Diesel NOx sensor technology

Jan. 1, 2020
  Now that SCR has been embraced by all sectors of the diesel transportation industry, it is that much more important for automotive service professionals to understand how SCR works.

It’s taken some time, but the diesel world appears to have reached consensus on how to meet the Environmental Protection Agency’s (EPA) 2010 NOx standards. EPA 2010 was a major milestone in diesel emission control, calling for a 90 percent reduction in nitrogen oxide (NOx) emissions over the 2007 standards. The magnitude of the reductions required major changes in diesel engine design, and three engineering approaches arose as possible solutions: cooled exhaust gas recirculation (EGR), NOx adsorbers (sometimes referred to as lean NOx traps), and urea selective catalytic reduction (urea SCR).

Like this article? Sign up for our enews blasts here.

Chrysler jumped into the fray early, using both cooled EGR and a NOx adsorber on its 2007.5 and newer Cummins-powered pickups. This approach, while expensive, got its trucks EPA 2010-certified three years early and proved to the world that it could be done. Ford and GM both hung back during this time and used cooled EGR and diesel particulate filters (DPFs) to meet the EPA 2007 standards. However, everyone knew that whatever Ford and GM were doing with their diesel pickups at that time wasn’t going to make it once 2010 came along.

Urea SCR (also known simply as SCR) was considered to be an effective means of achieving the necessary NOx reductions. The advantages of SCR were significant, including the potential for greater horsepower and increased fuel economy relative to an engine that relied on EGR alone. The EPA was skeptical, however, because SCR required the driver to purchase and install diesel exhaust fluid (DEF) to make the system work. Beyond that, the vehicle wouldn’t run any different if the driver let the DEF tank go dry. Clearly, DEF had to be freely available and there would have to be controls built into the vehicle that would limit its operation if the DEF ran out, or was of poor quality.

Most of the issues regarding DEF have now been worked out, and as of the 2013 model year Chrysler, GM and Ford are all using SCR in their diesel-powered pickups. Beyond that, the holdouts in the heavy-duty truck world have also converted to SCR. Now that SCR has been embraced by all sectors of the diesel transportation industry, it is that much more important for automotive service professionals to understand how SCR works and how it is monitored by vehicle onboard diagnostics.

Urea SCR Operation The heart of the urea SCR system is the catalyst itself, which is based on either an iron or copper zeolite material. As mentioned earlier, DEF is injected into the exhaust stream ahead of the SCR catalyst. DEF (known as AdBlue in Europe) is a mixture of automotive grade urea and deionized
water, which decomposes into ammonia and carbon dioxide when exposed to heat from the exhaust gases. Ammonia is the reductant in the SCR reaction, and it enters the SCR catalyst along with the NOx molecules entrained in the exhaust gases. The term “selective” in selective catalytic reduction means that the ammonia prefers to react with the oxygen in the NOx molecules instead of the oxygen in the exhaust stream. The SCR reaction with ammonia reduces the NOx molecules into molecular nitrogen (N2) and water, effectively returning the gases back to their original form when they entered the engine intake manifold.

The SCR process works very well, provided the catalyst is at the right temperature (570 to 750o F) and the correct amount of DEF is used. If too little DEF is injected, conversion efficiency drops and NOx emissions increase. Conversely, too much DEF will result in a phenomenon known as ammonia slip, where unprocessed ammonia exits the SCR catalyst. Thus, the SCR system will operate efficiently only if accurate measurements can be made of the exhaust temperature and the NOx in the exhaust gases.

NOx Sensor Operation NOx sensors have a great deal in common with wideband oxygen sensors, but typically have their own module that communicates with the vehicle ECM over a CAN bus. In most applications there will be two NOx sensors used; one at the turbocharger exhaust outlet and another downstream from the SCR catalyst. These two signals are then compared to determine conversion efficiency of the catalyst. It is also possible to have a single NOx sensor at the outlet of the SCR catalyst and utilize an ECM strategy to calculate engine-out NOx (used in the 6.7 liter Ford Power Stroke).

Like oxygen sensors, NOx sensors will not work unless they are at the correct temperature, so they have an integrated heater that is also module-controlled. The power supply to the NOx sensor module is critical to proper sensor operation. In the case of the 2011 Duramax LML (VIN 8) engine, the glow plug control module (GPCM) supplies constant voltage to both of the NOx sensor modules. If system voltage is low, an internal voltage boosting circuit in the GPCM will make up the difference to ensure proper NOx sensor operation.

During a cold start, moisture in the exhaust system can interfere with NOx sensor operation. In this scenario, the Duramax ECM will not turn on the NOx sensor heaters until exhaust temperatures reach a certain threshold and condensed moisture is evaporated. This can result in a delay of up to 5 minutes before both NOx sensors are fully operational.

Diesel NOx sensors are dual-purpose, in that they are used to measure both the oxygen level in the exhaust as well as NOx (NO and NO2) content. The measurements take place in two separate chambers in the sensor, and the exhaust gases flow sequentially through one and then into the other. Gases flow from the exhaust stream through a diffusion barrier to reach the first chamber, which “pumps” the free oxygen out using a Nernst cell (simple oxygen sensor). The electric current for operating this first Nernst cell is used to measure exhaust oxygen content (lambda). In the LML Duramax, exhaust oxygen content information is provided to the ECM by NOx sensor 2 to aid in diesel particulate filter (DPF) regeneration.

With the free oxygen removed, the NOx is then left to migrate through another diffusion barrier and into the second measurement chamber. At this point, the NOx molecules encounter a catalytic element, which breaks them into nitrogen and oxygen gases. A second Nernst cell is then used to pump the newly-generated oxygen out of the chamber, and this electric current is used to calculate the NOx levels in the exhaust. The residual nitrogen gases then flow out an exit port in the second measurement chamber.

OBDII Monitors When using two NOx sensors in a urea SCR system, the upstream sensor is used to estimate what amount of DEF needs to be injected ahead of the SCR catalyst to achieve optimal NOx conversion. The downstream (post-catalyst) sensor is then used to check the results. Under ideal conditions, NOx conversion will be high and very little ammonia slip will occur.

An adaptive strategy similar to long-term fuel trim (LTFT) is used to determine the duty cycle that the DEF injector should be operated at. Since there are numerous variables (“drift” in component operation) that can affect the amount of DEF that is injected for a given duty cycle, the table is always being rewritten to maximize NOx conversion efficiency. These calculations are made based on signals from the NOx sensors, similar to the way that oxygen sensors are used in a gasoline-engine emission control system. If the correction factor (trim) of the DEF quantity changes too far in either the positive or negative direction, the ECM will set DTCs and turn on the check engine light (MIL).

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.

Ammonia Sensing 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.

Final Thoughts
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.

Subscribe to Motor Age and receive articles like this every month…absolutely free. Click here