Why lambda is useful
Lambda avoids assuming one fuel's stoichiometric mass ratio. It is therefore useful when discussing petrol, ethanol blends and other fuels.
Command, measurement and correction
The ECU can command a target, measure exhaust oxygen and apply corrections. Sensor error, exhaust leaks, misfire and fuel-delivery faults can make the observed value misleading.
Related terminology
AFRAir-fuel ratio describes the mass relationship between air and fuel entering combustion. The meaningful target depends on fuel, engine type, load, temperature, catalyst strategy and whether lambda or mass ratio is being discussed.Ignition timingThe commanded spark event relative to piston position in a spark-ignition engine. Knock, fuel octane, charge temperature, combustion speed and component protection constrain the safe target.Injection timingThe timing of one or more fuel-injection events. It affects combustion phasing, pressure rise, noise, efficiency, temperature and emissions.Knock correctionAn ECU response that reduces ignition advance when knock-related signals exceed modeled thresholds. Interpretation requires cylinder, load, fuel and sensor context.