oxygen content of outside air vs exhaust
Air/Fuel ratio
Zirconia Sensor
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Zirconia Oxygen Sensors are made from a ceramic that is known as zirconium dioxide (ZrO2). This oxide is selected for use in the automotive industry due to its high flexibility, tensile strength, resistance to wear and corrosion, low thermal conductivity, and oxygen ion conductivity.
Zirconium oxide ceramic along with a platinum coated electrode and a heater make up the major internal components of the zirconia oxygen sensor. The zirconia sensor generates its own voltage and is not reliant on the ECU to operate. The main element is the zirconia ceramic, which becomes conductive for oxygen ions at about 310° C. At this temperature, zirconia dioxide develops an electrical charge as oxygen ions pass through it. Since nature is constantly seeking to balance itself, when you place zirconia ceramic between environments with different amounts of oxygen, as the oxygen passes through the zirconia to offset the balance, the zirconia will develop a slight charge. The strength of that charge will depend on how many oxygen ions pass through it. The greater the difference in oxygen between the two atmospheres, the greater the charge developed. The zirconia oxygen sensor then uses a thin platinum coating to accumulate that charge, carry it to the sensor wires and onto the ECU.
The ECU can tell whether the air/fuel ratio is rich or lean based on the voltage (signal) which is generated by the zirconia sensor. A rich air/fuel mixture will generate a higher voltage than a lean air/fuel mixture. This is because in a rich air/fuel mixture there is less oxygen in the exhaust, therefore more oxygen will try to migrate from the outside air across the zirconia creating a stronger charge. A lean air/fuel mixture will have more oxygen in it thus producing a lower voltage,
Your engine will produce the greatest power with the fewest emissions when the air/fuel mixture is kept at a ratio of 14.7 to 1. This is referred to as “stoichiometric”. The zirconia oxygen sensor, at this 14.7:1 mixture generates a nominal voltage of 0.45 volts (450 millivolts). Any voltage below .045 volts is read as a lean mixture, anything above is a rich air/fuel mixture. The ECU reads the output voltage and adjusts the fuel flow through the injectors accordingly. The zirconia sensor is a binary sensor in that it only reports a rich or lean air/fuel mixture, therefore the ECU is constantly cycling between rich and lean so as to try to maintain an average 14.7:1 air/fuel ratio. The speed with which the sensor can cycle between a rich and lean signal is referred to as it’s cross count. The higher or faster the cross count, the faster the ECU can adjust the air/fuel mixture.
The zirconia dioxide oxygen sensors are divided into three types, the thimble type (3 wire and 4 wire sensors only), the thick film planar type (4-wire only), and the Wide Band Oxygen Sensor type.
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