Engineering Clean Air: The Continuous Improvement of Diesel Engine Emission Performance

Engineering Clean Air:

The Continuous Improvement of Diesel Engine Emission Performance

The Technology of Clean Diesel Engines, Current and Future

March, 2001

One Dulles Tech Center 2191 Fox Mill Road, Suite 100

Herndon, Virginia 20171

Tel: 703/234-4411

Fax: 703/234-4420

Executive Summary

The diesel industry has made great progress on a path of continuous improvement to virtually eliminate the key pollutants associated with on-road diesel engines. This transformation is of critical importance, because diesel plays a central role in economic activity. This paper (1) explains the inherent performance advantages of diesel; (2) provides an emissions profile of diesel as the starting point for emissions reduction; (3) documents the major emissions reductions that have been made to date, and explains the technologies that have made them possible;

and (4) describes the technologies that will be employed over the next decade to achieve the virtual elimination of key pollutants.

Performance advantages of diesel. The transformation of diesel emissions performance is of critical importance to the economy. Diesel is the

world's most efficient internal combustion engine. It provides both more power and more fuel efficiency than alternatives such as gasoline, compressed natural gas or liquefied natural gas. Diesel accordingly plays a central role in a wide range of economic activities. In the United States, 94% of all freight is moved by diesel power, and diesel is a critical part of a wide range of activities in transportation, agriculture, construction, mining, electric power generation, and fire and rescue services. Diesel's inherent performance advantages include:

• More power at lower engine speeds;

• Better fuel efficiency;

• Greater safety;

• More durability; and

• More power from a given engine size.

Diesel emissions profile. It is not widely recognized that diesel has some environmental advantages over other types of engines. Of the five major emissions from internal combustion engines – carbon monoxide, hydrocarbons, carbon dioxide, particulate matter and nitrogen oxides – diesel emits only small amounts of the first three. The challenge for diesel is reducing particulate matter (PM) and nitrogen oxides (NOx), and it is in those two emissions that vast

improvements have been made.

Technologies found in today's clean diesel engines. Particulate

matter emissions of new on-highway diesel engines have been reduced 83% since

1988. Emissions of nitrogen oxides have been reduced by 63% during the same time

period. This has been accomplished largely through improvements in fuel delivery,

the design of combustion chambers, and turbocharging. Electronic fuel injection

has permitted engine manufacturers to control fuel injection independently of

engine speed, permitting injection to be optimized for emissions performance. Fuel

also is injected at very high pressures to ensure a more complete burn, and the timing of fuel injection can be varied to meet emissions goals under different operating conditions. Combustion chamber design has been enhanced in a variety of ways. Turbocharging has been widely adopted, and has been refined by the development of air-to-air charge cooling to reduce combustion temperatures.

Nitrogen oxides reductions for 2004. Emissions standards for 2004 will cut NOx emissions in half for on-highway diesel engines, effecting an 83% total reduction since 1988. Exhaust gas recirculation (EGR) technology and advanced fuel injection/combustion control systems will play a major role in this reduction.

EGR lowers the temperature of the fuel burn by recirculating oxygen-depleted exhaust gases into the cylinders, reducing the oxygen content of the air involved in the burn. Advanced fuel injection/combustion control systems permit fuel pressure to be controlled independently from engine speed, allowing fuel injection to be shaped to meet temperature control and other emissions objectives.

Technologies to virtually eliminate key pollutants. The next wave of emissions reductions will arise from recently adopted 2007-2010 highway diesel engine standards. The new regulations calls for the reduction of both PM and NOx by 98% from 1988 levels – virtual elimination of these emissions from on-

highway engines. These emissions improvements will be achieved through both

continued refinement of engine technologies, like advanced turbocharging, as well

as new exhaust aftertreatment technologies. The new aftertreatment technologies

will be based on use of low-sulfur diesel fuel, which will enable catalytic converters,

particulate filters and other exhaust treatment technologies to work. Catalytic

converters are the technology that has been used with great success to reduce

gasoline engine emissions. Oxidation catalysts have been used successfully to

reduce PM in diesel applications, and selective catalytic reduction devices have

successfully reduced both NOx and PM. Particulate filters are in development, the

key challenge being the automatic disposal of trapped particulate as the filter

approaches its capacity. Catalysts to convert NOx into harmless nitrogen also are

in development.

PERFORMANCE ADVANTAGES OF DIESEL

The diesel's status as the world's most efficient internal combustion engine – producing more power and utilizing less fuel than other comparatively sized engines – has been recognized by widespread application throughout the world. In the United States, 94% of all freight is transported by diesel power. In Europe, where fuel prices put energy conservation at a premium, diesel powers nearly 25% of all new passenger vehicles. In France, Belgium, Austria, and Spain, over 40% of new passenger vehicles are diesel. Diesel power has also been selected for use in prototype vehicles by the U.S. Department of Energy's Partnership For a New Generation of Vehicles - a public/private partnership charged with developing radically more fuel efficient and environmentally friendly passenger vehicles.

Diesel's nearly universal use in a wide range of heavy-duty industrial applications reflects the combination of all its advantages: power, fuel efficiency, safety, durability, and suitability for very large applications. When these factors are taken into account, there is no practical substitute for diesel power in a wide range of activities, including: trucking, rail transport, public transit, inter-city bus service, marine shipping, construction, mining, agriculture, standby electric power generation, and fire and rescue vehicles.

• More Power. Diesels produce more drive force at lower engine speeds. This superior drive force is the result of the diesel engine combustion process, known as "compression ignition". Compression ignition produces superior combustion force in the cylinder, which in turn provides more power or "torque."

The compression ignition process works as follows: a diesel engine subjects air in the combustion chamber to very high compression. This compression heats the air to over 600 degrees Celsius, which is well over the ignition point of diesel fuel. Diesel fuel is then sprayed into the superheated combustion chamber where it spontaneously ignites (without the aid of a sparkplug), generating the power to move the piston. The force achieved in this process is considerably greater than that achieved in lower compression spark ignition engines (like natural gas or gasoline). Diesels thus produce more horsepower at lower engine speeds.

• Better Fuel Efficiency. Light-duty diesels, such as automobiles, use 30-60% less fuel than similarly sized gasoline engines, depending on the type of vehicle and driving conditions. Comparative studies have found on-road heavy-duty diesels to be more than 60% more fuel efficient than similarly sized spark-ignited natural gas engines (both compressed natural gas "CNG" and liquefied natural gas "LNG"). These advantages come from both the greater efficiency of

compression ignition and the higher energy content of diesel fuel.

Diesel's compression ignition process results in greater thermal efficiency – more of the fuel's energy is harnessed. This improves fuel economy. Diesel holds this advantage over any spark-ignited engine, including not only gasoline, but also CNG, LNG, and propane ("LPG"). Like gasoline engines, these other spark ignition engines are less fuel efficient because they burn fuel at lower temperatures under lower compression.

Diesel's superior fuel efficiency is not only a result of compression ignition, but also a result of diesel fuel's higher energy content. A gallon of diesel fuel contains roughly 11% more energy than a gallon of gasoline, 67% more than a gallon of LNG and 250% more than a gallon of CNG (at 3600 psi). The relatively low energy density of natural gas can be addressed in part by using larger fuel tanks, but the added weight of the tanks imposes an additional fuel economy penalty, and the tanks may also reduce the amount of useable space in the vehicle. As a result, LNG and especially CNG engines have a shorter cruising range than diesels.

• Safer. Diesel is less volatile than gasoline or natural gas – that is, the fuel does not as readily vaporize in the air. In addition, diesel fuel ignites only at a much higher temperature. For these reasons, diesel fuel is far less likely to ignite if spilled or released as a result of an accident. Diesel also is safer because it need not be handled in pressurized vessels. This is a sharp contrast to CNG, which is stored in pressurized cylinders (up to 3600 psi). High pressure greatly increases the risk of leaks during loading, unloading, and storage.

• More durable. Diesel engines are more durable than spark ignition engines.

Diesels both run more miles before needing rebuilding, and also are more easily rebuilt to original specifications. Light-duty diesel engines generally last

between 200,000 and 600,000 miles, compared to 70,000 to 200,000 miles for comparable gasoline engines. It is not uncommon for heavy-duty diesel truck engines to enjoy a life of 1,000,000 miles or more before rebuilding, nor is it uncommon for heavy-duty engines to power city buses for 15 to 20 years.

• Very large applications. The fact that diesels produce less wasted heat makes

them ideally suited for very large applications: ocean-going ships, railroad

locomotives and earth movers. One of the biggest issues in designing large

engines is the need to provide cooling systems to prevent overheating. This is a

major challenge when dealing with the heat produced in very large combustion

chambers. Because diesels waste less energy as heat, they place lesser demands

on cooling systems than spark ignition engines. This permits diesels to be scaled

up to very large sizes – diesel engines in some applications have cylinders as

large as three feet across.

EMISSIONS PROFILE OF DIESEL POWER SYSTEMS

While the performance advantages of diesel are widely known, diesels have environmental advantages that are less well understood: diesel power systems emit very little carbon monoxide, hydrocarbons, and carbon dioxide. Internal

combustion engines all emit the same basic types of emissions regardless of the type of fuel or combustion cycle used. Only the relative quantities of each emission type vary with the type of engine. The five major mobile source emissions are carbon monoxide, hydrocarbon, carbon dioxide, nitrogen oxides, and particulate matter.

Diesels produce only small proportions of the first three.

• Lower Carbon Monoxide Emissions. Diesels produce very little carbon monoxide (CO). Among all on-road mobile sources, heavy-duty diesel vehicles account for only 3% of total CO emissions. Industries whose personnel operate engines in confined spaces have traditionally used diesels because they produce so little carbon monoxide.

• Lower Hydrocarbon Emissions. Hydrocarbons (HC), a key precursor for ground level ozone (smog), are a major air pollutant from mobile sources.

Diesels, however, yield only small amounts of hydrocarbons. This results both from diesel's high combustion efficiency and from the lower evaporation rate of diesel fuel. Among on-road mobile sources, heavy-duty diesel vehicles account for only 6% of total HC emissions.

• Lower Greenhouse Gas Emissions. Today's diesel engines are low in

emissions of carbon dioxide (CO 2 ). (CO 2 is not a pollutant regulated by the EPA, but is considered a greenhouse gas.) Diesel's performance on this emission stems in part from its fuel efficiency – CO 2 emissions are closely related to overall fuel consumption. Today's on-road diesels are 30-60% more fuel efficient than their light- and heavy-duty counterparts.

While diesel performs well with respect to the emissions just described, diesel combustion inherently tends to produce significant amounts of particulate matter (PM) and nitrogen oxides (NOx). The extreme heat associated with diesel's compression combustion process tends to generate more NOx than less efficient, cooler burning engine types. The low level of oxygen in the combustion zone around individual fuel droplets prevents complete combustion of the diesel fuel which

contributes to the emission of soot (unburned carbon from the fuel) also known as particulate matter (PM).

The reduction of both PM and NOx simultaneously presents a unique emissions control challenge: some techniques used to control one of these pollutants increase the production of the other. For example, a major technique to reduce PM is to cause more complete combustion of the fuel. This reduces PM but also

produces more heat, conflicting with the goal of lowering combustion temperatures

to reduce NOx. The technical challenge of emissions reduction is to reduce both PM and NOx at the same time, without degrading the overall performance of the

system.

EMISSIONS REDUCTION THROUGH CONTINUOUS IMPROVEMENT Diesel power systems are enjoying a revolution in technology that already has achieved dramatic reductions in emissions. Technology incorporated in new on-highway engines has reduced particulate matter ("PM") emissions by 83%, and nitrogen oxides ("NOx") emissions by 63%, compared with engines produced before 1989. Technology now in development will, when it is implemented, enable further improvements so that the reductions from 1988 levels will be 98% for both PM and NOx.

Diesel emissions reductions are being achieved in stages, and

reductions have focused principally on highway engines and urban buses. Charts 1 and 2, set forth on the following page, detail current and future reductions in

emissions, and list some of the technologies that have enabled the reductions.

Chart 1

Diesel PM Emission Reductions and Enabling Technologies (Heavy-duty Highway Diesel Engines)

0.60

0.10

0.01 g/bhp/hr

1988 1998

• Computer-controlled high pressure fuel injection

• Improved combustion chamber configuration

• Turbo charging

• Low sulfur fuel

• Oxidation catalysts

• Selective Catalytic Reduction

• Particulate filters 2007

Chart 2

Diesel NOx Emission Reductions and Enabling Technologies (Heavy-duty Highway Diesel Engines)

While this review is focused on heavy-duty on-highway engine

emissions, these technologies also are being used in other applications. Light duty diesel engines for use in passenger vehicles, for example, will be required to meet the same emissions standards as gasoline powered vehicles under EPA standards that will be phased in from 2004 to 2009. Many of these same technologies will be employed in those engines.

This section describes the emissions reductions and enabling technologies in three parts: (1) the technologies found in current engines, which reflect the first wave of PM and NOx reductions in the years 1990-1994; (2) the further reduction in NOx emissions for the 2004 model year; and (3) the latest reductions in PM and NOx set for implementation in 2007-2010. 1

1 / Standards for NOx emissions from heavy-duty diesel engines were first established in 1974, but emissions control in the 1970s and 1980s was focused principally on

automobiles and other light duty gasoline vehicles. Those vehicles account for over 93% of vehicle miles traveled in the United States. The following chart lists heavy-duty diesel engine standards from 1988 through 2007:

(Footnote continued on next page)

10.70

4.00

2.50

0.20 g/bhp/hr

1988 1998 2004

• Computer-controlled high pressure fuel injection

• Improved combustion chamber configuration

• Injection timing retard

• Air-to-air charge cooling

• Low sulfur fuel

• Oxidation catalysts

• Selective Catalytic Reduction

• Exhaust gas recirculation

2007

Technologies Found in Today's Clean Diesel Engines

The emissions reductions achieved from 1990 through 1994 were massive: PM emissions of on-highway engines were reduced by 83% during that time. This was an extremely significant improvement – as a consequence of this change, the old image of a diesel truck accelerating up a hill and projecting a cloud of dirty black soot into the air is a picture of the past. While there are still such engines on the road, no engine sold in the United States since 1994, properly maintained and burning the proper fuel, will smoke in this way.

The PM reduction was accomplished by improvements designed to ensure a more complete burn of fuel within the engine. The primary enhancements include improved fuel delivery systems, improved configuration of combustion chambers, and turbocharging.

The same wave of emissions reductions produced a 63% improvement in NOx emissions of on-highway engines. These reductions also were achieved primarily through modifications in the engine. The modifications helped control combustion temperature, including offsetting the temperature increases caused by systems adopted to reduce PM emissions. NOx reductions have been achieved principally through improved fuel delivery, including electronic fuel injection and variable injection timing, and though air-to-air charge cooling, which reduces the higher temperatures created by turbocharging.

• Electronic Fuel Injection. The development of electronic fuel injection systems for diesel engines has played a central role in reducing both PM and NOx. Electronic systems calibrate fuel injection based on information from

Heavy-duty Diesel Engine Emission Standards (g/bhp-hr)

NOx PM

Model Year Standard %Change

Standard % Change

Standard % Change

1988 10.7 - 0.60 0.60 -

1990 6.0 -44% 0.60 0% 0.60 0%

1991 5.0 -53% 0.25 -58% 0.25 -58%

1993 5.0 -53% 0.25 -58% 0.10 -83%

1994 5.0 -53% 0.10 -83% 0.07 -88%

1996 5.0 -53% 0.10 -83% 0.05

-92%

1998 4.0 -63% 0.10 -83% 0.05 -92%

2004 2.0

-81% 0.10 -83% 0.05 -92%

2 20 00 07 7 0. 0 .2 2 - -9 98 8% % 0. 0 .0 01 1 - -9 98 8% % 0.01 -98%

a

Compared to the base model year 1988.

Beginning in 1996, the certification level was .05 but the in-use level was .07.

electronic sensors that monitor engine performance and vehicle activity. They are used both to ensure a more complete fuel burn to reduce PM, and to control temperature to reduce NOx. In contrast, older diesel fuel injection systems used mechanical means to control the quantity and timing of fuel injection. With those systems, rapid ramp-up of engine speed – such as acceleration with a heavy load – led to excess fuel being injected. Much of this fuel was not burned and was emitted as soot, which created the black exhaust that many associate with old diesel engines.

• High pressure fuel injection. PM emissions are reduced through more complete combustion of fuel injected into the combustion chamber. More

complete combustion can be achieved by improving the mix of air and fuel in the chamber. Modern high-pressure fuel injection systems force fuel into the

combustion chamber through smaller diameter holes at higher pressure – in excess of 25,000 pounds per square inch. This causes the fuel to break down into tiny droplets, thereby improving the air-fuel mix to achieve a more complete burn.

• Variable injection timing. NOx emissions can be reduced by a delay in the start of fuel injection, which reduces the temperature at which combustion takes place. This technique, known as injection timing retard, requires precise control of the beginning of injection into a cylinder in relation to the position of the piston in that cylinder. Most electronic fuel injection systems allow independent control of the timing of injection to optimize emissions performance. Reduction in NOx through this technique is combined with other measures such as high injection pressure and improved combustion chamber design, to minimize the loss of fuel economy and potential increase in PM emissions that otherwise would result from a delay in fuel injection.

• Improved combustion chamber configuration. More complete fuel

combustion, and reduced PM emissions occur when fuel and air are mixed more evenly in the combustion chamber. Engine manufacturers have invested great effort in optimizing the features of combustion chambers to ensure the best possible mix. Modern combustion chamber design reflects extensive modeling of several design elements, including: (1) the shape and depth of the combustion chamber and the piston bowl (the small area at the top of the piston into which fuel is injected); (2) spiral-shaped intake ports that cause air to swirl as it enters the chamber; (3) the number of cylinder valves; and (4) the placement of fuel injectors in the combustion chamber.

• Turbocharging. Turbocharging can both reduce PM emissions and improve

fuel economy. A turbocharger compresses the air that enters the cylinder,

forcing more air into the combustion chamber. The compressor is driven by a

turbine, which in turn is powered by the engine's own exhaust. The increase in

air in the combustion chamber offers two key advantages. First, it enables fuel

to burn more completely, reducing PM. Second, it permits more fuel to be added to the chamber, generating more power than a similarly-sized engine without turbocharging. By generating more power from a smaller displacement engine, turbocharging reduces engine weight and improves fuel economy.

• Air-to-air charge cooling. This is a further advance in turbocharging that reduces NOx emissions. Turbochargers deliver (or "charge") air at higher pressure, and therefore also increase the temperature of the air delivered for combustion. Air-to-air charge cooling reduces the temperature of the charged air, thereby lowering NOx emissions. Ambient air, which averages about 75° F, is used to cool the air to be charged in the combustion chamber. This is an improvement on water-based cooling systems that had been used in the past.

Those systems were limited in their effectiveness by their use of water at temperatures that could run as high as 210° F.

Nitrogen Oxides Reductions for 2004

Emissions standards for 2004 will cut current NOx emissions in half for heavy-duty on-highway engines. This reduction will effect an 81% total

reduction from 1988 levels. This next step will rely not only on the technologies already described, but also two additional technologies: exhaust gas recirculation, and advanced fuel injection control technologies.

• Exhaust Gas Recirculation. Exhaust gas recirculation ("EGR") will play a central role in achieving NOx reductions for 2004 generation engines. EGR reduces NOx by reducing the temperature at which fuel burns in the combustion chamber. Engines employing EGR recycle a portion of engine exhaust back to the engine air intake. The oxygen-depleted exhaust gas is mixed into the fresh air that enters the combustion chamber, which dilutes the oxygen content of the air in the combustion chamber. The reduction in oxygen produces a lower

temperature burn, and hence reduces NOx emissions by as much as 50%. The recycled exhaust gas can also be cooled, which further reduces NOx emissions.

• Advanced Fuel Injection. Advanced fuel injection systems provide much greater control of fuel injection to improve emissions and overall engine performance. In these systems injection pressure and injection rate can be controlled independently of engine speed and load, which is a departure from traditional fuel systems. Two of the most promising advanced fuel injection technologies are Common Rail Systems and Hydraulic Electronic Unit Injection systems.

In a Common Rail System (CRS) fuel is held in a reservoir, or "rail", that serves

maintained under pressure, and that pressure does not vary with engine speed.

Instead, pressure can be controlled independently to achieve emissions objectives.

Hydraulic Electronic Unit Injection (HEUI) systems also provide for lower emissions while improving fuel economy and performance. In these systems, individual unit injectors are actuated hydraulically by a high pressure oil pump, rather than mechanically. This high pressure oil controls the rate of injection, while electronics control the amount of fuel injected. All of this is done

independently of engine speed.

Traditional systems triggered fuel injection by mechanical means, using camshafts and plungers that were driven by the engine. This caused injection rates to rise and fall along with engine speed, and prevented independent regulation of injection pressure.

These advanced systems enable a number of emissions-reducing fuel injection techniques that previously were infeasible. For example, to reduce NOx emissions, fuel injection can be geared independently to control burn

temperature. In order to reduce particulate emissions, the main fuel injection can be split into two, causing a more complete burn of fuel. Other goals that can also be achieved through this technology include the reduction of engine

combustion noise by causing one or more small injections of fuel in advance of the main injection.

Common rail systems already are used widely in European passenger cars.

These systems are a large part of the reason that diesel vehicles enjoy wide consumer acceptance in Europe. Diesels are no longer noisier than gasoline vehicles, and show better driving characteristics, especially the delivery of superior power at low speed. HEUI systems are widely used in North America in medium duty trucks to improve emissions and performance.

Technologies to virtually eliminate key pollutants

In a departure from the past, EPA's 2007-2010 standards focus on reductions that can only be achieved through refinements to diesel power system as a whole: engine, fuel, and exhaust treatment technology addressed together as a system. The new standards will reduce both PM and NOx emissions from on-

highway engines to a level that is 98% below 1988 levels. The emissions reductions

will result primarily from advanced exhaust treatment technology, which will be

enabled by reductions in fuel sulfur content.

Exhaust treatment will involve particulate traps, which capture engine emissions before they leave the tailpipe, or catalytic converters that convert

emissions to harmless substances. (Catalytic converters have been used with great success to reduce emissions from automobiles.) Lower sulfur diesel fuel is required to enable use of advanced diesel aftertreatment technology. Sulfur prevents the use of the most aggressively formulated catalysts, and in some cases degrades the

effectiveness of the systems. The most promising technologies under development for meeting the 2007-2010 standards for new engines are discussed below. Many of these technologies may also be used to retrofit older engines in the existing diesel fleet to provide even more substantial fleet-wide emissions reductions.

• Oxidation Catalysts. Manufacturers report that flow-through oxidation

catalysts can reduce total PM by 25 – 50%. (Reductions of carbon monoxide and hydrocarbons in the range of 60-90% can also be achieved.) Oxidation catalysts are a proven technology. Over 1.5 million units have been installed on heavy- duty highway trucks built since 1994 and have operated successfully for

hundreds of thousands of miles. The catalysts also have been used on off-road diesels around the world for over 20 years, with over 250,000 units installed in the mining and materials handling industries. They also have been used extensively in retrofit applications, where they have been installed on U.S.

urban buses and on European highway trucks, with over 10,000 units installed over the last two years.

Oxidation catalysts initiate a chemical reaction in the exhaust stream, oxidizing pollutants into water vapor and other gases, such as sulfur dioxide and carbon dioxide. A typical oxidation catalyst consists of a stainless steel canister

containing a honeycomb-like structure called a substrate. The interior surfaces of the substrate are coated with catalytic precious metals, such as platinum or palladium. Oxidation catalysts are sensitive to the sulfur in diesel fuel, which tends to reduce the effectiveness of the catalyst. Lowering fuel sulfur content allows catalysts to be formulated more aggressively to achieve greater emissions reductions.

• Selective Catalytic Reduction Devices. Selective catalytic reduction ("SCR")

is another technology being actively developed. It has been found to produce

simultaneous reductions of NOx (75-90%), hydrocarbons (50-90%) and PM (30-

50%). SCR has been used to control NOx emissions from stationary sources for

over 15 years. More recently, the technology has been demonstrated in retrofit

applications on mobile sources. SCR is similar to an oxidation catalyst in that it

initiates chemical reactions to eliminate pollutants without itself being changed

or consumed. SCR goes beyond catalytic activity, however. An SCR system adds

a reducing agent to the exhaust stream in order to convert NOx to nitrogen and

oxygen. As the exhaust gases, along with the reducing agent (usually ammonia

or urea) pass over a catalyst coated substrate, NOx, HC and PM are converted to

• Particulate Filters. Diesel particulate filter systems are currently in

development to produce 80-90% PM emissions reductions. (Some versions of this new generation of filters currently are being marketed in Europe, where low sulfur fuel is readily available.) These systems consist of a filter positioned in the exhaust stream to collect particulate emissions as the exhaust gases pass through the system. The key challenge is posed by the volume of particulate trapped by the filter: over time the filter becomes clogged. Development work is currently focused on disposal of the trapped particulate, such as by burning or oxidizing the particulate in the filter. (This is known as filter regeneration.) Work is continuing to improve both filter efficiency and filter regeneration.

• NOx Catalysts. Two catalyst technologies are being developed specifically to reduce NOx emissions by up to 90%. The first, so-called "lean NOx catalyst"

works like SCR in that it adds a reducing agent to the exhaust stream to facilitate catalytic conversion. Systems using lean NOx catalysts inject diesel fuel into the exhaust gas to add hydrocarbons. The hydrocarbons act as a

reducing agent to facilitate the conversion of NOx to nitrogen and water vapor in the catalyst.

The second technology, "NOx Adsorbers" operates in two stages. First, the NOx is converted and adsorbed into a chemical storage site within the system. Then when the NOx adsorber becomes saturated, it is regenerated by adding extra diesel fuel to the exhaust stream. The addition of the fuel causes the NOx adsorber to work like a lean NOx catalyst -- it converts the collected NOx into simple nitrogen and oxygen which is emitted from the system.

• Advanced Turbochargers. In addition to aftertreatment technologies, continued improvements in engine technologies, like advanced turbocharger systems, may be used to meet the 2007-2010 standards. The next generation of turbocharging systems will feature increased use of variable geometry

turbochargers and electrically assisted turbochargers.

Variable geometry turbochargers work by adjusting the size of the air passage at the turbine wheel inlet in order to optimize turbine power. At low engine

speeds, when the exhaust gas flow at the turbine wheel inlet is low, the air passage at the inlet is focused by a nozzle. This causes the turbine wheel to spin faster and increases the turbocharger's boost pressure. In contrast, at high engine speeds and loads, which create greatly increased exhaust flow, the inlet nozzle opens to moderate turbine speed and turbocharger boost pressure.

Variable geometry turbochargers thus have a quicker response time during vehicle acceleration, and at the same time prevent over-boosting at high speeds.

This allows the vehicle to burn fuel more efficiently over the full range of operation, producing less emissions and achieving better fuel economy.

Electrically Assisted Turbochargers use a high speed electric motor to provide

additional turbo boost during short periods of acceleration, such as initial

acceleration, passing, and hill climbing. These systems use sophisticated electronics to monitor the demand for power and instantly supply additional boost air to the engine during these transient increases in engine load. This provides more air for combustion during these fuel-rich operating periods. The increased air permits more complete combustion, resulting in reduced emissions and better fuel economy.

• Diesel Fuel Sulfur Content. The primary purpose of lower sulfur fuel is to enable or improve the performance of aftertreatment technologies. However, reduced sulfur will also provide a emissions benefit by reducing sulfate PM and sulfur oxide emissions from existing engines directly. Sulfur in diesel fuel contributes to a small portion of particulate formation, so reducing the sulfur content of diesel fuel has the potential to reduce total PM emissions from the existing fleet by 3-5% without the addition of any aftertreatment device.

• Diesel-electric Hybrids. Moving in parallel with work on 2004 and 2007 technologies is the development of diesel-electric hybrids. Diesel-electric hybrids use electric motors to drive the vehicle wheels, while employing a diesel engine to generate electricity. This use of the diesel engine permits it to run at a

relatively constant speed and temperature, a state that is favorable to both NOx and PM control. The latest hybrid designs also are capable of achieving 40%

better fuel economy than conventional diesel power systems. Prototype diesel- electric hybrids currently are being operated in several urban transit bus fleets.

They also are being used in the federal government's Partnership for a New

Generation Vehicle ("PNGV"), in which each of the big three automakers has

developed a prototype mid-sized passenger vehicle that offers performance and

utility of production models while also providing fuel efficiency of up to 80 miles

per gallon.