1. Automotive News, April 12, 1982.
2. Source: German Federal Environment Agency.
3. K. Becker, D. Jost, K.U. Martin, H. Neidhard, Recommendations to Reduce the Emission of Pollutants-Motor Vehicle Exhaust Emissions, Report 7/76, German Federal Environment Agency 1976
4. D. Gruden, U. Markovac, G. Höchsmann, P.-F. Küper Schadstoffarme Antriebssysteme - Entwicklungsstand, Wirtschaftlichkeit, Kosten, Report 2/80, German Federal Environment Agency 1980.
5. Source: German Federal Traffic Agency.