The exhaust gas recirculation control has the task of favourably setting the fresh air mass fed to the engine with regard to clean combustion. As the total of fresh air mass and recirculated mass of exhaust gas (so-called engine throughput) is more or less constant in every operating point of the engine, the air mass also indirectly sets the recirculated mass of exhaust gas. The exhaust gas recirculation calculates a target position for the exhaust gas recirculation valve and for the throttle valve.
In the present engine, this is an exhaust gas recirculation valve with electrical activation.
The DDE control module controls the EGR valves by means of a square-wave signal with pulse duty factors (i.e. variable pulse widths) between 0 % and 60 %. With the present diameter of the opening of the EGR, exhaust gas recirculation of 0 to 60 kg/h is possible.
With an increase in the exhaust gas recirculation rate to around 20 %, the nitrogen oxide emissions can be reduced significantly. With a further increase in the exhaust gas recirculation rate, these emissions continue to drop, but carbon monoxide emissions and hydrogen chloride emissions rise. In addition, another increase in the exhaust gas recirculation rate increases the specific fuel consumption of the engine. An increase above the value of approx. 40 % leads to a rapid increase in carbon monoxide emissions.
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 controller drive is controlled in such a way that the specified fresh air mass for the operating point concerned is drawn in.
Depending on the engine speed and the injected volume of fuel, a base target value for the air mass is determined. This value is corrected depending on atmospheric pressure, air temperature, coolant temperature and engine speed. The stationary target value determined in this way is overridden by the desired fuel injection rate.
Exhaust gas recirculation calibration (service function) also affects the specified figure for exhaust gas recirculation.
Diagnosis instruction:
When checking the reference values for air mass in the control module functions <Diagnosis requests>/<Engine operating values>/<Air mass target>, bear in mind that this reference value is only meaningful when the EGR is switched on!