Tell me your understanding of how an oxygen sensor works and its function in an emissions control system.

"I don't claim to be an expert on oxygen sensors, but I do remember reading about them in my Internal Combustion Fundamentals course. There are two main types of oxygen sensors: the switching sensor, which was the first one used widely in production, and the linear sensor. If you think we have time, I can describe both, but I'll start with the switching sensor.

The switching-type sensor is basically a Galvanic cell with a solid electrolyte, usually zirconia doped with yttria. It is structured to have one side in contact with exhaust gas, and another side, sealed from exhaust, which is in contact with the ambient air. When the sensor is hot enough to conduct electricity, the CO, HCs and hydrogen (if any) on the exhaust side adsorb on the exhaust-side electrode and create a demand for oxygen ions, which are supplied at the air reference side. This sets up a voltage that is dependent on the air/fuel ratio through the Nernst equation. The voltage is low when there is an excess of oxygen on the exhaust side, which we call 'lean,' and is high when there is not enough oxygen, which we call 'rich.' The relationship between air/fuel ratio and the voltage developed by the sensor is highly non-linear. The slope is very steep near stoichiometry, and very shallow at the rich and lean ends of the curve. This is why it is called a switching sensor, and it is mostly used to identify whether the exhaust environment is rich or lean, without trying to identify how much rich or lean.

There are a lot of competing demands on the fuel control system. But for the purposes of this discussion, we can focus on the need for the fuel controls to maintain the air/fuel ratio of the exhaust gas near stoichiometry, which allows the catalytic converter to operate at high efficiency. The fuel control system uses the sensor voltage, which has been identified as either rich or lean, to make a decision to increase the amount of fuel injected (if it is lean) or decrease it (if the air/fuel ratio is rich). Obviously, the control system is more complicated than that - it makes an immediate proportional response, but it will have short and long-term adaptive terms that are learned over time if the signal keeps coming back either rich or lean over a period of time. This sensor is positioned just after the confluence of all the exhaust runners, usually near the outlet of the exhaust manifold. Another switching sensor is positioned after the catalytic converter.

Would you like me to describe how the rear sensor is used, as well?"