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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