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.
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.