Exhaust Gas Recirculation: Description and Operation

GF14.20-P-3000OGG Exhaust Gas Recirculation, Function
ENGINES 642.8 in MODEL 164.1 as of model year 2009 / AEJ (YoM) 08 model refinement package
ENGINES 642.8 in MODEL 164.8, 251.1 as of model year 2009 / AEJ (YoM) 08
ENGINES 642.8 in MODEL 251.0 /1 as of model year 2011 / AEJ (YoM) 10 model refinement package

Function requirements for exhaust gas recirculation (AGR) (EGR), general points
^ Circuit 87M ON (engine control ON)
^ Engine running
^ Engine in partial-load range

Exhaust gas recirculation, general
AGR (EGR) lowers the nitrogen oxide (NOX) content in the exhaust gas. This occurs by means of the following 3 processes:

- Reduction of the oxygen (O2) concentration in the combustion chamber
- Reducing exhaust flow
- Reduction of the combustion temperature as a result of the improved heat collection capacity of the components (e.g. water and carbon dioxide (CO2)) which are not actually involved in combustion but do influence the combustion process

Function sequence for exhaust gas recirculation
AGR (EGR) is active in the lower load and rpm range. The AGR (EGR) rate depends on engine load and engine speed.

For this purpose, the CDI control unit (N3/9) reads signals from the following components:

- Left hot film mass air flow sensor (B2/6)
- Intake air temperature sensor (B2/6b1)
- Right hot film mass air flow sensor (B2/7)
- Intake air temperature sensor (B2/7b1)
- Charge pressure sensor (B5/1)
- Charge air temperature sensor (B17/8)
- Pressure sensor downstream of air filter (B28/5)
- Accelerator pedal sensor (B37)
- Crankshaft Hall sensor (B70)

After evaluating the input signals, the CDI control unit uses a stored performance map to actuate the left exhaust gas recirculation positioner (Y27/9) using a pulse width modulated (PWM) signal. The quantity of recycled exhaust in the charge air manifold is increased or decreased via the exhaust gas recirculation actuator according to the actuation.

To increase the effect of AGR (EGR), the exhaust gas is passed through 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.
The bypass flap is controlled by the AGR (EGR) cooler bypass flap switchover valve (Y85) (except code (U42) BlueTEC (SCR) diesel exhaust treatment and code (494) USA version) in combination with a vacuum unit.