The condition of the SCR catalyst itself is monitored using signals from the NOx sensors. Over time, aging of the SCR catalyst will result in increased NOx emissions. The SCR efficiency monitor takes place over longer periods of time, watching for signs of progressive degradation in SCR catalyst performance. When the catalyst can no longer pass the tests, the ECM will set DTCs and turn on the MIL.
The SCR system cannot work correctly without clean, fresh DEF. The ECM can determine the quality of the DEF based on signals from the NOx sensors and the DEF tank level sensor. Whenever the vehicle DEF tank is filled, the ECM looks at how much of the new DEF mixture is required to minimize NOx output and compares it to the previous injection rate under similar conditions. If there is little difference between the new injection rate and where it was before the tank was filled, the normal adaptive strategy continues. However, if significantly more DEF is required, the ECM may conclude that the DEF is of poor quality (diluted or contaminated). A series of driver warnings are set into motion, which can eventually result in engine derating if the warnings are ignored.
A DEF quality warning will require a flushing of the DEF system and replacement with fresh reductant at a minimum. If the DEF is contaminated with diesel fuel or another chemical, the entire DEF system may have to be replaced.
One limitation in current NOx sensor technology is the sensor’s inability to distinguish between ammonia (NH3) and NOx. This has an impact on the DEF dosing strategy, because there is a point where continuing to increase the amount of injected DEF will cause the signal from the NOx sensor to bottom out and then start to rise as ammonia slip commences. Fortunately, technology will soon be available that directly measures ammonia levels in exhaust gases.
An ammonia sensor is under development by Delphi and is targeted for production in 2013. The ammonia sensor is not intended to replace the NOx sensor, but to work alongside it downstream from the SCR catalyst.
According to J.D. Ward, Delphi’s chief engineer, exhaust sensors and air meters, “The NH3 sensor is complimentary to a NOx sensor. Directly measuring ammonia allows for more precise urea dosing control. Therefore the engine can run leaner and achieve better fuel economy.”
While the ammonia sensor has similar materials and components to a wideband oxygen sensor (such as an integrated heater in the sensing element), the sensing cells are very different. The Delphi ammonia sensor is constructed with a pair of electrodes, one of which is made of a proprietary material that is sensitive to ammonia. The sensor is packaged with its own module, which communicates with the vehicle ECM over the CAN bus. When ammonia slip occurs, a voltage proportional to the amount of ammonia in the exhaust gas is generated across the electrodes. This signal (along with the NOx sensor signal) is used by the ECM to adjust DEF dosing for maximum SCR catalyst efficiency.
Benefits of the Delphi ammonia sensor include optimizing performance of the SCR catalyst, enabling the use of an optimally-sized SCR catalyst, and helping to provide SCR system diagnostics.
While selective catalytic reduction is now a mainstream technology, there is always room for improvement. As time goes on, better sensor technology will increase SCR system performance and make it more cost effective. This, in turn, will prolong the diesel engine’s position as the workhorse of the transportation industry.
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