Daihatsu F300 Service Manual page 1364

EMISSION CONTROL SYSTEM
• OXYGEN SENSOR
•
•
•
The oxygen sensor is a compact sensor installed at the exhaust pipe or exhaust manifold, which detects the
oxygen concentration (air-to-fuel ratio). When the air-to-fuel ratio is greater (leaner) than the stoichiometric
air-to-fuel ratio, there exists excessive amount of air compared with the amount of air required for the fuel to
be burnt. Hence, oxygen remains in the exhaust gas. Conversely, when the
aiHo-fuel
ratio is smaller (richer)
than the stoichiometric air-to-fuel ratio, no oxygen remains in the exhaust gas. It is. therefore, possible to
know whether the actual air-to-fuel ratio is richer or leaner than the stoichiometric air-to-fuel ratio by detecting
the oxygen concentration in the exhaust gas.
The oxygen sensor is so constructed that both sides of a measuring-tube-shaped solid electrolyte (Zirconia
element) are coated with thin films of platinum.
Flange (Body ground)
Platinum electrode at atmosphere side
Solid electrolyte (Zirconia element)
Platinum electrode at exhaust gas side
Coating (ceram·lc)
Protective cover
Exhaust gas
The Zirconia element has such characteristics that an
electromotive force is generated when the oxygen concentra-
tion differs between both sides of the element. Furthermore,
when the temperature of the oxygen sensor is high, its
electromotive force changes suddenly in the neighborhood of
the stoichiometric air-to-fuel ratio due to platinum catalysis.
(See the right figure.)
The exterior of the oxygen sensor is exposed to the exhaust
gas, whereas the interior is introduced
with
the atnosphere.
Utilizing the aforesaid characteristics, the oxygen sensor ac-
curately detects whether the oxygen concentration, Le. the
air-to-fuel ratio, is richer or leaner than the stoichiometric air-to-
fuel ratio. When the air-to-fuel ratio is richer than the
stoichiometric air-to-fuel ratio, the electromotive force of the
oxygen sensor becomes high, thus sending a rich signal to the
computer. Conversely, when the air-to-fuel ratio is leaner than
the stoichiometric air-to-fuel ratio, the electromotive force of the
oxygen sensor becomes low, thus sending a lean signal to the
computer.
The oxygen sensor begins its operation when the temperature
r'1ses above about 400°C.
EC-25
Great
Rich signal
i
WFf90.EC074
Judgment of
"too rich"
Comparative
1 voltage
g_ - -
-tt--·-
~
Judgment of
"too lean"
I
Lean
'-~--'..:'g_nal
Rich
*""
Stoichiometric
--t
Lean
air-to-fuel ratio
wFE90-EC075