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The application of Selective Catalytic Reduction (SCR) to control nitric oxides (NOx) in diesel engines (2010, Tier 2, Bin5) introduced significant amounts of Ammonia (NH3) and Urea to the NOx exhaust gas analyzers and sampling systems. Under some test conditions, reactions in the sampling system precipitate a white powder, which can accumulate to block sample lines, rendering the exhaust emission sampling inoperable. NOx gas analyzers used for exhaust measurement are also susceptible to precipitation within the sample path and detector components. The contamination requires immediate maintenance for powder removal to restore baseline performance. The results of experiments to eliminate the powder are presented. Analysis of the powder identifies it as ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4), which is consistent with the white crystalline precipitate.

A 2.5-liter light-duty diesel van certified to Euro 4 emission standards was tested in a chassis dynamometer test cell, which included a modal FTIR exhaust gas analyzer with the capability of measuring 22 separate gas species. The engine was equipped with a cooled Exhaust Gas Recirculation (EGR) system, which controls the nitrogen oxide emissions (NOx) to less than the 390 mg/km limit required by Euro 4 regulations. The vehicle was tested by dynamometer with the New European Drive Cycle (NEDC) sequence, and found to exceed the 390 mg/km NOx limit. The FTIR was applied as a diagnostic tool for the engine EGR function. The FTIR monitored N₂O, NO, NO₂, and NH₃ over the NEDC test cycles. The linear-control EGR valve failed abruptly during a subsequent test, and the relative concentration of the reduced and oxidized nitrogen species showed significant changes.