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The automotive industry has made great efforts in reducing fuel consumption. The efficiency of modern spark ignition (SI) engines has been increased by improving the combustion process and reducing engine losses such as friction, gas exchange and wall heat losses. Nevertheless, further efficiency improvement is indispensable for the reduction of CO2 emissions and the smart usage of available energy. In the previous years the Atkinson Cycle, realized over the crank train and/or valve train, is attracting considerable interest of several OEMs due to the high theoretical efficiency potential. In this publication a crank train-based Atkinson cycle engine is investigated. The researched engine, a 4-stroke 2 cylinder V-engine, basically consists of a special crank train linkage system and a novel Mono-Shaft valve train concept.

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.