Lambda probe

LAMBDA PROBE

The lambda sensor informs the injection-ignition control unit about the progress of the combustion of the air-fuel mixture and enables the system to keep the stoichiometric ratio of the mixture as close as possible to the theoretical value.

In order to obtain an optimal mixture the amount of air taken in by the engine should be the equivalent of the theoretical amount that would be needed to burn all the fuel injected.

In this case, it is said that the lambda factor (λ) is 1, in fact:

Therefore:

1. 1 = 1 IDEAL MIXTURE
2. λ > 1 LEAN MIXTURE
3. λ < 1 RICH MIXTURE

The lambda sensor set in contact with the exhaust gas, generates an electric signal, the voltage rating of which depends on the concentration of oxygen contained in the gas. This voltage is characterised by a sharp variation when the composition of the mixture departs from λ = 1.

1. Ceramic body
2. Protective tube
3. Metal body
4. Electric resistance

The sensor works on the basis of the fact that, with temperatures of above 300 °C, the ceramic material used becomes a conductor of oxygen ions.

Under these conditions, if the quantity of oxygen at the two sides (A and B) of the sensor is in different percentages, a voltage difference between the two ends is originated, which is the index of measurement for the difference of the amount of oxygen in the two environments (outside air side and exhaust gas side) and it informs the control unit that the oxygen remaining in the exhaust gas is not in sufficient percentage to warrant combustion with a low quantity of harmful by-products.

For temperatures below 300 °C the ceramic material is not active, therefore the sensor does not send utilisable signals and a special circuit in the control unit prevents loop mixture adjustment while the sensor is warming.

To ensure that the sensor quickly reaches its operating temperature, it is fitted with an electrical resistance appropriately supplied by the battery; this also makes it possible to install the sensor in cooler areas of the exhaust manifold.

REMOVAL/REFITTING

• Disconnect the battery (-) terminal.

1. Disconnect the lambda sensor electrical connection.

The lambda sensor comprises a ceramic body, based on zirconium dioxide, coated with a fine layer of platinum closed at one end, inserted in a protective tube and housed in a metal container which offers further protection and makes assembly on the exhaust manifolds possible.

The outer part of the ceramic is exposed to the flow of the exhaust gas, while the inner part communicates with the environment.