Diesel Exhaust Fluid (DEF) System
The DEF system consists of the following components located at the reductant reservoir:
- An electrically-operated reductant pump
- A reductant temperature sensor
- A reductant pressure sensor
- An integrated reductant level, quality, and temperature sensor
- Reductant system heaters
- Reductant control module
The remaining reductant system component, an electrically-controlled reductant injector, is external to the reservoir.
The on-board reservoir holds approximately 20 liters (5 gallons) of reductant. A pump within the reservoir supplies pressurized reductant to the reductant injector located upstream of the SCR. A reductant level sensor within the reductant reservoir provides the reductant control module a signal indicating reductant level. The reductant pressure sensor provides the reductant control module with a voltage signal proportional to the reductant pressure generated by the reductant pump. The reductant control module varies the duty-cycle of the pump voltage to maintain reductant pressure within a calibrated range.
When the ignition is turned Off, the reductant pump is run in reverse for about 45 seconds in order to purge the supply line of reductant. There is a one minute delay between ignition off and the start of purge to allow the exhaust system to cool in order to prevent hot exhaust gas from being drawn into the reductant line. The ECM also commands the reductant injector open during the purge process. Purging prevents the reductant from freezing in the pump or supply line to the reductant injector.
The ECM energizes the reductant injector to dispense a precise amount of reductant upstream of the SCR in response to changes in exhaust NOx levels. Feedback from NOx sensors 1, 2, and 3 allow the ECM to accurately control the amount of reductant supplied to the SCR. If more reductant is supplied to the SCR than is needed for a given NOx level, the excess reductant results in what is called ammonia slip where significant levels of ammonia exit the SCR. Since the NOx sensors are unable to differentiate between NOx and ammonia, ammonia slip will cause NOx sensor 2 to detect higher NOx levels than actually exist.
Cold Weather Operation
As reductant will freeze at temperatures below 0°C (32°F), there are 3 reductant heaters. Reductant heater 1 and 3 are in the reductant reservoir and reductant heater 2 is in the supply line to the reductant injector. The reductant control module monitors the reductant temperature sensor located within the reservoir in order to determine if reductant temperature is below its freeze point. If the module determines that the reductant may be frozen, it energizes the reductant heaters.
Reductant pump operation is disabled for a calibrated amount of time to allow the heaters time to thaw the frozen reductant. Once the thaw period expires, the module energizes the reductant pump to circulate warm reductant back to the reservoir to speed thawing. The ECM looks for an increase in the reductant temperature to verify that the reductant reservoir heater is working.
Reductant Sensor Module
The Quality Sensor, Reductant Level Sensor, and Reductant Temperature Sensor 2 are integrated into one module. Fluid level, quality, and temperature are communicated to the reductant control module using serial data.