An effective measure to reduce the noxious emissions of the exhaust gas is a post-treatment of the exhaust gas in the three way catalytic converter.
This transforms the three noxious components of the exhaust gas CO, HC and NO_x to H₂O, CO₂ and N₂.
(See the relevant lesson on the Catalytic post-combustion).

The transformation of all the three above mentioned components of the exhaust gas can take place only in a very narrow field: the Lambda window (Lambda = 0.99....1).
This can be obtained only through the Lambda regulation.

The Lambda regulation controls the air-fuel mixture at exactly Lambda = 1.

To this purpose the Lambda Probe overlooking the flow of the exhaust gas continuously provides a signal thanks to which the electronic control board controls the existing non burnt air-fuel mixture, extending or reducing, when necessary, the injection time.
The Lambda regulation is superimposed to the base control of the mixture formation system.

The Lambda regulation is effective only with a Lambda probe ready tomoperate.
The Lambda probe transfers a signal that can be used only at temperatures higher than approximately 350 °C.
The regulation is inhibited until this temperature is reached.
With cold probe, interruptions or short-circuits in the electric line, non plausible voltage values, impossible to evaluate, are generated.
In most of the cases the Lambda probes are heated, so that after just 30 seconds they are ready to operate.
Cold engines need for a regular operation a richer mixture (Lambda lower than 1).
Therefore, the Lambda regulation can be enabled only above a given engine temperature threshold.
With the Lambda regulation active, the signal from the Lambda probe is transformed in the control board through a comparator into a two point signal.
The transmitted signal (Lambda higher than 1, poor mixture or Lambda lower than 1, rich mixture) leads the regulator downstream connected to modify its regulation quantities (with a "jump" and then a "ramp behavior").

Figure 14 shows the behavior of the voltage of the signal of the Lambda probe as a function of time.

Fig. 14: Signal provided by the Lambda probe

At each passage from a rich mixture to a poor mixture, as well as from a poor mixture to a rich mixture, the Lambda regulator is activated.
The Lambda correction factor is used to influence the control times of the electro-injector.
With Lambda values higher than 1.0 (low voltage of the probe), we have an increase of the measurement of fuel; with Lambda values lower than 1.0 (high voltage of the probe), we have a reduction of the measurement of fuel.
With a voltage jump of the Lambda probe, the mixture is immediately changed by a given value to produce as soon as possible a correction of the mixture.
Then, the parameter follows a programmed adaptation function up to the next voltage jump of the Lambda probe.
The air-fuel mixture, through these jumps, continuously changes its composition in a very narrow window around Lambda = 1 toward the "rich" or "poor" direction.
This continuous and practically immediate regulation of the mixture at Lambda = 1 is the essential condition to obtain a high efficiency of the catalytic post-treatment of the noxious emissions.

Lambda regulation