3 Series E46 318ti (N42) COMP
This system can be used on "stereo", "mono" and Y-shaped exhaust systems.
A model for the exhaust temperature integrated into the DME satisfies, among other things, the following
requirements:
•
The converter heater ensures that the catalytic converter is warmed up quickly after the engine is started,
and so becomes capable of converting the gases quickly
•
The catalytic-converter protection ensures that the exhaust-gas temperatures, especially at full load,are
regulated so that thermal overloading of the catalytic converter can be ruled out.
Fast operational readiness of the oxygen sensors is essential to maintain the strict exhaust limits. To this end,
the oxygen sensors have integrated heating elements that are actuated as early as possible. At the same time,
the maximum heating current is not enabled until the oxygen sensors are free from condensation. The ceramic
materials in the oxygen sensors could otherwise be damaged beyond repair.
Other important tasks of the DME are to check the ability of the catalytic converter to convert the exhaust gases,
to monitor the oxygen sensors and to indicate any fault that may develop.
To do this, the DME carries out the following electrical and functional diagnosis:
•
Identifies interchanged oxygen sensors
•
Electrical diagnosis of oxygen sensors,including oxygen sensor heating
•
Identifies leaks in exhaust line as far as the catalytic converter
•
Checks the ability of the catalytic converter to convert exhaust gases
•
Identifies ageing of oxygen sensors
•
Functional check of oxygen sensor heating
•
Monitors fuel mixture adaptation
System function
The catalytic converter needs a closed-loop Lambda control system to convert pollutant emissions. Closed-loop
Lambda control is realised by the digital engine electronics and the oxygen sensors.
Closed-loop Lambda control system
Complete and perfect combustion requires an air/fuel ratio of approx. 14.7 kilograms of air to 1 kilogram of
fuel. This corresponds to a volume of air of approx. 11 cubic metres.
The ratio of air actually drawn in to the amount of fuel injected is known as Lambda. In normal vehicle operation,
the Lambda value will fluctuate. The engine generates its best output when there is less air (Lambda approx. 0.9
= rich mixture). The engine has its lowest fuel consumption when there is more air (Lambda approx. 1.1 = lean
mixture).
The catalytic converter achieves maximum reduction of pollutant emissions when the mixture is in the region of
Lambda = 1.
With modern catalytic converters, the conversion rate, i.e. the proportion of pollutants converted, is 98 to almost
100 percent.
The best possible composition of the fuel-air mixture is controlled by the digital engine electronics (DME). The
oxygen sensors supply the key information about the composition of the exhaust gas.
The oxygen sensor continually measures the residual oxygen in the exhaust gas. The fluctuating residual
oxygen values are converted into a voltage signal for the DME control unit. The DME corrects the mixture
composition by adjusting the fuel injection.
Behind the catalytic converter there is a second oxygen sensor (monitor sensor). The catalytic converter has the
ability to store a large amount of oxygen. This means that there is very little oxygen behind the catalytic
converter. The monitor sensor outputs an almost constant (damped) voltage signal.
With increasing age, the catalytic converter's ability to store oxygen deteriorates.
SBT Exhaust system E46, E83, E85, E87, E90, E91 N40, N42, N45, N46
BMW AG - TIS
12.02.2013 21:19
Issue status (12/2007) Valid only until next DVD is issued
Copyright
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