Controlling Diesel Emissions
Beginning in the 1960s, exhaust emissions from gasoline-powered internal combustion engines were increasingly reduced by government mandate. To reduce “smog” in California cities, particularly Los Angeles, the California Air Resources Bureau (CARB) imposed regulations on cars to be sold in that state and slowly those regulations were tightened and also spread to larger geographic regions of North America, then to Europe and finally the remainder of the globe. Digital engine management systems, as well as catalytic converters and electronic oxygen sensors were the primary systems and components used for automotive emission reductions.
In 1996, the US Environmental Protection Agency (EPA), in collaboration with CARB, required that all gasoline engines sold in the United States be fitted with second-generation on-board diagnostics (OBD2), a sophisticated system for electronic engine management and monitoring. Similar requirements were instituted in Canada and western Europe. With this system in place, gasoline vehicle electronics were sufficiently advanced so that state-mandated emission testing using a tailpipe probe was no longer necessary. Hooking up a scan tool to the mandated 16-pin OBD2 connector under the dashboard and running the engine was presumed to give a snapshot of the health of the engine and its emissions.
Year by year, increasingly tighter requirements for cleaner running, as well as better fuel mileage and higher power output, have improved gasoline motor vehicle efficiency and output and reduced exhaust emissions.
Early on, diesel powered vehicles were exempt from many emission reduction regulations, partly due to the mechanical nature of diesel injection. By the 2000s however, electronically controlled diesel fuel injection as well as diesel catalysts were developed to the point that a number of manufacturers, particularly in Germany, began offering highly efficient and emission controlled diesel engines in their car lines.
Once fuel mixture could be precisely metered electronically by the engine control module (ECM), It became possible for the EPA to impose clean-running regulations on diesels that were the same or similar to those for gasoline engines.
In broad terms, diesel exhaust can be cleaned in the following three steps.
Step One: Diesel Oxidation Catalyst (DOC)
The automotive catalytic converter is a metal chamber, installed in the exhaust stream. The role of the diesel oxidation catalyst (DOC) is twofold. First it converts and oxidizes hydrocarbons into water and carbon dioxide. Second, the catalyst provide additional heat for the exhaust system, increasing the conversion efficiency of the downstream subsystem(s) in reducing emissions. This step is essentially the same as is used in gasoline powered vehicles.
The catalytic converter chamber contains a ceramic honeycomb matrix. The ceramic has a precious metal coating (usually platinum, palladium or rhodium) which acts as a catalyst to combine unburned hydrocarbons and carbon monoxide in the exhaust with ambient oxygen to produce water vapor and carbon dioxide. The catalyst only works efficiently when it reaches “light-off” temperature (about 250° – 300°C or 480° – 570°F).
Step Two: Selective Catalytic Reduction (SCR)
The SCR module is another catalyst with a different array of catalyzing metals (platinum, vanadium, fe-zeolite). In the SCR chamber, nitrogen oxides (NOx) in the exhaust stream are converted into water, inert nitrogen and carbon dioxide, which are harmless (to humans) and present in earth’s atmosphere. Before exhaust gas enters the SCR chamber, it is dosed with diesel exhaust fluid (DEF), also known as urea, an aqueous solution that is approximately 67.5% water and 32.5% pure urea. When heated, DEF splits into ammonia and carbon dioxide. These molecules are atomized, broken up and vaporized, then enter a mixer that resembles a corkscrew. This twist mixer evenly distributes the ammonia within the exhaust flow. The mixture then enters the SCR module. Catalyst action combines and converts NOx and ammonia into harmless inert nitrogen and water.
The SCR module does not operate efficiently at low temperatures; therefore DEF (urea) is not injected into the exhaust stream until the stream temperature is between 200°C and 500°C (390°F and 930°F).
Ammonia and water components of DEF freeze at -11°C (12°F). Different manufacturers have devised various techniques for maintaining an unfrozen supply of DEF.
SCR has been adopted in Europe and Japan for trucks and passenger vehicles. Worldwide it is used in many diesel motor applications. It has been adopted by the US EPA for its EPA 2010 regulations. Almost every U.S. diesel engine manufacturer plans to adopt SCR technology, further proving its reliability.
Step 3: Particulate Filter (DPF)
The particulate filter (DPF) traps any soot remaining in the exhaust stream, which is then periodically burned away when sensors detect the trap is full. This process, known as regeneration, produces temperatures in excess of 600°C (1112°F) to burn away soot.