Subaru 2002 Impreza Service Manual page 46

CONTROL SYSTEM
Fuel Injection (Fuel System)
3. AIR-FUEL RATIO FEEDBACK FACTOR
The ECM creates this factor utilizing the front oxygen sensor signal. When the signal voltage is low,
the air-fuel ratio is richer than the stoichiometric ratio. The ECM then makes the fuel injection dura-
tion shorter by modifying the factor. When the voltage is high showing that the mixture is lean, the
ECM modifies the factor to make the injection duration longer. In this way, the air-fuel ratio is main-
tained at a level close to the stoichiometric ratio at which the three-way catalyst acts most effective-
ly.
(1 ) Front catalyst
(2)
Exhaust gas
(3)
Front oxygen (NF) sensor
(4) Fuel injector
(5) Combustion chamber
(6)
ECM
B2H0989B
A: Injection duration increment signal
B: Injection duration decrement signal
C: High oxygen density
D: Low oxygen density
E: Lean signal
F:
Rich signal
4. LEARNING FEATURE
The air-fuel ratio feedback control includes a learning feature which contributes to more accurate
and responsive control.
0
In the air-fuel ratio feedback control, the ECM calculates the necessary amount of correction
based on data from the oxygen sensor and adds the result to the basic duration (which is stored
in the ECM's memory for each condition defined by the engine speed and various loads.)
0
Without a learning feature, the ECM carries out the above-mentioned process every time. This
means that if the amount of necessary correction is large, the air-fuel ratio feedback control be-
comes less responsive and less accurate.
The learning feature enables the ECM to store the amount of correction into memory and add
it to the basic fuel injection duration to create a new reference fuel injection duration. Using the
reference duration as the basic duration for the injection a few times later, the ECM can reduce
the amount of correction and thus make its feedback control more accurate and responsive to
changes in the air-fuel ratio due to difference in driving condition and sensor/actuator characteris-
tics that may result from unit-to-unit variation or aging over time.
FU-30