Woven Metal Fiber Diesel Particulate Filter

Fabrizio Rinaldi ■ Tenneco Automotive Abstract Diesel particulates filtration has been achieved by a variety of filtering media, from Cordierite to Silicone Carbide and Sintered Metal materials. A brief investigation into a different filtering media was led not only to achieve varying filtration efficiencies and low back pressure characteristics, but also to have good serviceability and ease of packaging. The filtering media chosen consisted of steel fibers of various thicknesses woven into a filtering mesh. Varying the weave pattern and size of the fibers generates different average porosities of the filter media. Pleated constructions of varying average porosity were tested to measure the filtration efficiencies, soot loading capabilities and regeneration times. General rules of filtration efficiencies and general soot loading capabilities as a function of the surface area were empirically determined.

Introduction

Particulate matter arising from diesel engine exhaust has been filtered, over the years, by making use mostly of “Wall Flow” type filters. Figure 1 shows the typical filtering mechanism for a wall flow ceramic (Cordierite or Silicone Carbide) filter.

Fig. 1

Particulate containing exhaust gases are forced to flow through the walls of the filter, by plugging alternate cells along the face of the substrate. An open cell at the front face of the substrate is also plugged at the opposite end. The materials that make up the walls of the filter affect the filtration and as such characteristics like porosity, surface area and sintering temperature are of key importance in an overall practical and durable design.

The filtration mechanisms that are normally at play within these filter constructions are particle size dependent and can be briefly summarized as follows: (Note: The particle size ranges noted in the figures are rule of thumb estimates)

(a) Impaction or Inertial Deposition (Figure 2a): This is where big particles are intercepted due to inertia forces (b) Diffusional Deposition (Figure 2b): This is where very small particles reach the walls (obstruction) due to diffusion (brownian movement). (c) Flow-line Interception (Figure 2c): There is a size range in between the two extremes above where a particle in the flow-line just touching the surface is intercepted, this mechanism is also called blockage. The “obstruction to the flow” shown in all of the mechanisms described can be due to the porous walls of ceramic filters, but also to various other materials as the following examples illustrate.

Blocked Cells Filtered Gases Exiting Filter

Particulate Containing

Exhaust Gases Entering Filter

Obstruction to the Flow

Direction of Exhaust Flow

Particulates (<10 nm)

Obstruction to the Flow

Particulates (>1 micron)

Direction of Exhaust Flow

Direction of Exhaust Flow

Obstruction to the Flow

Particulates (>10 nm, <1 micron)

Fig. 2a

Fig. 2b

Fig. 2c

Sintered Metal Porous Material: Figure 3, shows a

TEM picture of a sintered metal filtering material created by sintering a fine wire mesh coated with metal powder and other additives.

Companies like HJS, PUREM and Bosch use this proprietary process to manufacture varying average porosity sheets and construct diesel particulate filters for a variety of applications LDV to HDV, OEM and/or retrofit.

Figure 4 shows a PUREM filter constructed by packing a finite number of sintered metal filtering sheets in a can that constrains the flow of exhaust gases, so as to achieve maximum filtration capability

Sintered Fiber Filtering Media:

Various companies (Rypos, Bekaerdt) make use of sintered metal fiber sheets as the main filtering media. This media is usually manufactured by using stainless steel fibers of varying thickness and length that are overlaid in a random fashion and sintered by the application of pressure and heat. (see Figure 5 as an example)

Various constructions are known, the preferred being a column arrangement of small pleated filtering elements as shown by Figure 6

Filtration of diesel particulates can also be achieved

in a “Flow Through” substrate configuration. A technology patented by EMITEC consists of alternate layers of sintered metal filtering sheets and ribbed metal foil sheets in a flow through substrate configuration. “Shovels” along the length of the ribbed metallic foil layer, partially force the particulate containing gases through the sintered metal filtering layers as shown in Figure 7

Fig. 3

Fig. 4

Flow of Exhaust

Gases

Pack of Sintered Metal

Sheets

Fig. 5 Example Of the Bekaerdt Sintered Metal Fiber Filtering Sheet

Fig. 6 A Type of Rypos DPF Made Up Of Sintered Metal Fiber Filtering Media

Individual Filtering Column Made up of Small Pleated Filtering Units

Fig. 7 Emitec Flow Through DPF Filtering Principle

Variations in the number and size of the “shovels”, among other, will contribute to varying filtration and loading capabilities. The filtration efficiencies (40%-70% on average) are not as high as for a “Wall-Flow” filter, but the back pressure characteristics induced in an exhaust system by this DPF, tend to be much lower (throughout the operating range) than for non Flow-Trough filters

In summary, many technologies have already been applied to filter particulates stemming from diesel engines exhaust. There is always a compromise between filtration efficiency, soot and ash loading capabilities, regeneration capabilities and back pressure induced in the system. This compromise is also affected by the material choice, which is further restricted by the useful life range required and the manufacturing capabilities of both the substrate manufacturer and the packager

This paper presents a brief initial investigation into a

metal filtering media that aims at maximizing the filtration efficiencies, while minimizing back pressure and regeneration time.

Experimental Test Setup:

The commercially available material chosen is shown in Figure 8

The make up of the material is as follows: Stainless steel fibers ( 2 thickness values per average porosity media) are weaved (pattern of weave is intrinsic in the filtration and permeability characteristics of the media). The sheet is then sandwiched between 2 fine wire meshes and can be sintered by applying pressure and heat. This is done to improve the mechanical properties of the final filtering media. The material is available with an average porosity ranging from 1 to 15 microns. Also two woven metal fiber sheets of different porosities can be applied together to give “Dual Laminates” filtering media , for example 8/3.5 DL – which would mean an 8 microns and a 3.5 microns average porosity woven metal fiber sheets coupled between the two fine wire meshes. The latter could help with fine tuning of filtration efficiencies and aid the soot loading capabilities.

To be able to test a variety of different porosity media, the construction of the DPF would have the following characteristics: (1) durable, (2) interchangeable filter units and (3) easy access to the filter material. Applying basic engineering judgment and with the aid of the material supplier, the filter unit geometry and housing shown in Figure 9 was developed.

Fig. 8 Close up of Woven Metal Fiber Filtering Media (15 microns Average Porosity)

Inlet Can

Outlet Can and

Filter Unit Mounting Support

Fiber Mat Seals

Exhaust Flow

Fig. 9 Filter Unit Geometry (top) and General DPF Assembly

Outlet Flange

Inlet Flange

Main Filtering Unit

In brief, a sheet of woven metal fiber filtering media was pleated and sealed over a perforated tube, to increase the mechanical strength of the pleated unit. The pleated unit was then welded to an inlet and outlet flange. This construction is referred to as the Main Filtering Unit shown in Figure 10.

To focus on the porosity only of the material the geometry of the filter unit was fixed at: 5.5” outer diameter x 11.5” long having 186 pleats (0.25” pleat height) made from a single sheet of 7.1 ft2 total surface area Various main filter units were then produced with different average porosities and with dual laminate woven metal fiber filtering media. When referring to the 8/3.5 microns dual laminate media, the larger porosity section of the filtering media will always face the inlet side of the pleated arrangement, in this case 8microns

Testing was done in an engine dynamometer cell using an ISUZU 2.8lt Turbo (MY 2000) generically referred to as I2.8T or an ISUZU 2.8lt (MY 2001) engine with a mechanical fuel pump, generically referred to as I2.8M.

To test whether the filter can be successfully regenerated, secondary fuel injection over a DOC (cordierite, 400 CPSi, 5.66” x 3”, 90g/ft3 Pt only) was employed.

The generic test cell setup is shown in Figure 11.

Note that to minimize changes in PM due to changes in geometry of the exhaust system, when testing raw PM emissions without the DOC or DPF unit, a straight length of pipe (all 2.5”diameter) was used, so as to keep the distance between exhaust outlet (turbo out or manifold pierce point) and the PM measuring equipment probe, equal.

The PM measuring equipment used was a Quartz Crystal Microbalance (single Element) supplied by Sensors Inc. The QCM unit measures, in real time, the mass of PM within a diluted sample (micro dilution was used). The probe transferring the sample to the QCM was heated and maintained at 200°C, to limit the amount of heavy hydrocarbon condensations on the particles. Figure 12 shows the main QCM unit.

Only steady state type testing was performed. The

generic schedule followed was: steady state time at each rpm level of 40-60 seconds, ramp up between rpm levels of 40 seconds. Usually idle either 850 or 800 rpm, while maximum rpm 3750 or 3000 depending on whether I2.8T or I2.8M were used. Increments of 250rpm from 1000 rpm were applied to all tests. Schedules to determine filtration efficiencies were kept identical before and after the application of the DPF. Results:

Various units were tested on a cold flow bench to get a feel for the back pressure characteristics of this filter as compared to other commercially available units. Figure 13 shows a chart that compares various ceramic filters and a competitor’s sintered metal fiber filter.

The chart above indicates that unloaded filters made with these woven metal fiber filter media will give lower back pressure characteristics, as compared to ceramic and sintered

Fig. 10 Main Filtering Unit Before Testing

Fig. 11 Complete DOC and DPF units Coupled to the Exhaust outlet of 2.8lt Engine

Exhaust Inlet

Exhaust Outlet

DOC DPF containing Woven

Metal Fiber Units

Secondary Fuel Injector Inlet

Fig. 12 Single Element Quartz Crystal Microbalance

Fig.13- Cold Flow Comparisons of Various Clean Filters

Operating Range of I2.8T and I2.8M Engines

metal fiber filters. Also the difference between the single layer unit (3.5 microns) and the dual laminate (8/3.5 microns) is very small, which indicates that very little back pressure increase will be introduced even if the most flow restrictive woven metal fiber media is used.

Figure 14 indicates the values of engine parameters collected during the standard steady state test schedule.

Maximum torque was 162lb/ft at 1750 rpm, while the maximum power is 100 hp at 3750 rpm (not shown here). All values shown are at a 100% throttle setting

The PM emitted was then tested before and after the introduction of the DOC, to gage the PM reduction due to the DOC. Figure 15 shows the PM emitted by the I2.8T engine with and without the DOC.

The maximum PM out (with DOC in the system) was achieved at 1750 rpm (1.02g/bhp-hr); while the lowest PM emitted was at 3000 rpm (0.047g/bhp-hr). At idle (850 rpm) the PM emitted was 0.87g/bhp-hr

Having a baseline of the PM emissions, the 3.5microns and subsequently the 8/3.5 microns dual laminate filter was introduced in the exhaust system and the PM emissions after the DPF were measured. Both filters gave essentially 100% filtration efficiencies as indicated by the photos in Figure 16, showing a clean (also to the touch) filter outlet.

The graph of PM emissions for both filters is not

shown here as the PM out of these filters fell well below the measuring capabilities of the QCM. Shortly after completion of the above tests, the ISUZU 2.8 Lt Turbo, suffered from mechanical failure and was replaced by an ISUZU 2.8lt (MY 2001) non-Turbo, fueled by a mechanical fuel pump.

Below, Figure 17 shows the values of this engine’s

parameters, collected using the same standard schedule as before, and at a 100% throttle setting.

Fig.14- I2.8T Engine Parameter values

Fig. 15- I2.8T Engine PM Emissions

Fig.16- Inlet and Outlet of Woven Metal Fiber DPF, After Filtration Tests

Inlet of DPF Assembly

Containing the WMF unit

3.5 microns Outlet 8/3.5 microns DL Outlet

Fig. 17- I2.8M Engine Parameter values

The exhaust was also fitted (in the exhaust manifold) with a wide band oxygen sensor, to monitor the Lambda and %oxygen in the exhaust stream.

The PM emissions over the standard schedule were then investigated. Figure 18 shows the raw PM (g/bhp-hr) emissions for each steady state engine speed, with and without the same DOC used before.

The maximum PM emitted with the DOC in the system, was achieved at idle (850 rpm, 4.31g/bhp-hr, λ = 0.95, %O2 = 13.9); while the lowest PM was achieved at an engine speed of 1500 rpm (0.08g/bhp-hr, λ = 1.22, %O2 = 17.6). Two filter units were constructed and tested, one being made from 15 microns and the other from 15/8 microns dual laminate woven metal filter media. Both exhibited partial filtration of the PM emitted, with the 15 microns having a fresh filtration efficiency of 24.1%, while the 15/8 microns DL showing a higher filtration efficiency of 57.1%. Figure 19 shows a photo of the outlet of the 15 microns filter unit. The soot accumulation on the outlet of both filter units was evident.

Of Particular interest was the increase in filtration exhibited for both the 15 microns and the 15/8 microns DL units, after each subsequent regeneration and testing sequence. Figure 20 indicates how the rate of accumulation of PM on the QCM varied for the 15/8 Microns DL filter unit. The 15 microns unit exhibited a similar trend. An accumulation period (time left engine at idle – 850 rpm) of 10 minutes was used for all tests. The regeneration sequence used after the accumulation period consisted of : ramping engine speed to 2750 rpm, secondary fuel injection (see later for details) to achieve soot light-off, continued fuel injection till the back pressure was decreased to below 2 kPa, immediate switch off of engine. On average the filtration efficiency improved by 10% from fresh to after the first regeneration. The next subsequent regenerations only marginally improved the filtration efficiencies and only at high engine speeds (2250 rpm – 3000 rpm). This can be explained by the accumulation of ash in the filter, and this deposit aided the filtration of PM.

Final testing was done to see how the 8/3.5 microns DL filter unit withstood loading and regeneration sequences. Note: The fuel injection sequence, although kept identical for all regeneration testing, was not optimized (uniformity across the face of the DOC, position of injector, etc.) for this system. The injector was controlled (pulse/width modulation) to achieve a 10000ppm fuel injection (as measured with a gas analyzer) at an engine speed of 2750 rpm. This quantity of fuel was arrived at after various fuel injection tests over the DOC, indicated the best exotherm that could be achieved with this system geometry at 2750 rpm.

Figure 21 shows photographs of the 8/3.5 microns filter unit shortly after having accumulated some PM and immediately after a regeneration sequence. Note how well the soot has been combusted off the filter.

R1

R2

R3

Fig. 18- I2.8M Engine PM Emissions

Fig. 19- 15 Microns Filter Unit Outlet

Fig. 20- QCM Mass of PM Emitted behind the 15/8 Microns DL filter Unit For 4 Std. Testing Schedules, With Regeneration Sequences (R1,R2,R3) in-Between

It was already understood from previous tests that the effective filtration geometry (0.74lt, 0.65m2) of these filter units, was by no means suited for commercial application to these engines. The following tests were also performed to gage the improvement necessary to reach commercial viability of this filter media. Figure 22a shows the back pressure variations of this exhaust system as a function of time, while performing PM loading and regeneration sequences at various engine speeds. For each load and regeneration sequence shown above, the loading was allowed to proceed so as to reach a maximum of

~12in.Hg (40kPa), while the regeneration was maintained until the back pressure dropped to ~1.5in.Hg(5kPa). The average temperature at which regeneration occurred was 1240°F (671°C). Figure 22b indicates both the average PM loading at each sequence number and the average time taken to regenerate the filter to a suitable back pressure value There are rpm ranges (2000 – 2250 rpm) where the loading and regeneration of this filter unit is excellent. Nevertheless the total filtration volume needs to be at 4 x the current volume, with the filtration surface area doubled, to strive for a marked improvement in back pressure characteristics, while loading. Due to the lower thermal inertia of these units, compared to uncoated ceramic DPF’s, the regeneration of such filters will always be of shorter duration and less energy intensive.

Conclusions

This investigation showed the feasibility of a different metal filtering media that can be used to construct DPF’s with varying filtration efficiencies. Furthermore these units can be easily assembled in an exhaust system and can be regenerated with considerable ease. The dynamic relationships between average porosity, total surface area and system geometry needs to be further investigated, to give a DPF with acceptable PM loading capabilities. Further studies must also take into consideration the regeneration strategy applicable to this type of DPF.

Fig. 21- 8/3.5 Microns DL Filter Unit Before and After Regeneration

REGENERATION

Fig. 22a- Load and Regeneration Test on 8/3.5 Microns DL Filter Unit Before

Fig. 22b- Load and Regeneration Test Data (Note: sequence numbers mentioned here refer to numbers in Figure 22a )

References

1. A. Mayer “Particulate Filter Systems, Particle Traps” Encyclopedic Article, May 2005.

2. http://www.dieselnet.com/technical.html 3. http://www.purem.de/eng/start.php 4. http://www.bekaert.com/corporate/products/Metal%20fibre

s.htm 5. http://www.emitec.com/download/library/en/Endversion.pd

f 6. T. Jacobs, S. Chatterjee, R. Conway, and A. Walker

(Johnson Matthey ECT); J. Kramer and K. Mueller-Haas (Emitec Inc.) “Development of Partial Filter Technology for HDD Retrofit”, SAE Technical Paper 2006-01-0213

7. http://www.sensors-inc.com/about.htm 8. http://www.sensors-inc.com/pdfs/qcm_aei_0304.pdf

Biographies Dr. Fabrizio Rinaldi

Received his PhD in organic chemistry in 1994, while working for a chemical company in South Africa that produced specialty chemicals. Joined Engelhard Corp. and between 1996 and February 2003 worked as process engineer, manufacturing manager and finally as technical and quality manager of various divisions. Worked as Chief Engineer Emissions technologies for Tenneco Inc. from March 2003 till present.