## 2 Materials and Methods
As shown in Figure 2, the pressure sensor is mounted on the cylinder head through a 5 mm threaded hole, and then a clamp nut fixes the cylinder head on an adjusting nut. The adjusting nut could be screwed into or out of the cylinder through the thread on the outer wall of the cylinder. When the adjusting nut is screwed down, the cylinder head moves downward with the adjusting nut and the clamp nut, the volume of the combustion chamber decreases, and the compression ratio (CR) increases. When the adjusting nut is screwed up, the adjusting nut, fastening nut, and cylinder head move upward together, the combustion chamber volume increases, and the compression ratio decreases. The limiting position of the downward movement of the adjusting nut is set as a reference zero point, where the combustion chamber volume is minimized. Owing to a 9.88 mm\({}^{3}\) channel volume in front of the pressure sensor, the zero point position corresponds to a maximum theoretical compression ratio of 56.4. The thread pitch of the adjusting nut is 0.8 mm, so for every 45 degrees (1/8 turn) of rotation of the adjusting nut, the cylinder head will move up or down 0.1 mm. The combustion chamber volumes and compression ratios as the adjusting nut is turned upward are shown in Table 3.  
The test bench utilizes a centrifugal fan for forced air cooling of the test engine. The cylinder block temperature can be controlled to a preset value by adjusting the cooling air flow rate. In addition, the test bench is equipped with an intake pressurization unit, including a pressure-stabilizing tank, an air pump, a pressure regulator valve, and a pressure regulator box. Boosted by an air pump downstream of the pressure-stabilizing tank, the intake air can be adjusted to a preset pressure in the pressure regulator box by controlling the opening of the pressure regulator valve. Intake flow is measured between the pressure-stabilizing tank and the air pump, and intake pressure is measured at the pressure regulator box. During the tests, the engine speed was set to 6000 r/min, the throttle was fully open, and the carburetor was calibrated to make the test engine present \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Name** & **Maker** & **Type** & **Measurement Range** & **Accuracy** \\ \hline Pressure transducer & KISTLER Ltd. & 6052B & 0–25 MPa & \(\leq\)\(\pm\)0.3\%/FSO \\ Crank angle sensor & ONO SOKKI & MP-935 & 0–360 deg CA & \(\leq\)\(\pm\)0.1 deg CA \\ K-type thermocouple & VICTOR Technology & DM6801A & \(-\)50–1300 \({}^{\circ}\)C & \(\leq\)\(\pm\)0.4 \({}^{\circ}\)C \\ Photoelectric & SUWEI Electronic Technology & DT2234B & 3–99,999 r/min & \(\leq\)\(\pm\)1 r/min \\ Flow meter & SIARGO & MF5706-N-25 & 0–25 L/min & \(\leq\)2.5\% \\ Dial indicator & HENGLIANG Measuring Tools & A-33 & 1–10 mm & \(\leq\)\(\pm\)0.01 mm \\  & YOUTENG Weighing & & & \\ Fuel weighing balance & Equipment & BS600L & 0–600 g & \(\leq\)\(\pm\)0.15 g \\  & (Hangzhou, China) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical data of the measuring instruments. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Type** & **Kerosene** & **Diethyl Ether** & **Castor Oil** & **Isopropyl Nitrate** \\ \hline Chemical formula & C\({}_{12}\)H\({}_{26}\) & C\({}_{4}\)H\({}_{10}\)O & C\({}_{18}\)H\({}_{34}\)O\({}_{3}\) & C\({}_{3}\)H\({}_{7}\)NO\({}_{3}\) \\ Heat value (MJ/kg) & 41.8 & 33.9 & 37.2 & - \\ Hexadecane value & 41.4 & \(>\)125 & - & 1200 \\ Ignition temperature (\({}^{\circ}\)C) & 210 & 160 & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Properties of fuel components. the maximum \(\mathrm{p_{mi}}\) and an approximate fuel/air equivalence ratio of 1.2. Keeping the same baseline operating condition unchanged, the in-cylinder combustion at different compression ratios, thermal statuses, and intake pressures was investigated.