## 3 Results and Discussions  
### Compression Ratio  
The combustion pressure and heat release rate curves averaged over 200 consecutive test cycles at the compression ratios of 12.4, 14.1, 16.5, 19.9, and 25.2 are given in Figure 3. The cylinder block temperature was controlled at 120 \({}^{\circ}\)C during the tests. In Figure 3a, the maximum combustion pressure (\(\mathrm{p_{max}}\)) tends to increase, and the crank angle related to the maximum combustion pressure (\(\mathrm{A_{pmax}}\)) tends to advance as the compression ratio increases. When the compression ratio rises gradually from 14.1 to 19.9, \(\mathrm{p_{max}}\) increases from 3.47 MPa to 5.47 MPa, and \(\mathrm{A_{pmax}}\) is moved from 10.3 deg after top center (ATC) to 1.3 deg ATC. In Figure 3b, the combustion duration is shortened, and the heat release time is advanced as the compression ratio gradually increases from 14.1 to 19.9. At the same time, a low-temperature exothermic reaction indicated by the bumps at the beginning of the heat release curves can be observed, which are advanced but weakened as the compression ratio increases. However, when the compression ratio rises to 25.2, the low-temperature exothermic reaction is no longer apparent; instead, an endothermic reaction with a negative heat release rate occurs. Meanwhile, the heat release time is delayed, and \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Number of Turns**} & **Movement Distance** & \multicolumn{2}{c}{**Combustion**} & \multirow{2}{*}{**Compression Ratio**} \\  & **(mm)** & **Chamber Volume** & **(mm\({}^{3}\))** & \\ \hline
0/8 & 0 & 9.88 & 56.4 \\
1/8 & 0.1 & 16.24 & 34.7 \\
2/8 & 0.2 & 22.61 & 25.2 \\
3/8 & 0.3 & 28.97 & 19.9 \\
4/8 & 0.4 & 35.33 & 16.5 \\
5/8 & 0.5 & 41.69 & 14.1 \\
6/8 & 0.6 & 48.05 & 12.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Combustion chamber volume and compression ratio with the displacement of the adjustment nut. Figure 2: Pressure sensor mounting and compression ratio adjusting. the heat release rate and cumulative heat release energy are reduced. Theoretically, the low-temperature exothermic reaction at the pre-combustion stage generates reactive atoms that help the primary heat release reaction in the combustion process. The inadequate low-temperature exothermic reaction can suppress the primary heat release reaction and may lead to abnormal in-cylinder combustion. It is considered that the tendency of the low-temperature exothermic reaction in the pre-combustion stage to weaken with increasing compression ratio may be related to the pre-compression air-fuel mixture temperature. As can be seen, the combustion pressure and temperature at the end of the expansion stroke tend to decrease with the increasing compression ratio (see the in-cylinder pressure variation in Figure 3a), which indeed results in the temperature decrease of the residual exhaust gas in the cylinder and the tendency of the temperature of the pre-compression air-fuel mixture in the next cycle to decrease [28]. The decrease in the temperature of the pre-compression air-fuel mixture in the next cycle should lead to an insufficiently low-temperature exothermic reaction. When the compression ratio is increased to 25.2, abnormal combustion with many partial burning and misfire cycles occurs (see Figures 4 and 5), which results in a drastic change in the chemical composition and thermal state of the residual exhaust gases in the cylinder. In this case, the low-temperature exothermic reaction does not proceed normally and presents as an abnormal heat absorbing reaction, contributing to a more severe abnormal combustion phenomenon. In addition, at the compression ratio of 12.4, the heat release rate curve is close to the horizontal line, and no evident heat release phenomenon can be observed. At too low compression ratios, the increment of internal energy converted from the compression work in the compression stroke is not enough to cause the spontaneous combustion of the air-fuel mixture.