Millions of small displacement single-cylinder engines are used for the propulsion of scooters, motorcycles, small boats and others. These SI-engines represent the basis of an affordable mobility in many countries, but at the same time their efficiency is quite low. Today, the limited fossil fuel resources and the anthropogenic climate require a sustainable development of combustion engines, the reduction of fuel consumption being an important factor. A variety of different strategies (turbo-charging, cylinder deactivation, direct injection, etc.) are investigated here to increase the efficiency of multi-cylinder engines. In the case of small displacement single-cylinder engines, other strategies are required because of their special design and the high pressure on costs.
In the context of this paper different layout parameters which have an influence on the working process are investigated, with the aim of increasing the efficiency of small displacement single-cylinder engines. This includes parameters such as compression ratio, lambda, external EGR and others. The thermodynamic losses from ideal engine efficiency to effective efficiency are presented and the calculations are supported by experimental data. Both two-stroke and four-stroke engines are investigated and the subject is limited to single-cylinder engines with a displacement of up to 150 cm3. Furthermore, achievable efficiencies by means of air-cooling and water-cooling are presented. The focus is put on representative operation points at PL (part load) and WOT of typical motorcycle applications in Asia. The calculations are mainly of theoretical importance, but the possibilities of transferring the outcome into real applications are also discussed. Finally, the paper concludes with an evaluation of appropriate strategies to reduce fuel consumption.