APPENDIX D

REVIEW OF PM EMISSION REDUCTION TECHNOLOGY
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In September 2001 and March 2002, staff updated the Board on the status of

advanced aftertreatment technology for particulate matter (PM) and oxides of

nitrogen (NOx). In this Appendix, staff summarizes and discusses the status of

these technologies. The discussion pertains specifically to availability and how

retrofit technologies reduce NOx and PM, although other pollutants such as

carbon monoxide (CO) and HC may also be significantly reduced through these

emission control systems.

AFTERTREATMENT TECHNOLOGY STATUS UPDATE

A. Advanced PM Aftertreatment Technology

There are several available emission control technologies, including engine

modifications, that can reduce diesel PM emissions from diesel-fueled engines.

Many of these emission control technologies are already being used today in a

variety of engine applications to reduce diesel PM emissions. Below is a

summary of the emission control technologies that will play a key role in reducing

exposures to diesel PM.

1. Diesel Particulate Filter

A diesel particulate filter (DPF) is positioned in the exhaust stream to trap or

collect a significant fraction of the particulate emissions while allowing the

exhaust gases to pass through the system. Since the volume of particulate

matter generated by a diesel engine is sufficient to fill up and plug a reasonably

sized filter over time, a means of disposing of the trapped particulate

(“regeneration”) must be provided. The most common means of disposal is to

oxidize or burn the particulate in the filter. To facilitate filter regeneration on

diesel engines in real operations, the exhaust gas temperature has to be

increased or the soot ignition temperature has to be lowered using a catalyst.

DPFs can incorporate either passive or active regeneration techniques. Passive

systems rely on the heat of the exhaust, usually with the aid of a catalyst, to

combust the PM at a higher average rate than the rate at which the PM is

accumulated. Thus, the applicability of passively regenerating diesel particulate

filters may be limited to applications with moderate to low engine-out PM

emissions and higher exhaust temperatures. The use of low sulfur fuel (15 parts

per million) with DPFs minimizes sulfate formation and in some cases is

necessary for proper catalytic operation.

Programs are underway to evaluate the correlation between levels of sulfur in

diesel fuels and the effectiveness of retrofit (both PM and NOx) devices. In one

demonstration program, BP/ARCO is testing its low sulfur diesel fuel, ECD-1, on

catalysts and particulate filters made by Johnson Matthey and Engelhard.

ECD-1 contains a maximum of 15 ppm sulfur. The first round of emission results

from the BP/ARCO demonstration program indicate that PM, HC, and CO are
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reduced by greater than 90 percent (LeTavec 2001). A second round of

emission tests one year later supports the same conclusion.

The Clean Diesel Demonstration Program conducted by New York City Transit

(NYCT) tested the results of using PM retrofits on urban buses (MTA NYCT

2001). The program was designed to test the emissions using these systems:

(1) original equipment manufacturer (OEM) diesel oxidation catalyst (DOC) using

350 ppm sulfur diesel fuel; (2) OEM DOC using 30 ppm sulfur diesel fuel; and (3)

Johnson Matthey’s Continuously Regenerating Technology (CRTTM) particulate

filter using 30 ppm sulfur diesel fuel.

The conclusions drawn from this study were: (1) the use of the ultra-low sulfur

diesel fuel alone resulted in a 76 percent average reduction in the total HC, 29

percent average reduction in CO, and 29 percent average reduction in PM; and

(2) the CRTTM resulted in 93-98 percent reductions in total HC, CO, and PM,

using the New York Bus Cycle. The CRTTM testing continued until November

2001. Confident in the results of this program, NYCT has contracted for ultra low

sulfur diesel fuel for its entire fleet for three years starting from September 2000.

NYCT has also committed to retrofitting 100 percent of its fleet of 3500 buses by

the end of 2003 (MTA NYCT 2001). Diesel particulate filters have been

retrofitted onto 1150 buses as of August 2002. All buses retrofit so far have had

1994 or later model year engines; all remaining buses with pre-1994 engines are

expected to be retired by the end of 2003 (Dana Lowell, personal

communication, 2002). 

Another issue that has arisen with regards to passive DPFs is that in these

systems, the catalyst oxidizes NO to NO2 and uses the produced NO2 as an

oxidant to remove the PM trapped in the filter material. Measurements of NOX

emissions (NO and NO2) from heavy-duty diesel vehicles equipped with passive

catalyzed filters have shown an increase in the NO2 fraction, though total NOX

emissions remain approximately the same. Passive catalyzed filters oxidize NO

to NO2, which burns soot captured in the filter. More NO2 is created than is

actually used in the regeneration process; and the excess is emitted. In fact, the

NO2 to NOX ratios could range from 20 to 70 percent, depending on factors such

as the diesel particulate filter systems, sulfur level in diesel fuel, and the duty

cycle (DaMassa, 2002). To minimize the possible effects on population

exposure to ozone and NO2, the ARB has established a cap of 20 percent of NO2

to NOx emission ratio for all diesel emission control technologies. To ensure that

the cap does not penalize retrofit strategies that reduce total NOx emissions, the

20 percent cap is determined from the baseline (pre-control) emissions.

The applicability of passively regenerating diesel particulate filters may be limited

to applications with moderate to low engine-out PM emissions and higher

exhaust temperatures, because passive DPF systems rely on the heat of the

exhaust, to combust the PM at a higher average rate than the rate at which the

PM is accumulated. Thus, although these conditions typically encompass late-
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model buses, they do not include all buses. For example, older two-stroke

engines are likely to require different control strategies. For those and other

applications in which the engine-out PM level is relatively high and the exhaust

temperatures are relatively cool, actively regenerating systems are more

appropriate. Active systems typically use an external source of heat to oxidize

the particulate matter. The most common methods involve electrical

regeneration by passing current through a heating element, injecting fuel to

provide additional heat for particle oxidation, or the use of a fuel-borne catalyst or

other reagent to initiate regeneration. Off-road applications of these active

systems have been implemented in Europe since the early 1990’s (Mayer and

Wyser 2001). However, it should be recognized that passive systems are more

attractive from a user's standpoint as they are expected to require less

maintenance and to be less expensive.

2. Diesel Oxidation Catalyst

A diesel oxidation catalyst (DOC) converts pollutants into harmless gases by

promoting chemical oxidation. The catalyst, which is similar in design to catalysts

used on passenger cars, oxidizes CO, gaseous HC, and the liquid HCs adsorbed

on the carbon particles present in diesel exhaust gases. The liquid and gas

phase HCs are referred to as the soluble organic fraction (SOF). The SOF is one

component of the total PM in exhaust emissions. Oxidation catalysts can reduce

the SOF by 90 percent under certain operating conditions (MECA 1998), and

according to staff estimates, could reduce total particulate emissions by greater

than 30 percent.

Additional benefits of the DOC include oxidation of several HC-derived

emissions, such as aldehydes or polynuclear aromatic hydrocarbons (PAHs), as

well as reduction or elimination of the diesel exhaust odor. The DOC also

oxidizes sulfur dioxide (SO2), which is present in diesel exhaust from the

combustion of sulfur containing fuels, generating sulfate particles and may

increase total particulate emissions. Reducing the sulfur content of the fuel will

reduce the sulfate particles formed. Oxidation catalysts have proven effective in

achieving modest PM emission reductions on older buses. Under the U.S. EPA’s

urban bus rebuild/retrofit program, five manufacturers (Detroit Diesel

Corporation, Engelhard, Johnson Matthey, Twin Rivers Technologies, and

Engine Control Systems) have certified diesel oxidation catalysts as providing at

least a 25 percent reduction in PM emissions (U.S. EPA 2001; MECA 1998).

The Diesel Emission Control – Sulfur Effects (DECSE) Program is a joint

government/industry program created to investigate the effects of diesel fuel

sulfur levels on emission control systems such as diesel oxidation catalysts. Two

DOCs (low and high temperature catalysts) were tested before, during, and after

a 250-hour aging cycle using four different sulfur level diesel fuels (DECSE

2001). The reduction efficiencies for HC, CO, and PM were evaluated. Results

from this study indicated that fuel sulfur level does not significantly affect
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performance degradation. Some performance loss, however, was noted as early

as 250 hours after initial installation. Other results from the same study showed

that low sulfur diesel fuel is needed if a DOC is to be used as an efficient

emission control device.

Some potential adverse environmental impacts of DOCs have been identified.

First, as is the case with most processes that incorporate catalytic oxidation, the

formation of sulfates increases at higher temperatures. Depending on the

exhaust temperature and sulfur content of the fuel, the increase in sulfate

particles may offset the reductions in SOF emissions. Using low sulfur (15 parts

per million) diesel fuel can minimize this effect. Second, a DOC could be

considered a “hazardous waste” at the end of its useful life depending on the

material(s) used in the catalytic coating. However, DOCs can be manufactured

with catalytic coatings such that the product would not be considered a

hazardous waste. Finally, because the oxidation process converts NO to NO2,,

the emissions of NO2 could increase. However, these concerns are relatively

minor, and DOCs have been successfully used as original equipment in

numerous diesel engines for many years.

3. Fuel Additives

Fuel additives are substances added to the fuel to reduce the total mass of PM,

with variable effects on CO, NOx and HC production. When additives are used

alone, the PM reduction range from 15 percent to 50 percent, although reduction

as high as 99 percent can be seen when additives are used with a DPF. Some

additive-based systems reduce polynuclear aromatic hydrocarbons by around 80

percent.

Fuel-borne catalysts (FBCs) are fuel additives added to diesel fuel to aid in soot

removal in diesel particulate filters by decreasing the ignition temperature of the

carbonaceous exhaust. Additionally, FBC can improve fuel economy, aid other

retrofit systems, and decrease mass PM emissions. FBC/DPF systems are

widely used in Europe for on-road and off-road, mobile and stationary

applications. Typical FBC materials include cerium, platinum, iron, strontium,

and sodium. Most additives act to reduce soot combustion temperature, thereby

facilitating filter regeneration and potentially preventing excessive filter loading

and/or uncontrolled regeneration. Additives can be used with both passive and

active systems.

The additive is added to the fuel in one of three methods: dosing the bulk fuel,

incorporating an on-board dosing system in the vehicle, or allowing consumers to

add the additive directly. The last method is least attractive due to high likelihood

of user error, thus allowing for situations where the vehicle may run with an

inappropriate additive dose. The formulation concentration of the additive, as

well as the actual base constituent of the additive, will profoundly affect the

behavior of the FBC. When used with a DPF, approximately 1% of metal
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consumed is emitted in the tailpipe exhaust. Although some similarities exist

among FBCs, it is inappropriate to draw generalizations between additives.

Additionally, additives with similar active ingredients can have significant

differences.

A question left unanswered about FBCs is the potential long-term health effect of

metals used in FBCs. When FBCs are used at high treatment rates without

filters, there is a potential for high levels of metal emissions and an increase in

ultra-fine particles. Other concerns associated with the use of FBCs include

incompatibility with other applications and interference with normal engine

functioning. Further investigations are needed to address these potential

concerns.

4. Alternative Diesel Fuels

Alternative diesel fuels, such as biodiesel, synthetic diesel, and water emulsions,

have also shown to reduce diesel PM emissions. Biodiesel fuel, a renewable

energy source, is derived from vegetable oils or animal fats. Biodiesel can be

used for combustion in a diesel engine either in the pure state or blended with

diesel fuel. Biodiesel fuel blends that include oxygenate additives can decrease

PM emissions, but can also increase NOx emissions. In addition, in some cases

biodiesel may not be compatible with alloys and elastomers commonly used in

diesel engines necessitating special engine material design considerations.

Synthetic diesel fuels are manufactured using carbon-containing feedstocks,

such as natural gas or coal. The most widely known synthetic diesel fuel

technology is the Fischer-Tropsch process. Synthetic diesel fuels are attractive

because they do not require modifications to existing diesel engines. The fuels

can be designed to provide both good engine performance and emission

reductions. The high cetane numbers and low sulfur content of synthetic fuels

promote emission reductions of several diesel exhaust pollutants. Significant

reductions in diesel emissions, including NOx and PM, have been reported when

using synthetic fuel. Because the sulfur content is very low, synthetic fuels are

compatible with a range of sulfur-sensitive aftertreatment technologies, such as

lean NOx catalysts or passive filters.

Water-fuel emulsions are a third type of alternative diesel fuel. The various

emulsifying technologies being developed utilize chemical additives (surfactants),

high pressures, or electrical phenomena and have 20-30% water in their

formulations. NOx and PM emissions have been reduced 40-50% using waterfuel emulsions. One drawback to this method is the increase of HC and CO

emissions. However, this increase can be mitigated by the use of a DOC.

Although water emulsions appear to be a promising diesel emission control

technology, they have been known to alter the fuel lubricity and corrosion

properties. The stratification of the emulsified fuel during storage is another
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problem associated with technology. Further development is needed to eliminate

these undesirable properties of the fuels.

B. Technology Evaluation

1. Retrofit Requirements

Following is the summary of the urban transit bus fleet rule requirements for

retrofitting diesel bus engines to reduce diesel PM emissions in use, which are

being proposed for modification. Title 13, CCR, section 1956.2 (f) requires that

older engines be retrofitted to reduce diesel PM earlier than newer engines.

Specifically, 100 percent of pre-1991 MY (Tier 1) diesel engines must be

retrofitted with technology that will reduce diesel PM by 85 percent by January 1,

2003. The same requirement applies to a lower percentage of MY 1991 through

1995 (Tier 2) engines by January 1, 2003, under a phase-in period. The

deadline for full compliance for all 1995 and older models is January 1, 2004, for

transit agencies on the diesel path and January 1, 2005, for transit agencies on

the alternative-fuel path.

For Tier 3, or 1996 through 2002 MY, engines, the rule specifies that these be

retrofitted under a phase in schedule as follows: for diesel path transit agencies,

20 percent by January 1, 2005; 75 percent by January 1, 2006; and 100 percent

by January 1, 2007. Transit agencies on the alternative-fuel path have two

additional years to begin and conclude retrofits, as follows: 20 percent by

January 1, 2007; 75 percent by January 1, 2008; and 100 percent by January 1,

2009. Included in the retrofit requirements are the following exemptions:

(1) MY 1990 and earlier engines that were originally certified to 0.6 g/bhp-hr PM

and have been retrofitted to 0.1 g/bhp-hr PM with an ARB certified retrofit

device are exempt from further retrofits;

(2) Tier 2 and 3 buses, operated by transit agencies on the alternative fuel path,

that are within two years of retirement are exempt from the retrofit

requirements; and

(3) Tier 2 and 3 buses, operated by transit agencies on the diesel path, that are

within one year of retirement are exempt from the retrofit requirements.

2. Requirements for Technology Verification

Prior to its use in any transit bus, ARB requires that a retrofit device be verified to

reduce diesel particulate matter emissions by 85 percent or, alternatively, to

levels of 0.01 g/bhp-hr or below (title 13, CCR, section 1956.2(f)(7)). The ARB

staff is developing regulations regarding the evaluation of retrofit devices. In

recognition of the major role that retrofit technologies must play in the reduction
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of public exposure to diesel PM, the Board has adopted a verification procedure

to verify emission reductions.

The verification process is intended to ensure that retrofit devices provide the

necessary reductions while remaining durable. Prior to a device being verified,

the manufacturer must provide a general description of the emission control

system, including the principles of operation; effects on engine performance and

fuel consumption; any fuel requirements (e.g., diesel fuel with a sulfur level of 15

ppm or less); and maintenance requirements. In addition, the manufacturer must

provide emissions test data (including NO2 measurements), and durability data.

Devices intended for heavy-duty engines, such as those used in transit buses,

must show a durability of 50,000 miles or 1,000 hours.

Installation of emission control equipment in the exhaust system of a vehicle may

result in increases in backpressure. The manufacturer must therefore

demonstrate that the resulting backpressure is within the engine manufacturer’s

specified limits, or will not result in any damage to the engine. Acknowledging

this, the verification procedure requires that a backpressure monitor be installed

for all filter-based systems.

To ensure acceptability to the user, the manufacturer must provide a warranty

that covers defects and damage to the vehicle. In addition, the manufacturer

must clearly specify in the owner’s manual the following information:

• Warranty statement including the warranty period over which the

manufacturer is liable for any defects;

• Installation and maintenance requirements;

• Fuel consumption penalty, if any;

• Fuel limitations, if any (e.g., sulfur content);

• Contact information for the manufacturer of replacement components and

maintenance supplies.

• Safety considerations.

3. Verified Technologies

As of August, 2002, the ARB staff has verified the following particulate control

strategies for retrofit use:

Level 3 - 85 percent or greater PM reduction

• Clean Air Partners - diesel particulate filter for use with specific Power

Systems Associates natural gas/diesel bi-fuel engines and Caterpillar engines

which have been converted to bi-fuel operation using the Power Systems

Associates and Clean Air Partners bi-fuel retrofit systems.
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• Engelhard - DPXTM diesel particulate filters for use with most 1994-2002

model year diesel engines in on-road applications using 15 ppm or less sulfur

fuel. All of these engines are four-stroke, turbocharged, and were certified in

California to the 0.1 gram per brake horsepower-hour (g/bhp-hr) PM emission

standard when new.

• Johnson Matthey - CRTTM diesel particulate filters for use with most 1994-

2002 model year diesel engines in on-road applications using 15 ppm or less

sulfur fuel. All of these engines are four-stroke, turbocharged, and were

certified in California to the 0.1 gram per brake horsepower-hour (g/bhp-hr)

PM emission standard when new.

Level 3 - 85 percent or greater PM reduction with 25 percent NOx reduction

• Cleaire - Flash and CatchTM system for use with specific 1994 through 1998

model year Cummins M11 engines, for steady state application (long haul

truck), operating on low-sulfur fuel in on-road applications. The nature of the

control place additional restrictions on the calibrations for which the controls

will function. These other restrictions are detailed in the verification letters,

which may be found on the ARB web site at

http://www.arb.ca.gov/diesel/verifieddevices/verdev.htm.

Level 1 - 25 percent or greater PM reduction with 25 percent NOx reduction

• Cleaire - Flash and MatchTM oxidation catalyst system for use with specific

1994 through 1998 model year Cummins M11 engines, for steady state

application (long haul truck), in on-road applications. The nature of the

control place additional restrictions on the calibrations for which the controls

will function. These other restrictions are detailed in the verification letters,

which may be found on the ARB web site at

http://www.arb.ca.gov/diesel/verifieddevices/verdev.htm.

4. Availability of PM Emission Reduction Technology

At this time, two particulate control devices that achieve 85 percent control are

available and ARB-verified for urban bus use (the Engelhard DPXTM and the

Johnson Matthey CRTTM). Those devices are verified only for 1994 and newer

four-stroke engines. The devices are passive diesel particulate filters that utilize

exhaust gas heat and a catalyst to regenerate. No system has been verified as

of August 1, 2002, to reduce diesel PM emissions from older and two-stroke bus

engines, at any level of PM reduction. In general, two-stroke bus engines are

more technologically challenging to retrofit with a passive DPF because PM

emissions tend to be higher than four-stroke engines. Furthermore, the exhaust

gas temperature may not meet the minimum temperature required for
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spontaneous regeneration. Perhaps as importantly, the number of engines that

are available to be retrofitted is relatively small and unlikely to grow, thus

providing a disincentive for manufacturers to bring such a technologically

challenging product to market.

Staff reported to the Board in March 2002 that the technology to reduce diesel

PM emissions by 85 percent would not be available for pre-1994 MY engines in

time to meet the January 2003, regulatory deadline. As a result, staff’s proposal

in this report revises the current PM retrofit requirement to allow transit agencies

more flexibility to reduce in-use diesel PM emissions to the same level as

envisioned by the original regulation. Transit agencies would use the funds

already earmarked for the retrofit of the Tier 1 and Tier 2 engines to reduce their

diesel PM emissions.