## 4 Analysis of Hybrid Systems with CVUs  
For simplicity, the above three system characteristics can be expressed by three letters, as shown in Table 4. The first letter denotes the direction of CVU manipulation: \"O\" for one-way manipulated systems and \"D\" for dual-way manipulated systems. The second letter denotes the power coupling pattern: \"D\" for direct-coupled systems and \"P\" for planetary gear-coupled systems. The third letter denotes the number of transmission paths parallel to the CVU: \"S\" for single-path systems and \"D\" for dual-path systems. For example, a \"D-D-D\" system has a dual-way manipulated CVU and directly coupled power input units and is a dual-path system. By combining the above three characteristics, the structures of the 28 studied hybrid systems [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31] can be classified into eight different categories, as shown in Table 5. Most of the current systems are O-D-S systems, whereas D-P-S and D-P-D are both null sets. There are only three dual-way manipulated systems: D-D-S and D-D-D systems. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**System Category: X-Y-Z**} \\ \hline First letter (X) & Direction of power flows through & O & One-way manipulated \\  & the CVU & & Dual-way manipulated \\ \hline Second letter (Y) & Coupling pattern of the power inputs & D & Direct coupled \\ \hline Third letter (Z) & Number of transmission paths & S & Single path \\ \cline{2-3}  & parallel to the CVU & D & Dual path \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Syntax of system categories. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Step** & **BPD** & **Effects on the System Property** \\ \hline
7 & Add the required the gearset(s) into the system. &
\begin{tabular}{c} The actual ratio of the system \\ in different driving modes, \\ and the total ratio coverage of \\ the system are determined. \\ \end{tabular} \\ \hline
8 & \
\begin{tabular}{c} Determine the types of the power \\ inputs. \\ of the system is \\ formed. \\ \end{tabular} &
\begin{tabular}{c} The powering units of the \\ system in each driving mode \\ are determined. The designer \\ needs to check again whether \\ the desired power flow \\ patterns of every driving \\ mode can be realized by the \\ finished clutches and brakes \\ arrangement. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._The BPD of the three dual-way manipulated systems are shown in Figure 13. The three systems all have bidirectional bridge structures, which are highlighted in red. Therefore, the power flow can be transmitted through the CVU in opposite directions under different driving modes. The system proposed by General Motors (GM) and TU/e has two loop structures, whereas the TUM system has only one loop. Although the GM system is a single-path system (D-D-S), the existence of the loop structure shows the possibility of it being redesigned as a dual-path system under proper clutch and gearest arrangement. The other two D-D-D systems, which were proposed by TU/e and TUM, are both bypass-type dual-path systems due to the absence of MDOF units in their loops. The torque is transmitted solely by the CVU during the seamlessly variable ratio modes. Therefore, the transmitted torque of the system cannot exceed the maximum allowable torque of the CVU or it will cause damage to the CVU, such as belt slippage for a V-belt CVU or transmission fluid leakage for a hydrostatic CVU.