## 3 Classification of Systems
### Summary of System Classification  
Among the three properties of the system, the first property, the direction of power flows through the CVU, determines the ability of the system to exploit the ratio coverage of the CVU. The third property, the number of power transmission paths parallel to the CVU, determines whether the system can share the torque load passing through the CVU with other transmission paths. The above two properties of the system can be identified by the connection pattern of the connection lines around the CVU: A bidirectional bridge structure for dual-way manipulated systems and a loop structure for dual-path systems. Therefore, the existence of the bidirectional bridge structure and the loop structure is the required condition for the system to exploit twice the ratio coverage of the CVU and reduce the torque load of the CVU with other mechanisms. Proper clutch and gearset arrangements are still required to provide the two features.  
The required structures for dual-way manipulated systems and the two types of dual-path systems are listed in Table 2. When synthesizing the BPD of a desired system, the required structures need to be selected first and then combined with other basic units, which are linked by connection lines. The complete layout of the system in the FPG can then be formed by adding clutches and gearsets into the BPD. In the example shown in Table 3, the BPD of a one-way manipulated, direct-coupled, and bypass-type dual-path system is synthesized in the beginning. The complete layout of the system is presented by the FPG after adding other units, such as coupling points, clutches, and a gearset, into the BPD.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**System** & **Required Structure** & **Required Structure in the BPD** \\ \hline Dual-way manipulated system & Bidirectional bridge structure & **CVU** \\ \hline Power-split-type dual-path system & Loop structure with a CVU and MDOF unit & **2 dof** \\ \hline \hline \end{tabular}
\end{table}
Table 2: Required structures of dual-way manipulated systems and dual-path systems.  
Figure 12: Example of loop structures for bypass-type dual-path systems in the BPD.  
_Appl. Sci._**2021**, _11_, 2500  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Step** & **BPD** & **Effects on the System Property** \\ \hline \multirow{4}{*}{1} & Select the loop structure with only the CVU as the base of the system. & The key component of the system is selected, and the features that the system may be able to achieve are determined. \\ \hline \multirow{4}{*}{2} & Add the inputs and output into the system. & The number of the power inputs and output is determined. \\ \hline \multirow{4}{*}{3} & Connect the inputs and output with a loop structure. & The connecting pattern among the inputs, output, and other units determines the power flow patterns that the system can achieve. \\ \hline \multirow{4}{*}{4} & Rearrange the layout of the connection lines. & After finishing the complete BPD, the possible power flow path of each driving mode can be formulated. If the desired power flow paths cannot be achieved, the designer can return to the previous step and reconnects the inputs and output to the system. \\ \hline \multirow{4}{*}{5} & Determine the position of the coupling points. & The arrangement of the coupling points determines the positions where the power flows will split or merge in the system. Therefore, it can affect the final layout of the system. \\ \hline \multirow{4}{*}{6} & Add the required clutches into the system. & The clutches and brakes serve as switches in the electronic circuit. They affect the load carrying and idling transmission paths in different driving modes, and this can affect the parasitic loss of the system. \\ \hline \hline \end{tabular}
\end{table}
Table 3: Example of synthesizing a system with a BPD.