Selective Catalytic Reduction: Delivery: Notes

The urea-water solution is delivered from the active reservoir to the metering module. This task is performed by a pump that is integrated in the delivery unit. The delivery unit additionally contains:

• Heater
• Pressure sensor
• Filter
• Return throttle
• Reversing valve.
  Fig 1: Identifying Delivery Unit Components

  Courtesy of BMW OF NORTH AMERICA, INC.
  INDEX EXPLANATION CHART
  
  

  Index	Explanation
  1	Metering line
  2	Delivery module
  3	Pump
  4	Reversing valve
  5	Filter
  6	Restrictor
  7	Pressure sensor
  8	Filter
  9	Level sensor
  10	Filter
  11	SCR catalyst
  12	Exhaust system
  13	Metering module

The pump is actuated by a pulse-width modulated signal (PWM signal) from the DDE. The PWM signal provides a speed specification for the purpose of establishing the system pressure. The value for the speed specification is calculated by the DDE based on the signal from the pressure sensor.

When the system starts up, the pump is actuated with a defined PWM signal and the line to the metering module is filled. This is followed by pressure build-up. Only then does pressure control take place.

When the metering line is filled, the opened metering valve allows a small quantity of the urea-water solution to be injected into the exhaust system.

During pressure control, i.e. during normal operation with metering, the pump is actuated in such a way that a pressure of 5 bar is applied in the metering line. Only a small part of the urea-water solution delivered by the pump is actually injected.

The majority of the solution is transferred via a throttle back into the active reservoir. This means, the delivery pressure is determined by the pump speed together with the throttle cross section.

Fig 2: Identifying Delivery Unit Components

Courtesy of BMW OF NORTH AMERICA, INC.

The solution is injected four times per second. The quantity is determined by the opening time and stroke of the metering valve. However, the quantity is so low that there is no noticeable drop in pressure in the metering line.