Diesel Particulate Filter Maintenance: 

Current Practices and Experience 

June 2005 

Manufacturers of Emission Controls Association 

Washington, 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 Fuel 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 

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EXECUTIVE SUMMARY 

Diesel engines are important power systems for on-road and off-road vehicles. These 

reliable, fuel-efficient, high torque engines power many of the world’s heavy-duty trucks, buses, 

and non-road vehicles. While diesel engines have many advantages, they have the disadvantage 

of emitting significant amounts of particulate matter (PM) and oxides of nitrogen (NOx) into the 

atmosphere. Diesel engines also emit toxic air pollutants. Health experts have concluded that 

pollutants emitted by diesel engines adversely affect human health and contribute to acid rain, 

ground-level ozone and reduced visibility. Studies have shown that exposure to diesel exhaust 

causes lung damage and respiratory problems and there is increasing evidence that diesel 

emissions may cause cancer in humans. 

In response to these health concerns, a number of countries worldwide (including the 

United States, the European Union, and Japan) have established emission limits for new diesel 

engines, with recent regulatory programs for new diesel engines requiring significant reductions 

in diesel particulate emissions in the 2005-2010 timeframe. Health concerns associated with 

diesel exhaust have also focused attention on emissions from the large population of existing 

diesel vehicles. As a result, local, regional, and national voluntary and mandatory diesel retrofit 

programs have been initiated in a variety of world markets. To date these retrofit efforts have 

focused on the application of diesel oxidation catalysts and diesel particulate filters for the 

reduction of diesel PM on in-use diesel engines operated in both on-road and off-road 

applications. These retrofit programs are expanding rapidly with more cost-effective technology 

solutions being verified for use on in-use diesel engines and a greater variety of technology 

options available for application on a wider range of in-use engines. 

Diesel particulate filters (DPFs) remove particulate matter in diesel exhaust by filtering 

exhaust from the engine. They can be installed on diesel-powered vehicles, off-road equipment, 

or stationary diesel engines. Wall-flow type filters can reduce diesel PM by more than 85% 

compared to an unfiltered diesel exhaust. A growing experience base is available for the 

application of filters on diesel engines, including more than 200,000 retrofit applications of 

filters worldwide and more than 1 million applications of filters on new light-duty diesel 

passenger cars in Europe. Successful application of diesel particulate filters on new or existing 

diesel engines requires a robust filter regeneration scheme that periodically oxidizes the 

collected soot present on the filter to maintain engine backpressure characteristics within 

specified limits. These regeneration methods include both passive (e.g., methods that rely 

strictly on the available exhaust temperatures in a given application) and active (e.g., 

supplemental heat inputs to the exhaust required to initiate soot combustion) systems. The 

effectiveness and durability of catalyst-based DPFs is maximized with the use of ultra-low sulfur 

diesel fuel. 

In addition to collecting soot, filters also collect inorganic-based exhaust constituents that 

are derived from several sources, including the combustion of engine lubricants, products of 

normal engine wear and/or corrosion, and materials associated with fuel-borne catalysts in DPF 

applications that use these catalysts to assist in the filter regeneration process. Over extended 

operation on the vehicle, these ash species slowly accumulate within the filter and gradually 

increase the pressure drop across the filter. Since excessively high backpressure on the engine 

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will result in a degradation of engine performance, the accumulated ash material within the filter 

needs to be periodically removed. This ash removal or cleaning operation is a necessary filter 

maintenance operation. Engine oil consumption characteristics, the total ash content of engine 

lubricant formulations, vehicle duty cycles, filter designs, and fuel-borne catalyst dosing rates all 

impact ash accumulation profiles and required filter maintenance cleaning intervals. 

Retrofit applications of wall-flow filters on heavy-duty diesel vehicles and original 

equipment applications of wall-flow filters on passenger cars in Europe provide information on 

current filter maintenance interval recommendations from manufacturers of filters or new lightduty vehicles equipped with filters. In retrofit applications on existing heavy-duty diesel 

highway vehicles, filter manufacturers recommend that filters be cleaned of ash after either 12 to 

24 months of operation, or at mileage intervals that range from 60,000 to 100,000 miles. It 

should be noted that actual maintenance intervals are based on factors such as engine lubricant 

consumption rates, duty cycles, and actual engine/vehicle applications. Older vehicles with 

higher lubricant consumption rates, for example, may need maintenance intervals that are more 

frequent than the maintenance intervals stated above. Late-model heavy-duty diesel engines 

with low oil consumption characteristics operated in over-the-road applications with retrofit, 

catalyst-based DPFs have successfully operated for more than 200,000 miles before filter 

cleaning was initiated. In Europe, passenger car filter maintenance intervals with systems using 

fuel-borne catalysts were initially designed for 80,000 km (ca. 50,000 mi) of operation but now 

have been extended up to 200,000 km (ca. 120,000 mi) with lower engine-out PM levels, lower 

fuel-borne catalyst dosing rates, and new filter designs featuring higher ash storage capacities. 

Manufacturers of filter technologies have developed safe, reliable, and effective 

procedures for removing the ash from particulate filters. These practices have been put in place 

in both retrofit wall-flow filter applications and wall-flow filter-equipped passenger cars in 

Europe. The ash cleaning practices include well-documented procedures for cleaning filters and 

the development of stand-alone filter cleaning stations or facilities that automate many of the 

necessary procedures. These procedures include combinations of pressurized dry air streams 

directed at the exit side of the filter with industrial vacuum devices used on the inlet side to 

safely collect ash removed from the filter. In some cases very controlled high temperature 

treatments of the filters are used before or after air cleaning procedures to remove organic 

materials and soot that may 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 first large-scale application of diesel particulate filters 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 manufacturers have indicated their intent to use diesel particulate filters to 

comply with the U.S. EPA’s 2007 PM emission regulation of 0.01 g PM/bhp-hr. In public 

forums to date, the engine manufacturers have indicated that complying with EPA’s 150,000 

mile minimum filter maintenance interval for heavy heavy-duty vehicles is part of their 

development plan for 2007 engines. The emissions control industry is committed to work with 

their customers in the engine manufacturing industry to make the 150,000 mile filter 

maintenance interval a reality for as wide an application base on new heavy-duty diesel engines 

as possible.

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1.0 INTRODUCTION

Diesel engines provide important fuel economy and durability advantages for large 

heavy-duty trucks, buses, and non-road equipment. They are often the power plant of choice for 

heavy-duty applications. While they have many advantages, they also have the disadvantage of 

emitting significant amounts of particulate matter (PM), oxides of nitrogen (NOx), and lesser 

amounts of hydrocarbon (HC), carbon monoxide (CO), and toxic air pollutants. 

Particles emitted from diesel engines are small – in most cases less than 2.5 microns in 

aerodynamic diameter. The particles are complex, consisting of an uncombusted carbon core, 

adsorbed hydrocarbons from engine oil and diesel fuel, adsorbed sulfates, water, and inorganic 

materials such as those produced by engine wear. Because of their extremely small size and 

composition, the particles emitted by diesel engines have raised many health concerns. Health 

experts have expressed concern that diesel PM may contribute to or aggravate chronic lung 

diseases such as asthma, bronchitis, and emphysema. 

There is growing evidence that exposure to diesel PM may increase the risk of cancer in 

humans. A comprehensive assessment of available health information, carried out by the 

International Agency for Research on Cancer (IARC) in June 1988, concluded that diesel 

particulate is probably carcinogenic to humans. The term “carcinogen” is used by the IARC to 

denote an agent that is capable of increasing the incidence of malignant tumors. In August 1998, 

California’s Air Resources Board identified PM emissions from diesel-fueled engines as a toxic 

air contaminant. In 2000, the U.S. EPA declared diesel PM to be a “likely human carcinogen.” 

A recent report, “Diesel and Health in America: 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 

impacts of diesel particulate emissions 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 damage from diesel PM estimated to total $139 billion in 2010. 

Despite health and environmental concerns, the diesel engine remains a popular means of 

powering trucks, buses, and other heavy equipment. 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 most 

impressive attributes of the diesel engine is its durability. In heavy-duty trucks, some engines 

have achieved operating lives of 1,000,000 miles or more while some engines power city buses 

for up to 15-20 years. 

A number of countries worldwide have established emission limits for new diesel 

engines, with recent regulatory programs for new diesel engines requiring significant reductions 

in diesel particulate emissions in the 2005-2010 timeframe. In the United States, the 

Environmental Protection Agency finalized in 2000 a comprehensive regulatory program to 

reduce emissions from on-road heavy-duty engines (diesel and gasoline) in the 2007-2010 

horizon (see www.epa.gov/OMSWWW/diesel.htm#hd2007 for details on EPA’s 2007 highway 

diesel program). Specifically, this EPA highway program requires new heavy-duty diesel 

engines to meet a 0.01 g/bhp-hr PM standard starting in 2007, a 90% reduction in PM emissions 

compared to the current 0.10 g/bhp-hr PM standard for new diesel engines that heavy-duty 

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engine manufacturers began to meet in 1994. Japan also has put in place emission standards 

starting in 2005 that require new heavy-duty diesel engines to meet a 0.027 g/kW-hr (0.02 g/bhphr) PM standard. These ultra-low PM emission 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, discussions are on-going concerning the next round of 

emission standards for light-duty (Euro 5) and heavy-duty (Euro 6) vehicles. These next sets of 

standards will likely include significantly tighter standards for PM emissions from all new 

diesel-powered vehicles sold in Europe starting perhaps as early as 2010. 

Since some diesel engines have very long operating lives, older uncontrolled diesel 

vehicles will continue to make up a significant portion of the heavy-duty vehicle fleet in world 

markets for years to come. Given the health and environmental concerns associated with diesel 

engines, there is increasing interest to retrofit older, “dirtier” diesel engines while newer, 

“cleaner” diesel engines enter the marketplace. Diesel particulate filters for controlling PM 

emissions from existing diesel cars and trucks are seeing growing retrofit applications in many 

world markets, including the U.S., Europe, Japan, and Korea, with pilot programs targeted for 

Mexico, Thailand, China, and other developing countries. These retrofit applications of DPFs 

have provided the first experiences with respect to filter maintenance practices. Filter 

maintenance of high efficiency particulate filters (e.g., ceramic wall-flow filters) is necessary 

due to the accumulation of inorganic material (ash) within the filter media over time that stems 

primarily from the combustion of engine lubricants. Filter maintenance procedures and 

equipment have been developed to serve the growing retrofit application of DPFs. This report 

summarizes the present filter maintenance experience base and highlights development trends in 

filter design, engine design, and lubricant formulations that will impact filter maintenance 

practices on new heavy-duty diesel engine applications as they come on-stream during this 

decade in the United States and Japan. The maintenance experience detailed in this report is 

based exclusively on filter designs employing ceramic wall-flow filters. These wall-flow filters 

currently dominate both the retrofit and original equipment-based applications of diesel 

particulate filters in all world markets. A brief overview of diesel particulate filters provides a 

backdrop to filter maintenance practices and experience. 

2.0 DIESEL PARTICULATE FILTERS 

2.1 Operating Characteristics and Performance 

As the name implies, diesel particulate filters remove particulate matter in diesel exhaust 

by filtering exhaust from the engine. They can be installed on diesel-powered vehicles, off-road 

equipment, or stationary diesel engines. Since a filter will fill up over time, engineers that 

design filter systems must provide a means of burning off, or removing, accumulated particulate 

matter. A convenient means of removal of accumulated particulate matter is to burn or oxidize it 

on the filter when exhaust temperatures are adequate. By burning off trapped material, the filter 

is cleaned or “regenerated.” Filters that regenerate in this fashion cannot be used in all 

situations, primarily due to insufficient exhaust gas temperatures associated with some types of 

diesel engines, the level of PM generated by a specific engine, and/or application operating 

experience. To ensure proper operation, filter systems are designed for the particular 

engine/vehicle application. In retrofit applications of filters, this application design process 

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

temperature history available during their normal operating conditions. 

In some non-road applications, disposable filter systems have been used in retrofit 

applications. A disposable filter is sized to collect particulate for a working shift or some other 

predetermined period of time. After a prescribed amount of time, or when backpressure limits 

are approached, the filter is removed and discarded. Disposable filters target retrofit applications 

and would not meet original equipment requirements. 

 

Filter Material A number of filter materials have been used in diesel particulate filters, 

including: ceramic (e.g., cordierite, aluminum titanate, or mullite formulations) and silicon 

carbide materials, fiber wound cartridges, knitted silica fiber coils, ceramic foam, wire mesh, 

sintered metal substrates, and temperature resistant paper in the case of disposable filters. 

Collection efficiencies of these filters range from 40 to over 90 percent. Filter materials capture 

particulate matter by interception, impaction, and diffusion. Filter efficiency has rarely been a 

problem with the filter materials listed above, but work has continued to: 1) optimize filter 

efficiency and minimize back pressure, 2) improve the radial flow of oxidation in the filter 

during regeneration, and 3) improve the mechanical strength of filter designs. 

Figure 1 provides a diagram of a typical ceramic wall-flow filter system. These wallflow filter designs are currently the design of choice for high efficiency (> 85%) diesel 

particulate matter reductions. In Figure 1, particulate-laden exhaust enters the filter from the 

left. Because 

Figure 1. 

Wall-Flow Diesel Particulate Filter 

 Trapped PM 

Plugged 

Cells 

CO2

H2O 

PM 

CO 

HCs 

PAHs 

 (Note: PM = particulate matter, HCs = hydrocarbons, PAHs = poly-aromatic hydrocarbons) 

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the 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 

matter is deposited, or trapped, on the upstream side of the cell wall. Cleaned exhaust gas exits 

the filter to the right.

Regeneration Many techniques can be used to regenerate a diesel particulate filter. 

Some of these techniques are used together in the same filter system to achieve efficient 

regeneration. Both on- and off-board regeneration systems exist. The major regeneration 

techniques are listed below. 

• Catalyst-based regeneration using a catalyst applied to the surfaces of the filter. A base 

or precious metal coating applied to the surface of the filter reduces the ignition 

temperature necessary to oxidize accumulated particulate matter. 

• 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 filter to facilitate oxidation of nitric oxide (NO) to nitrogen dioxide (NO2). The 

nitrogen dioxide oxidizes the collected particulate substantially reducing the temperature 

required to regenerate the filter. 

• Fuel-borne catalysts. Fuel-borne catalysts reduce the temperature required for ignition of 

trapped particulate matter through direct contact between the catalyst and particulate that 

is collected together on the filter media. In the case of wall-flow filters, the catalyst 

remains in the filter media and adds to the inorganic ash that accumulates within the 

filter. 

• Air-intake throttling. Throttling the air intake to one or more of the engine cylinders can 

increase the exhaust temperature and facilitate filter regeneration. 

• Post top-dead-center (TDC) fuel injection. Injecting small amounts of fuel in the 

cylinders of a diesel engine after pistons have reached TDC introduces a small amount of 

unburned fuel in the engine’s exhaust gases. This unburned fuel can then be oxidized 

over an oxidation catalyst upstream of the filter or oxidized over a catalyzed particulate 

filter to combust accumulated particulate matter. 

• Post injection of diesel fuel in the exhaust upstream of an oxidation catalyst and/or 

catalyzed particulate filter. This regeneration method serves to generate heat used to 

combust accumulated particulates by oxidizing fuel across a catalyst present on the filter 

or on an oxidation catalyst upstream of the filter. 

• On-board fuel burners or electrical heaters. Fuel burners or electrical heaters upstream of 

the filter can provide sufficient exhaust temperatures to ignite accumulated particulate 

and regenerate the filter. In some cases regeneration can be accomplished while the 

vehicle is in operation, whereas in other cases the vehicle must be out of service for 

regeneration to proceed. 

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• Off-board fuel burners or electrical heaters. Off-board regeneration stations combust 

trapped particulate matter by blowing hot air through the filter system. 

 The experience with catalyzed filters indicates that there is a virtually complete 

reduction in odor and in the soluble organic fraction of the particulate, but some catalyst 

formulations may increase sulfate emissions. Companies utilizing these catalysts to provide 

regeneration for their filters have modified catalyst formulations to reduce sulfate emissions to 

acceptable levels. Low sulfur fuel (e.g., 0.0015 wt.% or 15 ppm S maximum) is becoming 

available in the U.S. and other world markets and has greatly facilitated these efforts. 

In some situations, installation of a filter system on a vehicle may cause a slight fuel 

economy 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, some filter regeneration methods involve 

the use of fuel burners or fuel injection strategies and, to the extent those methods are used, there 

will be an additional fuel economy penalty. Many filter systems, however, have been optimized 

to minimize, or nearly eliminate, any noticeable fuel economy penalty. For example, in a 

demonstration 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 economy impact was also 

reported as a part of DPF retrofit program in California involving late model tractor/trailer 

delivery trucks (13). During the required retrofit technology verification protocols established 

by either the U.S. EPA or the California Air Resources Board, fuel penalties have been 

documented at about 1% for high efficiency filter systems. 

Filter systems do not appear to cause any additional engine wear or affect vehicle 

maintenance. Regarding maintenance of the filter system itself, manufacturers are designing 

systems to minimize maintenance requirements during the useful life of the vehicle. In 

applications that make use of high efficiency, wall-flow filters, accumulated lubricating oil ash 

will have to be periodically removed. Manufacturers provide the end-user with appropriate ash 

removal procedures. Current practices and experience with filter maintenance of wall-flow 

filters will be discussed in detail in the following sections of this report. 

Filter systems have been designed so that vehicle drivability is not affected, or at least 

effects can be minimized, most notably by limiting exhaust backpressure. Diesel particulate 

filter systems, which replace mufflers in retrofit applications, have achieved sound attenuation 

equal to a standard muffler. 

2.2 The Impact of Sulfur in Diesel Fuel on Diesel Particulate Filters 

Sulfur in diesel fuel significantly affects the reliability, durability, and emissions 

performance of catalyst-based diesel particulate filters. Sulfur affects filter performance by 

inhibiting the performance of catalytic materials upstream of, or on the filter. Sulfur also 

competes with chemical reactions intended to reduce pollutant emissions (e.g., the oxidation of 

NO to NO2) and creates particulate matter through catalytic sulfate formation. (Sulfate 

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formation is sometimes referred to as “sulfate make.”) Catalyst-based diesel particulate filter 

technology works best when fuel sulfur levels are less than 0.0015 wt.% (15 ppm S). Meeting 

the most stringent emission standards requires both the use of catalysts and ultra-low sulfur 

diesel fuel. 

Particulate filter technology can be successfully used in applications where the fuel sulfur 

level is greater than 15 ppm S, but only after filter application engineers make a careful 

assessment of the fuel sulfur level, the engine, the type of filter system, the operating conditions, 

and the emission reductions desired. PM emissions for catalyst-based filters operated with 

higher fuel sulfur levels will be higher than operation with ultra-low sulfur diesel fuel.

2.3 Operating Experience 

Limited diesel particulate filter retrofit demonstration programs began in the 1980s and 

continued in the early 1990s. The number of vehicles retrofitted, the number of programs, and 

the interest in new programs has grown significantly over the past few years with approximately 

200,000 DPFs installed as retrofits to date in a variety of world markets. Nearly all of these 

retrofit applications to date have utilized wall-flow filters like the design shown in Figure 1. 

Today, second and third generation retrofit filter systems can reduce PM emissions by 80 to 

more than 90 percent. 

In Europe, new light-duty vehicles equipped with diesel particulate filters are being 

offered commercially. Filters were introduced on new diesel passenger cars in Europe in mid2000, with more than 1,000,000 filter-equipped cars sold since that first introduction. These 

European light-duty vehicle applications have used ceramic wall-flow filters exclusively thus far. 

No performance or maintenance issues have been reported in Europe with passenger car DPFs. 

Peugeot (PSA) was the first manufacturer to introduce a DPF system for European diesel cars in 

2000 (16, 17, 20, 23, 25). This system employs a fuel-borne catalyst combined with advanced 

engine controls available with a modern, high pressure, common rail fuel injection engine for 

filter regeneration. Other European automobile manufacturers (e.g., Audi, Fiat, Ford, VW, 

BMW, Mercedes) are now offering DPF systems based on the PSA system and the use of fuelborne catalysts, or catalyzed filter systems that do not employ a fuel-borne catalyst. 

Sweden’s Environmental Zones program resulted in the commercial introduction of 

diesel particulate filters on urban buses (14). More than 4,000 buses have been retrofit with 

passive wall-flow filter systems in Sweden. Some of these buses have accumulated more than 

250,000 miles of service. Most recently in the summer 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 combined with selective catalytic reduction (SCR) systems using urea 

as the reducing agent to control NOx emissions. This bus fleet is claimed to be the cleanest 

operating diesel bus fleet in the world. Transit fleets in other major European cities such as 

London and Paris have also been successfully retrofit with DPFs. Renault Trucks have 

introduced an active regeneration DPF system in combination with a thermal management 

strategy as an original equipment option on one of their Euro 3 certified engine families used in 

urban transit, refuse hauler, and other heavy-duty on-road applications (4). A particulate filter 

based on sintered metal foils is now available in Europe for retrofit on certain models of existing 

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diesel passenger cars. A similar sintered metal foil-based particulate filter is being used on some

new heavy-duty diesel engines offered by one heavy-duty engine manufacturer in Europe to 

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

Transit bus retrofits using DPFs have also occurred in many urban fleets across the U.S., 

including New York City (26), Boston, Philadelphia, Washington, D.C., Seattle, and many 

transit fleets in California. DPF retrofits are an important 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 Emissions 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 emissions 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 refrigeration units (TRUs), portable diesel engines, 

and stationary engines that include DPF retrofits on existing engines (where applicable and 

verified) along with engine repowers and replacements as options for reducing PM emissions 

from these existing engines. Future ARB regulatory measures will also require significant 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 programs in the U.S., 

with a significant number of DPFs equipped on school buses since 2000 in conjunction with the 

use of ultra-low sulfur diesel fuel (15 ppm S max.). Additionally, several U.S. engine 

manufacturers offer a “green” school bus engine fitted with a catalyzed DPF for use with ultralow sulfur diesel fuel. Retrofits with DPFs on existing vehicles is also occurring in Japan as part 

of efforts by the Tokyo Metropolitan Government and other local government agencies to 

significantly reduce PM emissions from diesel trucks traveling through urban areas. 

As noted in the introduction, all engine manufacturers will be equipping their new onroad heavy-duty diesel engines sold in the U.S. with DPFs beginning in 2007 in order to meet 

EPA’s 2007 PM standards. These heavy-duty systems are expected to be integrated with 

advanced engine controls to ensure filter regeneration under all vehicle operating conditions and 

environments. An example of a heavy-duty engine equipped with a catalyst-based DPF system

that includes active filter regeneration strategies using integrated engine controls was recently 

detailed by Hino Motors on a 12.9 liter engine targeting PM emission levels below Japan’s 2005 

heavy-duty diesel PM emission requirements (8). Additional presentations concerning the 

integration of DPFs on new heavy-duty diesel engines can be found in the recent proceedings for 

the U.S. Department of Energy’s Diesel Engine Emission Reduction Conferences (see 

www.eere.energy.gov/vehiclesandfuels/resources/conferences/deer/index.shtml). Since 2001 

some new heavy-duty diesel engines have already been certified with DPFs in the U.S. These 

original equipment DPF applications have been targeted at urban bus or school bus applications. 

 

Diesel particulate filters have been installed on non-road equipment since 1986. Over 

20,000 active and passive systems have been installed as either original equipment or as a retrofit 

worldwide. Some non-road filter systems have been operated for over 15,000 hours, or over 5 

years, and are still in use. Examples of non-road equipment equipped with filters include: 

mining equipment, material-handling equipment such as cranes and forklift trucks, street 

sweepers, construction equipment, stationary power producing engines, and utility vehicles. 

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Germany, Austria, and Switzerland have established mandatory filter requirements for 

underground mining equipment (15). EPA finalized new emission standards for off-road diesel 

engines in mid-2004 that are expected to provide 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 

timeframe (see www.epa.gov/nonroad-diesel/2004fr.htm for details on EPA’s off-road diesel 

program). 

Diesel particulate filters can be combined with exhaust gas recirculation (EGR), NOx 

adsorber catalysts, or SCR (using hydrocarbons, urea/water, or ammonia 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 filter or NOx 

adsorbers and a DPF can achieve NOx reductions of 60 to 90 percent and PM reductions greater 

than 90 percent (31). Combined NOx and PM reductions can also be achieved by recalibrating 

the engine to minimize NOx while using a filter to capture increased PM emissions. A lean NOx 

catalyst added to an exhaust system using a particulate filter (30) can reduce NOx emissions 

from 10 to 25 percent using diesel fuel as the reductant for NOx (NOx performance of such a 

lean NOx catalyst is generally strongly tied to the fuel reductant use and reductant dosing 

strategy.). 

 

Toyota recently commercialized their DPNR (diesel particulate NOx reduction) system

that directly applies a NOx adsorber catalyst formulation to a wall-flow ceramic filter for 

combined PM and NOx reduction. This system is available in limited production on a diesel 

passenger car sold in Europe (2.0 liter engine displacement, (12)) and a light-duty truck 

application sold in Japan (4.0 liter engine displacement, (11)). 

3.0 FILTER MAINTENANCE PRACTICES 

Typical wall-flow particulate filters like those shown in Figure 1 remove both particulate 

matter that is produced from the combustion of fuel in a diesel engine and assorted other 

inorganic materials that may be present in the engine exhaust. These assorted inorganic 

materials are generally referred to as “ash.” Under normal filter operation, the filter collects 

particulate matter and periodically combusts the particulate matter during conditions that favor 

the oxidation of the largely carbon-based particulate (i.e., filter regeneration). The inorganic 

material that also collects on the filter does not completely combust into gaseous species that can 

exit through the filter but rather forms oxide and sulfate materials that remain in the filter. Over 

extended operation on the vehicle, these ash species slowly accumulate within the filter and 

gradually increase the pressure drop across the filter. Since excessively high backpressure on the 

engine will result in a degradation of engine performance, the accumulated ash material within 

the filter needs to be periodically removed. This ash removal or cleaning operation is a 

necessary filter maintenance operation. This section of the report will describe what makes up 

the ash species that collect in diesel particulate filters, what factors influence the rate of ash 

build-up in the filter, what the typical filter maintenance intervals are in current retrofit 

applications of wall-flow filters on heavy-duty diesel engines or wall-flow filters used on diesel 

passenger cars in Europe, and describe filter cleaning practices that are used today with wallJune 2005

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flow filters in either retrofit applications or in original equipment applications with European 

passenger cars. 

3.1 Sources of Filter Ash 

Inorganic material or ash that accumulates in a diesel particulate filter stems from three 

main sources. A primary source of ash is inorganic additives that are present in engine 

lubricants (3, 5, 18, 19). Engine oils are a complex mixture of a largely organic, refinery-derived 

“base stock” with an “additive package” that includes inorganic elements such as magnesium, 

calcium, zinc, phosphorus, and sulfur. These additives are used to impart important 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 make up the majority of the oil ash species with zinc, magnesium, and phosphorus 

generally found at lower levels in the oil ash. It is important to note, however, that there is a 

wide variation in lubricant compositions including ash constituents used in the marketplace 

today. 

All four-stroke diesel engines consume oil as part of their normal lubrication 

requirements. (Note that in some applications two-stroke diesel engines are still in use. Twostroke engines intentionally mix fuel with lubricating oils prior to combustion and generally have 

significantly higher oil consumption rates than four-stroke engines.) This consumed oil 

generally is combusted in the combustion chamber along with fuel. The products of the oil 

combustion process are contained in the engine exhaust gas and reach the particulate filter where 

the inorganic, ash constituents are collected and remain in the filter even after the organic 

particulate matter is removed in filter regeneration steps. 

A second source of ash material comes from normal engine wear and exhaust component 

scale or corrosion. These ash species can contain iron, nickel, silicon, chromium, aluminum, 

copper, and/or tin. A third source of ash material is associated with the use of a fuel-borne 

catalyst. In some filter applications a fuel-borne catalyst is directly added to the fuel at relatively 

low levels (generally < 30 ppm in concentration) before combustion. After combustion, the 

inorganic catalyst species are carried by the exhaust to the filter where they collect along with 

the organic particulate and other inorganic ash materials. These catalytic species assist in the 

filter regeneration process by lowering the combustion temperature of the organic particulate 

below temperatures normally associated with the combustion of particulate by oxygen present in 

the exhaust. Like all other inorganic ash species, these inorganic fuel-borne catalysts remain on 

the filter and contribute to the total ash content of the filter. Fuel-borne catalyst inorganic 

species can include platinum, iron, and cerium (23, 25). Filter systems that do not make use of 

fuel-borne catalysts will not have this contributor to the ash content of the filter. 

In some cases, soot and ash constituents that accumulate in a filter over time may interact 

and influence the combustion characteristics of the soot present in the filter. Fang and Lance (2), 

for example, have recently shown that phosphorus constituents of lubricants that make up some

of the ash materials present in filters can form protective layers on some soot surfaces that in 

turn alters the combustion characteristics of soot. This ash-soot interaction can cause the soot to 

June 2005

11 
have a higher resistance towards oxidation and result in less complete regeneration of the filter 

under a given exhaust temperature history. 

Recent SAE papers (27, 28) provide information on ash compositions collected from

filters used without fuel-borne catalysts. Primary ash constituents included calcium oxide (CaO, 

29-35 wt.%), sulfate (SO3, 35-39 wt.%), magnesium oxide (MgO, 5-11 wt.%), phosphate (P2O5, 

13-16 wt.%), and zinc oxide (ZnO, 5-10 wt.%), with a trace of iron oxide (Fe2O3, < 1%) reported 

in one of these references. Ash is reported to be comprised of 5-50 micron sized agglomerates of 

sub-micron primary particles. Particles from engine wear or corrosion of exhaust components, 

however, can be substantially larger than this. 

In the case of applications that make use of fuel-borne catalysts, the portion of the ash 

stemming 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 consumption 

characteristics of the engine (16, 20, 27, 28). For example, in the initial Peugeot passenger car 

DPF applications, the fuel-borne catalyst component of the ash was approximately 80% by 

weight of the total ash produced using a fuel-borne 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 mass fraction was reduced to about 57% (23). 

3.2 Factors that Influence Ash Accumulation 

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

material collected by wall-flow filters. Therefore, factors associated with lubricant consumption 

in the engine have a large impact on the rate of ash accumulation in a filter. Two primary 

parameters are the rate of oil consumption of a given engine and the ash content of the oil 

consumed by the engine. Higher rates (e.g., liters of oil per miles traveled or hours of engine 

operation) of oil consumption by an engine at a constant oil ash content or consumption of oil 

with a higher ash content will result in faster accumulation of ash in the filter and an associated 

faster rise in engine backpressure relative to some base case operation. Engine oil consumption 

rates are influenced by basic engine design, engine maintenance practices, engine duty cycles, 

and the service life of the engine. (Diesel engines are often “re-built” after a significant 

operating time. This re-build procedure can also influence engine oil consumption rates.) More 

recently manufacturers have introduced improved engine designs with lower engine oil 

consumption rates and continue to work on new engine designs with even lower oil consumption 

rates. Well-maintained engines can often consume less oil than engines that are poorly 

maintained. Engines operating over relatively low temperature, low speed duty cycles, such as 

some transit buses and refuse haulers, can have higher lubricant consumption rates than engines 

operating over duty cycles with higher operating temperatures such as highway vehicles. 

Since diesel engines can have long operating lives, engines that have been in service over 

an extended time will generally consume more oil than low mileage or low hour usage engines 

due to normal engine wear. Oil consumption can also be intentionally influenced by the use of 

certain engine aftermarket products that deliberately add oil from an engine’s oil sump to the 

combustion chamber while replenishing the oil reservoir with fresh lubricant, or by other 

procedures that mix used engine oil with fuel as part of a used oil disposal process. These 

June 2005

12 
procedures accelerate oil consumption by the engine and accelerate ash accumulation 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 combustion 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 formulations will obviously 

result in a faster accumulation of ash material in the filter. In retrofit applications, filter 

manufacturers generally provide recommended maximum lubricant consumption rates as one 

parameter of whether an engine is an acceptable candidate for a filter retrofit – typical values can 

be in the range of one quart consumed per 500 to 1,000 miles of operation. 

In the case of filter systems that work with the use of a fuel-borne catalyst, the dosing 

rate or concentration of the catalyst in the fuel will have a direct impact on the rate of ash 

accumulation within the filter. Since the introduction of filters using fuel-borne catalysts to 

assist with filter regeneration in European diesel passenger cars in mid-2000, the manufacturers 

of the fuel-borne catalysts used in these vehicles have been able to reduce dosage rates from 25 

ppm to 8 ppm through compositional and particle size changes to the fuel-borne catalyst, as well 

as reduced engine-out PM emissions from continued improvements in diesel engine combustion 

characteristics (23). These lower fuel-borne catalyst dosage rates, in turn, reduce ash 

accumulation rates in the filter and help to extend the service interval of the filter without 

adversely impacting the regeneration characteristics of the filter. Continuing development 

efforts aimed at even more active fuel-borne catalysts have demonstrated 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 normal engine/vehicle operation. As 

this ash accumulates over time, the pressure drop through the filter increases and backpressure 

on the engine increases. Filter or engine manufacturers generally set a maximum backpressure 

limit on the engine that defines the service interval between ash removal maintenance events. 

These maximum backpressure criteria are based on engine performance versus backpressure 

characteristics. Particulate filter properties, including the total volume, length/diameter ratio of

the filter, cell density, wall thickness, and the porosity of the wall that acts as the filter media on 

a given engine, are all important factors in defining the initial engine backpressure of the filter 

system. Filter substrate manufacturers have been engaged in a continuous development effort 

aimed at optimizing inherent filter physical properties such as wall porosity, cell density, and 

wall thickness to minimize filter pressure drop for applications with or without the presence of a 

catalytic coating on the filter (21, 32). In general, this substrate optimization process also 

includes other key filter properties such as mechanical strength, thermal conductivity, and 

thermal expansion properties. Catalyst manufacturers also have been actively involved in 

engineering the properties of catalyst coatings that can be applied to filter substrates to aid in 

backpressure reductions. 

June 2005

13 
Ash storage capacity of a given filter with constant cell density and wall thickness is 

proportional to filter volume. Larger filter volumes will be able to accumulate more ash before 

reaching a set maximum engine backpressure criteria. However, the desire to maximize the filter 

cleaning interval is generally at odds with packaging limitations on-board the vehicle or 

maximum engine backpressure criteria. Filter volumes are designed to meet packaging 

constraints, with minimal engine backpressure, but with sufficient volume to allow for 

acceptable soot regeneration characteristics and an acceptable quantity of ash storage that 

defines the filter maintenance interval. 

More recently substrate manufacturers have modified ceramic wall-flow filter designs to 

significantly increase ash storage capacity at a constant filter volume. This design alteration 

involves enlarging the inlet exhaust gas channel dimensions (6, 7). This provides additional 

geometric area for the ash to accumulate in the filter and expands the ash capacity of the filter by 

up to 50%. Figure 2 provides schematics of two alternative filter substrate designs that have 

been developed to increase ash storage capacity. One employs a large octagonal inlet channel 

Figure 2. 

Asymmetric Particulate Filter Substrate Designs with 

Larger Inlet Channel Cross-Sections Relative to 

Outlet Channel Cross-Sections 

in out in

out

out

out

out

in

in

in

in

in in out out

in in out 

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

smaller outlet channel cross sections. Commercial 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, common-rail diesel 

June 2005

14 
engine 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 

maintenance over the useful life of the vehicle (up to 200,000 km, or more than 120,000 miles). 

Additional applications of these large ash-capacity filter substrates are expected in 2005. 

 

3.4 Filter Maintenance Intervals 

 

Retrofit applications of wall-flow filters on heavy-duty diesel vehicles and original 

equipment applications of wall-flow filters on passenger cars in Europe provide information on 

current filter maintenance interval recommendations from manufacturers of filters or new lightduty vehicles equipped with filters. In retrofit applications on existing heavy-duty diesel 

highway vehicles, filter manufacturers recommend that filters be cleaned of ash after either 12 to 

24 months of operation, or at mileage intervals that range from 60,000 to 100,000 miles. It 

should be noted that actual maintenance intervals are based on factors such as engine lubricant 

consumption rates, duty cycles, and actual engine/vehicle applications. Older vehicles with 

higher lubricant consumption rates, for example, may need maintenance intervals that are more 

frequent than the maintenance intervals stated above. 

As a condition of filter retrofit verification requirements in the U.S., and to assist 

operators or fleet owners in determining when filters should be cleaned, retrofit filter 

manufacturers include some type of electronic diagnostic module that is installed on-board the 

vehicle at the time the filter is installed. This diagnostic module can provide several functions 

including monitoring and logging exhaust temperatures, engine backpressure, and/or filter 

pressure drop. Engine backpressure is typically monitored to determine when sufficient ash has 

accumulated in the filter to require a cleaning operation. These diagnostic modules generally 

include some type of display module that is mounted in the driver’s compartment 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 sufficient safety margins to ensure that 

excessive engine backpressure conditions are not reached under normal operations. These 

diagnostic modules can also alert operators to abnormal engine operating conditions that may 

impair normal filter operation. Backpressure monitoring capabilities for DPFs are required by 

both the EPA and CARB as a part of their retrofit programs. 

The rather wide range of filter service intervals based on mileage or months of operation 

in retrofit applications are a reflection of the variety of engines, duty cycles of filter applications, 

and the age of existing vehicles. Retrofit filter manufacturers have established a large and 

growing experience base with filters in applications on buses, highway trucks, industrial 

vehicles, and off-road equipment that allows them to provide operators with guidelines for filter 

maintenance intervals along with diagnostic modules that monitor and alert operators when ash 

removal is required. 

There are examples of recent retrofit filter applications on heavy-duty diesel highway 

engines that have demonstrated much longer filter maintenance intervals than the typical 60,000 

– 100,000 mile recommended intervals mentioned above. In a multi-year demonstration 

program operating in Southern California, trucks and buses were retrofitted with catalyst-based 

diesel particulate filters and operated on ultra-low sulfur diesel fuel (< 15 ppm S) to assess the 

June 2005

15 
long-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 emission performance over a 3 1⁄2 year period with mileage accumulations exceeding 340,000 

miles for each of three test vehicles monitored by the program. Each of these tractors was 

powered by a 1998 model DDC Series 60 heavy-duty diesel engine (430 hp, 12.7 liters 

displacement). During the first 3 1⁄2 years and 340,000+ miles of operation for each of these 

three test trucks, filters were cleaned of ash only once - after two years and approximately 

220,000 miles of operation on each vehicle. Backpressure monitors on the test vehicles did not 

indicate that cleaning was required after two years of operation and the amount of ash collected 

from these filters was relatively small. An important contributor to the relatively low 

backpressure increase over time observed on these DPF-equipped vehicles is the low oil 

consumption characteristics observed on the engines equipped in these vehicles. Oil 

consumption rates on average of approximately 3 quarts per 10,000 miles of service were 

observed on these vehicles. These tractor-trailers equipped with retrofit filter systems continue 

in operation at the time this report was released. These results indicate that modern diesel 

engines equipped with filters and with low oil consumption characteristics are capable of

extended operations between filter maintenance intervals. 

In the case of new European passenger cars equipped with particulate filters, first 

generation systems employing fuel-borne catalysts are designed to operate for 80,000 km before 

filter maintenance is required. At 80,000 km, an owner is requested to bring his or her vehicle in 

for service at which time the filter is replaced with either an unused filter or a filter that has been 

cleaned of ash. Second generation systems employing lower dosage rates of fuel-borne catalysts 

have extended this maintenance interval to 120,000 km. Third generation systems introduced in 

2004 include lower fuel-borne catalyst dosage rates with new asymmetric filter designs that 

employ large octagonal inlet channel cross sections, allowing service intervals to be extended up 

to 200,000 km (up to 120,000 miles). Recent reports of DPF designs based on the use of sintered 

metal sheets indicate that these designs are capable of running more than 260,000 km (more than 

155,000 miles) on a 2.0 liter European diesel passenger car application employing a fuel-borne 

catalyst with a dosing rate of 10 ppm. First generation catalyzed DPFs available on some

European diesel passenger cars (e.g., BMW, Mercedes, Renault) are currently sized to require 

maintenance at approximately 80,000 km intervals. The filter systems used in European 

passenger car applications include diagnostic capabilities to monitor exhaust temperatures, 

engine backpressure, and filter pressure drop with provisions to alert the driver if abnormal 

conditions are encountered. 

Toyota and Hino recently reported on the ash accumulation characteristics of their DPNR 

technology that combines a NOx adsorber catalyst coating on a relatively high porosity, wallflow ceramic filter (11). In a 4.0 liter engine used in a Japanese light-duty truck application, they 

found only a modest increase in pressure drop across the DPNR unit after 200,000 km

(approximately 125,000 miles) 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 amount 

consumed by the engine during this test deposited on the filter. The balance of the ash was 

assumed to be emitted in the exhaust of the vehicle. The authors speculate that the continuous 

PM oxidation characteristics of the DPNR system promote separation of the ash from the filter. 

The relatively high porosity of ceramic wall-flow filter used by Toyota in these DPNR 

June 2005

16 
applications may also contribute to the release of ash from the filter during normal vehicle 

operation. 

3.5 Filter Cleaning Procedures and Equipment 

Manufacturers of filter technologies have developed safe, reliable, and effective 

procedures for removing the ash from particulate filters. These practices have been put in place 

in both retrofit wall-flow filter applications and wall-flow filter-equipped passenger cars in 

Europe. The ash cleaning practices include well-documented procedures for cleaning filters and 

the development of stand-alone filter cleaning stations or facilities that automate many of the 

necessary procedures. 

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

steps: 

(1) removal of the filter from the engine/vehicle; a filter weight before cleaning may be 

determined at this point

(2) removal of the ash with a combination of pressurized dry air gun (e.g., 50-100 psi; 

note that catalytically coated filters may require the use of air pressures on the lower 

end of this range to minimize damage to the catalytic coating. Suppliers of 

catalytically coated filters provide recommendations 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-efficiency particulate air] or ULPA [ultra-low 

penetration air] filter) on the filter’s inlet side that includes provisions for collecting 

the ash. This pressurized air/vacuum treatment should direct the pressurized air flow 

across all channels on the filters to make sure each channel is cleaned of ash. Total 

time for air cleaning will depend on the size of the filter but is typically 30-50 

minutes for a filter sized for a heavy-duty highway engine. In some cases specially 

designed pulsed-air equipment is used to do this air cleaning procedure.

(3) inspection of the cleaned filter by probing individual channels, and/or weight or flow 

characteristics (e.g., filter pressure drop measurements using ambient temperature 

airflows) to confirm that the cleaning has been successful

(4) re-installation of the filter 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 the filter in an industrial oven or a stand-alone 

cleaning unit that includes a heating element to burn off any organic soot remaining on the filter. 

 Baking temperatures may vary between 450 and 800o

C with total baking times (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 minimize thermal stresses that may occur in 

the filter due to the release of heat associated with the oxidation of any remaining soot. If the 

baking step is done after an air cleaning procedure, an additional air cleaning procedure may be 

done on the filter following the baking procedure to remove ash still remaining in the filter. The 

heating strategy in some cases has been found to improve the ash collection process, but 

acceptable levels of ash removal have also been demonstrated by procedures that only use 

June 2005

17 
pressurized air or pressurized air in combinations with vacuum. In some cases a brief 5-10 

minute treatment with pressurized air on the outlet side of the filter is recommended prior to the 

baking step. In all cases any pressurized air/vacuum treatments must be done using an 

acceptable dust collection strategy to prevent entry into the local environment and exposure of 

the maintenance technicians to the dust associated with the ash. Technicians employed to 

conduct ash cleaning procedures need to wear appropriate eye protection, dust masks, and 

appropriate gloves. 

A number of filter system manufacturers offer automated or semi-automated machines or 

stand alone workstations for filter cleaning operations. These stations or machines include 

provisions for pressurized air streams, vacuum collection, and/or heating of filters. In some

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 filter cleaning system is shown in 

Figure 3. 

Figure 3. 

Diesel Particulate Filter Cleaning Unit 

Additionally, a number of companies, including engine/vehicle original equipment 

dealers and distributors, are offering cleaning services 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 filter cleaning capability. These filter cleaning service providers offer a variety of

service options including on-site cleaning and reinstallation of the filter at a customer’s facility 

June 2005

18 
or “swap and clean” programs that substitute a clean filter element for an ash-containing filter, 

with subsequent cleaning of the ash-containing filter. 

In Europe, where significant numbers of diesel passenger cars have been sold equipped 

with particulate filters, a complete centralized filter cleaning facility has been built by the 

exhaust component company that supplies uncatalyzed, filter-containing exhaust systems to the 

auto manufacturers (33). This facility allows for multiple filters to be cleaned in parallel for a 

relatively high throughput of filters per day (up to 1,000 DPFs per day). This operation features 

a filter cleaning procedure based on a washing protocol that utilizes warm water with detergent. 

This facility has been commissioned in stages based on wastewater recycling capacity with the 

first stage limited to 100 filters per day and the current second stage capacity limited to 250 

filters per day. Through 2003 this facility had cleaned about 2000 filters. 

In this European facility, operational cleaning steps include: (1) visual inspection and 

weighing of incoming filter elements, (2) regeneration of the filter with hot air to burn off any 

organic-based particulate matter retained on the filter, (3) weighing the regenerated filter, (4) a 

cleaning procedure that alternates flowing warm water (with detergent) and pulsed air flow 

through the filter walls to remove accumulated 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 monitored with backpressure measurements and on-line measurement of 

the exhaust gas that exits the filter. This filter cleaning facility also includes provisions for the 

treatment and recycling of the wash water used to clean filters before the wash water is returned 

to the local sewer system. Recovered ash materials 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 filter cleaning procedures have only been used on uncatalyzed filter 

systems that make use of filter canning systems that have mounting materials compatible with 

the water cleaning operation. Some ceramic filter substrate mounting materials, such as 

intumescent-based mats (mats that contain vermiculite components that expand when heated), 

deteriorate when exposed to water and should not be cleaned using water. Deterioration of the 

mat mounting material can lead to permanent damage and failure of the filter in service. 

Similarly, catalyzed filter elements should not be water cleaned due to the potential for 

degradation or damage to the catalyst coating present on the filter element. 

The use of excessive air pressures (above 100 psi) to clean catalyzed filters of ash should 

also be avoided since high cleaning pressures may damage or remove the catalyst coating from

the filter substrate. Recommendations from suppliers of catalyzed filters regarding maximum air 

pressures used in filter cleaning procedures should be followed in all cases. 

Filter baking/regeneration temperatures associated with ash cleaning above 850o

C are 

generally not recommended due to high temperature sintering reactions that occur within the 

stored ash or the potential for high temperature reactions between the stored ash and the ceramic 

filter substrate or catalyst coating. These high temperature solid-state reactions can result in an 

ash structure that is difficult to remove or the potential for damage to the substrate or catalyst 

coating. 

June 2005

19 
Available filter cleaning machines are generally based on pressurized air/vacuum

cleaning procedures described above since this procedure is compatible with both intumescent 

and non-intumescent filter mounting materials. Steam cleaning is not practical for ash removal 

on any filter systems due to the potential for forming cement-like species from unremoved ash. 

In all situations associated with filter maintenance procedures, filter elements need to be handled 

with care to avoid damage to the filters. In the case of filters used on heavy-duty engines, these 

filters can be large in size and weight and technicians must use appropriate safety procedures to 

minimize personal risks and damage to the filters. 

Published studies of recommended ash cleaning procedures generally report that filter 

pressure drop is returned to acceptable levels but generally not equivalent to an unused filter 

(14). In the retrofit application of filters, filter manufacturers provide end users with training 

courses and complete documentation describing recommended filter cleaning practices and/or 

operation of specified filter cleaning equipment. 

As detailed in Section 3.1, the ash collected from filter cleaning procedures consists 

primarily of oxides and sulfates of inorganic materials associated with lubricant additives and 

exhaust system corrosion. Disposal practices for ash collected from filters are determined by 

local and/or state environmental regulations. A compositional analysis of representative 

collected ash may be necessary to determine legal disposal practices. In some jurisdictions ash 

may 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 environmental 

agencies to determine acceptable disposal methods. 

Currently, laws are inconsistent regarding how to classify ash from particulate filters. In 

the State of New York, the New York State Department of Environmental Conservation 

(NYSDEC) provided information to NESCAUM (Northeast States for Coordinated Air Use 

Management, an organization representing state air quality officials 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 hazardous waste. Without this hazardous waste 

designation, NYSDEC indicated to NESCAUM that ash should be treated as a solid waste. 

In California, laws may vary depending on location. For that reason, a statement is 

included in owners manuals of California-verified filters that ash disposal must be in accordance 

with all applicable Federal, state, and local laws governing waste disposal. In California, if zinc 

is present in the ash collected from a filter in high concentrations, it can make this material a 

hazardous waste. The California Code of Regulations (Title 22, section 66261.4) establishes two 

limits for zinc in waste: 250 milligrams per liter for the soluble threshold limit concentration, and 

5,000 milligrams per kilogram for the total threshold limit concentration. The presence of zinc 

at or above these levels would cause an ash material to be characterized as a hazardous waste in 

California. Under California law, the generator of the waste has the responsibility to determine 

whether their waste is hazardous or not. This, in general, would require a chemical analysis of 

the collected ash sample to determine the zinc content. There are facilities in California that 

June 2005

20 
accept hazardous waste from conditionally exempt small quantity generators. Additional 

guidance concerning acceptable disposable methods of ash collected from filters in California is 

available from the California Department 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 exempt small quantity 

generators that generate less than 220 lbs. per month of solid waste from federal hazardous waste 

regulations. Additional information concerning federal hazardous waste requirements are 

available from the EPA Office of Solid Waste (OSW) by either calling their Resource 

Conservation and Recovery Act (RCRA) hotline at 703-412-9810 or 800-424-9346, or by 

visiting the following website: www.epa.gov/epaoswer/general/orientat/. The EPA also provides 

information about state contacts regarding solid and hazardous waste disposal issues at: 

www.epa.gov/epaoswer/osw/stateweb.htm. 

It is also important to note that the amount of ash collected from a filter during a typical 

cleaning procedure is relatively small. For example, ARB reports that the amount of material 

collected from a single filter in a single cleaning procedure is in the range of 10-20 grams of ash. 

 ARB extrapolated this ash collection amount to 10-20 kg of ash collected per year for a fleet of 

1000 transit buses. Actual amounts of ash collected from a filter during a cleaning procedure 

will vary depending on variables including the oil consumption rate of the engine. Ash disposal 

costs will depend on the quantity of ash disposed of at any given time 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 marketplace concerning ash disposal practices. MECA encourages 

federal, state, and local agencies to communicate on this topic and reach a common position on 

accepted safe disposal practices for ash collected from filters. EPA is expected to issue their 

own fact sheet concerning filter maintenance procedures sometime in 2005. This fact sheet will 

be posted on the EPA Office of Transportation and Air Quality (OTAQ) website 

(www.epa.gov/otaq). 

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

 

The first large-scale application of diesel particulate filters 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 manufacturers have indicated their intent to use diesel particulate filters to 

comply with the U.S. EPA’s 2007 PM emission regulation of 0.01 g PM/bhp-hr. This 2007 

introduction of filters on new highway heavy-duty diesel vehicles will encompass close to one 

million vehicles annually for vehicle weight classes above 8500 lb. GVW. EPA stipulates a 

150,000 mile minimum maintenance interval for any emission control device, including 

particulate filters, used in the heaviest weight class of heavy-duty vehicles. This means that 

manufacturers will need to design filter systems that are capable of operating a minimum of

150,000 miles between ash removal procedures for these so-called heavy, heavy-duty vehicles. 

 In public forums to date, the engine manufacturers have indicated that complying with 

this minimum filter maintenance interval is part of their development plan for 2007 engines. 

June 2005

21 
Several factors will contribute to the success in meeting this requirement. These include engine 

designs with inherently low and stable oil consumption characteristics, sufficient filter ash 

capacity to allow for long maintenance intervals, and reformulated lubricants that have lower ash 

content than lubricants currently in use today. Filter ash capacity may include the use of

asymmetric filter 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 limit lubricant ash content to 1.0 wt.% relative to the current 1.5 

wt.% ash limit for current heavy-duty lubricants to help extend filter maintenance intervals. 

SAE paper no. 2003-01-3109 describes some of the on-going efforts to develop new lubricant 

formulations designed for use in diesel engines using DPFs or other advanced diesel engine 

emission control technologies. 

As demonstrated by the extended filter maintenance intervals reported by long haul 

tractor trailers evaluated in the BP-ARCO program (13), the 150,000 mile maintenance interval 

should be met by 2007 engines used in similar applications. Other applications such as transit 

buses or refuse haulers, where vehicles operate for many hours but accumulate miles at a 

relatively low rate, may present a challenge in complying with EPA’s 150,000 mile minimum

maintenance interval. The emissions control industry is committed to work with their customers 

in the engine manufacturing industry to make the 150,000 mile filter maintenance interval a 

reality for as wide an application base on new heavy-duty diesel engines as possible. 

REFERENCES 

1. L. Pace, R. Konieczny, and M. Presti, “Metal Supported Particulate Matter-Cat, a Low 

Impact and Cost Effective Solution for the FIAT 1.3 JTD Euro IV Engine,” SAE Paper 

no. 2005-01-0471 (2005). 

2. H. L. Fang and M. J. Lance, “Influence of Soot Changes on DPF Regeneration,” SAE 

Paper no. 2004-01-3043 (2004). 

3. M. S. Sutton, N. Britton, B. Otterholm, P. Tengstroem, C. Frennhelt, A. Walker, and I. 

Murray, “Investigations into Lubricant Blocking of Diesel Particulate Filters,” SAE 

Paper no. 2004-01-3013 (2004). 

4. J.-C. Fayard and T. Seguelong, “A New DPF System for Duty Cycle Vehicles,” SAE 

Paper no. 2004-01-1937 (2004). 

5. S. Nemoto, Y. Kishi, K. Matsuura, M. Miura, S. Togawa, T. Ishikawa, T. Hashimoto, and 

T. Yamazaki, “Impact of Oil-derived Ash on Continuous Regeneration-type Diesel 

Particulate Filter – JCAPII Oil WG Report,” SAE Paper no. 2004-01-1887 (2004). 

6. K. Ogyu, K. Ohno, S. Hong, and T. Komori, “Ash Storage Capacity Enhancement of 

Diesel Particulate Filter,” SAE Paper no. 2004-01-0949 (2004). 

7. D. M. Young, D. L. Hickman, G. Bhatia, and N. Gunasekaran, “Ash Storage Concept for 

Diesel Particulate Filters,” SAE Paper no. 2004-01-0948 (2004). 

June 2005

22 
8. H. Toorisaka, J. Minamikawa, H. Narita, T. Muramatsu, T. Kominami, and T. Sone, 

“DPF Developed for Extremely Low PM Emissions in Production Commercial 

Vehicles,” SAE Paper no. 2004-01-0824 (2004). 

9. S. D. Whitacre, B. J. Adelman, M. P. May, and J. G. McManus, “Systems Approach to 

Meeting EPA 2010 Heavy-Duty Emission Standards Using a NOx Adsorber Catalyst and 

Diesel Particulate Filter on a 15 L Engine,” SAE Paper no. 2004-01-0587 (2004). 

10. D. Tomazic, M. Tatur, and M. Thornton, “Development of a Diesel Passenger Car 

Meeting Tier 2 Emission Levels,” SAE Paper no. 2004-01-0581 (2004). 

11. A. Shoji, S. Kamoshita, T. Watanabe, T. Tanaka, and M. Yabe, “Development of a 

Simultaneous Reduction System of NOx and Particulate Matter for Light-Duty Truck,” 

SAE Paper no. 2004-01-0579 (2004). 

12. T. Mizuno and J. Suzuki, “Development of a New DPNR Catalyst,” SAE Paper no. 2004-

01-0578 (2004). 

13. K. Kimura, T. Alleman, S. Chatterjee, and K. Hallstrom, “Long-Term Durability of 

Passive Diesel Particulate Filters on Heavy-Duty Vehicles,” SAE Paper no. 2004-01-

0079 (2004). 

14. B. Ball, D. Hully, P. Lausseur, and N. Lyons, “Experience of Continuously Regenerating 

Particulate Traps on City Buses in Europe,” SAE Paper no. 2004-01-0078 (2004). 

15. A. Mayer, J. Czerwinski, J.-L. Petermann, M. Wyser, and F. Legerer, “Reliability of 

DPF-Systems: Experience with 6000 Applications of the Swiss Retrofit Fleet,” SAE 

Paper no. 2004-01-0076 (2004). 

16. N. Jeuland, J. B. Dernenthon, L. Gagnepain, G. Plassat, P. Coroller, J.C. Mornique, G. 

Belot, and D. Dalili, “Performance and Durability of DPF (Diesel Particulate Filter) 

Tested on a Fleet of Peugeot 607 Taxis: Final Results,” SAE Paper no. 2004-01-0073 

(2004). 

17. T. Campenon, P. Wouters, G. Blanchard, P. Macaudiere, and T. Seguelong, 

“Improvement and Simplification of DPF System Using a Ceria-based Fuel-borne 

Catalyst for Diesel Particulate Filter Regeneration in Serial Applications,” SAE Paper no. 

2004-01-0071 (2004). 

18. W. A. Givens, W. H. Buck, A. Jackson, A. Kaldor, A. Hertzberg, W. Moehrmann, S. 

Mueller-Lunz, N. Pelz, and G. Wenniger, “Lube Formulation Effects on Transfer of 

Elements to Exhaust After-Treatment System Components,” SAE Paper no. 2003-01-

3109 (2003). 

19. E. Bardasz, D. Mackney, N. Britton, G. Kleinschek, K. Olofsson, I. Murray, and A. 

Walker, “Investigations of the Interactions between Lubricant-derived Species and 

Aftertreatment Systems on a State-of-the-Art Heavy Duty Diesel Engine,” SAE Paper no. 

2003-01-1963 (2003). 

20. G. Blanchard, T. Seguelong, J. Michelin, S. Schuerholz, and F. Terres, “Ceria-Based 

Fuel-Borne Catalysts for Series Diesel Particulate Filter Regeneration,” SAE Paper no. 

2003-01-0378 (2003). 

21. K. Ogyu, K. Ohno, A. Kudo, Y.Oshimi, and H. Sato, “Characterization of Thin-Wall 

SiC-DPF,” SAE Paper no. 2003-01-0377 (2003). 

22. S. Chatterjee, R. Conway, S. Viswanathan, M. Blomquist, and S. Andersson, “NOx and 

PM Control From Heavy-Duty Diesel Engines Using a Combination of Low-Pressure 

EGR and Continuously Regenerating Diesel Particulate Filter,” SAE Paper no. 2003-01-

0048 (2003). 

June 2005

23 
June 2005

24 

23. T. Seguelong, C. Rigaudeau, G. Blanchard, O. Salvat, C. Y. Colignon, and C. Griard, 

“Passenger Car Series Application of a New Diesel Particulate Filter System Using a 

New Ceria-Based, Fuel-Borne Catalyst: From the Engine Test Bench to European 

Vehicle Certification,” SAE Paper no. 2002-01-2781 (2002). 

24. G. Saroglia, G. Basso, M. Presti, M. Reizig, and H. Stock, “Application of New Diesel 

Aftertreatment Strategies on a Production 1.9 L Common-Rail Turbocharged Engine,” 

SAE Paper no. 2002-01-1313 (2002). 

25. M. Quigley and T. Seguelong, “Series Application of a Diesel Particulate Filter with a 

Ceria-based Fuel-Borne Catalyst: Preliminary Conclusions after One Year of Service,” 

SAE Paper no. 2002-01-0436 (2002). 

26. S. Chatterjee, R. Conway, T. Lanni, B. Frank, S. Tang, D. Rosenblatt, C. Bush, D. 

Lowell, J. Evans, R. McLean, and S. J. Levy, “Performance and Durability Evaluation of 

Continuously Regenerating Particulate Filters on Diesel Powered Urban Buses at NY 

City Transit – Part II,” SAE Paper no. 2002-01-0430 (2002). 

27. J. Gieshoff, M. Pfeifer, A. Schaefer-Sindlinger, U. Hackbarth, O. Teysset, C. Colingnon, 

C. Rigaudeau, O. Salvat, H. Krieg, and B. W. Wenclawiak, “Regeneration of Catalytic 

Diesel Particulate Filters,” SAE Paper no. 2001-01-0907 (2001). 

28. G. A. Merkel, W. A. Cutler, and C. J. Warren, “Thermal Durability of Wall-Flow 

Ceramic Diesel Particulate Filters,” SAE Paper no. 2001-01-0190 (2001). 

29. K. N. Pattas, Z. Samaras, D. Sherwood, K. Umehara, C. Cantiani, O. Aguerre Chariol, P. 

Barthe, and J. Lemaire, “Cordierite Filter Durability with Cerium Fuel Additive: 100,000 

km of Revenue Service in Athens,” SAE Paper no. 920363 (1992). 

30. B. Edgar, “Emission Control Systems and Components for Retrofit and First-Fit 

Applications,” U.S. Dept. of Energy Diesel Engine Emission Reduction (DEER) 

Conference, Coronado, CA, August 2004. 

31. M. Khair, “Update on Progress of APBF-DEC EGR/DPF/SCR Demonstration Program at 

SwRI,” U.S. Dept. of Energy Diesel Engine Emission Reduction (DEER) Conference, 

Coronado, CA, August 2004. 

32. G. Merkel, “New Cordierite Diesel Particulate Filters for Catalyzed and Non-Catalyzed 

Applications, U.S. Dept. of Energy Diesel Engine Emission Reduction (DEER) 

Conference, Newport, RI, August 2003. 

33. R. Parmann, “Diesel Particulate Filter: A Success for Faurecia Exhaust Systems, U.S. 

Dept. of Energy Diesel Engine Emission Reduction (DEER) Conference, Newport, RI, 

August 2003.