Lean Mixture Sensor - Toyota COMPUTER-CONTROLLED SYSTEM Training Manual

ELECTRONIC CONTROL SYSTEM - Lean Mixture Senso r
LEAN MIXTURE SENSO R
The lean mixture sensor is constructed in
basically the same way as the zirconia element
type oxygen sensor, but its use differs .
Theoretical
air-fuel ratio
Engine EC U
Heate r
Protective
cove r
OPERATION
The zirconia element type oxygen sensor
operates on the principle that a voltage will be
generated if the difference in the oxygen
concentration inside and outside the sensor is
great .
In the lean mixture sensor, however, a voltage
applied to the zirconia element when the
temperature is high (650°C [1202°F] or greater),
results in a current flow with a value which is
proportional to the oxygen concentration in the
exhaust gas .
In other words, when the air-fuel mixture is rich,
there will be no oxygen in the exhaust gas, so
no current will flow through the zirconia
element . When the air-fuel mixture is lean, on
the other hand, there will be a lot of oxygen in
the exhaust gas and the amount of current
flowing through the zirconia element will be
large, as shown in the following graph .
The lean mixture sensor has been adopted to
assure that the air-fuel ratio is kept within a
predetermined range, thereby improving fuel
economy as well as drivability .
This sensor also comes with a heater to heat the
zirconia element . The heater is controlled in the
same way as the heater of the oxygen sensor .
For further details, see page 114 .
ELECTRICAL CIRCUITR Y
Lean
mixture
sensor
OHP 2 8
REFERENCE
When the air-fuel mixture is extremely lean
(about 20 :1), combustion will be accompanied
by reductions in NOx (oxides of nitrogen), CO,
and HC (hydrocarbon gas), as shown in the
graph below . This is a good thing up to a
point . However, if the air-fuel mixture is too
lean,
not only will HC concentrations
increase, but the engine will lose power
and/or misfire .
CO HC
(%)
I
(PP M
12 ~
8
d
4
60 0
40 0
20 0
0
\ \HC
\
\ \~
CO
\
\
NOx
NOx
(PPM)
300 0
200 0
100 0
10 12 14 16 18 20
2 2
0
Richer
-
Air-fuel ratio
~
Leane r
OHP 28
33