Unwanted reactions result in the formation of hydrogen sulfide and ammonia, which poison converters unlimited. Nickel or manganese is sometimes added to the washcoat to limit hydrogen-sulfide emissions. Sulfur-free or low-sulfur fuels eliminate or minimize problems with hydrogen sulfide.
For compression-ignition (i.e., diesel) engines, the most commonly used converters unlimited is the diesel oxidation catalyst (DOC). DOCs contain palladium and/or platinum supported on alumina. This converters unlimited particulate matter (PM), hydrocarbons, and carbon monoxide to carbon dioxide and water. These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping reduce visible particulates. These converters unlimited are ineffective for NOx, so NOx emissions from diesel engines are controlled by exhaust gas recirculation (EGR).
In 2010, most light-duty diesel manufacturers in the U.S. added catalytic systems to their vehicles to meet federal emissions requirements. Two techniques have been developed for the catalytic reduction of NOx emissions under lean exhaust conditions, selective converters unlimited reduction (SCR) and the NOx adsorber.
Instead of precious metal-containing NOx absorbers, most manufacturers selected base-metal SCR systems that use a reagent such as ammonia to reduce the NOx into nitrogen and water.[23] Ammonia is supplied to the catalyst system by the injection of urea into the exhaust, which then undergoes thermal decomposition and hydrolysis into ammonia. The urea solution is also referred to as diesel exhaust fluid (DEF).
Diesel exhaust contains relatively high levels of particulate matter.converters unlimited remove only 20–40% of PM so particulates are cleaned up by a soot trap or diesel particulate filter (DPF). In the U.S., all on-road light, medium, and heavy-duty diesel-powered vehicles built after 1 January 2007, are subject to diesel particulate emission limits, and so are equipped with a 2-way catalytic converter and a diesel particulate filter. As long as the engine was manufactured before 1 January 2007, the vehicle is not required to have the DPF system. This led to an inventory runup by engine manufacturers in late 2006 so they could continue selling pre-DPF vehicles well into 2007.[24]
Lean-burn spark-ignition engines
For lean-burn spark-ignition engines, an oxidation converters unlimited is used in the same manner as in a diesel engine. Emissions from lean burn spark ignition engines are very similar to emissions from a diesel compression ignition engine.