The paper analyzes a series of planetary mechanisms with 4 elements that can couple the power sources of a hybrid vehicle. For the structural optimization of the planetary mechanisms with two degrees of freedom, the response of the mechanism to the operating cycles of the hybrid vehicle is analyzed. The studied planetary mechanisms are compared with the Power Split Device power flow division mechanism used by Toyota on the Prius model. For the study of the motion of the mechanical system, the steady-state motion is studied. For this, the differential equations of motion are obtained with the help of Lagrange's equations. The results obtained are presented in the form of diagrams. At the end of the paper, the conclusions of the study are presented.