously reduce NOx, CO, and HC
• Temporary rich operation (λ≈0.8-0.9) only during high load for knock protection
• Lean operation (λ>1) limited to specific low-load conditions

Diesel Engines:

• Minimum λ≈1.4 at full load (prevents visible smoke)
• Idle λ≈4.0 (maximizes air for complete combustion)
• Require complex aftertreatment (DPF+SCR) due to oxygen-rich exhaust

Modern engines use wideband oxygen sensors for real-time λ monitoring. The engine control-unit adjust fuel delivery up to 100 times/second to maintain target lambda based on:

• Engine temperature
• Load demands
• Emissions requirements
• Aftertreatment status

* Petrol cold start: λ≈0.9 (rich for catalyst heating) * Diesel regeneration: λ<1.5 temporarily (raises exhaust temperature) * Hybrid engines: λ>1.0 during electric-assist phases

Lambda management remains critical for meeting Euro 7/ULEV standards, with petrol-diesel differences driving distinct emission control strategies.