Oxygen Sensor

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 (NO_X) 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.