Soot Distribution in DPF Systems. A Simple and Cost Effective Measurement Method for Series Development
Marco Ranalli, Juergen Klement
ArvinMeritor LVS-Air and Emission Technologies
Markus Hoehnen, Ralf Rosenberger
Additive GmbH
Abstract
The key feature of a reliable DPF system is the control over the amount of soot burnt during the regeneration. Since only an overall evaluation of the collected soot is possible on-board, only a DPF-system with homogeneous soot distribution can avoid areas of local overcharging which may lead to disastrous consequences during the filter regeneration. Hence, a system layout optimisation, which can ensure a good soot distribution also in the “worst-case” loading conditions, is necessary.
The major problem in the optimisation work is related to the lack of measurement methods which could be standardised for series development.
Indirect methods, such as the measurement of the velocity profile, provide only a rough estimation of the soot distribution, while other methods, like Computer Aided Tomography, are too complex and expensive for standard series investigations. For this reason an investigation method based on the integration of thermal emissions during a controlled regeneration was developed. Extensive investigations were conducted with DPF loaded on the engine bench in different operation points and loading conditions. The method was validated and two different cases of soot distribution for the same DPF were analysed and compared with LDA (Laser-Doppler Anemometry) measurements and CFD simulations. Thanks to the results, this investigation method is already being utilised in series development projects.
Introduction
Since the aim of the measurements was to investigate a real situation, a complete exhaust system upstream to the substrate was built. Components of this system were: pre catalyst, down-pipe, oxidation catalyst and SiC DPF substrate (FIG 1). Oxidation catalyst and DPF were separated by a connection flange with the same section. A different system was utilized for the measurements.
Figure 1: Exhaust System Layout
Measurement SET-UP
FIG 2 shows the measurement set-up utilized for this investigation. In order to isolate the effect of only the soot combustion, it was necessary to create ideal conditions of homogeneous velocity and temperature profile upstream the DPF. For this reason, an expansion cone and a long connection pipe were used.
Figure 2: Measurement set-up
In addition, the DPF was externally insulated in order to minimize the heat losses through the canning. The regeneration was recorded with an infra-red camera. The camera provides a sequence of infra-red pictures (spectral range 2-5 µm) with a 360x240 pixel resolution, displaying the outlet gas temperature with a sampling rate of 1 Hz. In order to record only the heat due to the soot combustion, the same heat up rump was initially performed and subtracted from the curve of the soot regeneration. The integration of this value through the complete measurement provided the final value, (see FIG 3). An in-house developed software made it possible to repeat this procedure for the 360X240 matrix provided by each snapshot of the regeneration measurements.
Figure 3: integration procedure
Validation
In order to validate this investigation tool, a test loading with a known soot distribution was performed. A cross-shaped metal sheet was placed in front of the substrate, in order to obtain a cross-shaped soot-free area (FIG 4). The filter was loaded with an overall specific soot quantity of 6 g/l, which makes a local density of around 8 g/l in the areas where the soot is actually collected (i.e. outside the cross).
Figure 4: the “X-Filter”
The mask was then removed and the distribution evaluated with different investigation methods. The soot distribution inside the “X-Filter” was first evaluated with computed aided tomographic equipment. This kind of measurement provided the radial soot distribution in 20 mm “slices”. The integrated distribution across the length is shown in FIG 5 and confirms the x-shaped area free of soot.
Figure 5: computer-aided tomography of the “X-Filter”
LDA and Thermographic measurements were then performed. The precise measurement of the flow velocity coming out of the “X-Filter” also revealed a soot-free area corresponding to the x-shaped region (see FIG 6).
Figure 6: LDA of “X-Filter”
Even a relatively small amount of soot collected in certain areas is sufficient to affect the velocity profile and make the soot distribution “visible” on the cross-section. It must be said that this kind of investigation can only be qualitative. On the other hand, once the measurement technique is well tuned, it is relatively simple to obtain fast and useful information concerning the soot distribution.
Right after the LDA measurement, the filter was regenerated under controlled conditions.
Figure 7: controlled regeneration for “X-filter”
As shown in FIG 7, the regeneration was very heterogeneous: the filter starts to regenerate in the center and then the regeneration propagates in the outer region. It is not possible to indicate with precision where the soot is collected. It is only with the integration of the heat emissions through the duration of the measurement, that the location of the soot on the cross-section becomes visible.
Figures 8 and 9: “soot mapping” of X-Filter
FIG 8 and 9 clearly show the expected distribution of the soot on the cross-section (soot mapping). Even where the metal sheet only covered half a segment, the accuracy of the measurement was not affected. It must be mentioned, that the resulting value is a soot index, which is the result of the integration of the temperature values due to the sole soot combustion over the duration of the regeneration. Under controlled conditions of homogeneous flow velocity and temperature in each point of the section, the soot index can be related to the actual specific soot distribution across the section. Validation investigation to verify this value by knowing the overall soot quantity trapped in the filter is ongoing. In the following section two different cases of real soot distribution will be analyzed.
Case Analysis
In the following example the same filter was loaded with approximately 9.5 g/l of specific soot loading at very high flow velocity (500 kg/h) with the initial and with an optimized layout.
Figures 10 and 11: CFD simulation of soot distribution and LDA measurement of the loaded filter (case 1).
In the first case the original exhaust system, composed by a pre-cat, an underfloor catalyst and a DPF, was used. Mainly because of the lack of optimized geometry of the downpipe and inlet cone, the radial soot distribution was not homogeneous, with an higher soot concentration in the central part of the filter. An in-house developed CFD module, allowed a fast prediction of the axial soot distribution without additional computational time. As already defined by Dr. Hossfeld [4], a soot distribution number can be characterized as a function of the local pressure loss coefficient linked to the soot deposition
The system was then built and the substrate loaded at the engine bench. The LDA measurement confirmed a higher soot collection in the central part of the filter section, which caused the otherwise homogeneous flow to be diverted to the outer area filter (FIG 11).
In the second case, the system layout was modified –mainly into the shape of the downpipe and the inlet cone- in order to achieve an homogeneous soot distribution.
Figures 11 and 12: CFD simulation and LDA measurement. (case 2)
The optimization of the system design was possible thanks to the CFD tool, which allowed the fine tuning of the system’s geometry within the constraints due to the underbody position of the DPF. With the initial lay-out, a higher soot concentration was present in the central region. On the contrary, the optimized system had, at the same loading conditions, a very homogeneous soot distribution. This was confirmed also by the LDA measurement of the loaded DPF (FIG 12). In the optimized system, the velocity profile was almost the same for all the filter units, with a coefficient of soot distribution ( d ) close to 1.
The differences in the soot distribution could be seen clearly during the regeneration, even if the flow parameters were exactly the same. With a non-homogeneous soot distribution, the regeneration is faster and stronger when the soot concentration is higher, i.e. in the central region (FIG 13).
Figure 13: regeneration of DPF in the initial (left) and optimized exhaust system.
It must be said that, during this phase, very dangerous temperature gradients occurred. In the following phase, when the central segments have finished regenerating, the outer segments started to regenerate, resulting in thermal gradients in the opposite direction. In the optimized system, the complete section regenerated at the same time with a reduced risk of filter cracking.
Figure 14: “soot mapping” for the DPF in the initial layout.
FIG. 14 shows the result of the integration procedure on the first measurement. The central area appears to be overloaded, while the efficiency of other areas remains poor.
Figure 15: “soot mapping” for the DPF in the optimized layout.
FIG.15 shows the result of the soot loading in an optimized layout. With the same overall sot amount, the maximum local value for soot loading is up to 40 % lower.
Conclusion
While the axial soot distribution is very much related to factors linked to engine and substrate characteristics (mass flow, temperature, engine displacement and operating point, substrate volume, cell density and wall thickness), the optimization of the radial soot distribution is strongly affected by the engineering of the exhaust system manufacturer. This method provides a valuable validation tool to optimize any DPF system layout, with relevant improvements in terms of durability, volume efficiency and/or cost reduction. Once measurement set-up and analysis software are fine tuned, this measurement method can be applied to any DPF shape with very small effort and limited expenses. As a result, the DPF-development methodology has already found application in the series development of all projects with DPF systems carried out at ArvinMeritor.
References
K. Ohno, K. Shimato, N. Taoka, H. Santae, T. Ninomiya, T. Komori and O.Salvat: Characterization of SiC DPF for Passenger Car. SAE Paper 2000-01-0185
M. Ranalli, P. Zelenka, S. Schmidt and G. Elfinger: An Active Regeneration Aid as a Key Element for Safe Particulate Trap Use. SAE_NA Paper 2001-01-062
J M. Ranalli, C. Hossfeld, R. Kaiser, S Schmidt and G. Elfinger: Soot Loading Distribution as a Key Factor for a Reliable DPF System: an Innovative Development Methodology. SAE 2002-01-2158
C. Hossfeld, R. Kaiser: Russbeladung in Dieselpartikelfilter-Systemen. MTZ 9/2003 (in German)
P. Kugland, E. Krieger and E. Santiago : Cleaner Diesels – Full Flow Soot Filter regeneration Systems SAE paper 910133
Contact
Marco Ranalli, COC Emissions ArvinMeritor Light Vehicle Systems
Marco.ranalli@arvinmeritor.com
ZEUNA STAERKER GmbH & Co.KG, Biberbachstrasse 9 D-86154 Augsburg , Germany
Definitions, Acronyms, Abbreviations
DOC Diesel Oxidation Catalyst
DPF Diesel Particulate Filter
LDA Laser Doppler Anemometer
|
