Referring to FIG. 1, an exemplary schematic diagram illustrates an inline six cylinder diesel engine 10 with exhaust gas recirculation (EGR) that is utilized to reduce NOx formation. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture. Because NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature and NOx formation progresses much faster at high temperatures, EGR reduces the amount of NOx the combustion generates. Nevertheless, NOx formation is an inherent part of combustion.
NOx formation has been known to have significant detrimental consequences on our environment. These consequences include acid rain, smog, and creation of harmful particulate matter. In order to combat these consequences, diesel engine manufacturers have implemented technologies to reduce NOx from diesel fuel combustion. One of these technologies involves downstream treatment of the diesel exhaust through selective catalytic reduction (SCR).
SCR uses a urea based diesel exhaust fluid (DEF) and a catalytic converter to significantly reduce oxides of nitrogen (NOx) emissions. Small quantities of diesel exhaust fluid (DEF) are injected into the exhaust upstream of a catalyst, where it vaporizes and decomposes to form ammonia and carbon dioxide. Ammonia (NH3) is the desired product, which in conjunction with the SCR catalyst, converts the NOx to harmless nitrogen (N2) and water (H2O).
FIG. 1 shows a common SCR implementation. In this circumstance, exhaust from the diesel engine 10 is directed through a particulate filter 12 and into a decomposition tube 14. In the decomposition tube 14, DEF (a blended aqueous urea solution and deionized water) from a supply tank 16 is injected into communication with the exhaust, which results in the urea decomposing to form ammonia prior to introduction to a catalytic converter 18. Within the catalytic converter 18, the ammonia and NOx components of the exhaust react in the presence of the catalyst to generate water, nitrogen gas, and oxygen gas. The water, nitrogen gas, and oxygen gas is thereafter conveyed to the open atmosphere via an exhaust stack or tailpipe 20.
As part of a common SCR implementation, decomposition tubes are formed by rolling metal sheets to form a circular cross-section tube. Thereafter, the tube is modified to include an injector fitting, commonly by welding the injector fitting over a preexisting opening in the rolled tube. Likewise, to the extent a mixer is provided with the decomposition tube, an entirely separate mixing structure is positioned within the tube and welded to the interior of the tube. The additional steps of mounting the injector fitting and mixing structure within the rolled tube add considerable cost and time to the overall fabrication process for producing a decomposition tube. At the same time, the welds may sometimes impart localized areas of weakness to the rolled tube that may eventually give way and create unintended orifices within the tube. Moreover, the seams between the welds and the decomposition tube and mixer are subject to attack by urea that flows through the decomposition tube, causing separation between the welds and the decomposition tube and mixer. This separation leads to attachment failure between the mixer and the decomposition tube.