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Excess-air ratio

This extreme voltage change can be used to control the mixture-formation system. The sensor only begins to function at a temper ature of approx. 300 °C, its ideal operational temperature being around 600 °C. In order to achieve a rapid response time and constant temperature, electrically-heated sensors are often employed.

Fig. 7.9 The zirconia oxygen sensor voltage in the region of lambda = 1

value which gives lambda = 1. The result is that the output trace of the zirconia oxygen sensor is of the form shown in Fig. 7.10. Points to note are as follows.

1 The maximum voltage should be between 800 mV and 1 V.

2 The slope on the rise and fall sides of the trace becomes less steep as the sensor ages, or is damaged by use of incorrect fuel or lubricants.

3 The peak-to-peak voltages should be at least 600 mV with an average of 450 mV.

4 The minimum voltage should be approximately 200 mV.

Fig. 7.10 Voltage trace for a zirconia oxygen sensor

As stated above, the sensor pattern changes as the sensor ages or is damaged in some way (poisoned) by leaded fuel or other contaminants. Figure 7.11 (P16 KTS 300) shows a comparison between the voltage trace for a good sensor

The ageing of the lambda sensor upstream of the catalytic converter is monitored by analysis of the sensor signal. An aged lambda sensor (dotted line) reacts more slowly to changes in the oxygen in the exhaust than a new lambda sensor (unbroken line).

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