The planetary gear hybrid powertrain (PGHP) is known as one of the most efficient configurations for hybrid electric vehicles. The system controls the speed and torque of each component so that it can produce adequate input and output speeds for the optimal operation of the total system. In this paper, the system mechanism is analysed to understand the relation between the input and output speeds. In addition, the transmission efficiency is expressed as a function of three parameters based on the calculation of the net output power. The characteristics of the transmission system showed a large difference from those of a conventional CVT. The speed ratio was the key parameter that affected the efficiency. Furthermore, the performances of the electric motor and generator were decisive factors owing to the loss in the energy conversion. In a simulation on an urban driving schedule, the optimal operation of the engine was able to be obtained, and the system showed an improvement of 2.2 per cent in fuel economy.
