Oxygen Sensor: Description and Operation
O2 Sensors
the MINI COOPER and MINI COOPER S are equipped with two oxygen sensors. One is positioned upstream and the other downstream of the catalyst. the sensors are of a zirconium dioxide type and input a signal to the EMS2000 relative to the oxygen content within the exhaust gas. This enables the EMS2000 to provide closed loop operation and maintain strict control of the air/fuel ratio around stoichiometry (14.7:1). This allows the catalyst to work efficiently and reduce emissions of Carbon Monoxide (CO), Hydrocarbons (HC) and oxides of Nitrogen (NOx) to acceptable levels. the post catalyst sensor is used to monitor the catalyst performance.
the pre-cat oxygen sensor measure the residual oxygen content of the exhaust gas. the sensor produces a low voltage (0-1000 mV) proportional to the oxygen content that allows the EMS2000 to monitor the air/fuel ratio. the sensor is mounted in the hot exhaust stream directly in front of the catalytic converter.
the "tip" of the sensor contains a micro porous platinum coating (electrodes) which conduct current. the platinum electrodes are separated by solid electrolyte which conducts oxygen ions. the platinum conductors are covered with a highly porous ceramic coating and the entire tip is encased in a ventilated metal "cage".
This assembly is submersed in the exhaust stream. the sensor body (external) has a small vent opening in the housing that allows ambient air to enter the inside of the tip.
the ambient air contains a constant level of oxygen content (21%) and the exhaust stream has a much lower oxygen content. the oxygen ions (which contain small electrical charges) are "purged" through the solid electrolyte by the hot exhaust gas flow. the electrical charges (low voltage) are conducted by the platinum electrodes to the sensor signal wire that is monitored by the EMS2000.
- If the exhaust has a lower oxygen content (rich mixture), there will be a large ion "migration" through the sensor generating a higher voltage (950 mV).
- If the exhaust has a higher oxygen content (lean mixture), there will be a small ion "migration" through the sensor generating a lower voltage (080 mV).
- This voltage signal is constantly changing due to combustion variations and normal exhaust pulsations.
- This conductivity is efficient when the oxygen sensor is hot (250° - 300° C). For this reason, the sensor contains a heating element. This "heated" sensor reduces warm up time, and retains the heat during low engine speed when the exhaust temperature is cooler.
the efficiency of catalyst operation is determined by evaluating the oxygen consumption of the catalytic converters using the pre and post oxygen sensor signals. A properly operating catalyst consumes most of the O2 (oxygen) that is present in the exhaust gas (input to catalyst). the gases that flow into the catalyst are converted from CO, HC and NOx to CO2, H2O and N2 respectively.
In order to determine if the catalysts are working correctly, post catalyst oxygen sensors are installed to monitor exhaust gas content exiting the catalysts. the signal of the post cat. O2 sensor is evaluated over the course of several pre cat. O2 sensor oscillations.
During the evaluation period, the signal of the post cat. sensor must remain within a relatively constant voltage range (700 - 800 mV). the post cat. O2 voltage remains high with a very slight fluctuation. This indicates a further lack of oxygen when compared to the pre cat. sensor.
If this signal decreased in voltage and/or increased in fluctuation, a fault code will be set for Catalyst Efficiency and the "Malfunction Indicator Light" will illuminate when the OBD II criteria is achieved.
O2 Sensor Failure Symptom (s)
- If the upstream sensor fails it may result in poor performance and rough idle.
- Poor emissions control will cause the MIL to illuminate.
- As soon as the EMS2000 has detected an upstream oxygen sensor failure, it will default the fuel control to open loop operation.
- Slow response of sensor due to ageing or possible contaminated sensor tip.
O2 Sensor Heating
the oxygen sensor conductivity is efficient when it is hot (250° - 300° C). For this reason, the sensors contain heating elements. These "heated" sensors reduce warm up time, and retain the heat during low engine speed when the exhaust temperature is cooler. OBD II requires monitoring of the oxygen sensor heating function and heating elements for operation.
the two oxygen sensor heating circuits receive operating voltage from the Main Relay when KL15 is switched "ON". Each of the sensor heaters is controlled through separate final stage transistors.
the sensor heaters are controlled with a pulse width modulated ground during a cold start. This allows the sensors to be brought up to operating temperature without the possibility of thermal shock. the duty cycle is then varied to maintain the heating of the sensors.
When the engine is decelerating (closed throttle), the EMS2000 increases the duty cycle of the heating elements to compensate for the decreased exhaust temperature.