Fiat 1998 Marea Weekend, Service Manual

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Marea- Marea Weekend
& Engine
99
update
Fuel system
10.
Control of the mixture strength (feed-back control)
NOTE The mixture strength is defined by the following ratio and is denoted by the greek letter a (alfa):
quantity of air drawn in by the engine
quantity of fuel injected
The stoichiometric ratio is defined by the following ratio and is denoted by
cc
st:
theoretical quantity of air required to burn all the fuel njected
quantity of fuel injected
The following ratio defines the mixture strength which is denoted by the greek letter
X
(lambda):
quantity of air drawn in by the engine
theoretical quantity of air required to burn all the fuel injected
Consequently a / a st =
X .
The stoichiometric ratio depends on the type of fuel: for current unleaded petrol this value is around 14.7 -
14.8 which corresponds to a Lambda of 1.
A mixture is spoken of as being rich () when the quantity of air is lower than that in the stoichiometric ra-
tio and in this case the Lambda < 1:
a mixture is described as
poor (or lean) when the quantity of air is greater than that in the stoichiometric
ratio and in this case the Lambda > 1.
The strategy is designed to correct the "basic" injection times so that the mixture strength continuously
fluctuates at high frequency between 0.98 and 1.02.
The fluctuation frequency varies according to the engine load and speed: it is in the order of tens of Hertz.
NOTE 1 Hz = 1 fluctuation per second
In the following conditions:
- cut-off,
- butterfly opening above 70° and high engine load,
- 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 possi-
ble differences between the basic map and the corrections imposed by the Lambda sensor which may take
place during operation. These differences (due to the ageing of the system and engine components) are
permanently memorized, allowing the operation of the system to be adapted to the gradual alterations in
the engine and the components compared with when they were new.
The strategy is disabled whilst the charcoal filter solenoid valve is open.
If the control unit is replaced, the vehicle must be road tested allowing the engine to reach operating tem-
perature and the control unit self-adjustment function to intervene (especially during idling stops).
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