581 The Voltaictype Ego Sensor

The voltaic, or zirconia (ZrO2), type oxygen sensor operates on the basis of a difference between the oxygen partial pressure of atmospheric air and the partial pressure of oxygen in the exhaust gas. At sea level, atmospheric air contains approximately 21% oxygen by weight, and this gives the oxygen a partial pressure of approximately 0.2 bar. The oxygen content of exhaust gas varies from zero in a rich mixture, to about 10% in a weak mixture, as shown in Fig.5.26. The partial pressure of the oxygen in the exhaust gas therefore ranges from near zero to approximately 0.01 bar.

High pressure oxygen in the atmosphere

Low pressure oxygen in exhaust gas

Platinum electrode

High pressure oxygen in the atmosphere

Low pressure oxygen in exhaust gas

Platinum electrode

(Connected to exhaust electrode)

Voltage from sensor is used by the ECM to keep lambda = 1

(Connected to atmospheric side electrode)

Exhaust pipe

Fig. 5.27 The EGO sensor as a voltaic cell

(Connected to exhaust electrode)

Voltage from sensor is used by the ECM to keep lambda = 1

(Connected to atmospheric side electrode)

Exhaust pipe

Fig. 5.27 The EGO sensor as a voltaic cell

Figure 5.27 shows that the sensor element is essentially a cell (battery). The plates are made from platinum and they have a layer of ceramic zirconia between them which acts as an electrolyte. The platinum plates act as catalysts for the oxygen which makes contact with them, and they are also used to conduct electricity away from the sensor. The catalyzing action that takes place when oxygen contacts the platinum plates causes the transport of oxygen ions through the electrolyte and this creates the electric current that gives rise to the e.m.f. (voltage) of the sensor. This sensor voltage is an accurate representation of the oxygen content of the exhaust gas.

In practice the sensing element is formed into a thimble shape as shown in Fig. 5.28. This type of construction exposes the maximum area of platinum to the exhaust gas on one side and to the atmospheric air on the other side. The platinum that is exposed to the exhaust gas is covered with a porous ceramic material. This allows the oxygen through to the platinum but protects the platinum against harmful contaminants in the exhaust products.

Fig. 5.28 Diagrammatic representation of the oxygen sensor in the exhaust pipe

The greater the difference in oxygen levels between the atmospheric air and the exhaust gas, the greater is the voltage produced by the EGO sensor. When the air-fuel ratio changes from slightly rich, say 14:1 (lambda = 0.93) to slightly weak, 16:1 (lambda = 1.06), there is a marked change in the oxygen partial pressure of the exhaust gas and this leads to a step change in the EGO sensor voltage because the ceramic electrolyte (zirconia) is very sensitive to oxygen levels, as shown in Fig. 5.29.

This sudden change in sensor voltage is used to trigger an action by the ECM, that will alter the fuelling, to maintain lambda = 1 (chemically correct air-fuel ratio). The result of this action is that the EGO sensor output cycles up and down, at a frequency that ensures that the engine runs smoothly and the exhaust catalyst is kept functioning correctly. The actual frequency is determined by the program that the designer places in the ROM of the ECM. All of this means that a voltaic-type EGO produces a standard type of output that can be measured by means of equipment that is readily available to vehicle repairers.

The approximate shape of the voltage waveform from the EGO sensor when in operation is shown in Fig. 5.30. This waveform arises from the way that the ECM alters the amount of fuel injected, i.e. lowering and raising the amount of fuel

Exhaust gas "Tfrom engine

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