The enhancement of efficiency will play a more and more important role in the development of future (small) internal combustion engines. In recent years, the Atkinson cycle, realized over the crank drive, has attracted increasing attention. Several OEMs have been doing investigations on this efficiency-increasing principle with in the whole range from small engines up to automotive ones.In previous publications, the authors stated that an indicated efficiency of up to 48% could be reached with an Atkinson cycle-based engine. However, these studies are based on 1D-CFD simulation. To verify the promising simulation results, a prototype engine, based on the Atkinson principle, was designed and experimentally tested. The aim of the present study is to evaluate and validate the (indicated) engine efficiency gained by experimental tests compared to the predicted simulation results. In order to investigate part load behavior, several valve timing strategies were also developed and tested.This publication gives a brief overview of the specific engine layout of the Atkinson prototype engine. The main part deals with experimental investigations of several defined load points and the thermodynamic evaluation of the measured data concerning heat release, wall heat and gas exchange etc.The results of this study indicate a high indicated efficiency at full load. However, the effective efficiency suffers, especially at higher engine speeds. For part load operation, the exhaust valve timing is essential to achieve high efficiency. The publication further indicates that small internal combustion engines using the Atkinson cycle could be an interesting alternative for future internal combustion engine applications.