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."
DPFs become plugged with either ash or soot or a combination thereof, says Cole Waldo, FSX's chief operating officer. As the engine operates, it exhausts unburned hydrocarbons in the form of soot and small amounts of engine oil that contains unburnable elements called ash. Ash is a combination of metals and minerals.
"Incoming ash and soot pass into the dead end holes on the inlet side of the DPF and lodge against the cell walls," he says. "The soot burns inside the hot cell and passes as a gas through the micro-pores in the cell wall. The unburnable ash stays in the cell and builds up as an accretion similar to a stalagmite in a cave.
"Ash build-up occurs over a long period of time and takes either a lot of mileage or higher consumption of engine oil. Eventually, the ash forms into a long square ash plug that is called an ash noodle. Ash plugs are difficult to remove and bad news if left to long in the DPF.
Constant thermal regenerations can cause some of the elements in ash such as potassium to react with the ceramic wall to form a low level grade of glass, adds Taylor. This is commonly called sintering or glazing of the cell walls. Glazing the cell walls is permanent damage.
Build up of ash in the cells is eventually detected by emission system sensors and the truck tries to correct the problem by going into active regeneration mode. This mode burns fuel, but doesn't cure the problem. Eventually, the truck will stop.
DPF can also plug with soot, notes Waldo. Soot plugging typically occurs quickly and is a result of an event or unfavorable operating conditions. A bad injector or too low of operating temperatures can cause the problem.
"While active regenerations can typically cure the symptom," he says, "the problem must be cured to stop the constant regenerations. Under extreme cases, the DPF can become so plugged with soot it will require removal of the DPF for cleaning.
"If an unfavorable operating condition exists and is not corrected, you should expect the DPF to quickly plug up again."
DPF CLEANING
Following the recommended DPF cleaning schedule will assure best operation in the majority of applications and duty cycles, says Navistar Engine's Shick. In applications requiring more frequent cleaning, the driver will know a cleaning is required if the vehicle regeneration process does not adequately clean the DPF.
As the DPF needs cleaning, he says, the DPF status indicator lamp on the instrument panel will change from off to solid on to flashing, and an audible alarm will beep. Normally, the vehicle regeneration process would address this but if regeneration is not adequate, the monitor lamp and audible alarm will remain active.
Another indicator that it is time to clean the DPF is the driver will notice ever-decreasing intervals between a system commands for an active regeneration. Some vehicles will have filter service monitors. Service technicians will have diagnostic tools that they can use to determine a DPF's "health."
For a typical Class 8 truck application, Cummins has recommended a DPF cleaning interval of 200,000 to 400,000 miles. The exact mileage at which a fleet will need to actually clean the DPF is impacted by several factors, including duty cycle.
There are various levels of cleaning for a diesel particulate filter. The first level of cleaning is a pulse air cleaning. The next level uses a thermal process.
Pneumatic cleaning is the first cleaning process a DPF needs to go through, says FSX's Taylor. The method is used to clean each cell by bi-directionally scanning high-pressure air into the filter from both ends.
The next cleaning stage is thermally processing the DPF to burn off any remaining soot and to loosen remaining ash from the cell walls, Waldo of FSX says. Once the soot is burned and the ash is loosened, it is important to do a second pneumatic cleaning to blow out the remaining ash.
"The key to successful thermal cleaning is using the correct temperature ramp rate, not exceeding the maximum temperature and avoiding pumping air through the DPF during processing," he says. "Even a little air pumped through the DPF is like fanning the flames. At higher temperatures the oxygen can cause the soot to flash leading to uncontrolled regeneration and damaging temperature spikes."
"Always thoroughly clean a DPF with a pneumatic cleaner before processing thermally," emphasizes Taylor. "Pneumatic cleaning removes as much soot (fuel) as possible so it cannot ignite."
Some DPFs do not need to go on to thermal cleaning, observes Waldo. "If testing proves the OEM specifications have been met with just the pneumatic cleaning, the DPF is ready to go."
Empty the DPF on a regular basis and uses good DPF cleaning equipment, advise all engine manufactures and DPF cleaning companies.
Anytime that the DPF housing is opened in normal dusty conditions, safe handling practice should be observed, says Donaldson's Schmidt. The EPA has determined that collected DPF ash is not a hazardous material, Mack's McKenna.
Never the less, when handling, capturing and containing extracted diesel soot and ash, technicians should use protective clothing and high quality dust collection equipment, recommends Taylor of FSX.
OBD SYSTEMS
For 2010 and later model year diesel engines in highway applications more than 14,000 pounds, the U.S. EPA requires OEMs to have dashboard malfunction indicator lights and diagnostic trouble codes to warn drivers of emission problems.
Similar to passenger car on-board diagnostics (OBD) systems, truck OBD systems monitor performance of the engine's major systems and emission control components, and notify the vehicle operator to any detected malfunction or deterioration with the emissions system prior to emissions exceeding a set of emissions thresholds.
All aftertreatment devices and emissions-related electronic sensors and actuators will have to be monitored for proper operation as well, says Navistar Engine's Shick.
When an issue occurs, diagnostic information must be stored in the engine's computer to assist in diagnosis and repair of the problem, condition or malfunction. EPA regulations define a "failure" as any change from as-built conditions which can raise the engine emissions beyond the regulated level.
The EPA also requires that one engine family per manufacturer be OBD-certified in the 2010 through 2012 model years, Shick points out. Beginning in 2013, all highway engines for all manufacturers must be certified to the OBD requirements.
DEF DETAILS
Diesel exhaust fluid is a simple, non-toxic and pre-mixed fluid composed of two-thirds pure water and one-third automotive-grade urea, say officials at Detroit Diesel. Experience indicates that average DEF consumption is between 2 and 3 percent of fuel consumption (2 to 3 gallons per 100 gallons of diesel fuel), depending on vehicle operation - duty cycle, geography, engine ratings, etc.
"We have found that lighter (engine) loads requires less DEF and we have seen the dosing rate drop to 2 percent," observes McKenna of Mack. "I tell everyone 3 percent. So if real world is closer to 2 percent than 3, the actual operating cost of SCR has been reduced by about 30% - not bad in today's money climate.
"We have found that heavier (engine) loads are getting markedly better fuel economy, and the dosing rate is closer to 3 percent - never above."
The manufacturers have taken precautions, at several levels, to prevent accidental filling of the DEF tanks with diesel fuel. All OEMs except Navistar are using a medium blue colored DEF fill cap and prominent markings and there are special fueling configurations. Navistar 2010 trucks will not have DEF tanks since DEF is not needed with its Advanced EGR emissions system.
"It is physically impossible to fit a standard diesel nozzle into a DEF tank, since the neck of the DEF fill tube is a smaller diameter than the diesel nozzle," Volvo's Saxman says. "The DEF inlet has been reduced to a 19-mm (about ¾-inch) diameter, from the commonly used 26-mm (about 1-inch) fuel dispenser nozzle," adds McKenna.
"As a third layer of protection, Mack offers as standard a magnetic interlock for the DEF dispenser," he continues. "We have located a magnet in the collar of the DEF tank and this releases the interlock in the DEF nozzle.
"Even when dispensing DEF, if the nozzle is removed from the filler neck, the valve release will close immediately. This alone should severely limit a ‘mis-fueling mis-adventure.'"
Other manufacturers, including DTNA, will also offer magnetic interlocks on DEF tanks as standard equipment on all vehicles.
Vehicles in 2010 that will use DEF will have two indicators on the instrument cluster that will alert the driver to the quantity of DEF on board, say Cummins officials. One will be a new DEF gauge, very similar to a fuel gauge today, which will indicate the level of DEF (i.e. full, half, quarter, etc.). The second indicator will be a new DEF low-level warning lamp that will illuminate when less than 10 percent of DEF is in the tank, advising driver that a DEF top off will be required.
When and if the tank is run dry, the engine will be power and speed limited until the DEF is replenished, say Detroit Diesel officials. Once the tank has been refilled, the engine will resume normal power levels.
Under no circumstances will the engine shut down or be prevented from restarting as a result of running out of DEF, stress all engine builders.
"At no time will we strand a driver with no DEF, as opposed to a no-fuel scenario - when you are going exactly nowhere," says McKenna.
The distribution infrastructure for DEF is in place. DEF is widely available at many locations, including truck stops, truck and automobile dealerships, engine distributors, diesel service stations, fueling stations, fuel distributors, auto parts stores and other retail locations. Pilot Travel Centers opened the first DEF dispensing at pumps at fuel plazas in several locations.
Engine manufacturers stress that DEF should not be "home brewed," as this can cause serious problems. DEF has strict requirements for maintaining concentration and purity of ingredients that is critical to the proper functioning and longevity of the SCR system. Furthermore, the OEMs require that DEF used with their SCR systems meet all ISO (International Organization for Standardization) specifications, as well as API (American Petroleum Institute) quality certification requirements.
COLD WEATHER
DEF is affected by cold weather, and as such, the DEF system requires a heating element, says DTNA's Thomas. During vehicle operation, SCR systems are designed to provide heating for the DEF tank and supply lines.
Paccar, by way of example, utilizes heat generated from the engine to ensure the solution remains at operational consistency at the coldest temperature extremes.
Because DEF is 67.5% deionized water, it will freeze into a crystalline-type (slushy) material, and begins freezing solid at extended periods below 12 degrees Fahrenheit," explains Mack's McKenna. If DEF freezes, the engine will start and run properly. The freezing and unthawing of DEF will not cause degradation of the product.
"Let's suppose we parked a truck overnight or over the weekend in the Upper Peninsula of Michigan in the middle of January where it is unarguably cold," he says. "Safe and good practice would have the truck either plugged in (block heater) or running a timed APU, so the engine coolant is already going to be about 60 degrees Fahrenheit. Upon engine start, coolant from the engine will be circulated through the DEF tank to thaw or re-liquefy product."
In addition to EPA allowances for cold weather starts, McKenna notes that cold engines produce an infinitesimal amount of NOx.
DEF stored at extremely high temperatures for extended periods of time will slowly degrade. Shelf-life for DEF is up to 18 months or greater under normal conditions. Storing in temperature-controlled conditions will add to its shelf life.
DEF, which weighs approximately 9 pounds per gallon, expands by approximately 7 percent when frozen. DEF packaging and tanks are designed to allow for expansion.
DEF is safe to handle and store, and poses no serious risk to humans, animals, equipment or the environment when handled properly, say DEF producers and suppliers. If spilled, the DEF should be contained and absorbed with an inert, non-combustible absorbent material such as sand. The material should be shoveled in to a suitable container for disposal.
Spills into a drain should be avoided. If DEF is spilled into a drain, the drain should be thoroughly flushed with water. If DEF is spilled on a vehicle, the area should be rinsed with water.
TECHNOLOGY CONSIDERATIONS
Manufacturers of SCR and EGR emissions technology each tout the advantages their systems provide. Among them: performance, durability and reliability, plus a reduced carbon footprint. SCR technologies have been proven to lower operating costs through improved fuel economy in most applications. Neither system impacts engine warranty.
And as you might expect, there will be an up-charge for the systems added into the price of a 2010 truck.
Biodiesel blends up to B20 are approved for MaxxForce engines today and this will continue in 2010. One of the drawbacks to certain biodiesel fuel stocks is the ability of the product to burn hotter, thereby increasing engine-out NOx, says McKenna of Mack. Engine manufacturers are working on that and the "fix" will be a software modification only.
Current commercially available engine lube oils that meet CJ-4 and CJ-4+ diesel engine specifications are recommended for use with both SCR and EGR emission technologies.
Donaldson's Schmidt points out there are a variety of retrofit options available for diesel engines to reduce toxic emissions. Some of the key considerations for retrofit technologies are to make sure the systems do not impair driving performance, determine maintenance requirements and compare the return on investment.
EMISSION CREDITS
There was been industry talk that Navistar will use NOx credits to make its 2010 MaxxForce engines with Advanced EGR compliant with the 2010 emissions standards. "We do not have to use credits," explains Shick. "We have them available, so we are choosing to use them.
"Using credits is a glide path to 0.2 g/bhp-hr of NOx which actually started back with the passage of the Clean Air Act. This gives us the opportunity to use the credits and tweak the performance so that our customers get optimum performance out of our 2010 engines."
Shick expects Navistar will not be the only company that will be using credits next year. "The fact is there will likely be other companies using credits to achieve 2010 emissions compliance," he says. "Virtually every other competitor out there has used credits at some point reach compliance.
"Use of credits in the truck market is nothing new," he goes on. "Frankly it's only became an issue for some reason when we happened to mention something about it."
