## 5 System Synthesis and Example  
To fulfill the need for both greater ratio coverage and torque capacity, a hybrid system will be designed with the aid of a BPD. To provide wider overall ratio coverage than the CVU, the new system is designed to be dual-way manipulated. To allow a higher torque capacity than the CVU during all continuously variable ratio modes, it is set to be a power-split-type dual-path system. The coupling pattern of the two input units can be either direct coupled or planetary gear coupled. Therefore, the category of the new system will be D-D-D or D-P-D. Section 3.1 showed that a bidirectional bridge structure is a requirement of a dual-way manipulated system, as shown in Figure 4. For a power-split-type dual-path system explained in Section 3.3.1, a loop structure comprising a CVU and an MDOF unit is required, as shown in Figure 10. To reduce the mechanical complexity, only 2 DOF-3E and 2 DOF-4E are taken into account in this paper. In Figure 14, four possible basic structures are formed by combining a bidirectional bridge and a loop structure. Among them, those shown in (a-d) are single-loop structures, whereas those in (e-h) are double-loop structures. Figure 14: Possible basic structures of the system. Figure 13: BPDs of the three dual-way manipulated systems, wherein the bidirectional bridge structures are highlighted in red. (**a**) General Motors two-mode concept (1986) [10]. (**b**) TU/e (1987) [11,12]. (**c**) Technical University of Munich (2006) [17,18]. The hybrid system has two inputs and one output. Therefore, by adding them to the structures shown in Figure 14, the complete BPD of the target system can be formed. To attain the goal of wider ratio coverage, higher torque capacity, and a continuously variable ratio, the system design follows the three principles below:  
1. The system has at least two driving modes, and each of them is driven by one of the two inputs. 2. In every driving mode, the power is transmitted by the power-split mechanism formed by the loop structure comprising the CVU and the 2 DOF unit to have a higher torque capacity than the CVU. 3. The transition between the two driving modes should be seamless. At the mode switching point, the CVU ratios in the two modes need to be the same. Therefore, the ratio range of the two modes should be joined together or have a small overlap region. To fulfill requirement (3), the system requires two loop structures, and each of them is used to transmit power in the two driving modes. As a result, only the four double-loop structures in Figure 14 e-h can be considered. In combination with different connection patterns among gearsets, inputs and output, various possible results can be generated. To demonstrate the effect of the synthesized system in a more understandable way, the BPD in Figure 15 is chosen as an example. The layout is based on the double-loop structure in Figure 14e, and its possible power flow patterns are shown in Table 6.  
To direct the power through the designed paths and adjust the ratio spans of different driving modes, the next step is to determine the position of the clutches and gearsets and the ratio of the gearsets. The result can be found through a global search method. One of the feasible solutions is shown in Figure 16. A M/G is used as IN\({}_{1}\) for low- to medium-speed driving, whereas an ICE is used as IN\({}_{2}\) for medium- to high-speed driving. The parameters of the example are shown in Table 7. \begin{table}
\begin{tabular}{c} Power source: IN\({}_{1}\) Power receiver: OUT \\ \hline Power source: OUT \\ \hline Power receiver: IN\({}_{2}\) Power receiver: OUT \\ \hline \end{tabular}
\end{table} 
Table 6: Possible power flow patterns of the synthesized system. Figure 15: BPD of the synthesized system. The synthesized system has two main driving modes: Electric mode and engine mode, as shown in Figures 17 and 18 respectively. In the electric mode, the input power of the M/G is split into two by planetary gearset PG\({}_{1}\).