Microwave sensing for determination of loading of filters

#This invention relates to determination of filter loading and more particularly to the use of microwave sensing to determine filter loading.

#In many realms there is a need for accurate sensing of the amount of material that has been captured by a filter. An example is the need to determine filter loading of soot on a diesel particulate filter (DPF). The amount of loading on a diesel particulate filter must be known in order to determine appropriate conditions for start-up of regeneration as well as monitoring conditions to determine when complete regeneration has been achieved. The level of loading is important in this context because regeneration of a DPF is often through an uncontrolled burn in which soot is ignited by the presence of free oxygen and a combustion wave is generated through the filter. Under certain conditions, it is possible that regeneration will produce temperatures that are very high resulting in large thermal stresses that can result in limited fatigue life of the filter and ultimately its destruction. Thus, the level of soot loading is important for successful filter regeneration.

#It is well known that there are several methods for determining the loading of a filter. The most commonly investigated method is by measurement of the pressure drop across the filter. This method can be combined with expert systems that calculate the amount of soot that has been captured through a cumulative calculation of soot production through an engine.

#FIG. 1 is a perspective view of a canned diesel particulate filter according to an embodiment of the invention.

#FIG. 2 is a cross-sectional view of another embodiment of the invention.

#FIG. 3 is a graph of experimentally determined S21 transmission as a function of frequency (magnitude of S21 is shown).

#The present invention is based on the recognition that microwaves can be used to determine the status of loading of traps or filters. The loading may be soot, particulates, ash or any solid/liquid. In addition to determining the total amount of loading, the microwave system to be described herein is useful in determining the distribution of the loading throughout the trap. The microwave sensing used in this invention can be inexpensive as inexpensive oscillators and detectors in the frequency range of interest are commercially available.

#In the case of a diesel particulate filter, the particulates are made from soot and other organic compounds (solid and/or liquid), and ash. For the purposes of this disclosure, the combination of carbon, organic compounds and ash will be referred to, for simplicity, as soot. Those of skill in the art will recognize that soot and organic compounds are removed through regeneration but ash loading will remain.

#Usually, diesel particulate filter units are made of cordierite material which has a dielectric constant at frequencies around S-band, slightly higher than 4, with a weak temperature dependence. The effective dielectric constant of the cordierite filter, which is mainly void with a fraction of cordierite, is around 1.5-1.7, and it is slightly anisotropic because of the orientation dependence of the trap. The presence of soot (which can be as much as 10 g/liter of trap, with the size of the trap being about two liters for 5.66 inch traps) changes the microwave characteristics of the microwave cavity, as the soot has a dielectric constant that is different from the gas (air or exhaust) that it displaces. Thus, the maximum soot loading for this trap could be as high as 20 g with a volume of about 20 cm3. This amount of soot corresponds to a substantial volume and a correspondingly large change in the dielectric characteristics of the trap. It is noted that the dielectric constant of some types of soot is approximately 2.