Oxygen Sensor Before Catalytic Converter
Oxygen Sensor Before Catalytic Converter
Oxygen sensor before catalytic converter
The oxygen sensor before catalytic converter is a broadband oxygen sensor (control sensor). The broadband oxygen sensor continuously measures the residual oxygen in the exhaust gas. The fluctuating values of the residual oxygen are sent to the engine control unit as a voltage signal. The engine control corrects the mixture composition by changing the fuel injection.
A new addition is the enhanced ADV LSU broadband oxygen sensor (Bosch). LSU stands for "universal oxygen sensor" and ADV for "Advanced".
Functional description
The sensor system of the broadband oxygen sensor consists of ceramic layers of zirconium dioxide (laminate). The heating element inserted in the laminate rapidly ensures the required operating temperature of at least 760 °C. The broadband oxygen sensor has 2 cells, a so-called measurement cell and a reference cell. The two cells are coated with electrode made of platinum.
The broadband oxygen sensor can be used to continuously measure an air/fuel ratio between 0.65 and pure air (constant characteristic curve). The broadband oxygen sensor is operational in a shorter time than the predecessor LSU 4.9 version.
For complete and perfect combustion, a air/fuel ratio of 1 kilogram of fuel and approx. 14.7 kilograms of air is necessary. The ratio between the air quantity that is actually being supplied and that required for a stoichiometric ratio is designated as lambda. During normal operation of the vehicle, the Lambda value fluctuates. The engine has its best performance with a lack of air (Lambda approx. 0.9 = rich mixture). The engine has its lowest consumption with excess air (Lambda approx. 1.1 = lean mixture). The catalytic converter can most effectively reduce pollutant emissions if the fuel-air mixture is in the region of Lambda = 1. The conversion rate, i.e. the proportion of converted pollutants, is 98 % to virtually 100 % in the case of modern catalytic converters. The optimum composition of the fuel-air mixture is regulated by the engine control. The oxygen sensors deliver essential information on the composition of the exhaust gas.
Structure and inner electrical connection
Voltage is applied at the at the pump cell. This means that oxygen ions are pumped into the measurement cell until a voltage of 450 mV settles between the electrode of the measurement cell. The resulting pump current is the measured variable for the fuel-air ratio. This enables the oxygen sensor emissions control to set any desired air/fuel ratio in the combustion chamber.
Characteristic curve and setpoint values
The new oxygen sensor characterized by an enlarged measuring range from lambda = 0.65. Additional benefits of the new oxygen sensor are its higher temperature stability, short response times of less than 30 milliseconds and high degree of signalling precision.
Observe the following nominal values for the broadband oxygen sensor:
Size Value
Lower voltage range of 10.7 to 16.5 V
oxygen sensor heating
Voltage of oxygen sensor 1.8 to 2 V
heating in warm-up phase
Voltage of oxygen sensor 12 V
heating up to 5 seconds
Voltage of oxygen sensor 9 V
heating up to 6.5 seconds
Operating temperature 760 °C
Heating resistance at 20 °C 2.0 to 3.2 ohms
Maximum pump current, air 1,5 mA
Diagnosis instructions
Failure of the component
In the event of failure of the broadband oxygen sensor, the following behavior is to be expected:
- Fault entry in the engine control unit
- Adaptation or emergency operation with substitute value
- The emissions warning light lights up in the instrument panel
General notes
The following diagnosis monitoring functions check the condition of the engine and exhaust emission system:
- Oxygen sensor adaptation
The purpose of oxygen sensor adaptation (mixture adaptation) is to compensate for component tolerances and ageing effects affecting the fuel-air mixture.
- Catalytic converter diagnosis
The diagnosis tests the oxygen storage capacity of the catalytic converter. The oxygen storage capacity provides an indication of the catalytic converter's conversion capability.
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