Function requirements for exhaust gas recirculation (EGR), general points
Circuit 87M (Engine management ON)
Engine running.
Engine in load range from neutral to the upper partial-load range
Exhaust gas recirculation, general
The EGR lowers the nitrogen oxide (NOx) portion in the exhaust. This occurs by means of the following 3 processes:
Reduction of oxygen (O2) concentration in combustion chamber
Reduction of the combustion temperature through reduction of the combustion speed
Reduction of the combustion temperature through a higher heat capacity of the recirculated exhaust compared to the intake air
Heat capacity
The heat capacity is a material property and indicates how much heat a substance can store per temperature change.
Function sequence for exhaust gas recirculation
The EGR is active from neutral through to the upper partial-load range. Determination of the exhaust gas recirculation rate takes place depending on the following variables:
Engine temperature, engine load and engine rpm
Ambient conditions such as the air intake or the charge air temperature
Exhaust temperature
Diesel particulate filter status (with code (474) Diesel particulate filter)
Exhaust pressure
Time limitation (in idle)
Boost pressure
Coolant temperature
For this purpose, the CDI control unit (N3/9) reads signals from the following assembly parts:
Engine oil temperature sensor (B1)
Left hot film mass air flow sensor (B2/6)
Left intake air temperature sensor (B2/6b1)
Right hot film mass air flow sensor (B2/7)
Right intake air temperature sensor (B2/7b1)
Boost pressure sensor (B5/1)
Coolant temperature sensor (B11/4)
Charge air temperature sensor (B17/8)
Temperature sensor upstream of catalytic converter (B19/7) (with CODE U77 (BLUETEC (SCR) diesel exhaust treatment))
Temperature sensor upstream of diesel particulate filter (B19/9) (with code (474) Diesel particulate filter)
Temperature sensor upstream of ATL (B19/11)
DPF differential pressure sensor (B28/8) (with code (474) Diesel particulate filter)
Pressure sensor downstream of air filter (B28/11)
Accelerator pedal sensor (B37)
Exhaust pressure sensor (B60)
Crankshaft Hall sensor (B70)
After evaluating the input signals, the CDI control unit uses the stored characteristics map to actuate the exhaust gas recirculation actuator (Y27/9) with a pulse width modulated signal. Depending on its actuation, the exhaust gas recirculation actuator increases or decreases the quantity of recirculated exhaust, which is led into the charge air distributor via a mixing tube downstream of the throttle valve actuator (M16/6).
To increase the effect of EGR, the exhaust gas is led to the exhaust gas recirculation cooler where it is cooled.
If the temperature of the recirculated exhaust gas is too low, the recirculated exhaust gas can be guided past the exhaust gas recirculation cooler via a bypass duct.
If the exhaust gas recirculation cooler bypass switchover valve (Y85) is actuated by the CDI control unit by means of a ground signal based on a characteristics map, then the vacuum stored in the vacuum reservoir acts on the exhaust gas recirculation cooler bypass vacuum unit and opens the bypass in the exhaust gas recirculation cooler via a linkage.
The exhaust temperature determined by the exhaust gas recirculation temperature sensor is used by the CDI control unit to monitor the cooling function of the exhaust gas recirculation cooler.