Small combustion engines can be found in various applications in daily use (e.g. as propulsion of boats, scooters, motorbikes, power-tools, mobile power units, etc.) and have predominated these markets for a long time. Today some upcoming competitive technologies in the field of electrification can be observed and have already shown great technical advances. Therefore, small combustion engines have to keep their present advantages while concurrently minimizing their disadvantages in order to remain the predominant technology in the future. Whereas large combustion engines are most efficient thermal engines, small engines still suffer from significantly lower efficiencies caused by a disadvantageous surface to volume ratio. Thus, the enhancement of efficiency will play a key role in the development of future small combustion engines.
One promising possibility to improve efficiency is the use of a longer expansion than compression stroke. This principle was invented and patented by James Atkinson at the end of the 19th century. Since the beginning of the 21st century the extended expansion has been enjoying renewed interest. This paper includes a theoretical overview of extended expansion with respect to feasible piston motion. An appropriate piston motion is used for a detailed 1D-CFD simulation to calculate indicated efficiency. Furthermore, thermodynamic losses between theoretical and indicated efficiencies are analyzed. Simulation parameters are based on experimental data of a conventional SI-engine (gross heat release, flow coefficients, etc.). Especially for small engines with comparatively low compression ratios, the Atkinson-cycle, if realized via the crank train, can be a very effective measure to enhance efficiency.
For DENSO, as supplier of components, it is of strong interest what the solutions for an efficiency increase could look like. In terms of the Atkinson cycle, it is crucial to know the potentials and limits in order to be prepared for specific needs of components used for this particular engine layout.