Exhaust treatment function - GF14.00-P-3000OGD

ENGINE 642.8 in MODEL 207 as of 6/1/13 with CODE U77 (Diesel Emission control BLUETEC (SCR)) 

ENGINE 642.8 in MODEL 212 (except 212.025) as of 9/1/14, 218 as of 9/1/14 with CODE U77 (Diesel Emission control BLUETEC (SCR)) 

Function requirements for exhaust treatment, general points 

• Circuit 87M ON (Engine management ON)
• Engine runs

Exhaust treatment, general 

The task of exhaust treatment is to reduce the exhaust emissions:

• Nitrogen oxides (NOx)
• Hydrocarbons (HC)
• Carbon monoxide (CO)
• Soot particles

Pollutant reduction is supported by the following subfunctions:

• Intake port shutoff (EKAS)
• Diesel particulate filter (DPF) preheating
• Exhaust gas recirculation (EGR)
• Injection of the AdBlue® reduction agent

The CDI control unit (N3/9) reads in the signals from the following sensors:

• Temperature sensor upstream of SCR catalytic converter (B16/15)
• Temperature sensor upstream of catalytic converter (B19/7)
• Temperature sensor upstream of diesel particulate filter (B19/9)
• Temperature sensor upstream of turbocharger (B19/11)
• Temperature sensor downstream of diesel particulate filter (B19/14)
• DPF differential pressure sensor (B28/8)
• Oxygen sensor upstream of catalytic converter (G3/2)
• Nitrogen oxides control unit downstream of diesel particulate filter (N37/7) (model 212, 218) or NOx sensor control unit downstream of diesel particulate filter (N37/7) (model 207), NOx sensor control unit downstream of diesel particulate filter (N37/7b1) (model 218) or NOx sensor downstream of diesel particulate filter (N37/7b1) (model 207, 212) via the drive train sensor CAN (CAN I)
• Nitrogen oxides control unit downstream of SCR CAT (N37/8) (model 212, 218) or NOx sensor control unit downstream of SCR CAT (N37/8) (model 207), NOx sensor control unit downstream of SCR CAT (N37/8b1) (model 218) or NOx sensor downstream of SCR CAT (N37/8b1) (model 207, 212) via the drive train sensor CAN
• Front SAM control unit with fuse and relay module (N10/1), outside temperature via chassis CAN 1 (CAN E1)

Function sequence for exhaust treatment 

The following subsystems are involved in exhaust treatment:

• Function sequence for oxidation catalytic converter 
• Function sequence for diesel particulate filter (DPF) 
• Function sequence for SCR catalytic converter 
• Function sequence for intake port shutoff 

Function sequence for oxidation catalytic converter 

The oxidation catalytic converter lowers the hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) and generates the required thermal energy for the DPF regeneration phase through afterburning.

Function sequence for diesel particulate filter (DPF) 

The diesel particulate filter consists of a ceramic honeycomb filter body made out of silicon carbide, which is coated with rare metal platinum.

The passages of the diesel particulate filter are opened alternately at the front and rear and are separated from each other through the porous filter walls of the honeycomb filter body.

The precleaned exhaust which has passed though the oxidation catalytic converter flows into the ducts of the DPF which are open to the front and passes through the porous filter walls of the honeycomb filter body into the ducts which are open to the rear. After this, the cleaned and filtered exhaust is dissipated through the exhaust system. The soot particles are retained in the honeycomb filter body of the DPF.

If the soot content exceeds a characteristics map-based value, the CDI control unit will start the regeneration phase provided the prerequisites for regeneration are given. The CDI control unit receives the information on soot particle content in the DPF via the DPF differential pressure sensor ().

Regeneration takes place by means of a periodical increase of the exhaust temperature. For this purpose, the following functions are initiated by the CDI control unit:

• One additional post injection via the fuel injectors (Y76)
• DPF glow function over the drive train LIN (LIN C1) over the glow output stage (N14/3) to the glow plugs (R9)
• Shift of shift characteristic curve via the drive train CAN (CAN C) by the fully integrated transmission control unit (Y38) (with code 427 (7-speed A/T)) or (code 421 (9-speed automatic transmission))

The reduction in the soot present is about 99%

The soot particles retained in the DPF are mostly burnt off to produce carbon dioxide (CO₂ ) by increasing the exhaust temperature. The ash produced remains in the DPF. During regeneration, the exhaust temperature is monitored by the temperature sensor upstream of the exhaust gas turbocharger and by the temperature sensor upstream of the diesel particulate filter.

Through the exhaust pressure lines upstream and downstream of the DPF, the DPF differential pressure sensor determines the pressure differential between the exhaust gas pressure upstream and downstream of the DPF. The soot content in the DPF is determined using a characteristics map on the basis of the pressure differential and the exhaust mass calculated by the CDI control unit. Necessary service/maintenance of the DPF is indicated by the engine diagnosis indicator lamp (A1e58) in IC (A1).

An interrupted regeneration is distributed for short stretches over a number of driving cycles. This means that more heating-up phases up to the required regeneration temperature will occur. Regeneration occurs unnoticeably by the customer.

Function sequence for SCR catalytic converter 

The exhaust gases expelled from the engine are cleaned in an oxidation catalytic converter, a diesel particulate filter (DPF) and a reduction catalytic converter (Selective Catalytic Reduction) SCR catalytic converter.

Oxidation in the oxidation catalytic converter converts the CO and HC to CO₂ and water (H₂ O). The diesel particulate filter consists of a ceramic honeycomb filter body made out of silicon carbide, which is coated with rare metal platinum.

The passages of the diesel particulate filter are opened alternately at the front and rear and are separated from each other through the porous filter walls of the honeycomb filter body.

The precleaned exhaust which has passed though the oxidation catalytic converter flows into the ducts of the DPF which are open to the front and passes through the porous filter walls of the honeycomb filter body into the ducts which are open to the rear. The soot particles are retained in the honeycomb filter body of the DPF. During the DPF regeneration phase, the higher exhaust temperature burns off the retained soot particles.

The AdBlue® reduction agent is injected upstream of the SCR catalytic converter and is converted to ammonia (NH₃ ) through thermal decomposition (heat-induced chemical reaction) and hydrolysis (water-induced chemical reaction).

There is a mixing element between the AdBlue® metering valve (Y129) and the SCR catalytic converter. This improves the hydrolysis of the AdBlue® reduction agent and ensures more uniform distribution of the AdBlue® upstream of the SCR catalytic converter.

In the SCR catalytic converter, the NOx contained in the exhaust is converted together with the NH₃ to molecular nitrogen (N₂ ) and H₂ O.

The CDI control unit calculates the quantity of reduction agent required based on a characteristics map and sends it via drive train sensor CAN to the AdBlue® control unit (N118/5). This control unit then initiates map-based injection of the calculated quantity of AdBlue® reducing agent through the AdBlue® metering valve.

If the "Reserve" fill level in the AdBlue® container is reached, the driver is informed parallel to an acoustic signal over the multifunction display (A1p13) that he must find a workshop and have the required maintenance work done.

If the "empty" fill level is reached in the AdBlue® container, the plausibility of the "empty" fill level is checked using a computer model. If the plausibility check also results in an "empty" fill level, an acoustic signal is given in parallel, an entry is made in the fault memory of the control unit (CDI), and the engine diagnosis indicator lamp is lit on the instrument cluster. The driver then has up to 20 engine starts available, with an assumed trip distance of 32 kilometers in each case. The number of standing starts is displayed in the instrument cluster. The vehicle can no longer be started after the last remaining "Start".

Function sequence for intake port shutoff 

The intake port shutoff (EKAS) achieves the best possible relation between eddy and air mass in all load conditions of the engine.

The CDI control unit additionally reads the following sensors for intake port shutoff:

• Oil temperature sensor (B1)
• Atmospheric pressure sensor, for the atmospheric pressure
• Accelerator pedal sensor (B37), for atmospheric pressure
• Crankshaft Hall sensor (B70), for engine speed

After evaluating the input signals, the CDI control unit actuates the intake port shutoff actuator motor (M55) by means of a pulse width modulated (PWM) signal. In the lower engine speed and engine load range, half of the intake ports (2 intake ports per cylinder available) are closed by means of the intake port shutoff flaps.

In the open intake ports, the flow rate is thus increased. This leads to a higher swirl which creates a better vortex. This improves combustion and also contributes to reducing the soot particles in the exhaust gas.

As engine speed and load increases, the closed intake ports open continuously, so that the best possible relation between air eddying and air mass is provided for each operating phase of the engine. In this way, the exhaust characteristics and the engine performance are optimized.

If there is a fault or discontinuity in the supply voltage, the flaps are opened by spring force.