Depending on the operating status, a certain quantity of exhaust gas is returned via both EGR valves into the exhaust tract in order to reduce pollutant emission. EGR valve 1 is mounted on the left and EGR valve 2 on the right of the intake tract.
The pressure converters connect the vacuum units with the vacuum supply by means of hoses. Depending on the activation status and controlled by the DDE slave control unit, they switch a variable vacuum to the vacuum unit. The pressure converters are activated by a square-wave signal with duty factors (= variable pulse width) between 5 and 95 %.
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. Since the mass of fresh air drawn in is known for all operating points with the EGR switched off, the reduction in the mass of fresh air caused by exhaust gas recirculation can be used as a measure for the quantity of recirculated exhaust gas. For the purpose of exhaust gas recirculation control, a set fresh air mass is defined in a characteristic map with following influencing variables for all operating conditions:
The current setting for exhaust gas recirculation matching (service functions) is an additional influencing variable.
During operation, the quantity of recirculated exhaust gas is controlled by the duty factor at the pressure converter such that the set mass of fresh air defined for the specific operating point is drawn in. With the aid of the mass air flow sensors the control units monitor deviations between the set and actual fresh air mass on both sides. The following faults can occur:
Note: The types of faults must be taken into consideration when evaluating stored fault codes as precise location of a fault is only possible based on the types of fault.
The status of exhaust gas recirculation can be read out of the DDE control unit. The status indicates whether EGR control is active or whether it has been deactivated due to one of the conditions listed below. The status is EGR = 0 if the EGR control is active. Refer to the list below for the statuses when EGR is deactivated. The highest status is output if there are several reasons for EGR activation.
and if following faults occur: