SCR System (US Diesel)
SCR System (US Diesel)
SCR system (US diesel)
As of 09/2008, the US diesel will be introduced in the USA (United States of America). The introduction will initially involve only the M57TU2TOP engine with DDE 73 in BMW X5, X6 and 3 Series vehicles. In order to meet the stricter legal emission reduction requirements for a sustained period, this engine will be equipped for the first time with an SCR system that sprays the carbamide-water mixture into the exhaust tract. As can be seen in the following illustration, this mixture is metered after the diesel particle filter into the exhaust tract in the form of a fine spray. The metered mixture reacts there with the hot exhaust particles that arise in the course of combustion inside the engine, reducing the overall content of harmful NOx emissions (nitrogen oxide) in the exhaust gas. The chemical reaction takes place in the SCR catalytic converter downstream of the particle filter. This chemical reaction can only take place when the SCR catalytic converter has reach a required minimum temperature. Before this temperature (around 200 °C in SCR catalytic converter) is reached, the metering is inactive.
SCR system overview (without passive tank):
Components of the SCR system (without passive tank):
Chemical reaction of the selective catalytic reduction:
After the carbamide-water mixture has been metered into the exhaust tract, this mixture reacts with the nitrogen oxides in the exhaust gas and with the oxygen molecules in the exhaust gas. The result is that there is a reduced amount of harmful nitrogen oxides in the emitted exhaust gas, as well as molecular nitrogen and liquefied water.
IMPORTANT NOTE: This catalytic reduction can only take place when the SCR catalytic converter is filled (loaded) with reducing agent.
Operating strategy and display concept for US diesel:
BMW US diesel models with SCR system can only be operated (driven) when the SCR system is operational. In order to establish operational status, there must be adequate carbamide-water mixture in the corresponding tanks. This tank system is divided into an active tank and a passive tank. The vehicle tanks are filled via the corresponding service hatches. There is a service hatch for the active tank and a separate opening for the passive tank. In general, the mixture that is metered directly into the exhaust tract is taken exclusively from the active tank. The active tank is heated. The passive tank is not heated. There is a pump device to deliver the carbamide-water mixture from the passive tank into the active tank. On a longer journey, to ensure continuous vehicle readiness, the mixture must neither age to any great extent nor freeze.
IMPORTANT INFORMATION: If the SCR system cannot be operated, the vehicle cannot be operated. The customer must be informed of the necessity to fill the system with the carbamide-water mixture.
As soon as the mixture stored in the vehicle reaches a critical fill level, an acoustic signal sounds and the remaining range is shown in the indicating instrument of the vehicle. Reaching a critical fill level in the active tank can be read out by querying the tank volume in the control-module function "mixture fill level active tank or total remaining tank volume".
IMPORTANT INFORMATION: If the critical fill level of the active tank is reached, the vehicle enters an automatic shutdown scenario. The customer must be made aware of the fact that this shutdown scenario can be prevented by topping up the active tank with new carbamide-water mixture. (In this situation, topping up the passive tank has only a limited effect.)
Temperature resistance and ageing stability of the mixture:
The carbamide-water mixture is not absolutely resistant to ageing and not absolutely temperature-resistant. At a temperature lower that the freezing point of water (i.e. -11 °C), the mixture freezes. To ensure permanent vehicle readiness, the mixture in the active tank is heated by the heaters in the system. The defrosting period of a frozen carbamide-water mixture varies and can take up to one hour. (Example: heating up a frozen mixture from -30 to melting temperature takes less than ten minutes.) During the defrosting period, the vehicle may be operated. The mixture and line heating is very powerful. The fatigue life of the mixture is continuously checked by monitoring mechanisms within the system. Any high inputs of energy due to high continuous temperatures in the SCR system lead to every carbamide-water mixture ageing prematurely. (In the case of sustained prevailing temperatures of the mixture above 60 - 70 °C, ageing damage has been shown to occur.) The temperature of the carbamide mixture can be read out in the control-module functions at
NOTE: An aged mixture cannot be used and must be replaced in service by skilled personnel with a new carbamide-water mixture.
Integration of the SCR system in the particle filter composite system:
The BMW SCR system is integrated in the composite system for effective particle filtering that has been used to date. The exhaust backpressure sensor used to date on US vehicles is replaced by the differential pressure sensor. In addition to the exhaust gas temperature sensors before the oxidation catalytic converter and diesel particle filter used to date, there is an exhaust gas temperature sensor after the diesel particle filter, before the SCR catalytic converter. The oxidation catalytic converter and particle filter are fitted in a shared housing. There are no major changes to the general function and configuration of the particle filter.
Interaction with MSA system (Automatic Start/Stop function):
On BMW diesel models with SCR system that have an Automatic Start/Stop (MSA) function, there are special features in the operating strategy. The operational readiness of the MSA function can be delayed by the heating operation of the SCR system. The SCR system, on the other hand, continues to work independently of any MSA stops to comply with the emission reduction requirements. In individual cases, the SCR system can set a shutdown inhibitor for the MSA function. As a general principle, operation of the SCR system has priority over the MSA function.
Diagnosis instructions:
All of the measured variables relevant to the new SCR system that concern the NOx sensor system and the catalytic converter can be seen in the control-module functions at