DDE 6 exhaust-gas recirculation control (EGR)

Depending on the operating status, a certain quantity of exhaust gas is returned via the EGR valve into the exhaust tract in order to reduce pollutant emission.

Important: On the M67TÜ engine, two EGR valves are fitted and these are activated individually by the DDE control units. What is described below applies to both of the EGR valves. EGR valve 1 is located in the direction of travel on the left.

Components

• EGR valve with vacuum unit
  
• Pressure converter
  
• Exhaust line
  

Function

The vacuum unit is connected to the vacuum supply via the pressure converter. The applies a variable vacuum to the vacuum unit according to the commands received from the DDE control unit. The DDE control unit controls the pressure converter by means of a square-wave signal with a duty factor (= variable pulse width) between 50 % and 75 %.

Control

The quantity of recirculated exhaust gas influences the mass of the fresh air drawn in: The more exhaust gas is recirculated, the less fresh air is drawn in. The mass of fresh air that passes through the engine at any point on the performance curve when the EGR is switched off is a known quantity. Thus the reduction in the mass of fresh air drawn in due to exhaust gas recirculation is a measure of the quantity of exhaust gas recirculated. During operation, the duty factor on the pressure converter is controlled in such a way that the specified fresh air mass for the operating point concerned is drawn in.

The DDE control unit calculates a specified fresh air mass figure for every operating point from the following variables:

• Engine speed
  
• Injection volume
  
• Coolant temperature
  
• Atmospheric pressure
  
• Intake air temperature
  
• Reduction in exhaust gas recirculation by idling for longer than 5 min
  

Exhaust gas recirculation calibration (service function) also affects the specified figure for exhaust gas recirculation.

The causes of cutoff of the exhaust gas recirculation in operation can be read out of the control unit. The causes of the cutoff indicate whether EGR control is active or whether it has been deactivated due to one of the conditions listed below.

Exhaust gas recirculation is switched off when:

• Coolant temperature ≤ 15 °C or ≥ 105 °C
  
• Rotation speed ≤ 750 rpm or ≥ 2800 rpm
  
• Injection volume too high (dependent on engine speed)
  
• Atmospheric pressure ≤ 880 mbar
  
• Battery voltage ≤ 9 V
  
• Intake-air temperature ≤ 25 °C or ≥ 105 °C
  
• When starting the engine
  
• Coasting
  
• Engine idling for too long
  

and if following faults occur:

• Fault codes 4500, 4506: exhaust gas recirculation, control deviation
  
• Fault codes 41B0, 41D0, 41E0: Exhaust gas recirculation
  
• Fault codes 3FB0, 3FC0, 3FD0: mass air flow sensor
  
• Fault code 4060: Atmospheric pressure sensor
  
• Fault codes 4180, 4190, 41A0: Turbocharger pressure adjuster
  
• Fault code 4680: Supply voltage 2
  
• Fault code 4140: mass air flow sensor
  
• Fault code 3F00: Turbo pressure sensor
  
• Fault codes 4520, 4530: Charge-air-pressure control, control deviation
  

Monitoring

During controlled operation of the EGR, the control unit monitors the mass of the intake air drawn in. If the control unit detects a discrepancy between the specified and actual intake air mass that is outside the permissible limits, the following faults may be stored:

• Fault codes 4500, 4506: exhaust gas recirculation, control deviation
  

If the control unit detects a fault in the control of the pressure converter, the following faults may be stored:

• Fault codes 41B0, 41D0, 41E0: Exhaust gas recirculation, activation