Toyota COMPUTER-CONTROLLED SYSTEM Training Manual page 86

NOTE
Air-fuel ratio learned control ;
EFI - Functions of Engine EC U
When engine condition changes over time, the
air-fuel ratio that is created from basic injection
duration calculated by the Engine ECU deviates
from the theoretical air-fuel ratio . When this
happens, time is required for the air-fuel ratio to
return to the theoretical air-fuel ratio by air-fuel
ratio feedback correction . The deviation may
also exceeds the correction range of air-fuel
ratio feedback correction .
Consequently, the Engine ECU remembers the
central value of the correction ratio and cor-
rects the amount of deviation from the central
value (a) for basic injection duration . This func-
Correction
ratio
1 . 2
1 . 0
0 .8
Lean mixtur e
Normal
conditio n
b . Lean Mixture Senso r
The ECU corrects the injection duration based on
signals from the lean mixture sensor to keep the
air-fuel ratio within the "lean" range . (This is
called a "closed-loop" operation . )
In order to prevent overheating of the catalyst
and assure good engine operation, air-fuel ratio
feedback does not occur under the following
conditions (open-loop operation) :
• During engine startin g
• During after-start enrichment
• During power enrichmen t
• When the coolant temperature is below a
predetermined leve l
• When fuel cut-off occurs
tion is referred to as air-fuel ratio learned con-
trol, and the value remembered by the Engine
ECU is referred to as the learned value .
As a result of this learned control, air-fuel feed-
back correction is constantly able to correct
the central value of the correction ratio with a
value of 1 .0 .
This enables the air-fuel ratio to return rapidly
within a narrow range near the theoretical air-
fuel ratio . Furthermore, learned control is per-
formed when feedback correction is being per-
formed .
Central feedback valu e
Over life
tim e
The ECU determines the target air-fuel ratio
based on signals from the sensors . It then
converts this ratio to an electric current and
compares this current with the current from the
lean mixture sensor . If the current from the lean
mixture sensor is larger than the target current,
it judges the air-fuel ratio to be leaner than the
target air-fuel ratio and increases the amount of
fuel injected . If the current from the lean
mixture sensor is smaller, it judges that the air-
fuel ratio is richer than the target air-fuel ratio,
and reduces the amount of fuel injected .
The correction coefficient used by the ECU
varies over a range of 0 .8 to 1 .2, and is 1 .0
during an open-loop operation .
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