Lambda & AFR

What is the air/fuel ratio and lambda?

For petrol engines, the air/fuel ratio is very important.
It is the quantity of air aspirated by the engine versus the quantity of fuel aspirated (carburetor) or injected (injection).

Lambda is a value that gives you an indication of the air/fuel ratio. A lambda sensor measures the quantity of oxygen in the exhaust. With a complete combustion, no oxygen is left at all. In this way the sensor can tell to us or the engine management system that the engine runs rich or lean and allows the ECU to make corrections if necessary.
The normal lambda sensor has a hysteretic characteristic between 0,9 and 1,1.
Now we also have wideband lambda sensors that have a linear characteristic between lambda 0,7 to 1,2. This is the ideal sensor for tuning.

* The ideal air/fuel ratio for minimum consumption is nearly 15,4 kg of air against 1 kg of fuel (lambda 1,05)
* The ideal air/fuel ratio for maximum power is 13,2 kg of air against 1 kg of fuel (lambda 0,9)
* The car manufacturers admitted the norm of lambda 1 as 14,7 kg air against 1 kg of fuel. This is between minimum economy and maximum power

Therfore, if we want maximum power, we have to make sure that the lambda is 0,9 at that point for an atmospheric engine. A turbo- or a supercharged engine has to run richer (at lambda 0,82) only for safety reasons, because a higher pressure and a higher exhaust temperature can destroy the engine. By making the mixture more rich, we lower the temperature in the combustion room and the exhaust to protect the engine.


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