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2-7 [M500]
FUEL INJECTION SYSTEM
5 . Sensor and Switch
1 . 02 SENSOR
The OZ sensor is used to sense oxygen concentration in
the exhaust gas. If the fuel ratio is leaner than the
stoichiometric
ratio in the mixture
(i .e . excessive
amount of air), the exhaust gas contains more oxygen .
To the contrary, if the fuel ratio is richer than the
stoichiometric ratio, the exhaust gas contains hardly
any oxygen .
Therefore, examination of the oxygen concentration in
exhaust gas makes it possible to show whether the
air/fuel ratio is leaner or richer than the stoichiometric
ratio.
The Oz sensor has a zirconia tube (ceramic) which
generates voltage if there is a difference in oxygen
concentration between the inside and outside of the
tube . Platinum is coated on the inside and outside of the
zirconia tube for the purpose of catalysis and electrode
provision. The hexagon screw on the outside is
grounded to the exhaust pipe, and the inside is con-
nected to the ECU through the harness.
A ceramic heater is employed to improve performance
at low temperature.
B2-286
Fig. 11
When rich air-fuel mixture is burnt in the cylinder, the
oxygen in the exhaust gases reacts almost completely
through the catalytic action of the platinum coating on
the surface of the zirconia tube . This results is a very
large difference in the oxygen concentration between
the inside and outside, and the electromotive force
generated is large.
When a lean air-fuel mixture is burnt in the cylinder,
oxygen remains in the exhaust gases even after the
catalytic action, and this results in a small difference in
the oxygen concentration . The electromotive force is
very small .
The difference in oxygen concentration changes greatly
in the vicinity of the optimum air-fuel ratio, and hence
the change in the electromotive force is also large . By
inputting this information into the MPFI control unit, the
air-fuel ratio of the supplied mixture can be determined
easily . The Oz sensor does not generate much electro-
motive force when the temperature is low. The charac-
teristics of the electromotive force stabilize at tempera-
ture of approximately 300 to 400°C (572 to 752°F) .
Electromotive y
Atmosphere
force
V
~
(
U
Exhaust
Exhaust
gases
gases
large
Theoretical air-fuel
ratio
0
(Rich signal)
To be judged
as rich
Comparison
f
voltage
0 0
o
To be judged
Ti x
as lean
m
w O
!
(Lean signal)
Small
Air-fuel
Large
Small
ratio
B2-287
Fig. 12
2. WATER TEMPERATURE SENSOR
The water temperature sensor is located on the water
pipe which is made of aluminum alloy. Its thermistor
changes resistance with respect to temperature . A wa-
ter temperature signal converted into resistance is
transmitted to the ECU to control the amount of fuel
injection, ignition timing, purge control solenoid valve,
etc .
Y 2.5
20°C 168°F) 2.5 kn
u
c
m
1 -20 0
40
80
Temp . °C (°F)
(-4) (32) (104)
(176)
20
60
(68)
(140)
B2-288
Fig . 13
10
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