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Increasing the efficiency of an internal combustion engine is possible thanks to the use of an extended expansion stroke relative to the suction stroke. This solution was first patented by James Atkinson in the construction of an internal combustion engine in the XIX century. The article presents a solution using the principle of extended expansion stroke. In the design of Szymkowiak's proprietary engine, a conceptual crank-piston system was used with additional elements enabling the Atkinson cycle to be obtained. A 3D model was created based on which forces acting in the system were tested. The generated piston path profile allowed to characterize its movement. A mobile mesh of the combustion chamber was created, thanks to this a detailed CFD simulation was performed in the AVL Fire software. An important criterion was the assumption of adiabatic characteristics of processes during the combustion.

The tests were carried out on an 3D engine model with an unconventional multiple linkage system. Compared to a classic crankset, the mechanism consists of more elements. In this multiple linkage system the camshaft, the piston rod and the main rod are connected to one common element. The camshaft rotating during operation at twice the speed of the crankshaft makes possible to achieve different piston stroke lengths with each revolution. With proper synchronization of the camshaft revolution with the crankshaft, the suction and compression stroke is smaller in relation to the expansion and exhaust strokes. For this reason, the Atkinson cycle was obtained without interfering with the variable valve timing. The thermal cycle is characterized by increased theoretical thermal efficiency. Due to the unique mechanism, the piston movement has different characteristics compared to classic solutions. Therefore, work was undertaken to analyze the distribution of forces in the system.