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SENSOR BUSINESS, MARKETING AND TECHNOLOGY DEVELOPMENTS: NGK HELPS DRIVE AUTOMOTIVE EXHAUST GAS SENSING.

Article Excerpt
Since its introduction in 1976 on Volvo vehicles, the exhaust oxygen sensor, which measures oxygen content in the exhaust gas, has become a ubiquitous, vital component in the gasoline-powered vehicle's emission control system. Exhaust oxygen sensors are used in most gasoline-powered vehicles to control and optimize the air/fuel mixture entering the engine and to help monitor the performance and efficiency of the three-way catalyst, which is used to oxidize carbon monoxide and hydrocarbon emissions into water and carbon dioxide and to reduce NOx emissions into nitrogen, CO2, and water. The oxygen sensor is required to sense and control the air/fuel ratio at, or in the vacinity of, the stoichiometric value (14.7:1), since the efficiency of the three-way catalyst in converting NOx, CO, and hydrocarbon emissions is highest at, or in the vacinity of, the stoichiometric air/fuel ratio (where the amount of oxygen is sufficient to consume the supplied fuel in the engine).

On-Board Diagnostics (OBD-II) requirements for detecting catalyst efficiency or deterioration (adopted by the U.S. EPA and CARB (California Air Resources Board) for implementation by January 1, 1996) have led to the widespread adoption of the dual oxygen sensor approach in North America for detecting the deterioration of the three-way catalyst. This approach for catalytic converter monitoring uses the air/fuel feedback control sensor in the front of the catalyst and a second oxygen sensor placed after the catalyst. The catalyst's oxygen storage capacity, which can be used as an indicator of the catalyst's performance, can be measured by comparing the oxygen sensor signals. The rear oxygen sensor (placed downstream from the catalyst) can also be used to monitor and correct for aging of the front oxygen sensor to maintain the stoichiometric air/fuel mixture at high mileage.

Four-cylinder vehicles with gas-powered engines typically have two oxygen sensors, while V-6 and V-8 gas-powered vehicles tend to have 3-4 oxygen sensors.

The most widely used automotive exhaust oxygen sensors are Nerst-type electrochemical cells that use a cup-shared zirconia oxide solid electrolyte ceramic sensing element doped with Y2O3, and two platinum electrodes deposited on the two sides of the ZrO2 (zirconia) ceramic. Stoichiometric air/fuel oxygen sensors are typically designed in the form of a thimble mounted on a spark plug shell. Exhaust oxygen sensors with a built-in heater are more effective for emission control immediately after start-up, since the heater enables the sensor element to reach its active temperature in a shorter period of time.

The resistive-type titanium dioxide oxygen sensor, comprised of either porous ceramic material or a thick-film on a insulating substrate, has also been used in vehicles. The TiO2 exhaust oxygen sensor, which reportedly can be susceptible to contamination, requires a heater or a thermistor for temperature compensation.

The typical exhaust oxygen air/fuel sensor is a "switching" sensor that shows an abrupt change at the stoichiometric air/fuel value and is not suitable for proportional feedback control. The switching sensor...

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