The U.S. Environmental Protection Agency (EPA) 2010 diesel emissions standards, which went into effect this month, are the world’s most stringent. To meet these standards, commercial truck and engine manufacturers have settled on two types of emissions control technologies: 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. Both SCR and EGR emissions systems control nitrogen oxide (NOx).
NOx is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts, formed in small amounts when fuel is burned at high temperatures and pressures during an engine’s combustion process.
To handle emissions of particulate matter (PM), SCR and EGR systems each employ diesel particulate filter (DPF) technology. PM, the visible emissions in diesel exhaust, are formed by the incomplete combustion of fuel in 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, Mack Trucks, Paccar, Hino, Ford, GM and Isuzu are going with SCR technology.
Selective Catalytic Reduction
The main components of the SCR system 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 diesel oxidation catalyst and diesel particulate filter are mounted together in one container and usually referred to as the diesel particulate filter. Exhaust gases pass through the DOC, where chemical processes occur, and then through the DPF, where the particulate matter is collected on the filter medium.
The SCR after-treatment catalyst also has two parts, the SCR catalyst and an ammonia catalyst, which also are packaged together and commonly called the SCR catalyst. The two components work in concert with DEF to reduce NOx to 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 after-treatment devices. The PM is trapped by the diesel particulate filter.
The 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 into the truck’s exhaust stream. Vehicle packaging for these components is dependent upon exhaust configurations specified and the application.
The diesel exhaust fluid 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 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. 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 nitrogen and water vapor that are released into the atmosphere through the vehicle’s exhaust pipe.
Exhaust Gas Recirculation
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. Since the oxygen has been burned, 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 capacity.
Both EGR and SCR are technologies used in a wide range of applications globally to meet emission requirements for diesel-powered commercial vehicles.
Diesel Particulate Filters
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. A catalytic reaction generally is used to regenerate the filter, simply by oxidizing the collected trapped particulate, or soot.
Over time, DPFs “fill up,” and need to be periodically cleaned by means of a regeneration process. 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.
The clogging increases the pressure drop (backpressure) within the exhaust system, causing a variety of problems from increased stress on many internal engine components. These include a lack of engine power and vehicle speed, poor fuel economy, overheating and ever-shortening intervals between system regeneration commands.
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 continually is burned off while a vehicle is driving, which uses a combination of exhaust heat and a catalyst to combust the soot.
Active regeneration uses fuel introduced through the seventh injector or a supplemental heat source such as an electrical heater, flame-based burner or precious metal catalytic burner to combust (oxidize) the soot. This only occurs when passive exhaust temperatures are either relatively low or high for short periods of time.
Active regenerations, which often require that the vehicle be stationary, generally 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.
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 lights up.
No new steps need to be included in a vehicle’s routine maintenance for 2010 DPFs. However, these filters need to be cleaned over time. Many truck and engine dealers have DPF cleaning technology and offer cleaning services to fleets of all sizes. In addition, some companies specialize in DPF cleaning services.
Some truck and engine OEMs offer DPF core exchange programs administered through their dealers. Through these programs, shops have certified clean DPFs available, which help minimize vehicle downtime.
On-Board Diagnostic Systems
For 2010 and later model year diesel engines in highway applications that weigh 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 of any detected malfunction or deterioration with the emissions system prior to emissions exceeding a threshold.
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. Beginning in 2013, all highway engines for all manufacturers must be certified to the OBD requirements.
Diesel Exhaust Fluid
Getting back to diesel exhaust fluid, it is an organic compound that is harmless to the environment. It is a pre-mixed fluid composed of two-thirds pure water and one-third automotive-grade urea.
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 data, including duty cycle, geography, engine ratings, etc.
Truck manufacturers have taken precautions to prevent accidental filling of the DEF tanks with diesel fuel. All OEMs, except Navistar, are using a medium blue DEF fill cap, prominent markings and special fueling configurations that make it physically impossible to fit a standard diesel nozzle into a DEF tank.
Navistar 2010 trucks will not have DEF tanks since DEF is not needed with its Advanced EGR emissions system.
Vehicles that use DEF will have two indicators on the instrument cluster to alert the driver to the quantity of DEF on board. One will be a new DEF gauge, 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 drivers that a DEF top-off will be required.
When and if the tank runs dry, the engine’s power will be diminished and speed limited until the DEF is replenished. Once the tank has been refilled, the engine will resume normal power levels.
Engine builders stress that under no circumstances will the engine shut down or be prevented from restarting as a result of running out of DEF.
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.
Features, advantages and benefits
Manufacturers of selective catalytic reduction and exhaust gas recirculation emissions technology each tout the various features, advantages and benefits their systems provide, including performance, durability and reliability, plus a reduced carbon footprint.
Neither system, it should be noted, impacts engine warranty.
And, as might be expected, there will be an up-charge for either system that will be added into the price of a 2010 diesel truck. BI