19-4
4. AIR/FUEL RATIO O
2
FEEDBACK CONTROL BY A/F SENSOR
Receiving the current value signal from the A/F (Air Fuel Ratio) sensor, the ECM controls the fuel injection duration (fuel
amount) so that the engine’s suction air fuel mixture is at or near the target air/fuel ratio determined by the main CPU.
EXHAUST GAS
ZIRCONIA ELEMENT
CERAMIC HEATER
PLATINUM ELECTRODES
DIFFUSED RESISTANCE
LAYER
TIP OF THE
SENSOR
ATMOSPHERE
A/F (AIR FUEL RATIO) SENSOR
The A/F sensor, mounted on the exhaust manifold, detects the oxygen density in the exhaust gas. The A/F sensor has the
built-in zirconia element coated with the platinum on the inner and outer surfaces and wrapped with the coarse diffused
resistance layer. The sensor is structured so that its inner surface is exposed to the atmosphere and the outer surface to
the exhaust gas.
When a given voltage is applied between the platinum electrodes, the current does not change by applying further
voltage between the electrodes. However, the current value changes in accordance with the oxygen density in the
exhaust gas that passed through the diffusion resistance layer. Utilizing this characteristic, the A/F sensor detects the
change in oxygen density in the exhaust gas linearly, and thereby it realized very precise control of the air/fuel ratio.
(mA)
i
A/F SENSOR AIR FUEL RATIO/CURRENT
CHARACTERISTIC CHART
A/F
10
0
10
15
20
5
–5
–10
MAP SENSOR
DECISION OF BASIC
INJECTION AMOUNT
ECM
BASIC INJECTION AMOUNT
ENVIRONMENT COPENSATION
TARGET A/F RATIO
COMPENSATION
DECISION OF ACTUAL
INJECTION AMOUNT
INJECTOR
DECISION OF TARGET
A/F RATIO
CKP SENSOR
CMP SENSOR
IAT SENSOR
ECT SENSOR
BARO SENSOR
A/F SENSOR
