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).
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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.