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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2000 TRAVERWOOD ROAD

ANN ARBOR, MI 48105

 

May 4, 2012

 OFFICE OF

AIR AND RADIATION

MEMORANDUM

SUBJECT: Diesel Particulate Filter Regeneration

FROM: Lauren Steele, Office of Transportation and Air Quality

TO: Docket EPA-HQ-OAR-2011-1032

Engine Combustion Characteristics and Emissions Formation

Particulate matter (PM) consists of many components. Carbonaceous soot and 

solid-phase hydrocarbons originate from incomplete combustion of fuel and lubricating 

oil. Metal and metal oxide PM (i.e. metallic ash) originates from engine wear and/or 

lubricating oil additives. Sulfur-containing compounds, including solid-phase sulfate 

particles, are formed when other compounds react with the sulfur present in the fuel and 

lubricating oil.

Within the combustion chambers of the cylinders in an operating engine, there 

exists an important trade-off between PM (soot and hydrocarbon) formation and nitrogen 

oxides (NOX) formation. To explain this trade-off, we describe the combustion as taking 

place in sequential stages. The initial formation of soot and hydrocarbon PM occurs 

during the early stages of combustion. However, most of the initially formed soot and 

hydrocarbon PM is actually oxidized in-cylinder, late within the combustion process, so it 

is never seen at an engine’s tailpipe. In contrast, NOX is formed in the middle stage of 

the combustion process when the highest combustion temperatures occur, and it is not 

largely eliminated late in the combustion process as is soot and hydrocarbon PM. This is 

significant because in-cylinder NOX control is achieved by lowering peak combustion 

temperatures, but these lower temperatures in turn decrease oxidation of in-cylinder soot 

and hydrocarbon PM; thereby increasing soot and hydrocarbon PM at the tailpipe. Thus 

we have a trade-off between engine combustion conditions that either optimize NOX

control or PM control, but not both. This requires crucial decision-making when 

designing an engine to meet EPA emissions standards. As is explained in the preamble to 

the proposed rule, this trade-off becomes even more important when designing an engine 

that will be successfully integrated with an emissions control system equipped with a 

diesel particulate filter (DPF).
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DPFs typically use a porous ceramic or cordierite substrate or metallic filter to 

physically trap particulate matter (PM) and remove it from the exhaust stream.1In some 

non-road applications such as in underground gassy mines, disposable paper-based or 

fiberglass-based filters may be employed downstream of an exhaust cooling system (e.g. 

a “wet scrubber”).

In contrast with soot and hydrocarbon PM, which is periodically oxidized from a 

DPF, most metallic ash and other noncombustible materials remain on the filter. This 

accumulated ash must be cleaned offline periodically by removing the DPF and sending 

it to a service center. Regarding sulfate PM, it should be noted that DPFs must be used in 

conjunction with ultra-low sulfur diesel fuel, which has a sulfur content of less than 15 

parts per million. Higher levels of fuel sulfur adversely affect sulfate formation 

downstream of the DPF, and sulfur also interferes with DPF regeneration. Sulfur can 

deactivate the catalytic materials within the DPF, which in turn can inhibit the oxidization 

of trapped soot and hydrocarbon PM at lower exhaust temperatures.

Engines operating over relatively low-temperature, low-speed duty cycles can 

have more difficulty achieving conditions sufficient for DPF regeneration, and may

require more frequent maintenance and driver interaction. When an engine manufacturer 

certifies a new engine for application in vocational vehicles, it is tested using EPA’s 

standard federal test procedure (FTP) which simulates urban stop-and-go driving. It is 

not the purpose or intent of the FTP to simulate conditions under which a DPF may

regenerate. Indeed, it is rare that an engine will experience DPF regeneration during an

FTP test cycle. Instead, manufacturers typically collect data in other ways to determine 

whether their control algorithms are appropriate for the engine and its intended 

applications.

When a truck engine operates at low speeds and loads, its temperature remains 

relatively low and this is typically a point of less efficiency on its power curve. When a 

diesel engine operates at higher loads, its exhaust temperature increases and it generally 

shifts to a place of higher efficiency on its power curve. This higher, more optimal 

temperature not only enables greater fuel efficiency and the activation of catalysts within 

the emission control system, it also can lead to the generation of less soot and more NOX

emissions. Because the principal NOX formation mechanism in an engine is thermal 

formation within the combustion chambers, many NOX reduction technologies have 

focused on ways to decrease combustion temperatures. For successful operation of a 

DPF, however, the highly reactive NOX compounds, especially NO2, are useful, acting as 

powerful oxidizers. When NO-rich engine exhaust is passed over a DOC, a large fraction 

of NO2 is formed, which increases the rate of oxidation of the deposited soot in the cells 

of the DPF, thus cleaning the filter through passive regeneration. When combustion 

temperatures are lower, NOX content tends to be lower, decreasing the rate of oxidation 

in the DPF, leading to an increased need for active regeneration.

Furthermore, the NOX-PM trade-off of most diesel combustion processes 

(homogeneous charge compression ignition (HCCI) being an exception) dictates that 

 
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when combustion temperatures are lower, in-cylinder soot oxidation is lower, and this 

increases the PM loading of the DPF. Engines operating at lower temperatures may also 

experience higher lubricating oil consumption than engines operating over duty cycles 

with higher operating temperatures, leading to higher rates of ash accumulation and 

eventual DPF removal for ash cleaning. Therefore, both the engine’s duty cycle and the 

overall control strategy of the engine’s emissions control system play a large role in the 

success of integrating a DPF with an engine to control PM emissions.

Types of Regeneration

Passive Regeneration

Passive regeneration refers to methods that rely strictly on the available exhaust 

temperatures and exhaust constituents to oxidize PM from a DPF in a given vehicle 

application. Passive regeneration is an automatic process that occurs without the 

intervention of an engine’s on-board diagnostic and control systems, and often without 

any operator alerts. Passive regeneration is often a continuous process, because of which, 

it is sometimes referred to as continuous regeneration.

DPFs 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. 

When a vehicle with only a passively regenerating DPF experiences unacceptable engine 

backpressure conditions, the driver may be alerted to take corrective action. Operation at 

higher vehicle speeds and loads typically leads to successful passive regeneration of a 

DPF. When driving the vehicle in this manner does not achieve passive regeneration, the 

vehicle must be taken to a service facility for maintenance, where the DPF is typically 

removed for offline regeneration.

Many DPF’s today are designed with a diesel oxidation catalyst (DOC) directly

upstream. The presence of oxidized compounds such as NO2 in the exhaust stream, in 

combination with sufficient temperatures, can enable the burn-off (oxidizing) of 

accumulated soot on a continuous basis while those conditions exist.

In designing a system to enable passive regeneration, engineers seek to achieve a 

rate of soot oxidation that is in equilibrium with the rate of soot deposition. When 

deposition and oxidation rates are in equilibrium, the pressure drop due to PM 

accumulated on the filter will remain constant.2 When the deposition rate is higher than

the oxidation rate, particles continue to accumulate and the pressure drop rises. For a 

given DPF configuration, catalyst, fuel, and engine, the temperature at which this 

equilibrium is attained is called the balance point temperature. An ideal DPF system will 

be optimized for its engine so that the balance point temperature is attained or exceeded 

frequently throughout the duty cycle.

 

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“Science and Technology of Catalytic Diesel Particulate Filters,” Barry A. A. L. van Setten, Michiel 

Makkee, and Jacob A. Moulijn, Catalysis Reviews, 43(4), 489–564 (2001), retrieved from 

http://koala.ece.stevens-tech.edu/sd/archive/07F-08S/websites/grp6/references/5.pdf.
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Scientists have studied the effects of modifying DPF systems, including 

alternative catalysts, to facilitate oxidation and reduce the balance point temperature. 

While most systems in use today do not incorporate the use of fuel borne catalysts, the 

study conducted by Jelles et al in 1999 illustrates the steady-state pressure drop that can 

be achieved.3 Figure 1 depicts results of a test with dosing of a fuel-borne catalyst to 

determine the lowest temperature at which equilibrium is attained. 

Figure 1 Illustration of Equilibrium with Passive Regeneration 

The experiment was conducted with a Corning EX80 wall-flow monolith filter, 

where the lowest balance point temperature for 100 ppm cerium catalyst additive was 

found to be between 430C and 440C. The curves are typical of a balance point 

temperature experiment. The positive slopes indicate normal operation with soot 

deposition. The horizontal lines at approximately five and 10 hours indicate when 

equilibrium is achieved. In the case of an in-use engine passively regenerating, 

theoretically these horizontal slopes would continue until operating conditions caused PM 

accumulation (and pressure drop) to increase. The periodic spikes in temperature and 

steep negative slopes in pressure drop shown in Figure 1 indicate active regenerations as 

part of the experiment, and would not be part of an in-use passive system operating 

profile.

Automatic Active Regeneration

In a vehicle whose normal operation does not generate temperatures needed for 

passive DPF regeneration, the system needs a little help to clean itself. This process is 

called active regeneration, and supplemental heat inputs to the exhaust are provided to 

initiate soot oxidation. The frequency of active regeneration depends on many

parameters, including engine emissions profile, engine loads, vehicle duty cycle, and 

ambient temperature. There are two types of active regeneration: those that may occur 

 

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Jelles, S.J.; Makkee, M.; Moulijn, J.A.; Acres, G.J.K.; Peter-Hoblyn, J.D. Diesel Particulate Control. 

Application of an Activated Particulate Trap in Combination with Fuel Additives at an Ultra Low Dose 

Rate. SAE paper 1999-01-0113, 1999, available at http://papers.sae.org/1999-01-0113/
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automatically either while the vehicle is in motion, while idling, or while operating in 

PTO mode, and those that must be driver-initiated and occur only while the vehicle is

stationary and out-of-service. The first type is discussed here, while driver-initiated 

parked regeneration is discussed below. During both of these types of active 

regeneration, NOX emissions may increase, especially when not otherwise controlled by a 

NOX aftertreatment device. An estimate of emissions during regeneration is presented in 

a memorandum to the docket entitled “NOX Emissions from DPF Regeneration.”

It is important to note that all DPF systems with active regeneration components 

also have the capability to passively oxidize soot accumulated on the filter, though some 

may rarely achieve a level of equilibrium and will continue to experience rising 

backpressures until an active regeneration is completed. Vehicle operating conditions or 

duty cycles determine whether passive oxidation may occur to the degree necessary to 

maintain engine exhaust backpressures within acceptable limits. In some cases, the small 

amount of passive oxidation merely serves to lengthen the time interval between active 

regenerations. In some extreme cases where the DPF system almost never achieves its 

balance point temperature, passive oxidation may be negligible. In those cases the 

system would rely almost exclusively on active regenerations to maintain a clean PM 

filter.

With actively regenerating DPF systems, the engine’s electronic control module 

(ECM) is programmed to detect operating conditions and produce driver signals that 

provide information about DPF conditions related to regeneration. When conditions 

indicate a need for active regeneration, and thus more heat, a number of things can occur. 

A variety of manufacturer approaches can be taken to produce the supplemental heat 

needed for active regeneration. Diesel engines with MY 2007 or newer emission control 

systems often incorporate approaches ranging from air intake throttling to fuel dosing. 

Some of these approaches are described further in Section IV.C of the preamble to the 

proposed rule, and estimates of the additional fuel use by a vehicle whose DPF 

regeneration system employs fuel dosing are described in a memorandum to the docket 

entitled “Fuel Use with Dosing for DPF Regeneration.”

Actively regenerating DPF systems typically require sufficient air flow, 

temperature and soot accumulation before an automatic active regeneration will be 

initiated. As mentioned above, this may occur either while the vehicle is in motion or 

parked, if pre-set engine operating conditions are met. When the engine’s ECM signals 

the initiation of an automatic active regeneration and the extra heat is generated, an ideal 

DPF system accomplishes this as a transparent process, with no effects perceivable by the 

driver. As a safety precaution, some vehicles will illuminate a dash lamp during this 

time, indicating when exhaust temperatures are higher than normal.

Vehicles with automatic active regeneration systems require operators to be alert 

to dash lamps and indicators. Written instructions are provided that explain what each 

lamp means and what action is needed. Some vehicle signals instruct drivers to change 

driving conditions to allow automatic active regenerations to occur. As an example, an 

illustration of a typical DPF dash lamp is provided in Figure 2. The left image indicates a 

steady illumination (usually amber), while the right image indicates the lamp is flashing. 
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Figure 2 Aftertreatment Diesel Particulate Filter Lamp

The instructions corresponding to these lamps, as provided in a Cummins Engine 

Company operator manual, are shown in Table 1.

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Table 1 Aftertreatment Diesel Particulate Filter Lamp Instructions

The AFTERTREATMENT DIESEL 

PARTICULATE FILTER lamp indicates, when 

illuminated or flashing, that the aftertreatment 

diesel particulate filter requires regeneration.

An illuminated AFTERTREATMENT DIESEL 

PARTICULATE FILTER lamp indicates that 

the aftertreatment diesel particulate filter needs 

to be regenerated at the next changing 

opportunity. This can be accomplished by:

1. Changing to a more challenging duty 

cycle, such as highway driving, for at 

least 20 minutes

2. Performing a stationary regeneration

A flashing AFTERTREATMENT DIESEL 

PARTICULATE FILTER lamp indicates that 

the aftertreatment diesel particulate filter needs 

to be regenerated at the next possible 

opportunity. Engine power may be reduced 

automatically.

When this lamp is flashing, the operator should:

1. Change to a more challenging duty 

cycle, such as highway driving, for at 

least 20 minutes

2. Performing a stationary regeneration

Because EPA emissions standards are performance based; and therefore, do not 

dictate any required emission control system technologies or configurations, each 

manufacturer has the discretion to program the timing and sequence of lamps as needed 

to inform drivers of the condition of its emission control systems. It is not uncommon in 

today’s heavy-duty fleet for an engine’s ECM to limit its maximum speed, torque or 

power when an overloaded DPF is detected. These engine and emission control system 

protection measures can alert drivers to the need to change driving conditions to enable 

active mobile regeneration or to make plans to allow for a manual active regeneration, 

described below. If a driver disregards such warnings, the risk of uncontrolled engine 

shutdown or catastrophic DPF failure increases. The preamble to the proposed rule 

includes additional discussion of DPF failure.

 

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101-048 Engine Indicator Lamps, 2007 ISL OM, last modified December 2011
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Manual Active Regeneration

When a vehicle’s driving cycle does not present conditions for regular passive or

automatic active regeneration (e.g. due to insufficient temperature or exhaust flow), a 

manual active regeneration becomes necessary. In contrast to the practice for an 

overloaded passively regenerating DPF system, filters with heavy soot buildup on 

vehicles with actively regenerating systems rarely need to be removed and regenerated 

offline. Instead most vehicles are equipped with switches and instructions for an operator 

to initiate a manual active regeneration while the truck is parked in a safe location and 

continuously monitored by the driver or a service technician.

A manual active regeneration allows the engine’s ECM to increase engine speed 

and exhaust temperature to a greater extent than what is typically allowed during an 

automatic active regeneration. Because the ECM takes full control of an engine during a 

manual active regeneration, the vehicle must remained parked and not used for other 

purposes, such as pumping water in PTO mode. While this type of regeneration does 

temporarily take a vehicle out of service, the duration to complete a parked manual active 

regeneration is typically less than 40 minutes, and the process may be interrupted at any 

time. This is important, because there are often cases where a vehicle, particularly an 

emergency vehicle, must be returned to service prior to completion of a manual active 

regeneration. As more time elapses before interrupting a manual active regeneration, the 

progression of warning lamps will resume earlier in the warning/DPF loading process, 

providing more opportunities for other types of regeneration to occur, or at least 

providing more operating time before the next manual active regeneration is demanded. 

EPA has information from one engine manufacturer indicating that interrupting a manual 

active regeneration (starting with a fully loaded filter) after as little as 10 minutes may

enable a vehicle to resume normal driving, which could then lead to the occurrence of an 

automatic active regeneration or at least to arrive safely at a local garage before reillumination of the warning lamp indicating a need for a manual active regeneration.

In the case where a manual active regeneration is not initiated when requested by

the engine’s controls, the DPF may become so overloaded that a manual active 

regeneration is no longer an available option. The reason for this is that an extreme level 

of soot loading requires a very carefully managed regeneration, under the supervision of a 

highly trained technician. A discussion of DPF failure is presented in Section IV.C of the 

preamble to the proposed rule, including a description of catastrophic failure, which is an 

uncontrolled, “runaway” regeneration. Technician-controlled regenerations may require 

towing the vehicle to a special service center, and may occur while the DPF is on the 

vehicle, or offline with the DPF removed from the vehicle. In such cases, if a spare DPF 

is not available, the vehicle could be out of service overnight.