2. Review of existing literature

Literature with relevance to the topic of this book comes from different fields. One naturally is the standard literature on internal combustion engines which is elaborate to an extent that it would go far beyond the framework of this book to give a complete list¹. The first Otto engines at the turn of the century were gas-fuelled engines. They are well covered in the standard literature on engines.

Another field is the literature on biogas, dealing mainly with issues concerning the biofermentation and the various plant designs for different biomaterials, plant sizes, etc. The greater portion of the literature was written within the last ten to fifteen years while the awareness of the role and potential of biogas as an energy gradually increased. In a standard sourcebook for renewable energies for developing countries from 1976 biogas did not yet receive any attention [1]².

Others, however, quoted biogas but mainly as an alternative energy for household use [2]. From the mid-seventies onwards a large number of papers in conferences and journals signalized the growing importance of biogas, not only for small-scale use in households but as a product of municipal and industrial waste treatment with anaerobic fermentation. To name only a few there is L. Sasse's standard book on biogas plants for rural applications [3], BORDA's Biogas Handbook [4] and more recent publications like Oekotop's "Biogas" on the more practical and implementation issues in developing countries [5] and the GTZ's "Production and Utilization of Biogas in Rural Areas of Industrialized and Developing Countries" [6].

The importance given to biogas in the developing countries themselves is documented in numerous publications and seminar proceedings like "Energy for Development in Eastern and Southern Africa" [7] and many others especially from India and China where the small-scale biogas technology development had gained momentum one generation before it became an international development issue.

With the increase in biogas production towards larger quantities the technical utilization like the transformation into mechanical energy became an issue to be researched on. While larger engines specifically designed for gas were on the market, smaller engines modified from standard Otto or diesel engines were seen to fill the gap for small to medium and decentralized applications. Indian [8] and Chinese [9] publications mainly dealt with the modification of small stationary diesel engines for dual fuel operation. Others went on to modify medium-sized diesel engines including their governors [10], or researched the performance paramters of dual fuel biogas engines in more detail [1 1].

Biogas as a fuel for vehicles has been an issue since the 1950's. While in Europe the use in tractors seems to be the issue [12, 13], in Brazil the aim is to substitute petrol and diesel fuel in the automotive sector using purified and compressed biogas or natural gas [14].

Much useful material and information have been contributed in recent years by publications of manufacturers of gas engines and modified engines or suppliers of equipment and modification kits for standard Otto and diesel engines. Some of their publications are named in the Literature Reference List.