A sensor-technology which focuses exclusively on one ideal point is outdated. Consider the direct petrol injection engines, which feature a stratified charge operation or engines in which the control device regulates a lean combustion. In these cases a broad-band Lambda oxygen sensor is necessary. By the way, other industrial branches make use of this technology as well.
The prerequisite for the development of the broad-band sensor, was the planar technology. An additional chamber is added with an opening to the exhaust gas pipe. This chamber also receives two conductive foils. Thereby the broad-band sensor has either 4 or 5 connections. Remaining are the two for the heating voltage and one (together with ground) for the Lambda-sensor voltage.
There are two additional connections: they regulate the admission of residual oxygen from the exhaust gas, via a small opening to the so-called pump-cell. The two conducting foils are positioned in such a way, that via a certain current flow, the force of attraction working on the negatively charged oxygen ions can be controlled. The aim is, that only the amoumt of residual-oxygen remains in the pump cell, that is necessary for a Lambda value of 1.
Should a Lambda value of 1 on the layer boundaries between the pump-cell and the measuring area be determined, nothing at all happens. At a value of over 1 (lean), oxygen is “pumped” outwards, if the value is less than 1, oxygen is extracted from the exhaust gas. Thereby, the pump-current is the measure for the Lambda during the combustion. Continuous co-operation is thus necessary between the sensor and the control unit, to determine the Lambda value of any combustion mixture.
In the meantime, there are Lambda-sensors with an access time of only a few milli-seconds. These are able to ascertain the mixture ratios in the individual cylinders.
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