Exhaust gas recirculation, function - GF14.20-P-3000OG

ENGINE 642.8 in MODEL 204, 212, 218 

ENGINE 642.8 in MODEL 207 

Function requirements for exhaust gas recirculation (EGR), general points 

• Circuit 87M ON (Engine management ON)
• Engine runs
• 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 property of the substance and specifies how much thermal energy is needed to raise the temperature of a substance by 1 °C.

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 pressure
• Time limitation

To do this, the CDI control unit (N3/9) reads in 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, 207, 218)
• Right hot film mass air flow sensor (B2/7)
• Right intake air temperature sensor (B2/7b1) (model 212), right intake air temperature sensor (model 204, 207, 218))
• Boost pressure sensor (B5/1)
• Charge air temperature sensor (B17/8)
• Temperature sensor upstream of SCR catalytic converter (B19/7) (with code U42 (BlueTEC (SCR) diesel exhaust treatment)) (model 204, 212 to 31. 08. 2014, 218 to 31. 08. 2014), (with code U77 (BlueTEC (SCR) diesel exhaust treatment)) (model 207, 212 as of 1. 09. 2014, 218 as of 1. 09. 2014)
• Temperature sensor upstream of diesel particulate filter (B19/9) (for code (474) Particulate filter)
• Temperature sensor upstream of turbocharger (B19/11)
• Temperature sensor downstream of diesel particulate filter (B19/14) (for code (474) Particulate filter)
• Pressure sensor downstream of air filter (B28/5)
• DPF differential pressure sensor (B28/8) (for code (474) Particulate filter)
• Accelerator pedal sensor (B37)
• Actual value potentiometer 1 throttle valve (M16/6r1)
• 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 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.