Editorial Feature

Exhaust Oxygen Sensor

Introduction

An exhaust gas oxygen sensor, also known as lambda sensor, is a device for measuring the oxygen proportion in the exhaust gas being analyzed.

The sensor is part of the emissions control system and provides data to the engine management computer. The input from the oxygen sensor is used to balance the fuel mixture.

It was developed by the Robert Bosch GmbH company in the 1960's. This sensor enables the engine to run as efficiently as possible and also reduce exhaust emissions.

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Description

The exhaust oxygen sensor includes a sensing portion that is exposed to the exhaust gas stream to detect residual oxygen in the exhaust gas and transmit the data to the control unit.

A control unit fine-tunes the electric pulses transmitted to the fuel injectors. It includes a zirconia ceramic tube that is covered by a louvred metal shroud to protect it from breaking. A wire contacts the inner surface of the tube through a spring and an electrode bush.

The inner and outer surfaces of the tube are covered with a porous platinum layer, which makes the tube act as a porous, solid electrolyte. The tube becomes a conductor at temperatures around 350°C, and detects the oxygen level in exhaust gas by creating a voltage.

Working Principle

The exhaust oxygen sensors produce a voltage signal that identifies the amount of unused oxygen in the exhaust. Heating a zirconia element in the tube generates a voltage that varies according to the amount of oxygen in the exhaust vessel to that residing in the outside environment.

The output voltage is compared to a preset level by the control unit. The output of the sensor ranges from 0.2 to 0.8V, and perfectly balanced fuel mixture of 14.7 parts of air to 1 part of fuel provides an average reading of around 0.45 V. However, the output of the sensor does not remain constant and varies from rich to lean.

Applications

The exhaust oxygen sensors are used in determining if the air-fuel mixture is rich or lean. Modern spark-ignited combustion engines employ oxygen sensors and catalytic converters to ensure that engines burn their fuel efficiently and cleanly in order to reduce exhaust emissions. Thus, these sensors also help in reducing the amounts of both unburnt fuel and oxides of nitrogen entering the atmosphere.

Since oxygen sensors are located in the exhaust stream, they do not measure the amount of air or the fuel entering the engine directly. However, when information from oxygen sensors is combined with information from other sources, it can be used to indirectly determine the air-fuel ratio.

References

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