Lambda sensors or oxygen sensors are used in the vehicle exhaust
system to analyse the proportion of oxygen in the fuel-air mixture used in a
car. They help in determining the quality of air-fuel mixture; whether it is
lean or rich. It is a vital component in a modern car to regulate the harmful
emission gases. The amount of emission from a vehicle can affect the life of
catalytic converters.
Why is an Oxygen sensor
necessary?
If an oxygen sensor, the ECU can no longer sense the sir-fuel
ratio. As a result, the engine might perform poorly if it doesn’t get the right
air-fuel mixture. Even the engine emissions would shoot up in this scenario.
When was Oxygen sensor
introduced?
Oxygen sensors were introduced by Bosch in the late 1960s. Volvo
240 was the first mass produced automobile to be installed with oxygen sensors
in 1976.
Where is Oxygen sensor used?
Lambda sensors are used widely in petrol powered cars, since air
flow can be controlled in petrol engines. Over the years it has become an
essential part of the engine management system and emission control system. It
is located in the exhaust system, between the exhaust manifold and catalytic
converter.
Without a Lambda
sensor, the modern electronic fuel injection is not possible. Even though, it
is located in the exhaust manifold, it indirectly measures the air-fuel ratio.
It measures the oxygen content in the exhaust gases and indicates the air-fuel
ratio.
Quality of air-fuel mixture:
The stoichiometric air-fuel ratio for a petrol engine is 14.7:1. Engine requires different
ratios at different conditions.
Less amount of oxygen in the exhaust indicates that it is a rich mixture, which results in wastage
of fuel and unburned fuel results in excess emission.
Higher proportion of oxygen in the exhaust indicates that is a lean mixture, which can be a problem at
higher engine operating temperature. At high temperature, lean mixture can
produce more nitrogen oxide (NOX) emissions and can lead to
misfiring of the engine.
DESIGN OF OXYGEN SENSORS:
Oxygen sensors calculate the difference between the oxygen content
in the exhaust and the amount of atmospheric oxygen. Based on the difference,
it generates a voltage and sends it to the ECU. The ECU senses the voltage
signal and regulates the air-fuel ratio to the optimum level.
An oxygen sensor is made of a ceramic material plated with
platinum porous electrode (both on the inside and outside of the ceramic body).
The ceramic body is placed inside a housing to protect the sensing element from
any mechanical damages.
The ceramic material is usually made of zirconium dioxide or
Zirconia. The surface of zirconia is plated with platinum electrode. The end of
the oxygen sensor is provided with cables to carry the voltage signal to the
ECU.
The zirconia element is covered in a steel shell provided with a
lot of openings or slots for the exhaust gases to enter the shell and flow
through the zirconia element. The Zirconia element side is placed inside the
exhaust system, whereas the other end with the cables is outside the exhaust
system.
WORKING OF OXYGEN SENSORS:
The exhaust gas flows through the zirconia element. Meanwhile, the
atmospheric air flows through the gaps between the cables at the other end. The
outside air is made to flow through an internal passage towards the zirconia
end.
The exhaust gas flows through the platinum electrode coated on the
outside of the zriconia material. The atmospheric air flows towards the
platinum electrode coated inside the zirconia element. Both the Platinum
electrodes are linked to a cable each which carries the voltage output. The difference
in the concentration of oxygen molecules in the exhaust and atmosphere creates
a potential difference.
The oxygen ions are driven from a platinum electrode with higher
concentration of oxygen to the platinum electrode with lower concentration of
oxygen. For example, if the ambient air has more oxygen molecules than the
exhaust gases, then the oxygen ions will flow from inner platinum electrode to
the outer.
The movement of oxygen ions generates a potential difference
between the two electrodes. Lean mixture
Will generate a potential difference of as low as 0.1 V. A rich mixture will
generate a potential difference of 0.9
V.
The voltage signal is supplied to the ECU. The ECU compares the
signal strength to the standard values set by the manufacturer and regulates
the air-fuel mixture to the optimal value.
A vehicle oxygen sensor, also known as an O2 sensor, is a critical component of a vehicle's engine management system. It is located in the exhaust system and measures the amount of oxygen in the exhaust gases, providing feedback to the engine control module (ECM) to adjust the fuel-to-air ratio for optimal engine performance.
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