Exhaust System
Exhaust System
Exhaust system
The 4-cylinder N12 petrol engine complies with the exhaust emission regulations EURO 4 or LEVII in the US version (LEV = Low Emission Vehicle). The engine has an engine-proximate catalytic converter. In the US version, there is an additional catalytic converter on the underbody.
Two oxygen sensors are deployed for oxygen-sensor control. A broadband oxygen sensor (Bosch: type LSU 4.9) serves as the control sensor before the engine-proximate catalytic converter. A jump sensor (NTK: type FLO) serves as monitor sensor after the catalytic converter.
Brief description of components
The following components are described for the exhaust system:
Broadband oxygen sensor
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 750 °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 steplessly measure a fuel-air ratio between 0.6 and 2.5 (constant characteristic curve). The broadband oxygen sensor works with a lower heat output than a conventional oxygen sensor. The broadband oxygen sensor is also operational more quickly. Current is applied at the measurement cell. This means that oxygen ions are pumped into the reference cell until a voltage of 450 mV settles between the electrode of the reference cell. The applied current at the measurement cell is the measured variable for the fuel-air ratio. This enables the oxygen-sensor control to set any desired fuel-air ratio in the combustion chamber.
Engine-proximate catalytic converter
The catalytic converter reduces the pollutant emissions:
- The carbon monoxide (CO) is converted with oxygen (O2) into carbon dioxide (CO2).
- The hydrocarbon (HC) is converted with oxygen (O2) into carbon dioxide (CO2) and water (H2O).
- The nitrogen oxide (NOx) is converted into nitrogen (N) and oxygen (O2).
At all times, the Digital Motor Electronics (DME) regulate the fuel-air mixture with regard to the following criteria:
- Exhaust emissions
- Consumption
- Power output development
- Catalytic-converter protection
Here, the DME picks up the oxygen content in the emissions via the oxygen sensors and corrects the injection rate on the basis of this data.
A model for the exhaust-gas temperature integrated in the DME meets (among others) the following specifications:
- The converter heater ensures rapid heating-up and the conversion capability of the catalytic converter after engine start-up.
- The effect of the catalytic-converter protection is that the exhaust-gas temperatures, in particular at full load, are regulated in such a way that a thermal overload of the catalytic converter is prevented.
System functions
The following system function is described for the exhaust system:
Oxygen-sensor control
For complete and perfect combustion, a fuel-air ratio of 1 kilogram of fuel and approx. 14.7 kilograms of air is necessary. The air volume corresponds to around 11 cubic metres. The ratio of the air volume actually delivered to stoichiometric air volume is referred to as the 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 an air surplus (Lambda approx. 1.1 = lean mixture). The catalytic converter achieves the best reduction in the pollutant emissions when the mixture is in the range 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 Digital Motor Electronics (DME) control the optimised composition of the fuel-air mixture.
The oxygen sensors deliver essential information on the composition of the emissions.
The front oxygen sensor continuously measures the residual oxygen in the exhaust gas. The fluctuation values of the residual oxygen are forwarded to the DME control unit as a voltage signal. The DME corrects the mixture composition by means of fuel injection. A second oxygen sensor (monitoring sensor) is built in behind the catalytic converter. The catalytic converter has a high oxygen accumulation capability. This means there is only a little air behind the catalytic converter. The monitoring sensor supplies a virtually constant (attenuated) voltage. With increasing age, the oxygen accumulation capability of the catalytic converter declines. The control sensor then reacts increasingly to lambda deviation with voltage fluctuations. These characteristics are used by a special diagnosis function for catalytic converter monitoring. A malfunction of the catalytic converter is indicated by the emissions warning lamp.
Notes for Service department
General information
IMPORTANT: Protect the plug-in connection of the broadband oxygen sensor against soiling.
The broadband oxygen sensor requires ambient air inside the sensor. The ambient air enters the interior via the plug-in connection through the cable. This is why the plug-in connection must be protected against soiling, e.g. by wax or preserving agent. In the event of faults in the oxygen-sensor control, the plug-in connection on the broadband oxygen sensor must be checked for soiling. If necessary, the plug-in connection must be cleaned.
Diagnosis instructions
The following monitoring functions check the state of the exhaust system:
CO matching
On vehicles without oxygen-sensor control, the carbon monoxide emissions at idle speed are set using the BMW diagnosis system. The adjustment values are specified.
Lambda adaptation
Lambda adaptation (fuel mixture adaptation) serves the purpose of compensating for component tolerances that influence the mixture and ageing effects. Factors such as excess air and fuel delivery pressure also affect the Lambda adaptation (partially compensation). For these reasons, no exact control limits can be specified for a fault.
In Lambda adaptation, a distinction is made as follows:
- additive fuel mixture adaptation
- multiplicative mixture adaptation
Additive fuel mixture adaptation is effective at idle speed and/or in the range close to idle speed. With increasing engine speed, the degree of influence declines. Multiplicative mixture adaptation affects the entire characteristic map. An important factor is e.g. the fuel delivery pressure.
The service function "reset adaptation values" can be used to reset the adaptation values and equipment variations to the status on delivery. Thereafter, the adaptation values have to be relearned. Longer operation between idle speed and partial load is necessary in order to learn the mixture adaptation values. Catalytic converter diagnosis
Catalytic converter diagnosis
The catalytic converter diagnosis works with constant oxygen sensors before and jump sensors after the catalytic converter. The diagnosis check the oxygen accumulation capability of the catalytic converter. The oxygen accumulation capability is a measurement for the conversion capability of the catalytic converter. To do so, a rich mixture is specified during the 1st phase of the catalytic converter diagnosis (approx. 3 seconds) until the oxygen sensor voltage has reached a specified value. As rich emissions are low in oxygen, the oxygen accumulated in the catalytic converter is reduced. In the 2nd phase, a lean mixture with emissions rich in oxygen is set. The longer it takes to reach the maximum oxygen accumulation capability the higher the conversion capability of the catalytic converter.
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