Circuit description: Notes

The three-way catalytic converter is used to efficiently purify carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) contained in the exhaust gas. The maximum performance of this three-way catalyst is reached when it functions at near stoichiometric air-fuel ratio. Therefore, unless the engine is precisely controlled to produce near this air fuel ratio, CO, HC, and NOx are not efficiently purified. Oxygen sensors are provided for the purpose of this control. When an oxygen sensor detects an oxygen level within the exhaust gas produced from proper combustion near the stoichiometric air-fuel ratio, its output voltage changes quickly. This characteristic is used to send a signal to the engine control computer for an accurate air/fuel control.

Under a leaner air/fuel ratio, oxygen level within the exhaust gas increases. The oxygen sensor sends this information (lean air/fuel ratio state) to the engine control computer. (works with a small electric power: < less than 0.45V)

Under a richer air/fuel ratio, oxygen level within the exhaust gas decreases. The oxygen sensor sends this information (rich air/fuel ratio state) to the engine control computer. (works with a large electric power: more than 0.45 V)

A zirconium element is used for the sensor element. The zirconium element is made of platinum plated zirconium, containing integrated heating component. The inside surface of the zirconium element is exposed to the atmospheric air, and the outside surface is exposed to the exhaust gas. When there is a significant difference in oxygen level between the atmospheric air and the exhaust gas, the sensor outputs a small amount of voltage. Efficiency of the oxygen sensor increases significantly when heated. When exhaust gas temperature is low, the oxygen sensor cannot generate a useful voltage signal without supplemental heating. This is the reason why the oxygen sensor is equipped with a heater that heats up its zirconium element. The engine control computer uses the duty cycle approach to control the supplemental heating, adjusting the average electric current supplied to the zirconium element.

Fig 1: Oxygen Sensor Construction With A/F Ratio Graph

Courtesy of SUBARU OF AMERICA, INC.