WIDE BAND SENSORS - an example

How the NTK Pump Cell Sensor Works


Briefly, the NTK sensor requires a controller because it is more complex than a standard switching type sensor. It's made up of a narrow band oxygen sensor (the Reference Cell in the image) coupled to a pump cell and a small diffusion chamber. The electronics (in the WB unit case) are represented by the yellow and green symbols.

The pump cell, in conjunction with a catalytic reaction at the surface of the cell's electrodes, can either consume oxygen or consumed hydrocarbon fuel in the pump cell cavity, depending on the direction of the Ip current flow.

In normal sensor operation, a small sample of the exhaust gas passes through the diffusion gap into the pump cell. That exhaust gas is either rich or lean and both conditions are sensed by the reference cell which produces a voltage Vs above or below the Vref signal (this voltage has the characteristics of a narrow band switching type sensor).

A rich exhaust will produce a high Vs voltage and the electronics produces a pump current Ip in one direction to consume the free fuel. A lean exhaust produces a low Vs and the electronics sends the pump current in the opposite direction to consume free oxygen.

When the free oxygen or free fuel has been neutralized, the Vs feedback signal goes to about 450 m Volts (the same as the Vref value). The pump current required to produce this equilibrium is a measure of the Lambda or Air Fuel Ratio. The electronics in the WB unit converts the Ip into a V out which is the output of the WB unit. Not shown is the Rcal, or calibration resistor, in the sensor's connector which compensates for manufacturing variations between sensors.


Here are some representations of the Vs or narrow band Sense Voltage (left) and Ip or Pump Current characteristics described above. Note that the narrow band graph's x-axis covers only a very narrow lambda range (+/- 0.02), whereas the wideband's x-axis covers the range 0.9 to 2.2.




This graph shows the actual Vout of the DIY-WB unit. Note that the curve takes the same general shape as the above Ip graph.


In particular, note that the rich and lean regions have a different slope. This occurs because the chemical reactions in the rich and lean regions are fundamentally different, and the magnitude of Ip has a different effect in each region.

For product information on the GTC Oxygen Sensor Tester, please visit www.gtc.ca