Synthesis of Efficient Powersplit CVT/IVT System

The target of the work is to find out the algorithm of definition of the ratios of mechanical part of complex powersplit CVT/IVT system with regard to the highest achievable efficiency. The presented synthesis is focused on powersplit systems, which will consist of a CVT part, differential and eventually by-pass gear. The algorithm will be programmed and become an integral part of the program Sungear developed on Czech Technical University in Prague for analyses and synthesis of planetary stepped transmissions and CVT/IVT powersplit systems. The article will mainly present the algorithm of definition of efficient powersplit system.

For the search of the efficient powersplit system we assume that the following parameters are given:

1. a
   Spread and efficiency of used CVT system.
2. b
   Total spread of the whole powersplit CVT/IVT mechanism.
3. c
   Optional: Ratios of the used CVT system.
4. d
   Optional: Ratios of the whole powersplit CVT/IVT system.

The more kinematic parameters will be given, the more constraints of mechanism are defined and the less optimization capabilities will remain. If all four points a) up to d) will be given for a known structure, the mechanism will be totally constrained, i.e. the ratios of mechanical part can be directly calculated. The search of the most efficient mechanism is replaced with search of the best combination of differential, bypass ratio and CVT system.

When only the parameters in the first two points a) and b) will be given (optionally with c) or d)) then not only the topology, but also the ratios of the remaining mechanical part are searched for. In our study the parameters are searched in order to satisfy only one target: to obtain the highest possible efficiency. For the total efficiency the amount of the power transmitted through the CVT is decisive. In the study is demonstrated, that the total spread depends exponentially on the amount of power transmitted through the CVT part, and the total efficiency depends linearly on the amount of power transmitted through the CVT part. The bigger total spread is demanded, the lower total efficiency of the system will be obtained. In case of a mechanism, where will be demanded a total spread bigger than the given spread of the CVT part a solution is searched which includes shiftable (more mode) mechanical part. For shifting are searched preferably such combinations, where the clutch will be actuated in the point when both shafts rotate at the same speed. In the current state the algorithm is capable to find out the best ratios for combination of CVT part, differential with 2 DOF and 4 shafts with eventual combination of by-pass gear ratio. The results will be presented with help of schemes of possible real configurations.