Checking mixture strength

Checking mixture strength

NOTE The following ratio is defined as the mixture ratio and is denoted by the Greek letter α (alfa):

The following ratio is defined as the stoichiometric mixture and is denoted by α_st:

The following ratio is defined as the mixture strength and is denoted by the Greek letter λ (lambda):

It can easily be deduced that α / α_st = λ .

The stoichiometric ratio depends on the type of fuel: for current unleaded petrols it is around 14.7 - 14.8 which corresponds to a Lambda strength of 1.

We talk of a mixture being rich when the quantity of air is lower than the stoichiometric ratio in which case the Lambda < 1:

we talk of a mixture being poor (or lean) when the quantity of air is higher than the stoichiometric ratio in which case the Lambda > 1,

The strategy has the function of correcting the "basic" injection times so that the mixture strength constantly oscillates at a high frequency between 0.98 and 1.02.

The oscillation frequency varies according to the engine load and speed: it is in the order of tens of Hertz.

NOTE 1 Hz = 1 oscillation per second

In conditions of:

• - cut-off,
• - butterfly opening above 70°,
• - engine temperature below 25°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 may occur during operation. These differences (due to the ageing of the system and engine components) are memorized permanently, allowing the adaption of the operation of the system to gradual alterations of the engine and the components in relation to the characteristics when new.

The strategy is disabled when the charcoal filter solenoid valve is open.

If the control unit is replaced it is necessary to carry out a road test which allows the engine to reach operating temperature and the control unit (above all during idling stops) to operate in the self-adjustment mode.