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Mixture Ratio Feedback Control (Closed loop control)
NFEC0014S04
SEF336WC
The mixture ratio feedback system provides the best air-fuel mixture ratio for driveability and emission con-
trol. The warm-up three way catalyst can then better reduce CO, HC and NOx emissions. This system uses
a heated oxygen sensor 1 in the exhaust manifold to monitor if the engine operation is rich or lean. The ECM
adjusts the injection pulse width according to the sensor voltage signal. For more information about the heated
oxygen sensor 1, refer to EC-240. This maintains the mixture ratio within the range of stoichiometric (ideal
air-fuel mixture).
This stage is referred to as the closed loop control condition.
Heated oxygen sensor 2 is located downstream of the warm-up three way catalyst. Even if the switching
characteristics of the heated oxygen sensor 1 shift, the air-fuel ratio is controlled to stoichiometric by the sig-
nal from the heated oxygen sensor 2.
Open Loop Control
NFEC0014S05
The open loop system condition refers to when the ECM detects any of the following conditions. Feedback
control stops in order to maintain stabilized fuel combustion.
I Deceleration and acceleration
I High-load, high-speed operation
I Malfunction of heated oxygen sensor 1 or its circuit
I Insufficient activation of heated oxygen sensor 1 at low engine coolant temperature
I High engine coolant temperature
I During warm-up
I After shifting from “N” to “D”
I When starting the engine
Mixture Ratio Self-learning Control
NFEC0014S06
The mixture ratio feedback control system monitors the mixture ratio signal transmitted from the heated oxy-
gen sensor 1. This feedback signal is then sent to the ECM. The ECM controls the basic mixture ratio as close
to the theoretical mixture ratio as possible. However, the basic mixture ratio is not necessarily controlled as
originally designed. Both manufacturing differences (i.e., mass air flow sensor hot wire) and characteristic
changes during operation (i.e., injector clogging) directly affect mixture ratio.
Accordingly, the difference between the basic and theoretical mixture ratios is monitored in this system. This
is then computed in terms of “injection pulse duration” to automatically compensate for the difference between
the two ratios.
“Fuel trim” refers to the feedback compensation value compared against the basic injection duration. Fuel trim
includes short term fuel trim and long term fuel trim.
“Short term fuel trim” is the short-term fuel compensation used to maintain the mixture ratio at its theoretical
value. The signal from the heated oxygen sensor 1 indicates whether the mixture ratio is RICH or LEAN com-
pared to the theoretical value. The signal then triggers a reduction in fuel volume if the mixture ratio is rich,
and an increase in fuel volume if it is lean.
“Long term fuel trim” is overall fuel compensation carried out long-term to compensate for continual deviation
of the short term fuel trim from the central value. Such deviation will occur due to individual engine differences,
wear over time and changes in the usage environment.
GI
MA
EM
LC
FE
CL
MT
AT
AX
SU
BR
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BT
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IDX
ENGINE AND EMISSION BASIC CONTROL SYSTEM DESCRIPTION
Multiport Fuel Injection (MFI) System (Cont’d)
EC-33
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