Exhaust Gas Recirculation: Description and Operation

GF14.20-P-3000OG Exhaust Gas Recirculation, Function
ENGINES 642.8 in MODEL 204.0 /2 /9, 207.3 /4, 212.0 /2, 218

Function requirements for exhaust gas recirculation (EGR), general points
^ Circuit 87M ON (engine control 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
EGR is active from idle through to the upper partial-load range.
Determination of the EGR rate takes place depending on the following variables:

- Engine load and engine rpm
- Intake and charge air temperature
- Exhaust temperature
- Diesel particulate filter status (with code (474) Particulate filter)
- Exhaust back pressure
- Time limitation

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

- Left hot film mass air flow sensor (B2/6)
- Left intake air temperature sensor (B2/6b1) (model 212), left intake air temperature sensor (B2/6b1) (model 204, model 207, model 218)
- Right hot film mass air flow sensor (B2/7)
- Right intake air temperature sensor (B2/7b1) (model 204, model 207, model 212, right intake air temperature sensor (model 218))
- Boost pressure sensor (B5/1)
- Exhaust gas recirculation temperature sensor (B16/14) (with code (U42) Blue TEC diesel exhaust treatment (SCR) and code (494) USA version)
- Charge air temperature sensor (B17/8)
- Temperature sensor upstream of catalytic converter (B19/7) (with code (U42) BlueTEC (SCR) diesel exhaust treatment)
- Temperature sensor upstream of diesel particulate filter (B19/9) (with code (474) Particulate filter)
- Temperature sensor upstream of turbocharger (B19/11)
- Pressure sensor downstream of air filter (B28/5)
- DPF differential pressure sensor (B28/8) (with code (474) Particulate filter)
- Accelerator pedal sensor (B37)
- Throttle valve actual value potentiometer 1 (M16/6r1)
- DPF differential pressure sensor for OBD (B28/16) (with code (U42) BlueTEC (SCR) diesel exhaust treatment and code (494) USA version)
- Exhaust back pressure sensor (B60)
- Crankshaft Hall sensor (B70)

After evaluating the input signals, the CDI control unit uses a stored characteristic map to actuate the exhaust gas recirculation actuator (Y27/9) by means of a pulse width modulation (PWM) signal. This actuation determines the quantity increase or decrease of recirculated exhaust fed through a mixing tube to the throttle valve actuator (M16/6) in the charge air manifold.

To increase the effect of 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.

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.

Exhaust emission standards
Engine 642.8 complies with the EU-5 exhaust emission standards.
Vehicles with code (U42) BlueTEC (SCR) diesel exhaust treatment fulfill the EU-6 exhaust emissions standard or the BIN5 LEVII exhaust emissions standard for the USA.