Function requirements for exhaust treatment, general points
Circuit 87M (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)
Exhaust gas recirculation (EGR)
Injection of the reduction agent AdBlue® (with code U77 (BLUETEC (SCR) diesel exhaust treatment))
The CDI control unit (N3/9) reads in the following signals:
Temperature sensor upstream of SCR catalytic converter (B16/15) (with code U77 (BlueTEC (SCR) diesel exhaust treatment))
Temperature sensor upstream of catalytic converter (B19/7)
Temperature sensor upstream of diesel particulate filter (B19/9)
Temperature sensor upstream of ATL (B19/11)
DPF differential pressure sensor (B28/8)
Oxygen sensor upstream of catalytic converter (G3/2)
Front SAM control unit (N10/6), outside temperature via interior CAN (CAN B), electronic ignition lock control unit (N73), chassis FlexRay (Flex E), powertrain control unit (N127) and drive train CAN (CAN C1)
NOx sensor control unit downstream of diesel particulate filter (N37/7), signal for NOx sensor downstream of diesel particulate filter (N37/7b1) via drive train sensor CAN (CAN I) (with code U77 (BlueTEC (SCR) diesel exhaust treatment))
NOx sensor control unit downstream of SCR catalytic converter (N37/8), signal for NOx sensor downstream of SCR catalytic converter (N37/8b1) via drive train sensor CAN (with code U77 (BlueTEC (SCR) diesel exhaust treatment))
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 (with code U77 (BLUETEC (SCR) diesel exhaust treatment))
Function sequence for intake port shutoff
Function sequence for oxidation catalytic converter
The oxidation catalytic converter reduces the amount of hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) and, on vehicles with diesel particulate filter, generates the required thermal energy for the DPF regeneration phase by means of 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 soot 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 via the drive train LIN (LIN C1) and the glow output stage (N14/3) to the glow plugs (R9)
Shift line shifting via the drive train CAN (CAN ) by the fully integrated transmission control unit (C1Y3/8n4) (with code 427 (7G-TRONIC) or (with code 421 (9G-TRONIC)
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 ATL 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 particle 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. When DPF maintenance is required, this is signaled via the drive train CAN, the powertrain control unit, the chassis FlexRay, the electronic ignition lock control unit and the user interface CAN (CAN HMI) by the engine diagnosis indicator lamp symbol on the instrument cluster (A1).
On short-distance trips, regeneration is interrupted and distributed over several 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 (with code U77 (BLUETEC (SCR) diesel exhaust treatment))
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 (H2O). The diesel particulate filter consists of a ceramic honeycomb filter body made out of silicon carbide, which is coated with rare metal platinum.
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 in the SCR catalytic converter together with the NH3 into molecular nitrogen (N2) and H2O.
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.
The conversion rate of the NOx-portion in the exhaust is dependent on the temperature and can be up 80%. The reduction in the soot present is about 99%.
If the "Reserve" fill level in the AdBlue® tank is reached, the driver is informed by means of an acoustic signal and at the same time over the instrument cluster that he/she should find a workshop and have the required maintenance work carried out.
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 while at the same time an entry is made in the fault memory of the CDI control unit and the engine diagnosis indicator lamp symbol is illuminated 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.
For intake port shutoff the CDI control unit also reads in the following signals:
Engine oil temperature sensor (B1)
Atmospheric pressure sensor (integrated in the CDI control unit), for the atmospheric pressure
Accelerator pedal sensor (B37), for load detection
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.
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