Sensors lambda oxygen

Doped zirconium dioxide is the solid electrolyte in lambda sensors (oxygen sensors used in the field of environmental protection). 

Household appliances can also benefit from improvements in other areas. For example, oxygen sensors that measure the 02-concentations in exhaust gas have been developed that combine a Nernst type lambda gauge (which can measure only the ( -concentration at one lambda-point) with an amperometric 02-pumping cell. 

Another important application is in the refining of steels when the oxygen content must be controlled at the parts per million level and monitored continuously on line and many oxygen sensors are currently used in the steel industry for this purpose. The principle of operation is as described for the lambda sensor and one form is shown schematically in Fig. 4.39. In this case the reference activity is established by a chromium metal/chromium oxide mix rather than being defined by air. 

Oxygen sensors — Device for measurement of oxygen concentration or activity. See - Clark cell, - lambda-probe. 

Sensors based on platinum are used in temperature measurement because of the substantial change of electrical resistivity with temperature. CO detectors are common safety features in homes and industrial buildings. Oxygen sensors, known as Lambda or Exhaust Gas Oxygen (EGO) sensors, use platinum and are a central component of the engine control system in a catalyst-equipped vehicle. 

The first concept is the closed-loop-controlled three-way catalyst. In this, one type of catalyst, which is placed in the exhaust gas stream, is able to promote all the main reactions that lead to the simultaneous removal of carbon monoxide, hydrocarbons and nitrogen oxides. To balance the extent of the oxidation and the reduction reactions, the composition of the engine-out exhaust gas is maintained at or around stoichiometry. This is achieved by a closed-loop engine operation control, in which the oxygen content of the engine-out exhaust gas is measured up-stream of the catalyst with an electrochemical oxygen sensor, also called lambda sensor. 

Oxygen sensors in automotive applications are used to measure the air fuel (A/F) ratio of engine exhaust gases and to control the optimum A/F ratio for perfect exhaust gas after-treatment by catalytic converters [1]. Therefore, they are also known as lambda or A/F ratio sensors. 

One of the great advantages of using the oxygen sensor in the stoichiometric mode to control the air/fiiel ratio at A= 1, is that there is a step change of emf at stoichiometric air/fuel ratio, as shown in fig. 36. This arises due to the fact that po changes sharply at A = 1, the main reason for referring to the sensors as lambda sensors. 

Recently on board diagnosis (OBD) system requires the detection of conversion efficiency change of a TWC and an oxygen sensor is installed even in the downstream of the catalyst. Rear oxygen sensor is installed in a cleaner exhaust gas than the front oxygen sensor. Therefore, the sensor itself is hardly deteriorated and it is utilized for lambda point compensation of the front oxygen sensor. 

The lambda sensor, which is found in cars with catalytic converters, is an example of an oxygen probe based on the principle of selective electrodes. This sensor, which looks like a spark plug, has a zirconium sleeve (Zr02) that behaves as a solid electrolyte. The external wall is in contact with emitted gas while the internal wall (the reference) is in contact with air. Two electrodes measure the potential difference between the two walls, which is indicative of the difference in concentration of oxygen. 

Amperometric mode An advantage of the lambda sensor as described above is its sensitivity close to the 2=1 point where the oxygen activity in the exhaust changes rapidly with burn conditions and consequently so too does the cell e.m.f. The disadvantage is that under lean-bum (oxygen-rich) conditions the log dependence on oxygen activity renders the cell insufficiently sensitive to provide effective control. This shortcoming has led to the introduction of sensors used in the amperometric mode discussed below and illustrated schematically in Fig. 4.40. 

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