1. Scope of this publication

It is the aim of this book to provide a source for the basic understanding, the planning and the execution of issues and ventures in relation to biogas engines. The scope therefore needs to comprise a range of information from the theory of internal combustion engines to the actual way. of modification and to a guide on the parameters influencing a useful and economic operation of biogas engines.

The readers of this book are likely to come from various fields with a non-uniform background of specific experience and knowledge of the matter. On the basis of experience gained in a number of biogas programs and activities within the last ten years the publishers (GATE) and the author came to the understanding that a certain minimum of technical knowledge on the reader's part shall be taken for granted. The very basics, e.g. of the way an internal combustion engine functions or of workshop technology, are therefore not elaborately explained. The book is mainly addressed to readers with some technical background and those who are eager to further-embark on biogas engine matters. Some will find it useful as a handbook and reminder while others may realize that there are more parameters to be considered for a successful implementation than just buying an engine.

Parts of the contents, especially the chapters on the essential theory of internal combustion engines and on the operation of an engine together with a driven machine, are naturally not only biogas-engine-specific. It was however felt that in many cases people are only coming into contact with engine and machine operation issues in connection with a possible use of biogas for mechanical/ electric power. Sufficient knowledge or expertise on engines and their operation can therefore not always be assumed.

Furthermore there is an additional quality in using an engine fuelled by biogas. Here the whole fuel generation and supply side becomes an integral part of the system. There is a direct interdependence between the management and operation of the biogas plant, its size and gas storage facilities and the size and operations of the engine cum driven machine. It was therefore considered essential to elaborate on the system character of an issue comprising the generation of fuel energy, its transformation into mechanical energy and the consumption of the energy in a useful and economic way. Biogasfuelled engines easily turn out to be less practical and economic than other alternatives or solutions if the system aspect is undervalued.

Two chapters, one on the utilization of the engine's waste heat and one on the use of biogas in vehicle engines, have been added. The use of waste heat plays an essential part in making an energy system economic which utilizes only about 30 % of the fuel energy but has the potential of exploiting a total of about 80 % of the biogas energy if a useful purpose for the heat energy can be found.

Utilization of biogas in a processed form, i.e. almost pure methane CH4, is becoming more and more important in vehicle applications. While an effort in plant investment and process energy is necessary, a specific fuel situation may well provide economic incentives to use biogas in tractos, lorries and smaller vehicles. Institutions in Brazil are presently running elaborate research and development programs on this issue.

The type and size of engine considered in this book were limited by two factors. One is the conception that the modification of the engines should be possible with "local" means and expertise, i.e. without sophisticated laboratory-type methods. The other factor is that the basic engines used for modification should be standard engine types from larger series for reasons of availability and the. access to spares and service. The idea of self-handling of engines and modification also limits the engine's size. From the experience of a larger number of biogas projects a power range of about 50 kW was found to be a good compromise. While the theory is valid for the larger engines also, they often incorporate more sophisticated technology such as turbocharging.

This publication cannot and does not attempt to meet the claim of a recipe book for all possible cases. It rather wants to explain and make understandable the various design, economic and other influential parameters and their function in biogas engine issues.

The given examples therefore provide proposals on how to use the given information in a specific situation in order to arrive at a meaningful solution. Proposing standard solutions or final answers does not appear to be appropriate in dealing with an energy system with too many variables which are situation-specific and not always primarily technical ones. A change of only one variable can easily result in a totally different solution.

Positively speaking there is sufficient room and incentive for the reader's own engineering which he will hopefully enjoy after having worked his way through the following chapters. There is after all a better chance of planning, implementing and running a successful project with a broader understanding of the issues concerned.