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 ...

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