1 **U NITED STA** 

**TES ENVIRONMENTA L PROTECTION A** **GENCY** NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2000 TRAVERWO OD ROAD ANN ARBOR, MI 48105

May 4, 20 12 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** 
P articulate matter (PM) consists of many components. Carbonaceous soot and solid

- phase h ydrocarbons originate from incomplete combustion of fuel and lubricating oil. Metal and metal oxide PM (i.e. metallic ash) originate s 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 chambe rs of the cylinders in an operating engine, there exists an important trade

- off between PM (soot and hydrocarbon) formation and nitrogen oxides (NO X ) formation. To explain this trade

- off, we describe the combustion as taking place in sequential stages . T he initial formation of soot and hy drocarbon PM occurs during the earl y 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 eng ine’s tailpipe. In contrast, NO X is formed in the middle stage of the combustion process when the highest combustion temperatures occur, and it is not largel y eliminated late in the combustion process as is soot and hydrocarbon PM. This is significant be cause in

- cylinder NO X 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 NO X 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 r

ule , this trade

- off becomes even more important when designing an engine that will be successfull y i ntegrated with an emissions control sy stem equipped with a diesel particulate filter (DPF)

.2 DPFs ty pically use a porous ceramic or cordierite substrate or m etallic filter to phy sicall y trap particulate matter (PM) and remove it from the exhaust stream. 1 Insome non - road applications such as in underground gassy mines, disposable paper - based or fiberglass

- based filters may be employ ed downstream of an exhaust cooling s ys tem (e.g. a “wet scrubber”).
Incontrast with soot and hydrocarbon PM, which is periodically oxidized from a DPF, most metallic ash and other noncombustible materi als 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 catal yt ic materials within the DPF, which in turn can inhibit the oxidization of trapped soot and h ydrocarbon PM at lower exhaust temperatures. Engines operating over relatively low

- temperature, low

- speed dut y cy cles can have more difficulty

achieving conditions sufficient for DPF rege neration, 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 pro cedure (FTP) which simulates urban sto p - 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 a n FTP test cy cle. Instead, manufacturers t ypicall y collect d ata in other way s 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 relativel y low and this is typicall y a point of less efficiency on its power curve. When a diesel engine operates at higher loads, its exhaust temperature increases and it generall y shifts to a place of higher efficiency on its power curve. This higher, more optimal temperature not only

enables greater fu el efficiency and the activation of cataly sts within the emission control sy stem, it also can lead to the generation of less soot and more NO X emissions. Because the principal NO X formation mechanism in an engine is thermal formation within the combustion chambers, man y NO X reduction technologies have focused on way s to decrease combustion temperatures. For successful operation of a DPF, however, the highly reactive NO X compounds, especiall y NO 2 , are useful, acting as powerful oxidizers. When NO

- rich eng ine exhaust is passed over a DOC, a large fraction of NO 2 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, NO X co ntent tends to be lower, decreasing the rate of oxidation in the DPF, leading to an increased need for active regeneration. Furthermore, the NO X - PM trade

- off of most diesel combustion processes (homogeneous charge compression ignition (HCCI) being an excep tion) dictates that3 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 oper ating over duty cy cles with higher operating temperatures, leading to higher rates of ash accumulation and eventual DPF removal for ash cleaning. Therefore, both the engine’s dut y cy cle and the overall control strategy

of the engine’s emissions control sy stem 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 consti tuents 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 sy stems, and often without any operator alerts. Passive regen eration 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 s ome ty pes of diesel engines, the level of PM generated b y a specific engine, and/or application operating experience. When a vehicle with only a passivel y regenerating DPF experiences unacceptable engine backpressure conditions, the driver may be alerted to take corrective action. Operation at higher vehicle speeds and loads ty picall y 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 fac ility for maintenance, where the DPF is ty picall y removed for offline regeneration. Many DPF’s today

are designed with a diesel oxidation catal ys t (DOC) directly upstream. The presence of oxidized compounds such as NO 2 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 s ys tem 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, catal ys t, fuel, and engine, the temperature at which this equilibrium is attained is called the balance point temperature. An ideal DPF s ys t em will be optimized for its engine so that the balance point temperature is attained or exceeded frequentl y throughout the duty cy cle.

2 “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

.4 Scientists have studied the effects of modifying DPF sy stems, including alternative catal ys ts, to facilitate oxidation and reduce the balance point temperature. While most sy stems in use today do not incorporate the use of fuel borne cataly sts, the study conducted by

Jelles et al in 1999 illustrates the stead y - state pressure drop that can be achieved. 3 Figure 1 depicts results of a test with dosing of a fuel

- borne cataly st 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 catal ys t additive was found to be between 430C an d 440C. The curves are t ypical 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 , theoreticall y 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 i n pressure drop shown in Figure 1 indicate active regenerations as part of the experiment, and would not be part of an in

- use passive s ys tem operating profile. Automat ic Active Regeneration In a vehicle whose normal operation does not generate temperatures needed for passive DPF regeneration, the sy stem 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 dut y cycle, and ambient temperature. T here are two t ypes of active regeneration: those that may occur

3 Jelle s, 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 htt p://p apers.sae.org/1999

- 01

- 0113/5 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 onl y while the vehicle is stationary and out

- of

- service. The first t ype is discussed here, while driver

- initiated parked regeneration is discussed below. During both of these types of active regeneration, NO X emissions may increase, especially

when not otherwise controlled by a NO X aftertreatment device. An estimate of emissions during r egeneration is presented in a memorandum to the docket entitled “NO X Emissions from DPF Regeneration

. ” It is important to note that all DPF sy stems with active regeneration components also have the capability

to passively

oxidize soot accumulated on the fi lter, 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 cy cles determine whether passive oxidation may occur to the de gree 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 sy stem al most never achieves its balance point temperature, passive oxidation may be negligible. In those cases the sy stem would rely

almost exclusively

on active regenerations to maintain a clean PM filter. With actively

regenerating DPF s ys tems, the engine’s ele ctronic 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 numb er of things can occur. A variet y of manufacturer approaches can be taken to produce the supplemental heat needed for active regeneration. Diesel engines with MY 2007 or newer emission control sy stems 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 e stimates of the additional fuel use by a vehicle whose DPF regeneration s ys tem employ s fuel dosing are described in a memor andum to the docket entitled “Fuel Use with Dosing for DPF Regeneration.” Actively regenerating DPF s ys tems ty pically 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 sy stem ac complishes this as a transparent process, with no effects perceivable by the driver. As a safet y precaution, some vehicles will illuminate a dash lamp during this time, indicating when exhaust temperatures are higher than normal. Vehicles with automatic a ctive regeneration sy stems 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 al low automatic active regenerations to occur. As an example, an illustration of a ty

pical DPF dash lamp is provided in Figure 2 . The left image indicates a steady ill umination (usually

amber), while the right image indicates the lamp is flashing.6 **Figure 2**  **Aftertreatm ent Diesel Particulate Filter Lamp**  The instructions corresponding to these lamps, as provided in a Cummins Engine Company operator manual, are shown in Table 1

. 4

**Table 1**  **Aftertreatm ent Diesel Particulate Filter Lam** **p Instructions**  The AFTERTREATMENT DIESEL PARTI CULATE 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 dut y cy cle, such as highway

driving, for at least 20 minutes 2. Performing a stationary regeneration A flashing AFTERTREATMENT DIESEL PARTICU LATE 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 dut y cy cle, 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 an y 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 sy stems. It is not uncommon in today ’s heavy

- duty

fleet for an engine’s ECM to limit its maximum speed, t orque or power when an overloaded DPF is detected. These engine and emission control sy stem 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 r egeneration, described below. If a driver disregards such warnings, the risk of uncontrolled engine shutdown or catastrophic DP F failure increases. The preamble to the proposed rule includes additional discussion of DPF failure.

4 101

- 048 Engine Indicator Lamps, 2007 ISL OM, last modified December 20117 Manual Active Regeneratio n 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 sy stem, filters with heavy soot buildup on vehicles with activel y regenerating s ys tems rarel y need to be removed and regenerated offline. Instead most vehicles are equipped with sw itches and instructions for an operator to initiate a manual active regeneration wh ile 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 ty picall y 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 ty pe of regeneration does temporaril y take a vehicle out of service , the duration to complete a parked manual active regeneration is ty

pically less than 40 minutes, and the process ma y be interrupted at an y 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 t ypes of regeneration to occur, or at least providing more operating time before the next manual ac tive 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, wh ich could then lead to the occurrence of an automatic active regeneration or at least to arrive safel y at a local garage before re

illumination of the warning lamp indicating a need for a manual active regeneration. In the case where a manual active regene ration 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 carefull y managed regeneration, under the supervision of a highl y 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” r egeneration. 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.