Comments on ``Analysis of circulating power within hybrid electric vehicle transmissions'', Mechanism and Machine Theory 64 (2013) 131-143, by A.K. Gupta, C.P. Ramanarayanan

The study of power recirculation within planetary gear trains is very important for designing new hybrid transmissions. Gupta et al. [1] presented a method which utilizes the algebraic difference between the splitting power flow ratios x1 and x2, for determining the direction of the power flow in two-DOF planetary drives. This method is an extension to the original method presented in Ref. [5] for analyzing power recirculation in one-DOF, two-stage epicyclic gear trains. Gupta et al. [1] seem ambiguous about the key points of the method and include some fundamental errors in their analysis. The purpose of this paper is to demonstrate clearly a simple algorithm to identify power recirculation in two-input one-output planetary gear trains. Also, the necessary and sufficient operating conditions for power recirculation in two-input transmissions are proven rigorously.