Exhaust Gas Recirculation: Description and Operation

High-Pressure Exhaust Gas Recirculation DDE 7.3 (US Diesel)

High-pressure exhaust gas recirculation DDE 7.3 (US diesel)
On diesel models for the US market with M57TU2TOP engines, the high-pressure exhaust gas recirculation is regulated by the electrically operated EGR actuator. The high-pressure exhaust gas recirculation in the normal mode is controlled on the basis of the EGR ratio. During regeneration of the diesel particle filter, it is controlled conventionally, i.e. via the air mass.
All of the exhaust gas recirculation systems in previous diesel generations were always regarded as high-pressure exhaust gas recirculation.
Depending on the operating condition, a certain quantity of exhaust gas is recirculated via the EGR valve into the intake area. This reduces the nitrogen oxide emissions. The EGR systems in BMW 3 Series and BMW X5 vehicles are different! Both systems have this high-pressure EGR system, but the low-pressure EGR system is present exclusively in BMW X5 vehicles. In wide engine operation ranges, both high-pressure and low-pressure EGR control are active. In the high torque and/or high speed ranges, however, the activity of the high-pressure EGR system dominates:

High-pressure exhaust gas recirculation 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 cycle on the pressure converter is controlled in such a way that the specified fresh air mass for the operating point concerned is drawn in.

Mean quantity adaptation
The 'Mean quantity adaptation' function that can be selected in the service functions is used to adapt the exhaust gas recirculation more precisely to tolerance of the fuel injection rates.
The air ratio measured by the oxygen sensor and the air mass measured by the HFM are used to determine a mean fuel injection rate across all cylinders. This value is compared with the fuel injection rate specified by the DDE control unit. If there is a deviation, the air mass is adapted to the actual fuel injection rate by adjusting the exhaust gas recirculation (EGR) valve in such a way that the correct air ratio is set. The MMA is not a rapid control operation, rather an adaptive learning procedure. That means that the fuel injection rate error is learned in an adaptive characteristic map that is stored permanently in the DDE control unit.

Oxygen sensor
An oxygen sensor is required for the mean quantity adaptation function. The oxygen sensor is fitted immediately after the turbine of the exhaust turbocharger. The oxygen sensor, familiar from petrol engines, is a broadband oxygen sensor that provides exact air ratios over a wide measuring range.

Notes for Service department

Diagnosis instructions

Resetting the mean quantity adaptation
The service function 'Mean quantity adaptation' must be run if one of the following components was replaced:
- Air mass flow sensor
- Rail pressure sensor
- Oxygen sensor
Description and sequence of the service function, see corresponding functional description.

Adaptation of EGR valve
If the exhaust gas recirculation (EGR) valve is replaced, the following service function must be run before it is replaced:
- Adaptation of EGR valve
Description and sequence of the adaptation, see corresponding functional description.