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Opt Express. 2017 Apr 3;25(7):8180-8191. doi: 10.1364/OE.25.008180.

Ammonia concentration distribution measurements in the exhaust of a heavy duty diesel engine based on limited data absorption tomography.


A multichannel tunable diode laser absorption spectrometer is used to measure absolute ammonia concentrations and their distributions in exhaust gas applications with intense CO<sub>2</sub> and H<sub>2</sub>O background. Designed for in situ diagnostics in SCR after treatment systems with temperatures up to 800 K, the system employs a fiber coupled near-infrared distributed feedback diode laser. With the laser split into eight coplanar beams crossing the exhaust pipe, the sensor provides eight concentration measurements simultaneously. Three ammonia ro-vibrational transitions coinciding near 2200.5 nm with rather weak temperature dependency and negligible CO<sub>2</sub>/H<sub>2</sub>O interference were probed during the measurements. The line-of-sight averaged channel concentrations are transformed into 2-D ammonia distributions using limited data IR species tomography based on Tikhonov regularization. This spectrometer was successfully applied in the exhaust system of a 340 kW heavy duty diesel engine operated without oxidation catalyst or particulate filter. In this harsh environment the multi-channel sensor achieved single path ammonia detection limits of 25 to 80 ppm<sub>V</sub> with a temporal resolution of 1 Hz whereas, while operated as a single-channel sensor, these characteristics improved to 10 ppm<sub>V</sub> and 100 Hz. Spatial averaging of the reconstructed 2-D ammonia distributions shows good agreement to cross-sectional extractive measurements. In contrast to extractive methods more information about spatial inhomogeneities and transient operating conditions can be derived from the new spectrometer.


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