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.
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.
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.
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.