How the humidity of a DPF effects the microwave based soot load determination

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Feulner, Markus ; Stöber, Ralf ; Fischerauer, Gerhard ; Moos, Ralf:
How the humidity of a DPF effects the microwave based soot load determination.
2014
Veranstaltung: 18th ETH Conference on Combustion Generated Nanoparticles , 22.- 25.06.2014 , Zürich, Switzerland.
(Veranstaltungsbeitrag: Kongress/Konferenz/Symposium/Tagung , Sonstige Präsentationstyp)

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Abstract

In today’s exhaust gas after-treatment systems, diesel particulate filters (DPF) are an essen-tial element to meet the stringent limits concerning emissions of soot-particles. To know the actual trapped soot mass is important for an efficient regeneration and engine control strate-gy. One novel approach for measuring the soot inside DPF is a contactless microwave-based method. With stub antennas, electro-magnetic waves are emitted inside the canning, and their reflection and transmission through the filter monolith is measured. Different measurands can be extracted of the reflection- and transmission spectra over frequency, like resonance frequencies, fres, or an averaged transmission factor. Accumulating soot changes the electric properties of the filter and therefore the resonance-spectra also change. In prin-ciple, the basic functionality of this system for soot load detection has been proven in labora-tory and dynamometer tests. In this study, influencing parameters, like the humidity of the filter and ambient atmosphere shall be examined.

An uncoated aluminum titanate wall flow filter, 6” x 5.66”, was soot loaded on a dynamometer test bench (3l TDI engine) under constant speed and load. Soot load was measured gravi-metrically and the microwave signals were recorded continuously. Afterwards the DPF was exposed to air of different temperature and relative humidity, r.h., (15-80 °C / 15-80 %) in a climatic chamber, while the microwave spectra were measured. This climatic exposure test has been conducted with the soot-loaded as well as a soot-free DPF and the empty canning.

During soot loading, resonance frequency (TE112-Mode) and the averaged transmission parameter shifted linearly towards lower values, as expected from literature and own meas-urements. As can be seen, exemplarily for a soot-free DPF, both, increasing temperature and humidity lead to decreasing microwave signals due to a higher conductivity. Thereby, the higher the temperature is, the larger is the influence of humidity. It has to be emphasized, that signal shifts are quite small compared to the shift during soot loading.

Relative signal shifts are shown for a soot accumulation of 1 gsoot/lDPF and an increase of humidity from 15 to 50 % at 80 °C. In case of a soot-free DPF or empty canning there is almost no influence of humidity, but for a soot-loaded DPF, ambient conditions need to be considered in real-world applications. However, the averaged transmission factor is by far less affected by noise factor than the reso-nance frequency.