Controlling the mixture strength

Controlling the mixture strength

NOTE The mixture strength is defined and the ratio is indicatd by the greek letter α (alfa).

The stoichiometric ratio is defined and the ratio is indicated by αst.

The mixture strength is defined and the ratio is indicated by the greek letter λ (lambda):

It can easily be deduced that αst= λ.

The stoichiometric ratio depends on the type of fuel: for current unleaded petrol it is about 14.7 - 14.8 and corresponds to a Lambda of 1:

A mixture is called rich when the quantity of air is lower than the stoichiometric figure and in that case the Lambda is λ<1:

A mixture is called poor (or lean) when the quantity of air is higher than the stoichiometric figure and in that case the Lambda is λ>1.

The strategy is designed to correct the "basic" injection times so that the mixture strength constantly oscillates at a high Hertz frequency:

NOTE 1 Hz = 1 oscillation per second

In

• cut-off conditions,

or

• in full load engine conditions,

or

• with the engine cold (coolant and air temperature below around 20 °C)

the strategy is disabled.

Self-adjustment

The control unit is equipped with a self-adjustment function which has the task of memorizing any differences between the basic map and the corrections set by the Lambda sensor which occur repeatedly during operation. These differences (due to the ageing of the system and engine components) are permanently stored in the memory, allowing the adjustment of the system operation to the gradual alterationsin the engine and components compared to when they were new.

The strategy is disabled during the opening of the charcoal filter solenoid valve. If the control unit is replaced, a road test must be carried out which allows the engine to reach operating temperature and the control unit to intervene (especially during stops whilst idling) imposing the self--adjustment function.