hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO x ) and particulate matter (PM) or number (PN) [1,2]. To meet upcoming European regulation standards for diesel engines and comparable regulations worldwide, like US Environmental Protection Agency's Tier 3 [3] for example, an improved exhaust aftertreatment approach is necessary. The combination of diesel oxidation catalysts (DOC), diesel particulate filters (DPF) and selective catalytic reduction (SCR) systems is a state of the art and promising exhaust gas treatment concept already in use on heavy-duty vehicles as well as passenger cars. The SCR technology uses ammonia (NH 3 ) as the reactant with NO x directly in the exhaust. Due to the risks and restrictions when handling gaseous ammonia, a non-toxic aqueous urea solution consisting of 32.5 % urea (AUS32/AdBlue) [4] is commonly used to provide a liquid pre-stage of ammonia.A modern diesel exhaust system is quite complex and equipped with several sensors for air-fuel ratio λ, NO x , temperature and pressure drop. For SCR, AUS32 is injected downstream of the DOC and DPF, forming H 2 O, CO 2 and NH 3 . The latter reduces the NO x to water vapor and molecular nitrogen in a subsequent catalyst [5]. With urea-SCR and an optimal NO/NO 2 ratio of 1, NO x conversion rates up to 100 % can be achieved [6].The drawbacks of a SCR system are evidently the increased complexity, higher weight and the need to refill an extra operating fluid. However, unlike the concurrent denox technology lean NO x trap (LNT), the SCR technology does not impede engine optimization concerning performance and efficiency. Thus, up to 10 % lower fuel consumption compared to vehicles using LNT can be achieved and even operational costs decrease [7], which is why SCR is the only exhaust aftertreatment system used by European heavy-duty vehicle manufacturers and also applied in midsize and bigger passenger cars [5].Abstract A new developed tunable diode laser spectrometer for the measurement of ammonia (NH 3 ) mole fractions in exhaust gas matrices with strong CO 2 and H 2 O background at temperatures up to 800 K is presented. In situ diagnostics in harsh exhaust environments during SCR after treatment are enabled by the use of ammonia transitions in the ν 2 + ν 3 near-infrared band around 2300 nm. Therefore, three lines have been selected, coinciding near 2200.5 nm (4544.5 cm −1 ) with rather weak temperature dependency and minimal interference with CO 2 and H 2 O. A fiber-coupled 2.2-μm distributed feedback laser diode was used and attached to the hot gas flow utilizing adjustable gas tight high-temperature fiber ports. The spectrometer spans four coplanar optical channels across the measurement plane and simultaneously detects the direct absorption signal via a fiber-coupled detector unit. An exhaust simulation test rig was used to characterize the spectrometer's performance in ammonia-doped hot gas environments. We achieved a temporal resolution of 13 Hz and temperature-dependent precisions of NH 3 mole fraction ranging from 50 to 70 ...
A multichannel tunable diode laser absorption spectrometer is used to measure absolute ammonia concentrations and their distributions in exhaust gas applications with intense CO2 and H2O 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 CO2/H2O 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 ppmV with a temporal resolution of 1 Hz whereas, while operated as a single-channel sensor, these characteristics improved to 10 ppmV 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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