The Advantages of Cavity Ring-Down Spectroscopy in the Analysis of Diesel Engine Emissions

Kathleen  Hartnett, Picarro, Inc.

With the drive toward reduced levels of NOx emissions from diesel vehicles, the need to measure non-traditional combustion gases such as hydrogen sulfide, ammonia and formaldehyde has emerged. To meet the emissions regulations for 2007 and 2010, development of exhaust gas after-treatment systems and advanced combustion strategies requires a better understanding of the combustion and emission reduction processes. The ability to monitor these non-traditional gas species is critical to the optimization of these processes and difficult for established technology to measure in real-time and without interference.

Cavity Ring-Down Spectroscopy (CRDS) is a laser-based, all-optical technique that utilizes a high finesse optical cavity to measure gas species at the parts-per-billion level. When coupled with a high precision wavelength monitor, the CRDS analyzer has a spectral resolution 1000 times better than a Fourier Transform Infrared Spectrometer (FTIR) thus allowing CRDS to distinguish individual absorption features and greatly reducing instrument sensitivity to contaminant gas species. For most applications, the CRDS technology enables analyzer operation for long periods of time without the need for human intervention and calibration gases. The capabilities of this new measurement method are highlighted.

The ability to measure hydrogen sulfide, without interference, at 1 Hz with a 50 ppbv lower detection limit enables the optimization of Lean NOx Traps. Measurement of ammonia at the ppb level permits dilute exhaust gas measurements from a vehicle with an emission reduction approach based on Selective Catalytic Reduction. To support development of advanced combustion strategies such as low temperature modes (homogeneous charge compression ignition or HCCI) the ability to measure formaldehyde at 1 Hz provides needed insight into combustion dynamics.

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