Currently more and more countries around the globe are legislating controls aimed at achieving safe concentrations of these pollutants by regulating their level of emissions from combustion sources, notably those discharged from the exhausts of automobiles. The maximum allowable levels of emissions from automobiles are already regulated by many governments using measures in grams per kilometre or grams per mile.
Emission control standards were initially established in the major centres of Japan, the United States, and the European Union. These regions have similar approaches to emission reduction, although each set of regulations is tailored to specific requirements. The US system, based on groups of regulations called Tiers, has decreasing emissions limits and increasing durability requirements. The European regulations also define increasing restrictions. Currently Euro IV sets limits to 2009 when Euro V will take effect. The most stringent emissions standards in the world are to be found in California which has become the benchmark for worldwide environmental legislation. The legislation refers to LEV (Low Emission Vehicle), ULEV (Ultra LEV), SULEV (Super Ultra LEV), and ZEV (Zero Emission Vehicle).
Catalytic Converters have been instrumental in reducing emissions of harmful gases from vehicles since their inception in response to the US Clean Air Act of 1970. Regulated emissions have been reduced approximately 1/3 while the number of cars on the road have more than doubled. Platinum, palladium and rhodium are essential components in automobile catalytic converters reducing engine-out emissions by well over 90%, and in some cases by over 99%.
The rest of the world is following the emission standards and technologies of the vanguard nations, allowing them to adopt increasingly stricter standards at a faster pace.
The evolution of the emission standards for criteria pollutants and for hydrocarbons which are implicated in the formation of ground level smog are shown in the graphs below.
Carbon Monoxide (CO)
The European Union initially set the benchmark for carbon monoxide emissions for both gasoline and diesel vehicles. In 2002, Japan reduced their carbon monoxide limits to match that of the EU and in 2005 adopted a new measurement method which effectively reduced this limit even further. Euro IV, adopted in 2005, saw EU limits reduced by 57% for gasoline and 22% for diesel vehicles. In the United States, with gasoline powering 95% of the cars, such drastic cuts in emissions were considered unrealistic and, as the US is less densely populated than the EU and Japan, CO reduction has not received the same attention.
Catalytic converters oxidize hydrocarbon exhaust into carbon dioxide and water. Hydrocarbon regulations in Japan and the US are the same for both gasoline and diesel vehicles. Japan’s hydrocarbon limits are the lowest in the world. Europe regulates the combination of HC + NOx for diesel vehicles where higher NOx is produced due to the combined ability of these chemicals to produce ground level ozone.
NOx includes Nitrogen Monoxide (NO), and Nitrogen Dioxide (NO²). In gasoline vehicles, reduction catalysts are used to decrease NOx by transforming it into Nitrogen and Oxygen. A Three Way Catalyst (TWC), converts NOx, HC, and CO simultaneously. This technology can only be used in gasoline engines because of its exhaust composition. NOx abatement in diesel engines is trickier, but is currently being met using NOx adsorbers, which utilise platinum as the primary catalytic agent.
Japanese NOx regulations are the same for both gasoline and diesel engines. By 2009, the NOx limit for the US will drop from the highest of the vanguard nations to the lowest and most stringent. In 2005, the EU’s Euro IV regulation for NOx reduces the limit by 50% for diesel vehicles and 47% for passenger vehicles. The adoption of Euro IV made the use of NOx adsorbers necessary for many vehicles, although engine improvements allowed some manufacturers to delay the inclusion of after treatment.
Particulate Matter (PM)
In the EU and Japan, particulate matter limits are currently only set for diesel vehicles. In 2009, Japan will introduce a particulate matter limit for gasoline engines as well. Diesel vehicles are also the primary target of PM legislation in the US, although there is legislation in place for gasoline vehicles.
The legislated levels of PM reduction require the use of catalyzed particulate filters which contain platinum. The platinum converts NO to NO2 which generates heat allowing the filter to self-clean. Although some manufacturers have found an alternative to costly particulate filters to meet Euro IV standard, in 2009 Euro V will require the use of particulate filters on all diesel vehicles. Diesel vehicles make up over half of the vehicles sold in Europe. In the US, particulate filters are required on most diesel vehicles to meet current standards. As the limits decrease in 2009, all vehicles will need to use particulate filters to meet Tier II standards. Furthermore, in 2007, durability requirements for the emissions systems increase from 100,000 miles to 120,000 miles useful vehicle life. The only metal known to perform to these stringent durability requirements is platinum.