Measuring A Broadband Oxygen Sensor - TiePie Automotive

The broadband lambda sensor or broadband oxygen sensor is a sensor which can measure the concentration of oxygen in the exhaust gases. The broadband oxygen sensor is based on the 4-wire version of the zirconia oxygen sensor with a modification for measuring the actual oxygen concentration instead of only producing a signal for a too rich or too lean mixture.

The sensor consist of three parts: pump cell, measurement chamber and measurement cell. The pump cell and measurement cell consist of a zirconium dioxide (zirconia) plate to which a thin layer of platinum is applied on both sides. When an oxygen concentration difference exists between the two sides, a voltage difference will be present between the two platinum plates. This voltage depends on the concentration difference and is about 450 mV for an ideal mixture.

The measurement cell is in contact with the outside air on one side and to the measurement chamber at the other. Opposite of the measurement cell, a pump cell is placed which can pump oxygen into or out of the measurement chamber by means of an electric current. A small amount of exhaust gases can flow into the measurement chamber through a small channel. This can change the oxygen concentration in the measurement chamber, changing the measurement cell voltage from its ideal value of 450 mV. To return then measurement cell back to 450 mV, the ECU sends a current through the pump cell. Depending on the direction and amount of current, oxygen ions can be pumped into or out of the measurement chamber to return the measurement cell voltage to 450 mV.

When a rich mixture is burned, the exhaust gases contain little oxygen and a current is sent through the pump cell to pump more oxygen into the measurement chamber. Conversely, when a lean mixture is burned, the exhaust gases contain a lot of oxygen and the current through the pump cell is reversed to pump oxygen out of the measurement chamber. Depending on the magnitude and direction of the current, the ECU changes the amount of injected fuel. When an ideal mixture is burned, no current flows though the pump cell and the amount of injected fuel remains unchanged.

For optimal performance the sensor needs to have a temperature of about 750°C. The sensor is equipped with a PTC resistance for electric heating, that is fed from the system relay or sometimes from the ECU. The negative side of the regulated heating is switched to ground by the ECU with a varying duty cycle signal.