# Type Synthesis of Variable Compression Ratio Engine Mechanisms
## 5 Selection of Suitable VCR Engine Mechanisms  
The detailed evaluation of the mechanisms enumerated in Section 4 is quite difficult since the relationship between the stroke variation and the kinematic structure of each mechanism cannot be foreseen before the dimensional synthesis and analysis are carried out. Besides the difficulties in the dimensional synthesis of 2-DOF mechanisms, the same kinematic structure may have completely different kinematic and dynamic characteristics depending on the dimensions and pivot positions of implemented mechanisms. For example, [PERSON] et al. [14] showed that by adjusting the link lengths and pivot position of an initially designed Stephenson III VCR engine mechanism within prescribed tolerances, the maximum of the vertical second harmonic acceleration sum at the joints was decreased by 58.333% without sacrificing a desired stroke. Another example is that the kinematic chain _02 Aa02_ shown in Table 5 was patented by five automotive companies and implemented differently. For these reasons, the following four practical evaluation criteria that consider kinematic structure, force transmissibility, side thrust, and reliability are applied in this research to determine appropriate candidate VCR engine mechanisms by excluding inadequate kinematic structures. The first criterion used to exclude unsuitable mechanisms is the presence of an unnecessary loop. Among the 87 enumerated mechanisms in Tables 5-10, some nine-link linkages contain an unnecessary loop such that removing two links in that loop does not affect the function of the VCR engine. For example, the mechanism _03 Bb01_ in Table 7 has four-link loop with links 2-3-4-CL, but removing links 3 and 4 does not affect the relative motion between the remaining links. There are 30 mechanisms corresponding to this case and they are listed in Table 11. In fact, removing two links from the listed mechanisms results in one of the seven-link VCR engine mechanisms shown in Table 5. For example, _03 Bb01_ in Table 7 becomes _02 Aa01_ in Table 5 when links 3 and 4 are removed, and _03 Bb02_ in Table 7 becomes _02 Aa02_ in Table 5 when links 4 and 5 are removed. \begin{table}
\begin{tabular}{c c} \hline \hline
**Criterion** & **Excluded Mechanisms in Tables 5-10** \\ \hline \multirow{7}{*}{Presence of an unnecessary loop} & _03 Bb01_, _03 Bb02_, _03 Ca01_, _03 Ca02_, _03 Ca03_, _03 Cc01_, _03 Cc02_, _03 Ce01_, _03 Ce02_, _03 Ce01_, _03 Dc02_, _03 Dc05_, _03 Dc06_, _03 Dc09_, _03 Dc10_, _03 Dc13_, _03 Dc14_, _03 Dd07_, _03 Dd08_, _03 Dd09_, _03 Dd10_, _03 Dd11_, _03 Dd12_, _03 Dd13_, _03 Dd14_, _03 Dc04_, _03 Dc05_, _03 Ea02_, _03 Ea03_, _03 Ea04_ \\ \hline \multirow{7}{*}{Three or more links are used to connect the piston and crankshaft} & _03 Cb02_, _03 Cb04_, _03 Cb01_, _03 Cc03_, _03 Cc04_, _03 Cd01_, _03 Cd02_, _03 Cf03_, _03 Cg01_, _03 Cg04_, _03 Cg04_, _03 Ch06_, _03 Dh05_, _03 Dh06_, _03 Dh07_, _03 Dh08_, _03 Dc04_, _03 Dh07_, _03 Dh09_, _03 Dh09_, _0 Considering the force transmissibility and power loss from the piston to the crankshaft, the second evaluation criterion used in this research is the number of links connecting the piston and the crankshaft. The mechanisms having three or more links between the piston and the crankshaft are classified as unsuitable mechanisms. There are 42 mechanisms corresponding to this case as listed in Table 11.  
The third criterion selected in this research is that the piston must be connected to a binary connecting rod to avoid excessive side thrust and piston friction [6]. There are nine mechanisms that have ternary or higher connecting rod as listed in Table 11 and they are excluded from the suitable VCR engine mechanisms. Lastly, the fourth criterion is chosen to consider the reliability and efficiency in varying the compression ratio.