On the pages of the article described cam-piston mechanism developed as an alternative to piston crank mechanism. The main feature of this mechanism is that the piston is dwell in the TDC by the 30°CA and thus allows combustion of the fuel in constant volume. Authors presented the process of designing an analytical cam. The starting point for the calculation was the displacement of piston graph and the limits of acceleration and velocity values obtained. The results of strength calculations of the cam with regard to fatigue problems are also presented. Calculations were carried out in order to reduce the weight of the cam. For this purpose, the Finite Element Method was used.