DDE 7.3 Exhaust Gas Recirculation Control, US Diesel

DDE 7.3 Exhaust Gas Recirculation Control, US Diesel

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

Components
- High-pressure or low-pressure EGR valve with vacuum unit
- Pressure converter (one per EGR component)
- Exhaust pipe
Exhaust gas cooler

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 activates the pressure converter by means of a square-wave signal with a pulse 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 pulse 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
- Fuel injection rate
- Coolant temperature
- Atmospheric pressure
- Intake air temperature
- Reduction in exhaust gas recirculation by idling for longer than 5 min

Exhaust gas recirculation adjustment (service function) also affects the target value 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.

The exhaust gas recirculation control is switched off at:
- 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