Overview of SCR Control
Overview Of SCR ControlHeater Control
When the ignition switch is turned on, the DEF control module determines whether the heater control is necessary or not. The heater control is the defrosting control or thermal control which is performed by the DEF control module if the signals from the DEF tank temperature sensor, DEF supply pump temperature sensor and outside air temperature sensor (MAF sensor) are at or below the specified values when the ignition switch is turned on.
Defrosting Control
The DEF control module determines that the DEF needs to be defrosted if any signal from the DEF tank temperature sensor, DEF supply pump temperature sensor or outside air temperature sensor (MAF sensor) is at or below the specified value, and opens the DEF heater valve to start circulation of the engine coolant into the DEF tank and DEF supply module. When the signals from the DEF tank temperature sensor, DEF supply pump temperature sensor and outside air temperature sensor (MAF sensor) all exceed the specified values, the DEF control module closes the DEF heater valve, and terminates the defrosting control. After the defrosting control is terminated, if the signal from the DEF tank temperature sensor or DEF supply pump temperature sensor falls down to or below the specified value, the defrosting control is started again.
Thermal Control
After a certain period of time has passed since the defrosting control was started, if only the signal from outside air temperature sensor (MAF sensor) is at or below the specified value, the DEF control module moves on to the thermal control, keeping the DEF heater valve open.
Startup Control
After the ignition switch is turned on, if the SCR catalyst temperature is at or above the specified value and the engine is in running status when the defrosting control under the heater control is completed (including the case that the defrosting control is determined as unnecessary by the DEF control module), the DEF control module activates the DEF supply module and the DEF injector to fill the DEF in the DEF piping between the DEF supply module and DEF injector and increase the DEF pressure up to 130 psi (900 kPa). After the pressure has been increased, the pressure is maintained at 130 psi (900 kPa) using the DEF pressure sensor built into the DEF supply module.
DEF Filling
DEF Pressurization
NOx Sensor Control
Immediately after the engine has started, moisture inside the exhaust pipe must be removed to protect the NOx sensors. Therefore, the DEF control module calculates the amount of heat required to evaporate moisture, based on the signal from the MAF sensor. After that, the DEF control module calculates the actual generated heat amount based on the exhaust gas temperature and exhaust gas flow amount, and when the generated heat amount exceeds the estimated heat amount as necessary for moisture evaporation, the dosing control by the NOx sensors is started.
Dosing Control
The DEF control module calculates the optimal DEF injection amount, and provides it to the DEF injector. The DEF injection amount is calculated based on either the information from the ECM or the measurement values on the NOx sensors, depending on the vehicle conditions.
Dosing Control through Calculation
The feedbacks from the NOx sensors are not available until the NOx sensor control is completed, so the DEF control module calculates and determines the DEF injection amount based on the ECM information (engine RPM, fuel injection amount, engine coolant temperature, exhaust gas flow amount).
Dosing Control based on NOx Sensor Information
After the DEF injection amount is calculated by the DEF control module based on the measurement values on the NOx sensors, the DEF injection amount is adjusted referring to the information such as the temperature before the SCR catalyst and ECM information to finalize the DEF injection amount.
NH3 Storage Control
The DEF control module finalizes the DEF injection amount by taking the actual amount of NH3 stored in the SCR catalyst into consideration against the self-calculated DEF injection amount (dosing control through calculation, dosing control based on NOx sensor information). Therefore, the DEF injection amount may not increase or decrease in proportion to the NOx concentration.
DPF Cooperative Control
When the storage amount of NH3 in the SCR catalyst is at or above the specified value, the DEF control module performs the DPF automatic regeneration start delay control and DEF injection stop control against the ECM to prevent ammonia slip phenomenon which occurs during the DPF automatic regeneration. In addition, the DPF cooperative control is not performed during the DPF manual regeneration, because protection of DPF is prioritized.
*Ammonia Slip: A phenomenon which induces ammonia odor around the vehicle due to ammonia that is emitted from the exhaust pipe without being purified through the reaction in the SCR catalyst while the temperature is high.
DPF Automatic Regeneration Start Delay Control
When the DPF automatic regeneration request is called in the ECM, the DEF control module calculates the NH3 storage amount in the SCR catalyst, based on the cumulative DEF injection amount (NH3 amount) and NOx senor information, and transmits the DPF automatic regeneration permission or prohibition signal to the ECM.
When DPF Automatic Regeneration is Prohibited
When the NH3 storage amount in the SCR catalyst is at or above the specified value, the DEF control module stops the DEF injection, and when the NH3 storage amount in the SCR catalyst falls down to or below the specified value, the DEF control module sends the DPF automatic regeneration permission signal to the ECM.
When DPF Automatic Regeneration is Permitted
When the NH3 storage amount in the SCR catalyst is at or below the specified value, the DEF control module sends the DPF automatic regeneration permission signal to the ECM.
After-Run Control
The DEF control module stops giving instructions to the DEF supply module to pressurize the DEF injector as the engine is stopped. Then, the DEF control module switches the reverting valve built into the DEF supply module to ON to return the DEF that is filled in the DEF injector, DEF supply module and the piping between those two back to the DEF tank for preventing the DEF from freezing while the engine is stopped. After the engine has been stopped, operating sounds are heard from the DEF tank and DEF supply module, but the operating sounds stop when the DEF is all returned to the DEF tank (approx. 2 minutes). Finally, the DEF control module switches the reverting valve to OFF, and shuts down.
