# Diesel Particulate Filter Maintenance: Current Practices and Experience

June 2005

Manufacturers of Emission Controls Association

W a shington, D.C. 

### www.meca.org

### Table of Contents

EXECUTIVE SUMMARY......................................................................... 1

1.0 INTRODUCTION............................................................................... 3

2.0 DIESEL PARTICULATE FILTERS........................................................ 4

2.1 Operating Characteristics and Performance...................................... 4 2.2 The Impact of Sulfur in Diesel Fu el on Diesel Particulate Filters............. 7 2.3 Operating Experience.................................................................. 8

3.0 FILTER MAINTENANCE PRACTICES...................................................10

3.1 Sources of Filter Ash....................................................................11 3.2 Factors that Influence Ash Accumulation..........................................12 3.3 The Impact of Filter Properties on Ash Accumulation...........................13 3.4 Filter Maintenance Intervals..........................................................15 3.5 Filter Cleaning Procedures and Equipment.......................................17

4.0 U.S. 2007 HEAVY-DUTY DIESEL FILTER MAINTENANCE.......................21

REFERENCES.......................................................................................22



### Figures

Figure 1. Wall-Flow Diesel Particulate Filter....................................................5

Figure 2. Asymmetric Particulate Filter Substrate Designs .................................14

Figure 3. Diesel Particulate Filter Cleaning Unit .............................................18

June 2005 iEXECUTIVE SUMMARY

Diesel engines are im portant power system s for on-road and off-road vehicles. These reliable, fuel-efficient, high torque engines pow er m a ny of the world’s heavy-duty trucks, buses, and non-road vehicles. W h ile diesel engines have m a ny advantages, they have the disadvantage of em itting significant am ounts of particulate m a tter (PM) and oxides of nitrogen (NOx) into the

atm o sphere. Diesel engines also em it toxic air pollutants. Health experts have concluded that pollutants em itted by diesel engines adversely af f ect hum an health and contribute to acid rain, ground-level ozone and reduced visibility. Studies have shown that exposure to diesel exhaust causes lung dam a ge and respiratory problem s and there is increasing evidence that diesel

em issions m a y cause cancer in hum ans.

In response to these health concerns, a num ber of countries worldwide (including the United States, the European Union, and Japan) have established em ission lim its for new diesel engines, with recent regulatory program s for ne w diesel engines requiring significant reductions in diesel particulate em issions in the 2005-2010 tim efram e . Health concerns associated with diesel exhaust have also focused attention on em issions from the large population of existing diesel vehicles. As a result, local, regional, and national voluntary and m a ndatory diesel retrofit

program s have been initiated in a variety of world m a rkets. To date these retrof it ef f o rts have f o cused on the application of diesel oxidation catalysts and diesel particulate f ilters f o r the reduction of diesel PM on in-use diesel engines operated in both on-road and off-road applications. These retrofit program s are expanding rapidly with m o re cost-effective technology solutions being verified for use on in-use dies el engines and a greater variety of technology options available for application on a wider range of in-use engines.

Diesel particulate f ilters (DPFs) rem ove particulate m a tter in diesel exhaust by f iltering exhaust from the engine. They can be installed on diesel-powered vehicles, off-road equipm ent, or stationary diesel engines. W a ll-f l ow type f ilters can reduce diesel PM by m o re than 85%

com p ared to an unf iltered diesel exhaust. A growing experience base is available f o r the application of filters on diesel engines, including m o re than 200,000 retrofit applications of f ilters worldwide and m o re than 1 m illion applications of f

ilters on new light-duty diesel passenger cars in Europe. Successf ul application of diesel particulate f ilters on new or existing diesel engines requires a robust filter regeneration schem e that periodically oxidizes the collected soot present on the f ilter to m a intain engine backpressure characteristics within specified lim its. These regeneration m e thods include both passive (e.g., m e thods that rely strictly on the available exhaust tem p eratures in a given application) and active (e.g., supplem ental heat inputs to the exhaust required to initiate soot com bustion) system s. The ef

f ectiveness and durability of catalyst-based DPFs is m a xim i zed with the use of ultra-low sulf ur diesel fuel.

In addition to collecting soot, f ilters also collect inorganic-based exhaust constituents that are derived from several sources, including the com bustion of engine lubricants, products of

norm a l engine wear and/or corrosion, and m a terials associated with fuel-borne catalysts in DPF applications that use these catalysts to assist in the f ilter regeneration process. Over extended operation on the vehicle, these ash species slowly accum u late within the filter and gradually increase the pressure drop across the f ilter. Since excessively high backpressure on the engine

June 2005 1will result in a degradation of engine perform a n ce, the accum u lated ash m a terial within the filter needs to be periodically rem oved. This ash rem oval or cleaning operation is a necessary f ilter

m a intenance operation. Engine oil consum ption characteristics, the total ash content of engine lubricant f o rm ulations, vehicle duty cycles, f ilter designs, and f u el-borne catalyst dosing rates all im pact ash accum u lation profiles and required filter m a intenance cleaning intervals.

Retrof it applications of wall-f l ow f ilters on heavy-duty diesel vehicles and original equipm ent applications of wall-f l ow f ilters on passenger cars in Europe provide inf o rm ation on current f ilter m a intenance interval recom m e ndations f r om m a nuf acturers of f

ilters or new lightduty vehicles equipped with f ilters. In retrof it applications on existing heavy-duty diesel highway vehicles, f ilter m a nuf acturers recom m e nd that f ilters be cleaned of ash af ter either 12 to 24 m onths of operation, or at m ileage intervals that range from 60,000 to 100,000 m iles. It should be noted that actual m a intenance intervals are based on factors such as engine lubricant

consum ption rates, duty cycles, and actual engine/vehicle applications. Older vehicles with higher lubricant consum ption rates, for exam ple, m a y need m a intenance intervals that are m o re frequent than the m a intenance intervals stated above. Late-m odel heavy-duty diesel engines with low oil consum ption characteristics operated in over-the-road applications with retrofit, catalyst-based DPFs have successfully operated for m o re than 200,000 m iles before filter cleaning was initiated. In Europe, passenger car f ilter m a intenance intervals with system s using fuel-borne catalysts were initially designed for 80,000 km (ca. 50,000 m i ) of operation but now have been extended up to 200,000 km (ca. 120,000 m i ) with lower engine-out PM levels, lower f u el-borne catalyst dosing rates, and new f ilter designs f eaturing higher ash storage capacities.

Manufacturers of filter technologies have developed safe, reliable, and effective procedures f o r rem oving the ash f r om particulate f ilters. These practices have been put in place in both retrof it wall-f l ow f ilter applications and wall-f l ow f ilter-equipped passenger cars in Europe. The ash cleaning practices include well-docum ented procedures f o r cleaning f ilters and the developm ent of stand-alone f ilter cleaning stations or f acilities that autom a te m a ny of the necessary procedures. These procedures include com b inations of pressurized dry air stream s directed at the exit side of the filter with indus trial vacuum devices used on the inlet side to saf e ly collect ash rem oved f r om the f ilter. In som e cases very controlled high tem p erature treatm e nts of the f ilters are used bef o re or af ter air cleaning procedures to rem ove organic

m a terials and soot that m a y be contained in the filter. Ash collected from used filters should be disposed of according to local, state, and/or federal solid waste disposal regulations.

The f i rst large-scale application of diesel particulate f ilters in the U.S. on new vehicles will occur starting in 2007 for heavy-duty diesel engines used in highway trucks and buses. All heavy-duty engine m a nuf acturers have indicated thei r intent to use diesel particulate f ilters to

com p ly with the U.S. EPA’s 2007 PM em ission regulation of 0.01 g PM/bhp-hr. In public forum s to date, the engine m a nufacturers have indicated that com p lying with EPA’s 150,000

m ile m i nim u m f

ilter m a intenance interval f o r heavy heavy-duty vehicles is part of their developm ent plan for 2007 engines. The em issions control industry is com m itted to work with their custom ers in the engine m a nufacturing industry to m a ke the 150,000 m ile filter

m a intenance interval a reality f o r as wide an application base on new heavy-duty diesel engines as possible.

June 2005 21.0 INTRODUCTION

Diesel engines provide im portant fuel econom y and durability advantages for large heavy-duty trucks, buses, and non-road equipm ent. They are often the power plant of choice for heavy-duty applications. W h ile they have m a ny advantages, they also have the disadvantage of

em itting significant am ounts of particulate m a tter (PM), oxides of nitrogen (NOx), and lesser

am ounts of hydrocarbon (HC), carbon m onoxide (CO), and toxic air pollutants.

Particles em itted from diesel engines are sm all – in m o st cases less than 2.5 m i crons in aerodynam i c diam eter. The particles are com p lex, consisting of an uncom busted carbon core, adsorbed hydrocarbons from engine oil and diesel fuel, adsorbed sulfates, water, and inorganic

m a terials such as those produced by engine wear. Because of their extrem ely sm all size and

com position, the particles em itted by diesel engines have raised m a ny health concerns. Health experts have expressed concern that diesel PM m a y contribute to or aggravate chronic lung diseases such as asthm a , bronchitis, and em physem a.

There is growing evidence that exposure to diesel PM m a y increase the risk of cancer in hum ans. A com p rehensive assessm ent of available health inform ation, carried out by the International Agency for Research on Cancer (IARC) in June 1988, concluded that diesel particulate is probably carcinogenic to hum ans. The term “carcinogen” is used by the IARC to denote an agent that is capable of increasing the incidence of m a lignant tum o rs. In August 1998, California’s Air Resources Board identified PM em issions from diesel-fueled engines as a toxic air contam inant. In 2000, the U.S. EPA declared diesel PM to be a “likely hum an carcinogen.” A recent report, “Diesel and Health in Am erica: The Lingering Threat,” issued in February 2005 by the Clean Air Task Force (see www.catf.us for a copy of this report), reviews the health im pacts of diesel particulate em issions in the United States. This report states that fine particulate pollution from diesel engines shortens the lives of nearly 21,000 people in the U.S. every year, with health-related dam a ge from diesel PM estim ated to total $139 billion in 2010.

Despite health and environm ental concerns, the diesel engine rem a ins a popular m eans of powering trucks, buses, and other heavy equipm en t. Most buses and heavy-duty trucks are powered by diesel engines for good reasons. Diesel engines are reliable, fuel-efficient, easy to repair, inexpensive to operate, and have a relatively low initial purchase price. One of the m o st im pressive attributes of the diesel engine is its durability. In heavy-duty trucks, som e engines have achieved operating lives of 1,000,000 m iles or m o re while som e engines power city buses for up to 15-20 years.

A num ber of countries worldwide have established em ission lim its for new diesel engines, with recent regulatory program s for ne w diesel engines requiring significant reductions in diesel particulate em issions in the 2005-2010 tim efram e . In the United States, the Environm ental Protection Agency finalized in 2000 a com p rehensive regulatory program to reduce em issions from on-road heavy-duty engines (diesel and gasoline) in the 2007-2010 horizon (see www.e p a .gov/ OM SWWW/ di e s e l .ht m#hd2007 for details on EPA’s 2007 highway diesel program ). Specifically, this EPA hi ghway program requires new heavy-duty diesel engines to m eet a 0.01 g/bhp-hr PM standard st arting in 2007, a 90% reduction in PM em issions

com p ared to the current 0.10 g/bhp-hr PM standa rd for new diesel engines that heavy-duty

June 2005 3engine m a nufacturers began to m eet in 1994. Ja pan also has put in place em ission standards starting in 2005 that require new heavy-duty di esel engines to m eet a 0.027 g/kW -hr (0.02 g/bhphr) PM standard. These ultra-low PM em issi on standards for new heavy-duty engines in both the U.S. in 2007 and Japan in 2005 are expected to be achieved through the application of diesel particulate filters (DPFs). In Europe, disc ussions are on-going concerning the next round of

em ission standards for light-duty (Euro 5) and hea vy-duty (Euro 6) vehicles. These next sets of standards will likely include signif i cantly tighter standards f o r PM em issions f r om all new diesel-powered vehicles sold in Europe starting perhaps as early as 2010.

Since som e diesel engines have very long operating lives, older uncontrolled diesel vehicles will continue to m a ke up a signif i cant portion of the heavy-duty vehicle f l eet in world

m a rkets for years to com e . Given the health and environm ental concerns associated with diesel engines, there is increasing interest to retrofit older, “dirtier” diesel engines while newer, “cleaner” diesel engines enter the m a rketplace. Diesel particulate filters for controlling PM

em issions from existing diesel cars and trucks are seeing growing retrofit applications in m a ny world m a rkets, including the U.S., Europe, Japan, and Korea, with pilot program s targeted for Mexico, Thailand, China, and other developing count ries. These retrofit applications of DPFs have provided the f i rst experiences with respect to f ilter m a intenance practices. Filter

m a intenance of high efficiency particulate filters (e.g., ceram ic wall-flow filters) is necessary due to the accum u lation of inorganic m a terial (ash) within the filter m e dia over tim e that stem s prim arily f r om the com bustion of engine lubricants. Filter m a intenance procedures and equipm ent have been developed to serve the growing retrofit application of DPFs. This report

sum m a rizes the present f ilter m a intenance experience base and highlights developm ent trends in filter design, engine design, and lubricant form ulations that will im pact filter m a intenance practices on new heavy-duty diesel engine appli cations as they com e on-stream during this decade in the United States and Japan. The m a intenance experience detailed in this report is based exclusively on f ilter designs em ploying ceram ic wall-f l ow f ilters. These wall-f l ow f ilters currently dom inate both the retrofit and original equipm ent-based applications of diesel particulate f ilters in all world m a rkets. A brief overview of diesel particulate f ilters provides a backdrop to f ilter m a intenance practices and experience.

2.0 DIESEL PARTICULATE FILTERS

2.1 Operating Characteristics and Performance

As the nam e im plies, diesel particulate f ilters rem ove particulate m a tter in diesel exhaust by f iltering exhaust f r om the engine. They can be installed on diesel-powered vehicles, of f -road equipm ent, or stationary diesel engines. Since a f ilter will f ill up over tim e, engineers that design filter system s m u st provide a m eans of burning off, or rem oving, accum u lated particulate

m a tter. A convenient m eans of rem oval of accum u lated particulate m a tter is to burn or oxidize it on the f ilter when exhaust tem p eratures are adequate. By burning of f trapped m a terial, the f ilter is cleaned or “regenerated.” Filters that regenerate in this fashion cannot be used in all situations, prim arily due to insufficient exhaust gas tem p eratures associated with som e types of diesel engines, the level of PM generated by a specific engine, and/or application operating experience. To ensure proper operation, filter system s are designed for the particular engine/vehicle application. In retrofit applications of filters, this application design process

June 2005 4includes extensive data logging of the vehicle(s) under consideration to quantify the exhaust

tem p erature history available during their norm a l operating conditions.

In som e non-road applications, disposable f ilter system s have been used in retrof it applications. A disposable f ilter is sized to collect particulate f o r a working shif t or som e other predeterm i ned period of tim e. After a prescribed am ount of tim e, or when backpressure lim its are approached, the f ilter is rem oved and discarded. Disposable f ilters target retrof it applications and would not m eet original equipm ent requirem e nts.

Filter Material A num ber of f

ilter m a terials have been used in diesel particulate f ilters, including: ceram ic (e.g., cordierite, alum inum titanate, or m u llite form ulations) and silicon carbide m a terials, fiber wound cartridges, knitted silica fiber coils, ceram ic foam , wire m e sh, sintered m e tal substrates, and tem p erature resistant paper in the case of disposable f ilters. Collection ef f i ciencies of these f ilters range f r om 40 to over 90 percent. Filter m a terials capture particulate m a tter by interception, im paction, and diffusion. Filter efficiency has rarely been a problem with the filter m a terials listed above, but work has continued to: 1) optim ize filter ef

f i ciency and m i nim i ze back pressure, 2) im prove the radial f l ow of oxidation in the f ilter during regeneration, and 3) im prove the m echanical strength of filter designs.

Figure 1 provides a diagram of a typical ceram ic wall-f l ow f ilter system . These wallf l ow f ilter designs are currently the design of choice f o r high ef f i ciency (> 85%) diesel particulate m a tter reductions. In Figure 1, particulate-laden exhaust enters the f ilter f r om the left. Because

Figure 1. Wall-Flow Diesel Particulate Filter

Trapped PM

Plugged Cells

CO 2 H 2 O PM CO HCs PAHs

(Note: PM = particulate m a tter, HCs = hydrocarbons, PAHs = poly-arom atic hydrocarbons)

June 2005 5the cells of the filter are capped at the downstream end, exhaust cannot exit the cell directly. Instead, exhaust gas passes through the porous walls of the filter cells. In the process, particulate

m a tter is deposited, or trapped, on the upstream side of the cell wall. Cleaned exhaust gas exits the f i l t e r t o t h e r i g h t

. Regeneration Many techniques can be used to regenerate a diesel particulate f ilter. Som e of these techniques are used together in the sam e f ilter system to achieve ef f i cient regeneration. Both on- and off-board regeneration system s exist. The m a jor regeneration techniques are listed below.

• Catalyst-based regeneration using a catalyst applied to the surfaces of the filter. A base or precious m e tal coating applied to the surface of the filter reduces the ignition

tem p erature necessary to oxidize accum u lated particulate m a tter.

• Catalyst-based regeneration using an upstream oxidation catalyst with either a bare or catalytically-coated filter. In this technique, an oxidation catalyst is placed upstream of the f ilter to f acilitate oxidation of nitric oxide (NO) to nitrogen dioxide (NO 2 ). The nitrogen dioxide oxidizes the collected particulate substantially reducing the tem p erature required to regenerate the f ilter.

• Fuel-borne catalysts. Fuel-borne catalysts reduce the tem p erature required for ignition of trapped particulate m a tter through direct contact between the catalyst and particulate that is collected together on the f ilter m e dia. In the case of wall-f l ow f ilters, the catalyst

rem a ins in the filter m e dia and adds to the inorganic ash that accum u lates within the f ilter.

• Air-intake throttling. Throttling the air intake to one or m o re of the engine cylinders can increase the exhaust tem p erature and facilitate filter regeneration.

• Post top-dead-center (TDC) fuel injection. Injecting sm all am ounts of fuel in the cylinders of a diesel engine after pistons have reached TDC introduces a sm all am ount of unburned fuel in the engine’s exhaust gases. This unburned fuel can then be oxidized over an oxidation catalyst upstream of the f ilter or oxidized over a catalyzed particulate filter to com bust accum u lated particulate m a tter.

• Post injection of diesel fuel in the exha ust upstream of an oxidation catalyst and/or catalyzed particulate filter. This regeneration m e thod serves to generate heat used to

com bust accum u lated particulates by oxidizing fuel across a catalyst present on the filter or on an oxidation catalyst upstream of the f ilter.

• On-board fuel burners or electrical heaters. Fuel burners or electrical heaters upstream of the filter can provide sufficient exhaust tem p eratures to ignite accum u lated particulate and regenerate the f ilter. In som e cases regeneration can be accom plished while the vehicle is in operation, whereas in other cases the vehicle m u st be out of service for regeneration to proceed.

June 2005 6• Off-board fuel burners or electrical heaters. Off-board regeneration stations com bust trapped particulate m a tter by blowing hot air through the filter system

. The experience with catalyzed f ilters indicates that there is a virtually com p lete reduction in odor and in the soluble organic fr action of the particulate, but som e catalyst f o rm ulations m a y increase sulf ate em issions. Com p anies utilizing these catalysts to provide regeneration f o r their f ilters have m odif i ed catalyst f o rm ulations to reduce sulf ate em issions to acceptable levels. Low sulfur fuel (e.g., 0.0015 wt.% or 15 ppm S m a xim u m ) is becom i ng available in the U.S. and other world m a rkets and has greatly f acilitated these ef f o rts.

In som e situations, installation of a f ilter system on a vehicle m a y cause a slight f u el

econom y penalty (on the order of 1% or less). This fuel penalty is due to the increase in backpressure with the filter system . As noted above, som e filter regeneration m e thods involve the use of fuel burners or fuel injection strategi es and, to the extent those m e thods are used, there will be an additional fuel econom y penalty. Many filter system s, however, have been optim ized

to m i nim i ze, or nearly elim inate, any noticeable fuel econom y penalty. For exam ple, in a dem onstration program in Athens, Greece, no noticeable fuel penalty was recorded when the filter was regenerated with a cerium fuel-borne catalyst (29). More recently, experience in the New York City Transit program and in the San Diego school bus program has shown that fuel penalties for filters are zero or less than one percent. Little or no fuel econom y im pact was also reported as a part of DPF retrofit program in California involving late m odel tractor/trailer delivery trucks (13). During the required retr ofit technology verification protocols established by either the U.S. EPA or the Calif ornia Air Resources Board, f u el penalties have been docum ented at about 1% for high efficiency filter system s.

Filter system s do not appear to cause any additional engine wear or af f ect vehicle

m a intenance. Regarding m a intenance of the f ilter system itself

, m a nuf acturers are designing system s to m i nim i ze m a intenance requirem e nts during the useful life of the vehicle. In applications that m a ke use of high efficiency, wall-flow filters, accum u lated lubricating oil ash will have to be periodically rem oved. Manuf acturers provide the end-user with appropriate ash

rem oval procedures. Current practices and experience with f ilter m a intenance of wall-f l ow f ilters will be discussed in detail in the f o llowing sections of this report.

Filter system s have been designed so that vehicle drivability is not af f ected, or at least effects can be m i nim i zed, m o st notably by lim iting exhaust backpressure. Diesel particulate filter system s, which replace m u fflers in retrofit applications, have achieved sound attenuation equal to a standard m u ffler.

2.2 The Impact of Sulfur in Diesel Fu el on Diesel Particulate Filters

Sulfur in diesel fuel signif i cantly af f ects the reliability, durability, and em issions perf orm a nce of catalyst-based diesel particulate f ilters. Sulf ur af f ects f ilter perf orm a nce by inhibiting the perf orm a nce of catalytic m a terials upstream of , or on the f ilter. Sulf ur also

com p etes with chem ical reactions intended to reduce pollutant em issions (e.g., the oxidation of NO to NO 2 ) and creates particulate m a tter through catalytic sulfate form ation. (Sulfate

June 2005 7f o rm ation is som e tim es ref e rred to as “sulf a te m a ke.”) Catalyst-based diesel particulate f ilter technology works best when fuel sulfur levels are less than 0.0015 wt.% (15 ppm S). Meeting the m o st stringent em ission standards requires bot h the use of catalysts and ultra-low sulfur diesel fuel.

Particulate filter technology can be successfully us ed in applications where the fuel sulfur level is greater than 15 ppm S, but only af ter f ilter application engineers m a ke a caref u l

assessm ent of the fuel sulfur level, the engine, the type of filter system , the operating conditions, and the em ission reductions desired. PM em issions f o r catalyst-based f ilters operated with higher f u el sulf ur levels will be higher than operation with ultra-low sulf ur diesel f u el.

2.3 Operating Experience

Lim ited diesel particulate filter retrofit dem onstration program s began in the 1980s and continued in the early 1990s. The num ber of vehicles retrofitted, the num ber of program s, and the interest in new program s has grown significantly over the past few years with approxim a tely 200,000 DPFs installed as retrofits to date in a va riety of world m a rkets. Nearly all of these retrof it applications to date have utilized wall-f l ow f ilters like the design shown in Figure 1. Today, second and third generation retrofit filter system s can reduce PM em issions by 80 to

m o re than 90 percent.

In Europe, new light-duty vehicles equipped with diesel particulate f ilters are being offered com m e rcially. Filters were introduced on new diesel passenger cars in Europe in m i d2000, with m o re than 1,000,000 filter-equipped cars sold since that first introduction. These European light-duty vehicle applications have used ceram ic wall-f l ow f ilters exclusively thus f a r. No perform a nce or m a intenance issues have been reported in Europe with passenger car DPFs. Peugeot (PSA) was the first m a nufacturer to intr oduce a DPF system for European diesel cars in 2000 (16, 17, 20, 23, 25). This system em ploys a fuel-borne catalyst com b ined with advanced engine controls available with a m odern, high pressure, com m on rail fuel injection engine for filter regeneration. Other European autom obile m a nufacturers (e.g., Audi, Fiat, Ford, VW

, BMW

, Mercedes) are now offering DPF system s based on the PSA systemand the use of fuelborne catalysts, or catalyzed f ilter system s that do not em ploy a f u el-borne catalyst.

Sweden’s Environm ental Zones program resulted in the com m e rcial introduction of diesel particulate filters on urban buses (14). More than 4,000 buses have been retrofit with passive wall-flow filter system s in Sweden. Som e of these buses have accum u lated m o re than 250,000 m iles of service. Most recently in the sum m e r of 2004 Volvo Bus launched a fleet of new diesel buses operating along the west coast of Sweden equipped with catalyst-based DPFs for controlling diesel PM com b ined with selective catalytic reduction (SCR) system s using urea as the reducing agent to control NOx em issions. This bus fleet is claim e d to be the cleanest operating diesel bus f l eet in the world. Transit f l eets in other m a jor European cities such as London and Paris have also been successfully retrofit with DPFs. Renault Trucks have introduced an active regeneration DPF system in com b ination with a therm a l m a nagem e nt strategy as an original equipm ent option on one of their Euro 3 certif ied engine f a m ilies used in urban transit, ref u se hauler, and other heavy-duty on-road applications (4). A particulate f ilter based on sintered m e tal foils is now available in Europe for retrofit on certain m odels of existing

June 2005 8diesel passenger cars. A sim ilar sintered m e tal f o il-based particulate f ilter is being used on som e new heavy-duty diesel engines offered by one h eavy-duty engine m a nufacturer in Europe to

com p ly with the Euro 4 heavy-duty PM standards that began in 2005 (1, 24).

Transit bus retrofits using DPFs have also occurred in m a ny urban fleets across the U.S., including New York City (26), Boston, Philade lphia, W a shington, D.C., Seattle, and m a ny transit fleets in California. DPF retrofits are an im portant facet of the California Air Resources Board’s (ARB) Diesel Risk Reduction Program (see ARB’s report entitled “Risk Reduction Plan to Reduce Particulate Matter Em issions from Diesel-Fueled Engines and Vehicles” available at: www.arb.ca.gov/diesel/dieselrrp.htm ). This broad based ARB regulatory effort is charged with reducing PM em issions by 75% from all existing diesel engines used in California by 2010 and by 85% by 2020. To date ARB has approved a series of hybrid PM reduction rules covering transit fleets, refuse haulers, transportation refr igeration units (TRUs), portable diesel engines, and stationary engines that include DPF retrof its on existing engines (where applicable and verified) along with engine repowers and replacem ents as options for reducing PM em issions f r om these existing engines. Future ARB regulatory m easures will also require signif i cant PM reductions from public and private on-road and off-road vehicle fleets.

School buses have also been a focus of both ARB and EPA retrofit program s in the U.S., with a significant num ber of DPFs equipped on school buses since 2000 in conjunction with the use of ultra-low sulfur diesel fuel (15 ppm S m a x.). Additionally, several U.S. engine

m a nufacturers offer a “green” school bus engine fitted with a catalyzed DPF for use with ultralow sulfur diesel fuel. Retrofits with DPFs on ex isting vehicles is also occurring in Japan as part of efforts by the Tokyo Metropolitan Governm e nt and other local governm e nt agencies to significantly reduce PM em issions from diesel trucks traveling through urban areas.

As noted in the introduction, all engine m a nufacturers will be equipping their new onroad heavy-duty diesel engines sold in the U.S. with DPFs beginning in 2007 in order to m e et EPA’s 2007 PM standards. These heavy-duty system s are expected to be integrated with advanced engine controls to ensure filter regeneration under all vehicle operating conditions and environm ents. An exam ple of a heavy-duty engine equipped with a catalyst-based DPF system that includes active f ilter regeneration strategies using integrated engine controls was recently detailed by Hino Motors on a 12.9 liter engine targeting PM em ission levels below Japan’s 2005 heavy-duty diesel PM em ission requirem e nts (8). Additional presentations concerning the integration of DPFs on new heavy-duty diesel e ngines can be found in the recent proceedings for the U.S. Departm e nt of Energy’s Diesel Engine Em ission Reduction Conferences (see www.eere.energy.gov/vehiclesandfuels/resources/conferences/deer/index.shtm l ). Since 2001

som e new heavy-duty diesel engines have already been certified with DPFs in the U.S. These original equipm ent DPF applications have been ta rgeted at urban bus or school bus applications.

Diesel particulate filters have been installed on non-road equipm ent since 1986. Over 20,000 active and passive system s have been installed as either original equipm ent or as a retrofit worldwide. Som e non-road filter system s have been operated for over 15,000 hours, or over 5 years, and are still in use. Exam ples of non-road equipm ent equipped with filters include:

m i ning equipm ent, m a terial-handling equipm ent such as cranes and forklift trucks, street sweepers, construction equipm ent, stationary power producing engines, and utility vehicles.

June 2005 9Germ any, Austria, and Switzerland have established m a ndatory filter requirem e nts for underground m i ning equipm ent (15). EPA finalized new em ission standards for off-road diesel engines in m i d-2004 that are expected to provi de significant opportunities for DPFs over a wide range of new off-road diesel engines and applications sold in the U.S. in the 2011-2015 tim efram e (see www.epa.gov/nonroad-diesel/2004fr.htm for details on EPA’s off-road diesel

program ).

Diesel particulate f ilters can be com b ined with exhaust gas recirculation (EGR), NOx adsorber catalysts, or SCR (using hydrocarbons, ur ea/water, or am m onia as the reductant for NOx) to achieve significant NOx and PM reductions (9, 10, 22, 31). Engines retrofit with a low pressure EGR system and a DPF can achieve NOx reductions of over 40 percent and PM reductions of greater than 90 percent (22). Engines equipped with SCR and a f ilter or NOx adsorbers and a DPF can achieve NOx reductions of 60 to 90 percent and PM reductions greater than 90 percent (31). Com b ined NOx and PM reductions can also be achieved by recalibrating the engine to m i nim i ze NOx while using a f ilter to capture increased PM em issions. A lean NOx catalyst added to an exhaust system using a particulate f ilter (30) can reduce NOx em issions from 10 to 25 percent using diesel fuel as the reductant for NOx (NOx perform a nce of such a lean NOx catalyst is generally strongly tied to the fuel reductant use and reductant dosing strategy.).

Toyota recently com m e rcialized their DPNR (diesel particulate NOx reduction) system that directly applies a NOx adsorber catalyst f o rm ulation to a wall-f l ow ceram ic f ilter f o r

com b ined PM and NOx reduction. This system is available in lim ited production on a diesel passenger car sold in Europe (2.0 liter engine displacem ent, (12)) and a light-duty truck application sold in Japan (4.0 liter engine displacem ent, (11)).

3.0 FILTER MAINTENANCE PRACTICES

Typical wall-f l ow particulate f ilters like those shown in Figure 1 rem ove both particulate

m a tter that is produced from the com bustion of fuel in a diesel engine and assorted other inorganic m a terials that m a y be present in the engine exhaust. These assorted inorganic

m a terials are generally referred to as “ash.” Under norm a l filter operation, the filter collects particulate m a tter and periodically com busts the particulate m a tter during conditions that favor the oxidation of the largely carbon-based particul ate (i.e., filter regeneration). The inorganic

m a terial that also collects on the f ilter does not com p letely com bust into gaseous species that can exit through the filter but rather form s oxide and sulf ate m a terials that rem a in in the filter. Over extended operation on the vehicle, these ash species slowly accum u late within the filter and gradually increase the pressure drop across the f ilter. Since excessively high backpressure on the engine will result in a degradation of engine pe rform a nce, the accum u lated ash m a terial within the f ilter needs to be periodically rem oved. This ash rem oval or cleaning operation is a necessary filter m a intenance operation. This section of the report will describe what m a kes up the ash species that collect in diesel particulate f ilters, what f actors inf l uence the rate of ash build-up in the f ilter, what the typical f ilter m a intenance intervals are in current retrof it applications of wall-f l ow f ilters on heavy-duty diesel engines or wall-f l ow f ilters used on diesel passenger cars in Europe, and describe filter cleaning practices that are used today with wallJune 2005 10f l ow f ilters in either retrof it applications or in original equipm ent applications with European

passenger cars.

3.1 Sources of Filter Ash

Inorganic m a terial or ash that accum u lates in a diesel particulate filter stem s from three

m a in sources. A prim ary source of ash is inorganic additives that are present in engine lubricants (3, 5, 18, 19). Engine oils are a com p le x m i xture of a largely organic, refinery-derived “base stock” with an “additive package” that includes inorganic elem ents such as m a gnesium

, calcium , zinc, phosphorus, and sulfur. These additives are used to im part im portant lubricating and stability properties to the oil. A lubricant used in heavy-duty diesel engines today in the U.S. can have a total inorganic ash content that ranges from 1.2 to 1.5 wt.%. Sulfur and calcium will generally m a ke up the m a jority of the oil ash species with zinc, m a gnesium , and phosphorus generally found at lower levels in the oil ash. It is im portant to note, however, that there is a wide variation in lubricant com positions including ash constituents used in the m a rketplace today.

All four-stroke diesel engines consum e oil as part of their norm a l lubrication requirem e nts. (Note that in som e applications two-stroke diesel engines are still in use. Twostroke engines intentionally m i x fuel with lubricating oils prior to com bustion and generally have significantly higher oil consum ption rates than four-stroke engines.) This consum ed oil generally is com busted in the com bustion cham ber along with fuel. The products of the oil

com bustion process are contained in the engine exhaust gas and reach the particulate filter where the inorganic, ash constituents are collected and rem a in in the f ilter even af ter the organic particulate m a tter is rem oved in f ilter regeneration steps.

A second source of ash m a terial com e s from norm a l engine wear and exhaust com ponent scale or corrosion. These ash species can contain iron, nickel, silicon, chrom i um , alum inum

, copper, and/or tin. A third source of ash m a terial is associated with the use of a fuel-borne catalyst. In som e f ilter applications a f u el-borne catalyst is directly added to the f u el at relatively low levels (generally < 30 ppm in concentration) before com bustion. After com bustion, the inorganic catalyst species are carried by the exhaust to the filter where they collect along with the organic particulate and other inorganic ash m a te rials. These catalytic species assist in the f ilter regeneration process by lowering the com bustion tem p erature of the organic particulate below tem p eratures norm a lly associated with the com bustion of particulate by oxygen present in the exhaust. Like all other inorganic ash species, these inorganic fuel-borne catalysts rem a in on the f ilter and contribute to the total ash content of the f ilter. Fuel-borne catalyst inorganic species can include platinum , iron, and cerium (23, 25). Filter system s that do not m a ke use of f u el-borne catalysts will not have this contributor to the ash content of the f ilter.

In som e cases, soot and ash constituents that accum u late in a filter over tim e m a y interact and inf l uence the com bustion characteristics of the soot present in the f ilter. Fang and Lance (2), for exam ple, have recently shown that phosphorus constituents of lubricants that m a ke up som e of

the ash m a terials present in f ilters can f o rm protective layers on som e soot surf aces that in turn alters the com bustion characteristics of soot. This ash-soot interaction can cause the soot to

June 2005 11have a higher resistance towards oxidation and result in less com p lete regeneration of the f ilter under a given exhaust tem p erature history.

Recent SAE papers (27, 28) provide inform ation on ash com positions collected from filters used without fuel-borne catalysts. Prim ary ash constituents included calcium oxide (CaO, 29-35 wt.%), sulfate (SO 3 , 35-39 wt.%), m a gnesium oxide (MgO, 5-11 wt.%), phosphate (P 2 O 5 , 13-16 wt.%), and zinc oxide (ZnO, 5-10 wt.%), with a trace of iron oxide (Fe 2 O 3, < 1%) reported in one of these references. Ash is reported to be com p rised of 5-50 m i cron sized agglom erates of sub-m i cron prim ary particles. Particles from e ngine wear or corrosion of exhaust com ponents, however, can be substantially larger than this.

In the case of applications that m a ke use of fuel-borne catalysts, the portion of the ash

stem m i ng from the fuel-borne catalyst will depend on the composition and reactivity of the fuelborne catalyst, the dosing rate of the fuel-borne catalyst, and the fuel and lubricant consum ption characteristics of the engine (16, 20, 27, 28). For exam ple, in the initial Peugeot passenger car DPF applications, the fuel-borne catalyst co m ponent of the ash was approxim a tely 80% by weight of the total ash produced using a fuel-bor ne catalyst dosing rate of 25 ppm in the fuel (25). In the second generation Peugeot DPF system that utilized a fuel-borne catalyst dosing rate of only 10 ppm , the fuel-borne catalyst ash m a ss fraction was reduced to about 57% (23).

3.2 Factors that Influence Ash Accumulation

As stated above, inorganic constituents of engine lubricants are the prim ary source of ash

m a terial collected by wall-f l ow f ilters. Theref ore, f actors associated with lubricant consum ption in the engine have a large im pact on the rate of ash accum u lation in a filter. Two prim ary param e ters are the rate of oil consum ption of a given engine and the ash content of the oil

consum ed by the engine. Higher rates (e.g., liters of oil per m iles traveled or hours of engine operation) of oil consum ption by an engine at a constant oil ash content or consum ption of oil with a higher ash content will result in faster accu m u lation of ash in the filter and an associated faster rise in engine backpressure relative to som e base case operation. Engine oil consum ption rates are influenced by basic engine design, engi ne m a intenance practices, engine duty cycles, and the service lif e of the engine. (Diesel engines are of ten “re-built” af ter a signif i cant operating tim e. This re-build procedure can also influence engine oil consum ption rates.) More recently m a nufacturers have introduced im proved engine designs with lower engine oil

consum ption rates and continue to work on new engine designs with even lower oil consum ption rates. W e ll-m a intained engines can often consum e less oil than engines that are poorly

m a intained. Engines operating over relatively low tem p erature, low speed duty cycles, such as

som e transit buses and refuse haulers, can have higher lubricant consum ption rates than engines operating over duty cycles with higher operating tem p eratures such as highway vehicles.

Since diesel engines can have long operating lives, engines that have been in service over an extended tim e will generally consum e m o re oil than low m ileage or low hour usage engines due to norm a l engine wear. Oil consum ption can also be intentionally influenced by the use of certain engine afterm arket products that deliberately add oil from an engine’s oil sum p to the

com bustion cham ber while replenishing the oil reservoir with fresh lubricant, or by other procedures that m i x used engine oil with fuel as part of a used oil disposal process. These

June 2005 12procedures accelerate oil consum ption by the engine and accelerate ash accum u lation by the filter. Beginning in 2007, blending of oil into diesel fuel will not be allowed unless the oil sulfur content is no greater than 15 ppm (consistent with EPA’s 15 ppm sulfur cap for on-road diesel fuel ).

SAE paper no. 2003-01-1963 (19) reports that ash deposition profiles and filter pressure drop profiles can also be strongly influenced by the com bustion of oil-doped diesel fuels, especially in the case when the oil doped into the fuel has a relatively high ash content. The use of engine lubricants with higher ash contents relative to other oil form ulations will obviously result in a faster accum u lation of ash m a terial in the filter. In retrofit applications, filter

m a nufacturers generally provide recom m e nded m a xim u m lubricant consum ption rates as one

param e ter of whether an engine is an acceptable candidate for a filter retrofit – typical values can be in the range of one quart consum ed per 500 to 1,000 m iles of operation.

In the case of f

ilter system s that work with the use of a f u el-borne catalyst, the dosing rate or concentration of the catalyst in the f u el will have a direct im pact on the rate of ash

accum u lation within the filter. Since the introduc tion of filters using fuel-borne catalysts to assist with filter regeneration in European diesel passenger cars in m i d-2000, the m a nufacturers of the fuel-borne catalysts used in these vehicles have been able to reduce dosage rates from 25

ppm to 8 ppm through com positional and particle size changes to the fuel-borne catalyst, as well as reduced engine-out PM em issions from con tinued im provem e nts in diesel engine com bustion characteristics (23). These lower fuel-borne catalyst dosage rates, in turn, reduce ash

accum u lation rates in the filter and help to exte nd the service interval of the filter without adversely im pacting the regeneration characteristics of the f ilter. Continuing developm ent efforts aim e d at even m o re active fuel-borne catalysts have dem onstrated the potential of further reducing dosage rates to 7 ppm (17).

3.3 The Impact of Filter Properties on Ash Accumulation

The filter serves as a “storage tank” for ash during norm a l engine/vehicle operation. As this ash accum u lates over tim e, the pressure drop through the filter increases and backpressure on the engine increases. Filter or engine m a nuf acturers generally set a m a xim u m backpressure lim it on the engine that defines the service interval between ash rem oval m a intenance events. These m a xim u m backpressure criteria are based on engine perform a nce versus backpressure characteristics. Particulate f ilter properties, including the total volum e, length/diam eter ratio of the f ilter, cell density, wall thickness, and the porosity of the wall that acts as the f ilter m e dia on a given engine, are all im portant f actors in def i ning the initial engine backpressure of the f ilter system . Filter substrate m a nufacturers have been engaged in a continuous developm ent effort aim e d at optim izing inherent filter physical properties such as wall porosity, cell density, and wall thickness to m i nim i ze filter pressure drop for applications with or without the presence of a catalytic coating on the filter (21, 32). In general, this substrate optim ization process also includes other key filter properties such as m echanical strength, therm a l conductivity, and therm a l expansion properties. Catalyst m a nufacturers also have been actively involved in engineering the properties of catalyst coatings that can be applied to f ilter substrates to aid in backpressure reductions.

June 2005 13Ash storage capacity of a given f ilter with constant cell density and wall thickness is proportional to filter volum e. Larger filter volum es will be able to accum u late m o re ash before reaching a set m a xim u m engine backpressure criteria. However, the desire to m a xim i ze the filter cleaning interval is generally at odds with packaging lim itations on-board the vehicle or

m a xim u m engine backpressure criteria. Filter volum es are designed to m eet packaging constraints, with m i nim a l engine backpressure, but with sufficient volum e to allow for acceptable soot regeneration characteristics and an acceptable quantity of ash storage that def i nes the f ilter m a intenance interval.

More recently substrate m a nufacturers have m odified ceram ic wall-flow filter designs to signif i cantly increase ash storage capacity at a constant f ilter volum e. This design alteration involves enlarging the inlet exhaust gas channel dim e nsions (6, 7). This provides additional

geom etric area for the ash to accum u late in the f ilter and expands the ash capacity of the filter by up to 50%. Figure 2 provides schem a tics of two alternative f ilter substrate designs that have been developed to increase ash storage capacity. One em ploys a large octagonal inlet channel

Figure 2. Asymmetric Particulate Filter Substrate Designs w i th Larger Inlet Channel Cross-Sections Relative to Outlet Channel Cross-Sections



## in in ou 

## t

out

out out

out

in in

in in

in in out ou 

## t

in in out

cross section and the second em ploys a large square inlet channel cross section relative to

sm aller outlet channel cross sections. Com m e rcial application of these larger inlet channel substrate designs was initiated in 2004, with the first application (using the large octagonal inlet channel filter substrate design) launched on a new 2.7 liter V6 high pressure, com m on-rail diesel

June 2005 14engine developed jointly by Peugeot and Ford for passenger car applications in Europe. The high ash capacity filter used in these applications allows the DPF to function without

m a intenance over the useful life of the vehicle (up to 200,000 km , or m o re than 120,000 m iles). Additional applications of these large ash-capacity filter substrates are expected in 2005.

3.4 Filter Maintenance Intervals

Retrof it applications of wall-f l ow f ilters on heavy-duty diesel vehicles and original equipm ent applications of wall-f l ow f ilters on passenger cars in Europe provide inf o rm ation on current f ilter m a intenance interval recom m e ndations f r om m a nuf acturers of f

ilters or new lightduty vehicles equipped with f ilters. In retrof it applications on existing heavy-duty diesel highway vehicles, f ilter m a nuf acturers recom m e nd that f ilters be cleaned of ash af ter either 12 to 24 m onths of operation, or at m ileage intervals that range from 60,000 to 100,000 m iles. It should be noted that actual m a intenance intervals are based on factors such as engine lubricant

consum ption rates, duty cycles, and actual engine/vehicle applications. Older vehicles with higher lubricant consum ption rates, for exam ple, m a y need m a intenance intervals that are m o re frequent than the m a intenance intervals stated above.

As a condition of f

ilter retrof it verif i cation requirem e nts in the U.S., and to assist operators or fleet owners in determ ining when filters should be cleaned, retrofit filter

m a nufacturers include som e type of electronic di agnostic m odule that is installed on-board the vehicle at the tim e the filter is installed. This diagnostic m odule can provide several functions including m onitoring and logging exhaust tem p erat ures, engine backpressure, and/or filter pressure drop. Engine backpressure is typically m onitored to determ ine when sufficient ash has

accum u lated in the filter to require a cleaning operation. These diagnostic m odules generally include som e type of display m odule that is m ounted in the driver’s com p artm ent and alerts the operator when engine backpressure has reached a level that requires the filter to be cleaned. Backpressure threshold criteria are generally set with suf f i cient saf e ty m a rgins to ensure that excessive engine backpressure conditions are not reached under norm a l operations. These diagnostic m odules can also alert operators to abnorm a l engine operating conditions that m a y im pair norm a l filter operation. Backpressure m onitoring capabilities for DPFs are required by both the EPA and CARB as a part of their retrofit program s.

The rather wide range of f

ilter service intervals based on m ileage or m onths of operation in retrof it applications are a ref l ection of the variety of engines, duty cycles of f

ilter applications, and the age of existing vehicles. Retrof it f ilter m a nuf acturers have established a large and growing experience base with filters in applications on buses, highway trucks, industrial vehicles, and of f -road equipm ent that allows them to provide operators with guidelines f o r f ilter

m a intenance intervals along with diagnostic m odules that m onitor and alert operators when ash

rem oval is required.

There are exam ples of recent retrofit filte r applications on heavy-duty diesel highway engines that have dem onstrated m u ch longer filter m a intenance intervals than the typical 60,000 – 100,000 m ile recom m e nded intervals m e ntioned above. In a m u lti-year dem onstration

program operating in Southern Calif ornia, trucks and buses were retrof itted with catalyst-based diesel particulate f ilters and operated on ultra-low sulf ur diesel f u el (< 15 ppm S) to assess the

June 2005 15long-term durability of passively regenerating diesel particulate filters (13). This program included 10 Class 8 tractor-trailers used for delivery operations, a subset of which was evaluated for em ission perform a nce over a 3 1⁄2 year period with m ileage accum u lations exceeding 340,000

m iles for each of three test vehicles m onitored by the program . Each of these tractors was powered by a 1998 m odel DDC Series 60 heavy-duty diesel engine (430 hp, 12.7 liters displacem ent). During the first 3 1⁄2 years and 340,000+ m iles of operation for each of these three test trucks, f ilters were cleaned of ash only once - af ter two years and approxim a tely 220,000 m iles of operation on each vehicle. Backpressure m onitors on the test vehicles did not indicate that cleaning was required after two years of operation and the am ount of ash collected f r om these f ilters was relatively sm all. An im portant contributor to the relatively low backpressure increase over tim e observed on these DPF-equipped vehicles is the low oil

consum ption characteristics observed on the engines equipped in these vehicles. Oil

consum ption rates on average of approxim a tely 3 quarts per 10,000 m iles of service were observed on these vehicles. These tractor-trailers equipped with retrof it f ilter system s continue in operation at the tim e this report was released. These results indicate that m odern diesel engines equipped with f ilters and with low oil consum ption characteristics are capable of extended operations between f ilter m a intenance intervals.

In the case of new European passenger cars equipped with particulate f ilters, f i rst generation system s em ploying fuel-borne catalysts are designed to operate for 80,000 km before filter m a intenance is required. At 80,000 km , an owner is requested to bring his or her vehicle in for service at which tim e the filter is replaced with either an unused filter or a filter that has been cleaned of ash. Second generation system s em ploying lower dosage rates of fuel-borne catalysts have extended this m a intenance interval to 120,000 km . Third generation system s introduced in 2004 include lower fuel-borne catalyst dosage rates with new asym m e tric filter designs that

em ploy large octagonal inlet channel cross sections, allowing service intervals to be extended up to 200,000 km (up to 120,000 m iles). Recent reports of DPF designs based on the use of sintered

m e tal sheets indicate that these designs are capable of running m o re than 260,000 km (m ore than 155,000 m iles) on a 2.0 liter European diesel passenger car application em ploying a fuel-borne catalyst with a dosing rate of 10 ppm . First generation catalyzed DPFs available on som e European diesel passenger cars (e.g., BMW

, Mercedes, Renault) are currently sized to require

m a intenance at approxim a tely 80,000 km intervals. The filter system s used in European passenger car applications include diagnostic capabilities to m onitor exhaust tem p eratures, engine backpressure, and filter pressure drop with provisions to alert the driver if abnorm a l conditions are encountered.

Toyota and Hino recently reported on the ash accum u lation characteristics of their DPNR technology that com b ines a NOx adsorber catalyst coating on a relatively high porosity, wallflow ceram ic filter (11). In a 4.0 liter engine used in a Japanese light-duty truck application, they found only a m odest increase in pressure drop across the DPNR unit after 200,000 km (approxim a tely 125,000 m iles) of operation using available engine oil with 0.42 wt.% Ca. Analysis of the aged DPNR revealed that only about 20% of the theoretical ash am ount

consum ed by the engine during this test deposited on the f ilter. The balance of the ash was

assum e d to be em itted in the exhaust of the vehicle. The authors speculate that the continuous PM oxidation characteristics of the DPNR system prom ote separation of the ash f r om the f ilter. The relatively high porosity of ceram ic wall-flow filter used by Toyota in these DPNR

June 2005 16applications m a y also contribute to the release of ash f r om the f ilter during norm a l vehicle operation.

3.5 Filter Cleaning Procedures and Equipment

Manufacturers of filter technologies have developed safe, reliable, and effective procedures f o r rem oving the ash f r om particulate f ilters. These practices have been put in place in both retrof it wall-f l ow f ilter applications and wall-f l ow f ilter-equipped passenger cars in Europe. The ash cleaning practices include well-docum ented procedures f o r cleaning f ilters and the developm ent of stand-alone f ilter cleaning stations or f acilities that autom a te m a ny of the necessary procedures.

Ash cleaning procedures used in retrofit filter applications can involve the following steps:

(1) rem oval of the f ilter f r om the engine/vehicle; a f ilter weight bef o re cleaning m a y be determ ined at this point (2) rem oval of the ash with a com b ination of pressurized dry air gun (e.g., 50-100 psi; note that catalytically coated f ilters m a y require the use of air pressures on the lower end of this range to m i nim i ze dam a ge to the catalytic coating. Suppliers of catalytically coated f ilters provide recom m e ndations on air pressures that can be used with their coated filters.) on the exit side of the filter with an industrial vacuum device (equipped with a HEPA [high-efficien cy particulate air] or ULPA [ultra-low penetration air] f ilter) on the f ilter’s inlet side that includes provisions f o r collecting the ash. This pressurized air/vacuum treatm e nt should direct the pressurized air flow across all channels on the filters to m a ke sure each channel is cleaned of ash. Total tim e f o r air cleaning will depend on the size of the f ilter but is typically 30-50

m i nutes f o r a f ilter sized f o r a heavy-duty highway engine. In som e cases specially designed pulsed-air equipm ent is used to do this air cleaning procedure. (3) inspection of the cleaned f ilter by probing individual channels, and/or weight or f l ow characteristics (e.g., filter pressure drop m easurem ents using am bient tem p erature airflows) to confirm that the cleaning has been successful (4) re-installation of the f ilter on the engine/vehicle

Modifications to the procedure described above include the addition of a filter baking/heating step either before or after the air cleaning procedure described in (2) above. A typical filter baking/heating step includes placing th e filter in an industrial oven or a stand-alone cleaning unit that includes a heating elem ent to burn of f any organic soot rem a ining on the f ilter. Baking tem p eratures m a y vary between 450 and 800 o C with total baking tim es (heat-up, hold, and cool-down) varying between 3 and 12 hours. These relatively long baking schedules include very controlled heat-up and cool-down schedules to m i nim i ze therm a l stresses that m a y occur in the f ilter due to the release of heat associated with the oxidation of any rem a ining soot. If the baking step is done after an air cleaning procedure, an additional air cleaning procedure m a y be done on the filter following the baking procedure to rem ove ash still rem a ining in the filter. The heating strategy in som e cases has been found to im prove the ash collection process, but acceptable levels of ash rem oval have also been dem onstrated by procedures that only use

June 2005 17pressurized air or pressurized air in com b inations with vacuum . In som e cases a brief 5-10

m i nute treatm e nt with pressurized air on the outlet side of the f ilter is recom m e nded prior to the baking step. In all cases any pressurized air/vacuum treatm e nts m u st be done using an acceptable dust collection strategy to prevent entry into the local environm ent and exposure of the m a intenance technicians to the dust associated with the ash. Technicians em ployed to conduct ash cleaning procedures need to wear appropriate eye protection, dust m a sks, and appropriate gloves.

A num ber of f

ilter system m a nuf acturers of f e r autom a ted or sem i -autom ated m achines or stand alone workstations f o r f ilter cleaning operations. These stations or m achines include provisions for pressurized air stream s, vacuum collection, and/or heating of filters. In som e cases these cleaning devices are relatively portable to allow for filter cleaning at any convenient location with available utilities. A picture of a stand-alone f ilter cleaning system is shown in Figure 3.

Figure 3. Diesel Particulate Filter Cleaning Unit

Additionally, a num ber of com p anies, including engine/vehicle original equipm ent dealers and distributors, are offering cleaning serv ices to end-users who either do not have an interest in establishing their own cleaning capabilities or do not have the internal resources to establish a f ilter cleaning capability. These f ilter cleaning service providers of f e r a variety of service options including on-site cleaning and reinstallation of the f ilter at a custom er’s f acility

June 2005 18or “swap and clean” program s that substitute a clean f ilter elem ent f o r an ash-containing f ilter, with subsequent cleaning of the ash-containing f ilter.

In Europe, where significant num bers of diesel passenger cars have been sold equipped with particulate f ilters, a com p lete centralized f ilter cleaning f acility has been built by the exhaust com ponent com p any that supplies uncatalyzed, filter-containing exhaust system s to the

auto m a nuf acturers (33). This f acility allows f o r m u ltiple f ilters to be cleaned in parallel f o r a relatively high throughput of filters per day (up to 1,000 DPFs per day). This operation features a f ilter cleaning procedure based on a washing protocol that utilizes warm water with detergent. This f acility has been com m i ssioned in stages based on wastewater recycling capacity with the first stage lim ited to 100 filters per day and the current second stage capacity lim ited to 250 filters per day. Through 2003 this facility had cleaned about 2000 filters.

In this European f acility, operational cleaning steps include: (1) visual inspection and weighing of incom i ng f ilter elem ents, (2) regeneration of the f ilter with hot air to burn of f any organic-based particulate m a tter retained on the f ilter, (3) weighing the regenerated f ilter, (4) a cleaning procedure that alternates flowing warm water (with detergent) and pulsed air flow through the filter walls to rem ove accum u lated ash, (5) a drying step using warm , dry air, and (6) a final weight check and inspection steps including inspection of the filter using an endoscope. The regeneration step is m onitored with backpressure m easurem ents and on-line m easurem ent of the exhaust gas that exits the f ilter. This f ilter cleaning f acility also includes provisions f o r the

treatm e nt and recycling of the wash water used to clean f ilters bef o re the wash water is returned to the local sewer system . Recovered ash m a terials are treated as a special industrial waste and disposed of in specific dedicated landfill sites. This cleaning process is ISO 14001 certified.

To date water-based f ilter cleaning procedures have only been used on uncatalyzed f ilter system s that m a ke use of filter canning system s that have m ounting m a terials com p atible with the water cleaning operation. Som e ceram ic filter substrate m ounting m a terials, such as intum e scent-based m a ts (m ats that contain verm iculite com ponents that expand when heated), deteriorate when exposed to water and should not be cleaned using water. Deterioration of the

m a t m ounting m a terial can lead to perm anent dam a ge and f a ilure of the f ilter in service. Sim ilarly, catalyzed filter elem ents should not be water cleaned due to the potential for degradation or dam a ge to the catalyst coating present on the f ilter elem ent.

The use of excessive air pressures (above 100 psi) to clean catalyzed filters of ash should also be avoided since high cleaning pressures m a y dam a ge or rem ove the catalyst coating from the filter substrate. Recom m e ndations from suppliers of catalyzed filters regarding m a xim u m air pressures used in filter cleaning procedures should be followed in all cases.

Filter baking/regeneration tem p eratures associated with ash cleaning above 850 o C are generally not recom m e nded due to high tem p erature sintering reactions that occur within the stored ash or the potential for high tem p erature reactions between the stored ash and the ceram ic filter substrate or catalyst coating. These high tem p erature solid-state reactions can result in an ash structure that is difficult to rem ove or the potential for dam a ge to the substrate or catalyst coating.

June 2005 19Available f ilter cleaning m achines are generally based on pressurized air/vacuum cleaning procedures described above since this procedure is com p atible with both intum e scent and non-intum e scent filter m ounting m a terials. Steam cleaning is not practical for ash rem oval on any f ilter system s due to the potential f o r f o rm ing cem ent-like species f r om unrem oved ash. In all situations associated with f ilter m a intenance procedures, f ilter elem ents need to be handled with care to avoid dam a ge to the f ilters. In the case of f

ilters used on heavy-duty engines, these f ilters can be large in size and weight and technicians m u st use appropriate saf e ty procedures to

m i nim i ze personal risks and dam a ge to the f ilters.

Published studies of recom m e nded ash cleaning procedures generally report that f ilter pressure drop is returned to acceptable levels but generally not equivalent to an unused filter (14). In the retrof it application of f

ilters, f ilter m a nuf acturers provide end users with training courses and com p lete docum entation describing recom m e nded f ilter cleaning practices and/or operation of specif i ed f ilter cleaning equipm ent.

As detailed in Section 3.1, the ash collected from filter cleaning procedures consists prim arily of oxides and sulf ates of inorganic m a terials associated with lubricant additives and exhaust system corrosion. Disposal practices for ash collected from filters are determ ined by local and/or state environm ental regulations. A com positional analysis of representative collected ash m a y be necessary to determ ine legal disposal practices. In som e jurisdictions ash

m a y be classified as a hazardous waste and can only be placed in landfill sites designated for hazardous waste. In general, states and/or local jurisdictions have landfills available that will accept collected filter ash, but end users need to consult with state and/or local environm ental agencies to determ ine acceptable disposal m e thods.

Currently, laws are inconsistent regarding how to classif y ash f r om particulate f ilters. In the State of New York, the New York State Departm e nt of Environm ental Conservation (NYSDEC) provided inform ation to NESCAUM (Northeast States for Coordinated Air Use Managem e nt, an organization representing state air quality of f i cials in the Northeast U.S.) in a letter dated October 11, 2001 concerning the classification of ash waste associated with particulate filters. In this letter, NYSDEC indicated that both New York state law [NYCRR 371.1(e)(2)(iv)] and federal law [40 CFR 261.4(b)(4 )] exclude residues from the burning of fossil fuels from being classified as a h azardous waste. W ithout this hazardous waste designation, NYSDEC indicated to NESCAUM that as h should be treated as a solid waste.

In California, laws m a y vary depending on location. For that reason, a statem ent is included in owners m a nuals of California-verified filters that ash disposal m u st be in accordance with all applicable Federal, state, and local laws governing waste disposal. In California, if zinc is present in the ash collected f r om a f ilter in high concentrations, it can m a ke this m a terial a hazardous waste. The California Code of Regulations (Title 22, section 66261.4) establishes two lim its for zinc in waste: 250 m illigram s per liter for the soluble threshold lim it concentration, and 5,000 m illigram s per kilogram for the total threshold lim it concentration. The presence of zinc at or above these levels would cause an ash m a terial to be characterized as a hazardous waste in California. Under California law, the generator of the waste has the responsibility to determ ine whether their waste is hazardous or not. This, in general, would require a chem ical analysis of the collected ash sam p le to determ ine the zinc content. There are f acilities in Calif ornia that

June 2005 20accept hazardous waste from conditionally exem pt sm all quantity generators. Additional guidance concerning acceptable disposable m e thods of ash collected from filters in California is available from the California Departm e nt of Toxic Substances Control.

Diesel fuel ash is not listed as a hazardous waste by EPA in the Code of Federal Regulations (40 CFR Part 261). Federal regulations also conditionally exem pt sm all quantity generators that generate less than 220 lbs. per m onth of solid waste from federal hazardous waste regulations. Additional inform ation concerning federal hazardous waste requirem e nts are available from the EPA Office of Solid W a ste (OSW ) by either calling their Resource Conservation and Recovery Act (RCRA) hotline at 703-412-9810 or 800-424-9346, or by visiting the f o llowing website: www.epa.gov/epaoswer/general/orientat/ . The EPA also provides inform ation about state contacts regarding solid and hazardous waste disposal issues at: www.epa.gov/epaoswer/osw/stateweb.htm

.

It is also im portant to note that the am ount of ash collected from a filter during a typical cleaning procedure is relatively sm all. For exam ple, ARB reports that the am ount of m a terial collected f r om a single f ilter in a single cleaning procedure is in the range of 10-20 gram s of ash. ARB extrapolated this ash collection am ount to 10-20 kg of ash collected per year for a fleet of 1000 transit buses. Actual am ounts of ash collected from a filter during a cleaning procedure will vary depending on variables including the oil consum ption rate of the engine. Ash disposal costs will depend on the quantity of ash disposed of at any given tim e as well as local/state regulations covering ash disposal.

MECA believes that uniform ash disposal regulations are needed nationwide before 2007 to avoid confusion in the m a rketplace concerning ash disposal practices. MECA encourages federal, state, and local agencies to com m unicate on this topic and reach a com m on position on accepted safe disposal practices for ash collected from filters. EPA is expected to issue their own fact sheet concerning filter m a intenance pro cedures som e tim e in 2005. This fact sheet will be posted on the EPA Of f i ce of Transportation and Air Quality (OTAQ) website ( www.epa.gov/otaq ).

4.0 U.S. 2007 HEAVY-DUTY DIESEL FILTER MAINTENANCE

The f i rst large-scale application of diesel particulate f ilters in the U.S. on new vehicles will occur starting in 2007 for heavy-duty diesel engines used in highway trucks and buses. All heavy-duty engine m a nuf acturers have indicated thei r intent to use diesel particulate f ilters to

com p ly with the U.S. EPA’s 2007 PM em ission regulation of 0.01 g PM/bhp-hr. This 2007 introduction of filters on new highway heavy-duty diesel vehicles will encom p ass close to one

m illion vehicles annually for vehicle weight classes above 8500 lb. GVW . EPA stipulates a 150,000 m ile m i nim u m m a intenance interval for any em ission control device, including particulate f ilters, used in the heaviest weight class of heavy-duty vehicles. This m eans that

m a nuf acturers will need to design f ilter system s that are capable of operating a m i nim u m of 150,000 m iles between ash rem oval procedures for these so-called heavy, heavy-duty vehicles.

In public forum s to date, the engine m a nufacturers have indicated that com p lying with this m i nim u m filter m a intenance interval is part of their developm ent plan for 2007 engines.

June 2005 21Several factors will contribute to the success in m eeting this requirem e nt. These include engine designs with inherently low and stable oil consum ption characteristics, suf f i cient f ilter ash capacity to allow for long m a intenance intervals, and reform ulated lubricants that have lower ash content than lubricants currently in use today. Filter ash capacity m a y include the use of

asym m e tric f ilter designs like those shown in Figure 2, with larger inlet channel cross sections for increased ash storage capacity.

The lubricant industry is currently finalizing the specifications for the next generation lubricants that will be used by 2007 and later heavy-duty diesel engines. This 2007 PC-10 lubricant category will likely lim it lubricant ash content to 1.0 wt.% relative to the current 1.5 wt.% ash lim it f o r current heavy-duty lubricants to help extend f ilter m a intenance intervals. SAE paper no. 2003-01-3109 describes som e of the on-going efforts to develop new lubricant form ulations designed for use in diesel engines using DPFs or other advanced diesel engine

em ission control technologies.

As dem onstrated by the extended filter m a intenance intervals reported by long haul tractor trailers evaluated in the BP-ARCO program (13), the 150,000 m ile m a intenance interval should be m e t by 2007 engines used in sim ilar applications. Other applications such as transit buses or refuse haulers, where vehicles operate for m a ny hours but accum u late m iles at a relatively low rate, m a y present a challenge in com p lying with EPA’s 150,000 m ile m i nim u m

m a intenance interval. The em issions control industry is com m itted to work with their custom ers in the engine m a nufacturing industry to m a ke the 150,000 m ile filter m a intenance interval a reality f o r as wide an application base on new heavy-duty diesel engines as possible.

REFERENCES

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