582 The Resistivetype Ego Sensor

The zirconia-type voltaic EGO is somewhat slow in operation and it is claimed that the titanium oxide (titania)-type EGO sensor has a faster response time and is, therefore, better for engine emission control purposes (Fig. 5.33).

Ego Sensor

The titania sensor reacts to changes in the partial pressure of oxygen in the exhaust gas. Changes in the concentration of oxygen in the exhaust gas cause the resistance of the sensor material to change. When the sensor is supplied with a set voltage from the control unit, the variation in current through the sensing element provides an indication of the oxygen content of the exhaust gas. In the sensing element, the titania is essentially a semiconductor whose resistive properties are affected by the concentration of oxygen that reacts with it. The reaction that occurs affects the resistance of the sensor element and the resultant sensor voltage is an accurate indicator of the partial pressure of the oxygen in the exhaust gas. The main differences between this sensor and the voltaic sensor are that the voltage levels are higher and that there is low voltage for a rich mixture and a high voltage for a weak mixture.

At the critical region, where the air - fuel ratio is chemically correct (lambda = 1), there is a marked change in the resistance of the sensor element which leads to it producing a waveform similar to that of the ZrO2 sensor, except that the voltage across the sensor is probably higher. The actual value is dependent on the voltage that is applied to the sensor.

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