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).