Introduction
1.1. Emission control regulations
Due to increasing mobility especially in large cities environmental protection has advanced to become a topic of central concern. Emission control regulations have been introduced in all industrialized countries in order to reduce the emissions of vehicles powered by internal combustion engines. The pollutants that are limited today are Hydrocarbons (HC), carbon monoxide. (CO), oxides of nitrogen (NOX) and particulate matter (PM) (Fig. 1).
.2. Carbon dioxide reduction
The carbon dioxide (CO2) contained in automotive exhaust is not classified as a pollutant. However, it is one of the substances responsible for the greenhouse effect and the global climate change. In the period since 1920 global atmospheric CO2 has risen continually from roughly 300 ppm to 380 ppm in the year 2005.
The amount of CO2 in the exhaust is a direct index of fuel consumption. Thus the only way to reduce CO2 emissions of internal combustion engines is to reduce their fuel consumption.
2. Emission control technology for gasoline and diesel engines
2.1. Three way catalyst system
A number of different catalytic converter concepts were applied in the past. For spark ignition engines with homogeneous mixture distribution operating at λ = 1, catalytic treatment of the exhaust gas using a three way catalyst system is at present the most effective emission control method ( Fig. 2). Included in this system is the Lambda closed loop control which monitors the composition of the Air/Fuel (A/F) ratio to feed the catalytic converter with the appropriate composition of exhaust gas. ( Fig. 3) Using the three way catalytic converter, the pollutant emissions of carbon monoxide, Hydrocarbons and oxides of nitrogen can be practically eliminated to low ppm levels provided the engine operates with the right A/F mixture distribution, which is obtained by partially adjusting a stoichiometric and modulated A/F ratio. Given the fact that it is not always possible to operate the engine under optimal conditions, an average pollutant reduction of more than 98% can be achieved [1].
2.2. NOX Storage Catalysts
On gasoline direct injection engines, the oxygen required to oxidize HC and CO is taken from the high proportion of oxygen remaining in the exhaust gas. As a result, a TWC is not sufficient, since the NOX cannot be converted under lean conditions.
The catalytic layer of the NOX accumulator type catalytic converter also contains substances which can store NOX (e.g. barium and alkaline oxides). All conventional NOX accumulator coatings also have the properties of a TWC, with the result that these catalysts operate like TWC at λ = 1 conditions.
In lean stratified-charge operation, NOX is converted in three stages. In the storage phase, NOX is first oxidized to NO2, which then reacts with the additives in the coating to become nitrates (e.g. barium nitrate).
As the quantity of stored NOX increases, the ability to continue binding NOX decreases. At a predefined laden state, the NOX accumulator must be regenerated, i.e. the nitrogen oxides stored must be removed and converted. For this purpose, the engine switches briefly to rich homogeneous operation (λ