# Type Synthesis of Variable Compression Ratio Engine Mechanisms
## 5 Selection of Suitable VCR Engine Mechanisms
If the links connected to the engine block by joints are only the piston, crankshaft, and control link, the force exerted by the piston would be imparted not only to the crankshaft, but a large portion of the piston force would also be transferred to the control link. This may require a large force or torque to vary and maintain the compression ratio. Consequently, it would be necessary to use a powerful actuator, and could even cause failure of the actuator. In order to reduce the force transmitted to the control link and force/torque required to alter and maintain the compression ratio, the ground connected control link can be installed in an independent loop in which neither the piston nor the crankshaft is included. By examining Figure 2, it can be concluded that the control link must be in an independent loop with another ground connected link other than the piston or crankshaft, and hence the engine block must be a quaternary link. The example can be found in the evolution of the Nissan VC-T engine which was initially designed with the mechanism [23] represented by the graph _02 Aa02_ shown in Table 5, then modified and produced in the form of the kinematic structure [35] with the graph _03 Dd05_ shown in Table 9. Applying the fourth criterion to the remaining six kinematic structures, three mechanisms are excluded as listed in Table 11.  
Excluding the aforementioned 84 cases, the remaining three mechanisms shown in Table 12 are selected as suitable VCR engine mechanisms in this research, only one of which has been patented. Examining the kinematic structures of the selected mechanisms in Table 12, each has an identical driving loop consisting of the engine block (_E_)--piston (_P_)--link _1_--link _2_--crankshaft (_CS_), but their control loops are different. In order to observe the variation principle of each mechanism, consider the case when the control link, _CL_, of each mechanism is actuated and positioned for a specific compression ratio. In the cases of the two mechanisms derived from graphs _03 Db04_ and _03 Dd05_, the four-link control loop (_E-CL-4-5_ or _E-CL-5-4_) in each mechanism becomes a structure, namely the engine block, and the two VCR engine mechanisms become Stephenson III six-bar slider-crank linkages. Hence, the piston displacement of each mechanism depends on the position of the revol  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Schematic diagram** & \(E\) & \(P\) \\
**Labeled graph** & \(E\) & \(P\) \\  
link 3 and link 4. In the case of the kinematic structure derived from graph _03 Dc03_, when the control link is fixed in a position, the five-link control loop (_E-CL-4-3-5_) becomes a four-bar linkage and the piston displacement is determined by the moving path of the joint connecting link 2 and link 3. In order to verify the compression ratio variability of the selected VCR engine mechanisms, kinematic analysis is carried out to find the piston displacement, cylinder volume, piston velocity, and piston acceleration of each mechanism at high and low compression ratio (C.R.) and compared with those of a 0.5 L conventional fixed-compression ratio single-cylinder engine mechanism with a crank length of 43 mm, connecting rod length of 140 mm, and piston offset of 0 mm. As mentioned above, the mechanisms in Table 12 have identical driving loops. For a fair comparison, the same dimensions are used for the links (1, 2, CS, and a piston offset of 0 mm) in the driving loop. In addition, since the mechanisms derived from _03 Db04_ and _03 Dd05_ are identical except their control loops, if the dimensions of links in the driving loop, link 3, and the position of the joint connecting link 3 and link 4 are the same, then the two mechanisms have the same piston motion. Hence, they are analyzed identically. Note that the following results are based on the dimensions of each mechanism determined empirically or from a previous study [14]. For detailed analysis, the dimensional synthesis and dynamic analysis must be followed to identify the pros and cons of each mechanism.