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

ENGINE 276.8 in MODEL 207, 212 as of model year 2014 with CODE 920 (Direct petrol injection with stratified charging) 

ENGINE 276.8 in MODEL 218 as of model year 2015 with CODE 920 (Direct petrol injection with stratified charging) 

Exhaust gas recirculation function requirements - general 

• Circuit 87M (engine management ON)
• Engine in partial-load range
• Vehicle in motion

Exhaust gas recirculation, general 

The exhaust gas recirculation lowers the combustion temperature during enrichment of the charge air with exhaust in stratified operation and in combined homogenous stratified operation, whereby the nitrogen oxide (NOx) development is reduced. In this way the NOx storage catalytic converter is loaded more slowly with NOx and stratified operation is enabled for longer periods.

The exhaust gas recirculation quantity can be up to 30% of the intake air and is calculated on the basis of the following sensors and signals by the ME-SFI [ME] control unit (N3/10):

• EGR pressure sensor 1 (B5/5), pressure upstream of the left exhaust gas recirculation valve (Y27/11)
• EGR pressure sensor 2 (B5/6), pressure upstream of the right exhaust gas recirculation valve (Y27/12)
• Charge air temperature sensor (B17/8), charge air temperature upstream of throttle valve
• Charge air temperature sensor downstream of throttle valve (B17/9), charge air temperature downstream of throttle valve
• Pressure sensor downstream of throttle valve (B28/7), engine load
• Left and right oxygen sensors upstream of catalytic converter (G3/3, G3/4), residual oxygen content
• Left and right oxygen sensors downstream of catalytic converter (G3/5, G3/6), residual oxygen content
• Left and right exhaust gas recirculation valve Hall sensors (Y27/11b1, Y27/12b1), position of the left and right exhaust gas recirculation valve rotary solenoids (Y27/11y1, Y27/12y1)

Function sequence for exhaust gas recirculation 

The following function sequence is described as an example based on the components of the EGR on the left side of the engine. The EGR on the right side of the engine behaves analogously.

The exhaust for the exhaust gas recirculation is taken out downstream of the turbocharger and led via an exhaust removal pipe to the exhaust gas recirculation valve (Y27/11). The exhaust gas recirculation rate is controlled by the opening cross-section of the exhaust gas recirculation valve. To do this the exhaust gas recirculation valve rotary solenoid that actuates the exhaust gas recirculation valve is actuated by the ME-SFI [ME] control unit depending on a characteristics map by means of a pulse width modulated signal (PWM signal). In order to keep the exhaust gas recirculation valve securely closed, the exhaust gas recirculation valve rotary solenoid is actuated with a duty cycle of about 15% and reverse polarity. In a deenergized condition, the exhaust gas recirculation valve is held closed by a spring closed by a spring in the rotary solenoids. The position of the exhaust gas recirculation valve rotary solenoids is recorded via the exhaust gas recirculation valve Hall sensor and transmitted to the ME-SFI [ME] control unit for control and diagnosis.

Downstream of the left and right exhaust gas recirculation valve the exhaust gases flow from both cylinder banks into the EGR line. From there the gases are led into the charge air distributor at a point shortly after the throttle valve, blend together with the fresh air and are led to the cylinders for combustion.

The quantity of recycled exhaust is determined on the base of the following values:

• The measured pressure in each exhaust gas recirculation valve
• Temperature in the charge air distributor upstream and downstream of the throttle valve
• The stored characteristics maps in the ME-SFI [ME] control unit for the CO ₂ portion in the combustion chamber

The respective exhaust gas recirculation valve, the exhaust gas recirculation valve rotary solenoid and the exhaust gas recirculation valve Hall sensor are combined in the exhaust gas recirculation housing. This is connected for cooling to the coolant circuit.

The left and right exhaust gas recirculation valves are closed in homogeneous mode.

Body 

Fig 1: Turbocharger Coolant Flow Diagram

Courtesy of MERCEDES-BENZ USA

Diagnosis of the EGR system 

A function test takes place for both exhaust gas recirculation valves shortly before the engine start and shortly after stopping the engine. To do this they are opened completely and then closed again. This test serves to reduce the tolerance and to increase the positioning accuracy.

A comparison of the positioning distance also takes place in decel mode of the engine for an opened exhaust gas recirculation valve. Here the functionality can also be checked.

If the ME-SFI [ME] control unit recognizes that a exhaust gas recirculation valve has not open or closes, a fault entry occurs. Furthermore in the case of non-closing of a exhaust gas recirculation valve, limp-home mode of the engine occurs with a reduction in power output.