Titania Element Type - Toyota COMPUTER-CONTROLLED SYSTEM Training Manual

ELECTRONIC CONTROL SYSTEM - Oxygen Sensor (02 Sensor )
2 . TITANIA ELEMENT TYP E
This oxygen sensor consists of a semiconductor
element made of titanium dioxide (Ti02, which
is, like Zr02, a kind of ceramic) . This sensor uses
a thick film type titania element formed on the
front end of a laminated substrate to detect the
oxygen concentration in the exhaust gas .
Protective
cover
OHP 2 7
OPERATION
The properties of titania are such that its
resistance changes in accordance with the
oxygen concentration of the exhaust gas . This
resistance changes abruptly at the boundary
between a lean and a rich theoretical air-fuel
ratio, as shown in the following graph . The
resistance of titania also changes greatly in
response to changes in temperature . A heater is
therefore built into the laminated substrate to
keep the temperature of the element constant .
x
T
Theoretical
8i air-fuel rati o
~
U
V
No air
into
exhaust gass
Much air
int o
exhaustgas s
Richer a Air-fuel
c*
Leaner
(no air) ratio ( much air)
OHP 2 7
This sensor is connected to the Engine ECU, as
shown in the following circuit diagram . A 1V
potential is supplied at all times to the OX ±i
terminal by the Engine ECU . The Engine ECU has
a built-in comparator* which compares the
voltage drop at the OX terminal (due to the
change in resistance of the titania) to a
reference voltage (0 .45 V) . If the result shows
that the OX voltage is greater than 0 .45 V (that
is, if the oxygen sensor resistance is low), the
Engine ECU judges that the air-fuel ratio is rich .
If the OX voltage is lower than 0 .45 V (oxygen
sensor resistance high), it judges that the air-
fuel ratio is lean .
*See page 37 for details on the comparator .
ELECTRICAL CIRCUITRY
Engine EC U
Check
connector
OX~ +
OX
OHP 2 7
1 V
0.45 V
32