SAE TECHNICAL
`PAPER SERIES
`
`2002-01-0322
`
`
`
`SiC and Cordierite Diesel Particulate Filters
`
`Designed for Low Pressure Drop and Catalyzed,
`Uncatalyzed Systems
`
`8. Hashimoto, Y. Miyairi, T. Hamanaka, R. Matsubara, T. Harada and S. Miwa
`NGK INSULATORS. LTD.
`
`Reprinted From: Diesel Exhaust Emission Control 2002:
`Diesel Particulate Filters
`
`(SP—1673)
`
`fl. E The Engineering Society
`"flfld‘gggflfigfij‘ggfl’;
`INTERNATIONAL
`
`SAE 2002 World Congress
`Detroit, Michigan
`March4-7,2002
`
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`1
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`JM 1007
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`JM 1007
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`2
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`

`

`SiC and Cordierite Diesel Particulate Filters Designed for Low
`Pressure Drop and Catalyzed, Uncatalyzed Systems
`
`3. Hashimoto, Y. Mlyalri, T. Hamanaka, R. Matsubara, T. Harada and S. Miwa
`NGK INSULATORS. LTD.
`
`2002-0141322
`
`Copyright to 2002 Society of Automotive Engineers‘ Inc.
`
`ABSTRACT
`
`DPFs (Diesel Particulate Filters) have been a primary
`technology utilized to purify diesel PM emissions. One of
`the major challenges of the DPF is to reduce pressure
`drop caused by PM and ash accumdation.
`
`This paper reports on the definition and investigative
`results of several major parameters Which determine the
`pressure-drop of Cordierite and SIC {Silicon Carbide)
`DPF's. After which, the successful material development
`for low pressure-drop Cordierite and SiC DPF's are
`presented.
`
`INTRODUCTION
`
`Diesel engines have predominance in heavy-duty trucks
`and buses. More recently, sales of diesel passenger
`vehicles have greatly increased in Europe due to the
`additional power and fuel efficiency olfered by diesels.
`Diesel engines are also considered to be one of the
`most realistic solutions for decreasing 002 emissions.
`
`A main concern of diesel vehicles is the high PM
`(particulate matter) emission. which has been identified
`as a potential carcinogenic. Tighter PM emission
`regulations will be introduced in Europe. United States
`and Japan on passenger cars and trucks over the next 7
`years. DPF (diesel particulate filter) has been the
`primary technology considered in meeting these tight
`PM emission limits. The Wall-flow Cordierite DPF's has
`been consistently used for folk-lift and mining equipment
`applications over the years and are now being applied to
`truck
`and
`bus
`retrofit
`applications.
`Furthermore.
`SiC(SiIicon Carbide) DPF's were introduced on a
`production, European vehicle In 2000.
`in this case. a
`Cerium fuel additive is used to reduce soot combustion
`temperature and forced regeneration is conducted
`periodically [1] [2]. The CeriumiSiC-DPF system purifies
`PM emissions by more than 90%. Japanese Tmck
`manufacturers are
`also seriously developing DPF
`
`systems to meet new short-term emission regulation
`effective in October 2003.
`
`The addition of the DPF exhaust system increases back
`engine pressure. which reduces engine power and
`increases fuel consumption. DPF accumulates not only
`PM but also ash from the engine oil and fuel. further
`increasing pressuredrop. Also, the available space for
`packing the DPF in the under-floor or toe-board position
`for passenger vehicles
`is
`limited. For heavy-duty
`vehicles
`the packaging envelope is
`also limited.
`Typically in this case, the muffler is modified to package
`the DPF. The limited DPF-packaging envelopes make it
`difficult to provide sufficient DPF cross sectional area or
`sufficient
`filtration area. Based on these conditions,
`optimizing
`pressure~drop
`and
`trapping
`efficiency
`simultaneously is a notable challenge for DPF systems.
`
`A catalyzed DPF system has been proposed as a future
`PM emission [3] control
`technology.
`In the case of
`catalyzed DPF system. the catalyst in the DPF further
`increases
`flow restriction.
`In
`addition
`to
`the
`aforementioned conditions. a DPF with low pressure-
`drop is very important
`from the aspect of engine
`performance.
`
`EQUATIONS OF DPF PRESSURE LOSSES
`
`GENERAL DlSCUSSION -
`
`As the first step of this study, the equations to define
`DPF pressure—drop were established. Pressure drop of a
`DPF consists of five different factors. These factors are
`shown in Figure 1. The first is contraction and expansion
`losses at the inlet and outlet faces of the DPF created by
`the plugged-cells. The second losses are the inlet and
`outlet channel frictional losses. The third and fourth are
`flow restriction when gas flow though porous wall of DPF
`and the soot layer. respectively. Pressure losses at the
`inlet and outlet cones of
`the converter are also
`considered.
`
`3
`
`

`

`9..
`
`Several equations of DPF pressu'e losses have already
`been proposed [4]. In this study. the equations of DPF
`pressure drop were developed for each of the live—
`tactors experimentally and independentiy, and these
`equations were used to determine the DPF's optimum
`cell stnicture to minimize pressure drop.
`
` 5
`
`3.4
`
`5
`
`1. Contraction and expansion Loss
`2. Channel flow less
`3. Wall flow loss without soot
`4. Wall flow loss with soot
`5. Duct loss
`
`Figure 1. Factors of DPF Pressure Losses
`
`The total pressure loss of a DF'F (AP) is expressed as a
`summation of these factors;
`the plug loss: oPm.
`the
`channel loss: APc. the restriction of clean wall:APw.¢1..n,
`the restriction through a soot layer. APwfld and the cone
`losses: APd.
`
`AP = APm 1- APc + APWmm + onm,t +APd
`
`CONTRACTION AND EXPANSION LOSSES -
`
`Contraction and expansion losses are generated at
`the plugged cells (on the inlet and outlet faces). To
`measure these losses. DPF's with various cell structures
`were sliced near the inlet and the outlet and pressure
`drops through these sliced test pieces were measured
`under various flow rates. Measured results are shown in
`the Figure 2. The Figure 3 shows the relationship
`between the ratio of open frontal area (OFA) and the
`pressure drop under the fixed flow rate of 7 Nm’imin.
`The general form for this pressure loss was employed
`with an empirical pressure coefficient a .
`
`FlowI Rate Smafmin
`
`Figure 2. Plug loss measurement
`
`Y = 3.6721 x “‘75
`
`
`
`o
`
`0.2
`
`0.4
`1-0FA
`
`0.6
`
`0.3
`
`Figure 3. Effect of OFA on Plug loss
`
`Apm=gpv°
`
`—-(1}
`
`CHANNEL FRICTION LOSSES —
`
`The or index was set to 2.2063. so the flow rate effect
`agreed with
`experimental
`results. The
`pressure
`coefficient g was expressed as a function of the ratio of
`open frontal area (OFA). The following
`equation
`matched the experimental data of
`the various cell
`structures.
`
`a = C1(1_OFA)2.TITB
`C1 = 0.02759
`
`_" (2}
`
`The Reynolds number of the channel flow in the DPF is
`in the range of laminar flow even at the maximum engine
`flow rate condition. The pressure loss of laminar flow
`through a channel is generally in proportion to viscosity.
`length and velocity. and inversely in proportion to a
`square of
`the hydraulic diameter. Pressure losses
`without the inlet and outlet plugs were measured for
`various cell structures. The measured results are shown
`in Figure 4. Here the channel friction loss was expressed
`by the following equation with the empirical index m and
`
`
`Sliced
`
`Sliced
`
`
`Length- start-I11
`Diameter n+1 10m
`Plug Length-10m
`Inlet Condition
`Tempelahsre-EILT-ZSS deg C
`
`1Tmii100cpsi
`
`
`
` PressureLoss.kPa
`
`12mili 300 cpsi
`
`4
`
`

`

`u: wall flow velocity (mls)
`p: viscosity (Pa 5 )
`u“: standard velocity {rnls}
`pa: viscosity under standard condition (Pa s}
`fttp}: Increase in the wall l'low pressure loss
`with soot loading under no and on
`
`
`
`Pent-Inability I
`
`(Air Velocity] (Sample Thickness) (Viscosity of Aid
`(Pressure Drop)
`
`Figure 5. Wall Permeability Measurement
`
`- -The u is constant- (u=uo)
`. Compensated under Room Temperature
`
`
`initial Pressure Drop
`
`{I We) -
`'"u‘flu
`fltn):
`
`I
`experimentally
`II
`decided
`
`Soot Amount per Unit Filtration Area
`
`9.
`
`3 5
`
`;
`3;
`a“.
`g
`E
`"Ii
`3
`
`Figure 6. Wall and Soot Layer Loss
`
`INLET AND OUTLET CONE LOSSES —
`
`The general form of contraction and expansion was
`employed:
`
`APd = 2 p v12 (1 - also“):
`vi: velocity in tube (mils)
`d.: Diameter of tube (In)
`D: diameter of DPF (m)
`
`— (5)
`
`It has been confirmed that the estimated pressure drop
`by these equations matched measured values of DPF
`pressure drop under various temperatures. flow rates.
`DPF dimensions. and soot loading rates.
`
`in. which is expected
`coefficient CZ. The parameter,
`theoretically to be a value near 1.0 was set to 1.0184 by
`curve-fitting.
`
`APc = czv’“ L pl (OFA) 1 (DH)2
`Cg = 0.012,
`rn = 1.0814
`
`— (a)
`
`
`
`.
`.
`Lawn“? .
`Dian-hr"- 110m
`
`Inlet Dorldltlm
`Tmmluru-MT- mod-g - -
`
`Pressure-twa- Fa
`
`Flow Rate (Srnalmin)
`
`Figure 4. Channel Loss Measurement
`
`WALL AND 800T LAYER PRESSURE LOSS-
`
`It is assumed that Darcy's law is applicable to these
`losses. The permeability of a clean wall can be
`measured and deterrnlned for each DPF material as the
`gradient of wall flow pressure plot as shown in the
`Figure 5. However. the permeability of the wall with soot
`loading will change with the soot loading-rate. Typical
`retationship between the soot
`loading-rate and the
`pressure loss is shown in the Figure 6. During the first
`stage of soot loading. the characteristics of the pressure
`drop with soot loading has a steep slope. shown as “I” in
`the figure. This is due to the soot being trapped inside
`the wall. as described later in this paper. The pressure
`drop curve has a transition-curve 'Il". then moves to the
`stable slope at a given soot-load,
`indicated as "III" in
`Figure 6.
`
`Pressure drop 01 the complete-DPF during soot
`loading was measured. and the resulting increase in flow
`resistance was determined as a function of the soot
`loading-rate for each material. Finally.
`the pressure
`losses through the wall and the soot layer are expressed
`as follows:
`
`APW = m.m +B W‘s“
`= (tolko) Us + flip) ”i” (no No)
`
`- l4)
`
`1! (ua tta). 6.3E06
`k: (well + soot) apparent permeability (m2)
`t: (wall + soot) thickness = to + t, (m)
`t,: apparent soot layer thickness (m)
`kn: permeability of clean wall (mg)
`to: wall thickness (m)
`
`5
`
`

`

`INVESTIGATION OF THE MAIN CONTRIBUTOR
`FOR PRESSURE DROP
`
`PRESSURE DROP WITHOUT SOOT LOADING
`
`The contribution rate of each parameter on both with-
`and without-soot-loading was
`evaluated with
`the
`pressure-drop equation. This study was carried out in
`terms of airflow rate at room temperature. The results of
`this study are shown in the Figure 7.
`It was also
`concluded that the main contributors for pressure-drop
`without soot loading were the expansion and contraction
`loss due to the cell-plugs and channel flow less.
`5
`
`
`
`PressueDrop.kPa
`
`GHQ-611
`5.66“Dx6'L
`12mitlSOOcpsi
`
`.3.
`
`OJ
`
`l0
`
`Flow Rate. Nm’i‘min
`
`Figure 2’. The effects of pressure-drop parameters
`on pressure-drop without soot loading
`
`This study also concludes the percentage contribution
`ratio of wall flow loss was relatively minor. According to
`equation (4). the permeability of the DPF material is the
`key factor for wall
`flow restriction. The relationship
`between permeability and pressure-drop without soot
`loading condition was examined at a 9 Nm’lmin of gas
`flow rate. These results are shown in Figure 8. Cordierite
`DPF‘s with several different levels of permeability were
`evaluated.
`
`4.66“Dx6'l..
`12mtl1300cpsl
`
`
`
`
`
` lnlialPressureDrop.kPa Qsurn’rmn
`
`0
`
`2x10“
`
`4x10“
`Penneablllty. n1a
`
`6x10"
`
`5x10“
`
`Figure 8.
`
`Influence of Permeability on pressure
`drop without soot loading
`
`The penneability of the DPF material has a relationship
`with pressure drop characteristics when the permeability
`of the material ls less than 4x 10"“ (me). It means that
`the permeability (accordingly. wall flow loss) has less
`impact on pressure drop without soot loading condition.
`if the material has sufficient permeability. This explains
`the reason why the coritribuljon of the wall flow loss
`(without soot loading) was small.
`
`PRESSURE DROP WITH SOOT LOADING
`
`The same pressure drop study as shown in Figure 8 was
`conducted at a soot-loading condition of 4 giL. Figure 9
`shows the evaluation results of the contribution of each
`pressure-drop parameter with the soot-loading condition.
`Contrary to the 'no soot loading" condition. the wall flow
`loss was the key contributor of pressure-drop, while the
`plug and the channel
`tosses had less impact. Also.
`Figure 9 shows the pressure-drop with soot- loading is
`much higher than that without soot loading. Accordingly.
`reducing wall flow loss is the key to a low pressure-drop
`DPF.
`
`40
`
`
`
`OHS-611
`5.56m‘L
`12m1vsoflcpsi
`Temp; 200 dog!)
`
`3
`
`_l D
`
` PressureDrop.kPa NO
`
`Air Flow Rate. Nm’rnin
`
`Figure 9. The effects of pressure-drop parameters
`on pressure-drop with soot loading
`
`PRESSURE DROP MEASUREMENT RESULTS
`
`The pressure-drop measurement of several Cordierite
`DPF's with different cell structures were conducted
`under without— and with-soot—loading conditions. Table 1
`shows the material properties and cell structures used
`for this study.
`
`Figure 10 outlines the pressure-drop evaluation test
`results without soot loading. The DPF sample size used
`for this study was it 5.66” (143.8mrn) and 6" (152.4mm)
`in length. Pressure-drop was measured in terms of gas
`flow rate at room temperature.
`
`The pressure-drop of material A with 17mil l 100cpsi
`was approximately 10% higher than material B with the
`same cell structure. Also. the higher the cell density. the
`higher the pressure-drop. These tendencies were the
`same for all the air-flow rates used for this study. As the
`
`6
`
`

`

`initial pressure-drop without soot at 2.27Nm3lmin. of
`airflow rate was approximately 0.4 kPa. 0n the other
`hand.
`the pressure-drop with 59 of soot loading was
`15kPa. approximately 37.5 times higher.
`
`The difference between material-A and -B increased
`under the soot-loading condition. Specifically, material-A
`had roughly 30% higher pressure-drop than the material-
`B above 29 of amount of soot loading. This is because
`the wall flow less has less impact on the Initial pressure-
`drop but is a key contributor with soot loading. as shown
`in Figure 7 and 9. In order to reduce the pressure-drop
`of the DPF, to analyze the material parameters on wall
`flow less is important.
`
`8-1
`1Tmi|l10 -.
`
`‘
`
`e-2
`12rrtili'm00psi
`
`3-3
`
`1
`
`
`
`
`
`
`
`
`2n1ll330opsi
`7
`
`NGK Soot Geriarator
`_ / Equpment :
`2m deg.c
`Gas Terrp. :
`Gas Flow Rae: 2.27 NmI’Jrrln
`DPF Size:
`5.EE"DXB'L
`
`
`
` 0
`
`2
`
`4
`
`6
`
`B
`
`10
`
`15
`
`
`
`PressureDrop.kPa a
`
`5
`
`Amomt of Soot Loadirg.g
`
`Figure 11. Pressure-drop of Cordierlte DPF with
`soot loading condition
`
`Also Figure 11 exhibits cell density dependency on
`pressure drop under
`soot-loading conditions was
`contrary to the without soot loading condition: the higher
`the cell density. the lower the pressure-drop. In the case
`of initial pressure-drop.
`the high cell density reduces
`hydraulic diameter.
`resulting in high pressure-drop.
`because the channel ftow loss is the key contributor. (in
`the other hand. pressure—drop with soot loading was
`mainly influenced by the wall flow loss. As the filtration
`area increases with cell density. the lower the wall flow
`loss. This is the reason the high cell density DPF has
`low pressure-drop under soot loading conditions.
`
`Based on these results. both material optimization and
`cell structure optimization are important
`to minimize
`pressure-drop of DPF.
`
`initial pressure-drop is primarily influenced by channel
`flow loss. small hydraulic diameter due to high cell
`density increases chamel flow losses.
`
`Table 1. Properties of Cordierite DPF
`for pressure drop evaluation
`
`Imam
`DHC-
`DHC-
`DHC-
`DHC-
`553
`61 1
`61 1
`61 1
`
`—uunn
`Mean Pore Size
`turn}
`
`Cell Density
`{cpsi}
`
`Wall thickness
`(mil)
`
`5
`
`
`DPF Size:5.66"D "L
`
`
`
` PressureDrop.ItPa
`
`
`
`Air Temp: Room Temp.
`
`0
`
`2
`
`6
`4
`Air Flow Rate. Nmaimin.
`
`8
`
`10
`
`Figure 10.
`
`Initial pressure drop of Cordierite DPF
`
`Alter measuring the initial pressure drop. the pressure-
`drop characteristics for the soot-loaded condition were
`evaluated. The same samples were used for this study.
`
`pertonnanoe with-soot was
`drop
`pressure
`The
`simificantly difierant than the initial. no-soot pressure-
`drop (shown in Figure 11). In the case of material-A. the
`
`7
`
`

`

`DESIGN OPTIMIZATION
`
`.
`
`DESIGN PARAMETERS ON BACK-PRESSURE
`
`These two effects seem compensate and wall thickness.
`which also has impact on wall flow loss, dominantly
`influences total DPF pressure-drop.
`
`According to the pressure drop Equations mentioned
`above. the followings can be listed up as the design
`parameters for DPF pressure losses:
`
`1) DPF Shape
`' Contour f Flowr surface area
`" DPF Length
`
`2) Cell Structure
`“‘ Wall thickness
`* Cell Density
`
`3) DPF Material characteristics
`' Pore Size
`' Mean pore size i Pore size distribution
`
`The available packaging envelope of the particular car
`and truck manufacturers influences the DPF shape
`(Contour and length). Therefore.
`the development
`focused on controllable parameters.
`such as
`cell
`structure and DPF material characteristics,
`to reduce
`DPF pressure-drop.
`
`CELL STRUCTURE OPTIMIZATION
`
`As shown in Figure 9. wall flow loss is a major influence
`on pressure-drop with-soot—Ioading. Greater filtration
`area by increasing call density is an effective approach.
`as shown in Figure 11.
`
`The impact of cell structure on pressure-drop with soot
`loading was calculated by using the pressure-drop
`equations. The conditions for this study are listed In
`Table w 2 and the calculated results are shown in Figure
`12 as isobars.
`
`Table 2. Conditions of pressure-drop calculation
`in terms of cell structure effect
`
`Material:
`Porosity:
`Mean Pore Size:
`DPF Size:
`
`Cordierite
`59 it.
`25 um
`1: 143.8mm x152.4mm
`(it 5. 66"):x6”)
`3 giL i3otal 7.59}
`Amount of Soot:
`2.27m l min
`Gas flow rate:
`Gas Temperature: 200 degree C.
`
`When the cell density is less than 200cpsi. the DPF
`pressure-drop with (ML of soot
`loading depended
`mainly on cell density. This is because higher cell
`density has a larger filtration area.
`
`On the other hand, when the cell density is greater than
`300cpsi. DPF pressure drop is primarily dependent on
`wall thickness and much less on cell density. Above 300
`cpsi.
`the cell structure increases filtration efficiency
`resulting in low wall flow drop. however channel friction
`loss becomes high due to small hydraulic diameter.
`
`Based on these results and considering the potential
`soot plugging of a higher cell density DPF. 300cpsi was
`selected as the optimum cell structure for the filter In
`order to have low pressure-drop.
`
`thickness is the other design consideration to
`lNail
`reduce pressure—drop. Thinner wall reduces wall flow
`loss. resuiting in low pressure-drop. But. thinner well
`also reduces thermal mass and mechanical strength of
`the filter. Lower
`thermal mass increases the DPF
`temperature during forced and
`uncontrolled
`soot
`regeneration. From these points of view.
`it
`is not
`advantageous to significantly reduce wall thickness.
`
`CellDensity.
`
`
`
`cpsi
`
`Web Thickness, mil
`
`Figure 12.
`
`Pressure-drop of Cordierite DPF
`with soot loading condition
`
`DPF MATERIAL OPTIMIZATION
`
`KEY MATERIAL PARAMETERS 0N BACK-PRESSURE
`
`In order to identify the key DPF material characteristics
`which effect pressure drop under soot loading conditions.
`21 different Cordierite and 4 different SIC DPF's were
`prepared for the evaluation.
`
`Basically. porosity and mean pore size were determined
`to be the key material characteristics of DPF materials.
`To
`investigate
`the
`influences
`of
`pore
`sizes
`characteristics further, several parameters were used for
`this study besides porosity and mean pore size.
`
`In order to investigate the effect of small pores. the
`parameter of "-10um porosity? was established. This
`parameter is defined as the integrated porosity of pores
`less than 10pm diameter. The parameter of “10-70pm
`porosity” and '+70p.m porosity" are defined as the
`
`8
`
`

`

`‘4
`
`the results of pressure-drop and filtration efficiency. the
`optimum pore size is between 10pm and 70pm.
`
`MECHANISMS OF PRESSURE DROP 0F DPF
`
`Further evaluation was conducted to confirm the soot
`trapping mechanism. The cross sectional area of the
`DPF wall was observed at 0.1. 0.3 and 0.5 git. levels of
`soot loading. As shown in Figure 6. the characteristics of
`the pressure-drop vs. soot loading are: a steep slope
`under the low soot loading (<0.1 gi'L) condition (region
`'1"); then transition to a moderate slope between a soot
`loading of 0.3 glL and 0.5 glL (region ‘Il") : then. above
`0.5 giL, the pressure-drop has a linear dependency on
`amount of soot loading (as shown “ill“ in the Figure 6}.
`
`shows the soot distribution. The top
`Photograph 1
`photographs are observation results on the surface of
`DPF wall. Letter “A” shows the surface of the inlet
`channel. letter “B“ shows the DPF wall. and letter “C”
`shows the surface of the outlet channel. The surfaces of
`the inlet channel had dark or black colors. which is the
`trapped soot.
`
`The lower photographs are the cross sectional area of
`the walls. Arrows In the photographs show the direction
`of gas flow. Photograph 1 {1] shows the cross sectional
`view of DPF material under 0.1 glL of soot
`loading
`condition. Pores inside the wall trapped soot. Near the
`entrance of the gas flow. the density of trapped soot is
`high compared with near exit side. in Photograph 1 (2).
`this phenomenon becomes clear. Finally in Photograph
`3. the soot layer developed on the DPF wall on the inlet
`channel. The top photograph in Photograph 1 (3) also
`shows the uniform black color. which is the developed
`soot layer.
`
`Based on these results. as the pores inside the DPF watt
`trap the soot. the pressure-drop rapidly increases. and
`the rate of increasing pressure-drop diminishes after the
`soot layer is developed. This rate seems to depend on
`the porosity of soot layer. It appears that porosity of the
`soot layer is relatively high.
`
`These observations did not prove if the smaller pores
`trap soot (less than 10m in diameter), due to difficulty
`to Identify unplugged small pores. However. according to
`these observation results.
`it
`is clear that pore-size
`characteristics are very important for pressure drop with
`soot loading. In addition to the influence of pores. soot-
`size influence and pore connectivity are items for further
`evaluations.
`
`integrated porosity from mum to 70pm and more than
`70pm respectively. Figure 13 is a schematic Illustration
`of these parameters. The relationship between these
`parameters and the pressure drop of DPF's with 1.5 git.
`of soot loading were investigated in order to assess the
`impact of the mid- and large size pores.
`
`10-?0pm Porosity :
`Integrate porosity
`l'rorn 10pm to 1'0er
`of pore diameter
`
`-10p.m Porosity :
`Integrate porosity
`under 10pm of
`pore diameter
`
`+70pm Porosity :
`Integrate porosity
`over 10pm of
`pore diameter
`
`Porosity 10 I
`
`Pore Diameter {um}
`
`Figure 13.
`
`Schernatlc Illustration of parameters'
`definition for the study of pressure
`drop mechanisms
`
`Figure 14 shows that generally the higher the porosity.
`the lower the pressure-drop, with the exception of an
`average mean pore size below 10pm. The total porosity
`is the key contributor for pressure drop under soot
`loading conditions. Also. DPF materials with a high
`concentration of mean pore size below 10pm have low
`porosity. This seems to be one of the reasons why the
`relationship between the porosity of below 10pm of
`pores and pressure-drop showed the opposite behavior.
`
`From these results, it was concluded that a material with
`high porosity and with high pore sizes is desirable to
`have low pressure drop performance.
`
`KEY MATERIAL PARAMETERS 0N FILTRATION
`EFFICIENCY
`
`Filtration efficiency is an equally key function of the DPF.
`The impact of these pore size and porosity on the
`filtration efficiency were also evaluated. Figure 15 shows
`these results.
`In this case.
`the relationship between
`filtration efficiency without soot loading condition was
`investigated.
`
`Figure 15 outlines very interesting results. In this case.
`the pore size of greater than 70pm has an adverse elfect
`on filtration efficiency. This tendency is not as obvious
`for the 10 pm to 70pm pore range. On the contrary. the
`porosity of less than 10pm diameter pores had a positive
`relationship with filtration efficiency. Soot can easily pass
`through large pores (more than 70pm) and can be easily
`trapped by the small pores (with less than 10pm
`diameter) at the initial soot loacfing condition. Based on
`
`9
`
`

`

`1) The porosity of pores below 10pm of pores
`
`4) The total porosity
`
`
`
`
`
`7 a
`
`:
`
`01
`
`
`
`Pressuredrop(kPa) (a)
`
`.5
`
`N
`
`a...
`
`“NI
`
`03
`
`ubU'l
`
`b)
`
`N
`
`0IIIII.onmac--
`
`.4
`
`0)
`
`
`
`Pressuredrop(kPa):-a.
`
`m M
`
`O
`
`.I. D
`20
`g '0O afl.9:..33
`
`mo
`
`30
`
`40
`
`60
`50
`Total porosity ($9)
`
`70
`
`80
`
`2) The porosity of pores from 10 to 70pm of pores
`
`5) The permeability
`
`d)
`
`0|
`
`
`
`
`
`(kPa) w
`.h Pressuredrop
`
`O
`
`20
`
`60
`40
`10-70pm pomsityt%)
`
`mD
`
`0
`
`2
`
`4
`
`.000
`
`6
`
`8
`
`For I MPSz(x1O'1° m2)
`
`3) The porosity of pores above 70pm of pores
`
`7 O
`
`il
`
`
`
`
`
`Pressuredrop(kPa}hat
`
`(.0
`
`N
`
`o
`
`20
`10
`+70prn porosity(%)
`
`o:O
`
`Figure 14. The correlation study between pore
`characteristics and pressure drop
`
`10
`
`10
`
`

`

`1) The porosity of pores below 10pm of pores
`
`4) The total porosity
`
`A oo
`
`
`
`
`
`Filtrationefficiency(56) J-D
`
`
`
`
`
`mD
`
`O)D
`
`hC)
`
`MCl
`
`
`
`
`
`Filtrationefficiency(9%)
`
`0
`
`20
`10
`4me porosilyt'i’u)
`
`30
`
`0
`
`asD
`
`40
`
`60
`50
`Total porosity (9’9)
`
`70
`
`80
`
`2} The porosity of pores from 10 to 70m of pores
`
`5) The permeability
`
`‘8‘
`
`100
`
`n1:-
`
`
`
`
`
`8
`
`Ea
`
`.2 60
`g{D
`
`540
`"E520LL
`
`0
`
`o
`
`2
`
`-
`
`4
`
`-
`
`6
`
`-
`
`8
`
`10
`
`For x Mps’(x1o"° m2}
`
`
`
`('56)
` Filtrationefficiency
`
`
`3 N
`
`D
`
`0
`
`60
`40
`20
`10-70pm porosity{%)
`
`80
`
`3) The porosity of pores above 100m of pores
`
`'8‘
`
`onO
`
`
`
`
`
`mD
`
`.h.0
`
`Filtrationefficiency(5%) NO
`
`
`
`
`
`U
`
`20
`1 0
`+70,um porosity(%}
`
`30
`
`Figure 15. The correlation study between pore
`characteristics and filtration efficiency
`
`11
`
`11
`
`

`

`110.191 of soot loading
`
`
`
`1 o
`
`APPROACHES FOR Low PRESSURE DROP
`up]:
`
`LOW BACK-PRESSURE TYPE CORDIERITE DPF
`MATERIAL
`
`the most
`Based on the aforementioned findings.
`etTective material
`improvement
`strategy to reduce
`pressure drop is to increase the material porosity and
`maintain the pore size between 10pm to 70pm. In this
`section. new Cordierite and SIC material developments
`are described.
`
`NGK Insulators. LTD has newly developed low pressure-
`drop type Cordierite DPF's. Material development has
`been completed which increases the porosity from the
`current mass production level (53%) to 65%.
`
`The current production material. DHC-558. has a
`porosity and mean pore size of 53% and 15pm.
`respectively. From this material. porosity and mean pore
`were increased as the initial step. This material. DHC-
`611, has a 59% porosity and 25pm mean pore size.
`
`approaches were
`different
`two
`From DHC-S11.
`conducted. The first was to increase porosity and
`maintain the 20m mean pore size. This material Is Cd-1
`with 65% porosity. The other direction was to increase
`the mean pore size to 35pm. This material. Cd-2. has
`59% of porosity. the same as DHC-611.
`
`these
`The Table 3 summarizes the properties of
`materials.
`and
`Figure
`16
`shows
`the material
`development approach schematically.
`
`Table 3. Properties of Low Pressure-drop
`type Cmdiertte DPF
`
`DPF II.“-
`
`mm
`
`
`
`
`
`
`
`
`1
`
`Mean
`
`
`
`
`.
`.
`.
`12
`17
`12 ITIII
`12 ml
`12 ml
`mil
`m“
`
`
`100
`cpsi
`
`300
`cpsi
`
`300
`cpsi
`
`300
`cpsi
`
`300
`cpsi
`
`Photograph 1.
`
`Cross sectional observation results
`of DPF wall
`
`The pressure drop performance of these materials was
`evaluated. and is graphed in Figure 17. The measuring
`conditions are shown in Table 4. The higher the porosity.
`
`12
`
`12
`
`

`

`it
`
`
`
`0
`
`15
`10
`5
`Amount of Soot Loading. 9
`
`20
`
`Filtration efficiency evaluation results
`Figure 18.
`of low pressure-drop type Cordierite DPFs
`
`these Cordien'te
`size distributions of
`The pore
`materials was determined and are plotted in Figure 19.
`The pore diameter plot of all the materials. exoept Cd-
`2. exhibited a statistically normal distribution. The
`peaks of these curves are between 20pm to 30pm.
`0n the other hand, Cd-2 had unique pore distribution
`with two peaks. One is around 20pm and the other is
`100um.
`
`
`
`
`.mnm
`
`1000
`
`1 00
`10
`Average Pore Diameter. urn
`
`
`1
`
`Figure 19.
`
`Pore size distribution of low pressure-
`drop type Cordierite DPFs
`
`According to the Figure 14 and 15. the optimum pore
`size is from10um to 70pm. DHC-6‘l1 and Cd-1 have
`sharp pore size distribution with Increasing porosity.
`Almost all pores of these materials are within this
`range. This explains the superior pressure-drop and
`filtration efficiency of these materials. Also. Figure 14
`concluded that DPF material with a porosity of 709m
`or more had low filtration efficiency. This is because
`particulate matter can easily pass through large pores.
`This explains why Cd-2 exhibited lower
`filtration
`efficiency.
`
`the lower the pressure-drop. The pressure-drop of Cd-1
`with 12mil l 300opsi are approximately 60% lower than
`OHS-558 with 17mil I 100 cpsi. and 25% lower than
`DHC-558 with 12mil it 300cpsi.
`
`70
`
`GI01
`
`
`
`Porosity('56) 8
`
`0|0!
`
`Mean Pore Size {um}
`
`Figure 16
`
`Schematic illustration of Cordierite DPF
`material development approaches
`
`N 01
`
`17mlir'1 00cpsi
`
`
`
`01
`PressureDrop.kPa_|-LMO01ID
`
`0
`
`15
`10
`5
`Amount of Soot Loading. 9
`
`20
`
`Figure 17. Pressure-drop evaluation results of low
`pressure-drop type Cordierite DPFs
`
`Tabie 4. Measuring condition of pressure drop
`
`DPF Size:
`
`it 143. 8mm x152.4mm
`(ii 5.66" x 6")
`2.27m3 I min
`Gas flow rate:
`Gas Temperature. 200 degree C
`Generating rate of Soot: 5.5g ! hour
`
`The filtration efficiency of these materials are also
`evaluated (Figure 18). The Cordierite materials with less
`than 3 25pm mean pore size had more than 90% of
`filtration efficiency with soot
`loading, while Cd-2 with
`351.011 of mean pore size had lower filtration efficiency.
`
`13
`
`13
`
`

`

`l L
`
`The pressure drop performance of Sic-1 and Sic-2 were
`measured and compared with Sic-3. The measuring
`conditions are the same as shown in Table 4.
`
`Figure 20 graphs the evaluation results. Sic-2 with 12mil
`l 3000psi has roughly 18% lower pressure-drop than
`SiC-‘l. Compared with the re-crystallized Sic-DPF with
`16mil of wall thickness and 180cpsi of cell density. Sic-2
`with 12mil and 300cpsi showed roughly 30% lower back-
`pressure.
`
`20
`
`i
`
`
`
`D
`
`5
`
`1D
`
`15
`
`20
`
`Amountot SootLoading {9}
`
`10
`
`5
`
`O
`
`.. 15
`it?
`1‘O.
`
`Eo
`
`d:
`
`at
`
`?
`
`.E
`
`Figure 20. Pressure-drop evaluation results of low
`pressure—drop type SIC DPF
`
`Based on these results. SEC-2 and Cd-‘l are excellent
`candldates for a low pressure-drop type DPF.
`
`LOW PRESSURE TYPE DPF FOR CATALYZED
`SYSTEM
`
`Recently. catalyzed DPF systems have been proposed.
`One example was presented in reference [3]. In the case
`of catalyzed DPF system. the wash coat. which contains
`catalyst for soot oxidation. is applied inside the pores. As
`a result. the porosity of the DPF material is reduced and
`the post-coated pore size distribution is changed.
`
`low
`As the total porosity is the key parameter for
`pressure-drop with soot
`loading.
`low pressure-drop
`Cordierite and SiC DPF are good candidates for the
`catalyzed DPF. Some catalyzed systems are also
`considering NOX adsorption catalyst. For this system.
`larger porosity is preferable.
`
`the
`on
`pressure-drop measurements
`Preliminary
`catalyzed DPF’s were conducted. For this evaluation.
`DHC-SSB and DHC-611 were used. Cell structure for
`this