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Lambda Sensors Lambda Sensors Q&A
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Q4: How is the air/fuel mixture adjusted?
A: When the Lambda Sensor detects the level of oxygen contained
in the exhaust gas, the ECU receives the Sensor’s output signal
and decides whether the mixture has an appropriate air/fuel ratio.
The amount of fuel injected is adjusted by means of ‘injection time
feedback compensation control’. When the mixture is found to be
too rich, fuel injection is decreased. When the mixture is found to
be too lean, fuel injection volume is increased (Fig. 2). The objective
is to achieve the ideal air/fuel ratio going into the engine (the
‘stoichiometric’ point).
Fig. 2: The Lambda Sensor output signal monitors the air/fuel ratio,
instructing the ECU how to adjust the fuel injection rate.
Q5: What is the ideal ‘stoichiometric ratio’?
A: For petrol, the stoichiometric, or ‘purification’ ratio is about 14.7
to 1 (this is equal to Lambda 1.00), meaning that for each pound of
fuel, 14.7 pounds of air will be burned (Fig. 3). The fuel mixture varies
from the ideal ratio quite a bit during driving, which is the reason
why it must be monitored and adjusted. Sometimes the mixture can
be ‘lean’ (an air-to-fuel ratio higher than 14.7); and other times the
mixture can be ‘rich’.
Fig. 3: Purification ratio of Catalytic Converter.
Q6: Why do some vehicles feature more than one
Lambda Sensor?
A: Many recently produced vehicles feature a second Lambda
Sensor mounted after the Catalytic Converter (post-cat Lambda
Sensor), as well as a Lambda Sensor placed before it (pre-cat
Lambda Sensor). The pre-cat Lambda Sensor is the control Sensor,
assisting the engine ECU to control the air/fuel ratio. The ‘rear’, post-
cat Lambda Sensor is the monitor Sensor, monitoring the function of
the Catalytic Converter.
Q7: How does the second Lambda Sensor analyse the
efficiency of the Catalytic Converter?
A: The post-cat Lambda Sensor monitors the function of the
Catalytic Converter by measuring the oxygen level in the exhaust
gas that leaves the Catalytic Converter. If the Sensor gives a high
voltage reading (Fig. 4), the Catalytic Converter is working effectively.
This is because if the converter is working correctly, all the oxygen in
the exhaust gases will be consumed by the chemical reaction taking
place inside between the oxygen and the harmful emissions. As
the Catalytic Converter wears down, however, some of the harmful
gases and oxygen will not be included in the chemical reaction and
will exit the Converter unchanged. The oxygen readings shown by
the post-cat Sensor will therefore increasingly mirror the signal of
the pre-cat Sensor, until both show the same signal (Fig. 5). This
indicates failure of the Catalytic Converter.
Fig. 4: Output from fully Fig. 5: Output from a failing
functional Catalytic Converter. Catalytic Converter.
DENSO
1st
Q8: Why have A/F (air/fuel) Sensors been developed?
A: A/F Sensor technology was a world-first innovation that was
developed by DENSO, offering a sensor with a linear output that
helps vehicles to meet the tightening emission regulations from
EURO 3 onwards. This new system uses the air/fuel ratio sensor
(A/F Sensor), replacing the conventional Lambda Sensor.