Design of a Zirconium dioxide lambda sensor
Zirconium dioxide element
The zirconium dioxide ceramic element is in effect a solid electrolyte.
The core of this lambda sensor is a hollow, thimble-shaped ceramic element.
At an operating temperature in excess of 350 °C it becomes permeable for oxygen ions. Early sensor designs relied on the exhaust gas to raise the temperature of the element but in order to reach and maintain the desired operating temperature rapidly, a built-in heater is connected to the element. In a complex process - depending on the residual oxygen content in the exhaust - a high or low electrical voltage is generated.
The protective metal tube is designed to allow good gas contact with the element whilst protecting the sensor element from solid particles in the exhaust. Importantly it also protects the element from contact with any water which may be present in the exhaust system. Water vapour, a product of the combustion process, may condense in the exhaust pipe and if contact with the hot ceramic element occurs damage can result due to thermal shock (rapid change in temperature).
An integrated heater ensures that the sensor element reaches the necessary operating temperature very quickly. This ensures that strict control of the fuelling system takes place within the first few metres of driving and thus make the vehicle operation more environmentally-friendly. This response time before operation commences is called the "light-off time". Modern sensors provide a signal within just a few seconds and thus have a very fast light-off time.
The ceramic holder ensures that the sensor element is located securely and protects it from vibrations.
The compressible gasket ensures a gas tight seal between the lambda sensor and exhaust system.
The hexagon is the contact point used by the appropriate tools to screw the sensor into the exhaust system. When this is carried out attention should be paid to the application of the appropriate tightening torque.
A special rubber sealing grommet is fitted between the wiring cables and the top of the sensor’s metal housing in order to prevent water ingress into the interior of the sensor. This rubber seal is crimped into the top of the sensor housing.
The two white cables (where fitted) are the electrical supply and earth for the integral heating device fitted inside the ceramic element.
This cable establishes the earth connection with the vehicle electrical system.
This cable carries the sensor signal to the engine control unit.