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To comply with U.S. EPA 2010 diesel emissions standards, the world's most stringent diesel emissions standards which will go into effect in January, commercial truck and engine manufacturers considered two types of emissions control technologies.

They are: selective catalytic reduction (SCR) and exhaust gas recirculation (EGR). SCR treats the engine exhaust downstream. EGR, also referred to as in-cylinder EGR and enhanced or advanced EGR, reduces emissions in the engine cylinder. These systems control nitrogen oxide (NOx). To handle particulate matter (PM), both systems employ diesel particulate filter (DPF) technology.

The 2010 emissions standards mandate calls for emissions no greater than 0.2 g/bhp-hr (grams per brake horsepower-hour) for NOx and 0.01 g/bhp-hr for PM.

On the federal level, the U.S. EPA has regulated on-highway diesel engine emissions standards through the Clean Air Act since 1970. Specific requirements for medium and heavy duty engine applications were established in 2001 to be phased in over a number of years, gradually reducing the legal levels of emissions for various sizes of diesel engines.

Navistar International, with its MaxxForce engines for its International brand trucks, is the only commercial truck OEM to offer an in-cylinder (Advanced EGR) solution for 2010 diesels. All other OEMs, including Daimler Trucks North America, Volvo Trucks North America, Paccar, Hino, Ford, GM and Isuzu are going with SCR technology.

Diesel engines emit a complex mixture of air pollutants, composed of gaseous and solid material. The visible emissions in diesel exhaust are the particulate matter, formed by the incomplete combustion of fuel in diesel engines. Nitrogen oxide, the generic term for a group of highly reactive gases all of which contain nitrogen and oxygen in varying amounts, is formed in small amounts when fuel is burned at high temperatures and pressures during an engine's combustion process.

NOx emissions contribute to the creation of smog and have a public health impact in excess of $175 billion, according to the EPA.

SCR SYSTEM

There are several main components of the SCR system, say officials at the Paccar Engines Group. These include diesel exhaust fluid (DEF) - often referred to simply by the name of its active component, urea; a DEF tank - used to store the fluid; a DEF dosing system - used to deliver the DEF; a diesel oxidation catalyst (DOC); a diesel particulate filter (DPF); and an SCR catalyst.

The DOC is a part of the diesel particulate filter system, the officials clarify. The diesel oxidation catalyst and diesel particulate filter are mounted together in one container. Exhaust gases pass through the DOC, where chemical process occur, and then through the DPF, where the particulate matter is collected on the filter medium.

The DOC and DPF are usually referred to as simply the diesel particulate filter or DPF.

The SCR aftertreatment catalyst also has two parts: the SCR catalyst and an ammonia catalyst, explain Paccar Engines Group officials. As with the DPF, the SCR catalyst and an ammonia catalyst are packaged together and are commonly referred to a just SCR catalyst. The two components work with in concert with DEF to reduce NOx into harmless levels of simple nitrogen and water vapor.

In essence, the SCR system works through a chemical reaction triggered by heat. As the exhaust leaves the engine with the NOx and PM pollutants, it travels downstream into the aftertreatment devices, explains Ed Saxman, product manager-drivetrain, Volvo Trucks North America.

Like 2007 models, the PM is trapped by the diesel particulate filter. Because SCR technology allows the engine to use fuel more efficiently, less PM is created. SCR systems will also use EGR, but to a much lesser degree than used in 2007 engine systems today.

These major components of the SCR system are all integrated into the exhaust system and vehicle chassis, along with extra wiring, hoses and sensors to manage the injection flow of DEF (urea) into the truck's exhaust stream.

Vehicle packaging for these components is dependent upon exhaust configurations specified and the application. For example, in a typical vehicle, the DPF and SCR catalyst, which can be similar in outward appearance to a muffler, will most likely be positioned within a box mounted under the cab.

The DEF is contained in a storage tank sized to minimize operator filling within the packaging and weight constraints of the vehicle. In most configurations, the tank will be mounted on the chassis adjacent to the fuel tanks on the driver's side, where refilling is simple and there is no interference with aftermarket-related modifications that occur behind the cab and/or sleeper.

The DEF dosing system is engineered so that the availability of ammonia is closely matched to the amount of NOx being produced by the engine in real time.

The NOx flows downstream of the engine passing through the DOC and DPF before a fine mist of diesel exhaust fluid is injected into the exhaust gas upon system demand only, explains David McKenna, director of powertrain sales and marketing for Mack Trucks. The high temperature degrades the DEF into ammonia (NH3), which mixes with the NOx laden exhaust.

This mixture flows across the SCR catalyst which assists in the quick process of converting essentially all the nitrogen oxide into harmless levels of simple nitrogen and water vapor that are released into the atmosphere through the vehicle's exhaust pipe.

Diesel exhaust fluid is an organic compound that is harmless to the environment. It is a non-toxic, non-polluting and non-flammable substance that is already widely distributed and used in different formulations for many industrial and agricultural needs.

EGR PROCESS

The EGR system captures a portion of the engine's exhaust gas and re-circulates it back into the engine's combustion cycle, reducing in-cylinder temperatures and thereby reducing NOx production, says Tim Shick, director-engine sales and marketing, Navistar Engine Group.

Since the oxygen has been burned out of it, the recycled exhaust serves to dilute the amount of oxygen in the intake charge air. This reduced amount of oxygen lowers the peak combustion temperature, which helps to reduce the formation of oxides of nitrogen.

The exhaust must be cooled, which requires an increase in cooling system capacity.

The challenge, Shick says, is to precisely control the combustion process. Navistar's MaxxForce Advanced EGR engines have increased injection pressure, improved combustion and refined calibrations for that purpose.

Unlike SCR, because NOx is totally handled in the engine cylinders, EGR systems avoid the addition of extra equipment and do not require the replenishing of an additive, he points.

IN COMBINATION

Both EGR and SCR are technologies that have been used in a wide range of applications around the world to meet emission requirements for diesel-powered commercial vehicles. In North America, SCR systems for 2010 will use EGR in combination with SCR. This differs from European emissions systems because North American requirements are more stringent, requiring the combination of SCR and EGR.

EGR alone can meet the requirements, Navistar's Shick says.

"The U.S. EPA historically has focused more on the reduction of PM than NOx," says McKenna. "In the EU, the concentration of heavy duty diesel engine emissions has largely been NOx based, employing SCR in the vast majority of applications."

However, two European manufacturers, Scania and MAN, offer EGR engines to meet current (Euro V) European emissions requirements, points out Shick.

TRAPPING PM

Diesel particulate filters (DPF), installed in place of mufflers, typically contain porous ceramic substrate to trap the fine particulate matter from the exhaust stream and prevent these particles from reaching the atmosphere, says Volvo's Saxman. A catalytic reaction is generally used to regenerate the filter, simply by oxidizing the collected trapped particulate, or soot. "This is not unlike the process used with a self cleaning oven," Mack's McKenna observes.

Over time, DPFs "fill up" and need to be periodically cleaned by means of a regeneration process, says Fred Schmidt, director of retrofit emissions business, Donaldson. The residue and ash produced from the burning of engine and fuel oil builds up and will eventually clog the DPF, adversely affecting the engine's performance and fuel economy.

"It would be like never emptying the bag on a vacuum cleaner, Mark Thomas, director electrical and electronic engineering, Daimler Trucks North America (DTNA), says. "This reduces the effectiveness of the vacuum's ability to pick up dirt."

The clogging increases the pressure drop (backpressure) within the exhaust system, causing a variety of problems from increased stress on many internal engine components, says Schmidt. Problems include a lack of engine power and vehicle speed, poor fuel economy, overheating and ever-shortening intervals between system regeneration commands.

There is also the risk of permanently damaging the DPF, adds Thomas.

REGENERATION METHOD

The regeneration process is typically achieved by burning off the trapped particulate matter. There are two types of regeneration: passive and active. With passive regeneration, particulate matter is continually burned off while a vehicle is driving using a combination of exhaust heat and a catalyst to combust the soot.

Active regeneration uses fuel introduced through the 7th injector or a supplemental heat source such as an electrical heater, flame-based burner or precious metal catalytic burner to combust (oxidize) the soot, says Saxman of Volvo. This only occurs when passive exhaust temperatures are either relatively low or high for short periods of time.

The active regeneration process often requires that the vehicle be stationary. It is much more rapid and occurs at temperatures of 1,100 degrees Fahrenheit or more, he notes.

Active regenerations typically won't be necessary for those applications where truck engines work hard enough to generate the heat necessary to continually burn off the trapped particulate matter. They may, however, be required for those applications where vehicles do a lot of stop-and-go operation or prolonged idling.

For 2010, a typical Mack highway tractor "may never see an active regen event as opposed to something around one every14 engine hours today," says Mack's McKenna. "Vocational trucks, dumps, mixers and roll-offs will see significantly reduced requirement for active regens - something around 300 to 400 engine hours."

Drivers will not notice passive regeneration, as the engine continues to operate normally. The only sign of any passive regeneration is an indicator light on the dashboard that illuminates.

DPF MAINTENANCE

No new steps need to be included in a vehicle's routine maintenance for 2010 DPFs. In fact, in SCR-equipped vehicles, the maintenance interval may be extended.

Over time, the DPF filter needs to be cleaned. Many truck and engine dealers have DPF cleaning technology and offer cleaning services to fleets of all sizes. There are also companies that offer DPF cleaning services.

Some truck OEMs offer DPF core exchange programs administered through their dealers. Through these programs, shops have readily available certified clean DPFs, which helps minimize vehicle downtime.

The Cummins distribution network, by way of example, has both new and ReCon diesel particulate filters in stock, as well as DPF cleaning machines.

A typical Class 8 DPF is a ceramic filter cartridge riddled with about 10,000 holes with 5,000 of them open and 5,000 plugged on each end forming a checkerboard, says Drew Taylor, national sales manager, FSX, a company that designs and manufactures diesel particulate filter cleaning machinery. The cells are deep dead end holes with a thin wall between the holes.

"The thin walls between the cells contain billions of micro-pores that allow gases to go through but not much in the form of particulates," he explains. "The filter runs at about 750 degrees Fahrenheit under ideal circumstances. When you take a filter off a truck, the inlet end should be black with soot and the outlet end should be so clean it will pass a white glove test."