## 3 Results and Discussions
### Thermal State  
The combustion pressure and heat release rate curves averaged over 200 consecutive test cycles at the cylinder block temperatures of 70, 90, 110, and 130 \({}^{\circ}\)C are given in Figure 6. The compression ratio was adjusted to 16.5 in the tests. In Figure 6a, as the cylinder block temperature increases from 70 to 130 \({}^{\circ}\)C, p\({}_{\text{max}}\) rises from 4.16 to 4.85 MPa, and A\({}_{\text{pmx}}\) is advanced from 10.4 to 4.3 deg ATC. In Figure 6b, the heat release time is advanced, the combustion duration is shortened, and the peak heat release rate increases with increasing cylinder block temperature; meanwhile, the low-temperature exothermic reaction also becomes more intense but does not change in its onset time. Theoretically, temperature strongly affects the chemical kinetic reaction rate in the HCCI combustion process. Under micro-scale conditions, the area-to-volume ratio increases dramatically, and the influence of the thermal state on the in-cylinder combustion should be more sensitive. In this situation, heating the cylinder and crankcase walls of the fresh intake mixture becomes the dominant factor influencing the thermal state of the pre-compression air-fuel mixture. Consequently, the low-temperature exothermic reaction in the pre-combustionstage becomes more intense as the cylinder block temperature increases, significantly influencing the subsequent combustion process. Figure 7 shows the cyclic variations in \(\mathrm{p_{mi}}\) over 200 consecutive test cycles for the cylinder block temperatures of 70, 90, 110, and 130 \({}^{\circ}\)C. At the cylinder block temperature of 70 \({}^{\circ}\)C, the excessively low cylinder temperature leads to abnormal combustion, and more misfire or partial burning cycles can be observed; \(\mathrm{p_{mi}}\) varies from 0.01 to 0.37 MPa with a mean value of 0.291 MPa. At the cylinder block temperature of 90 \({}^{\circ}\)C, \(\mathrm{p_{mi}}\) ranges from 0.22 to 0.30 MPa with a mean value of 0.262 MPa. At the cylinder block temperature of 110 \({}^{\circ}\)C, \(\mathrm{P_{mi}}\) varies from 0.16 to 0.25 MPa with a mean value of 0.203 MPa. At the cylinder block temperature of 130 \({}^{\circ}\)C, \(\mathrm{P_{mi}}\) ranges from 0.14 to 0.19 MPa with a mean value of 0.161 MPa. As the cylinder block temperature increases, combustion becomes more stable, and \(\mathrm{COV_{pmi}}\) decreases from 23.1% at 70 \({}^{\circ}\)C to 7.2% at 130 \({}^{\circ}\)C. Notably, \(\mathrm{p_{mi}}\) decreases significantly with increasing cylinder block temperature. On the one hand, a cylinder block temperature that is too high can lead to a too early heat release time and reduce thermal efficiency. On the other hand, high-temperature components heat intake gas and lower intake density, thus minimizing intake charge. The intake flow rate was measured to be 2.14 L/min at the cylinder block temperature of 70 \({}^{\circ}\)C, while it dropped to 1.96 L/min at 130 \({}^{\circ}\)C. The thermal state heavily influences the gas exchange process at the millimeter scale. Figure 6: Combustion pressures and heat release rates at different cylinder block temperatures: 1—70 \({}^{\circ}\)C (black); 2—90 \({}^{\circ}\)C (green); 3—110 \({}^{\circ}\)C (yellow); 4—130 \({}^{\circ}\)C (red). Figure 7: \(\mathrm{p_{mi}}\) at different cylinder block temperatures. Figure 8 shows the cyclic variations in CA10, CA50, and CA90 over 200 consecutive test cycles for the cylinder block temperatures of 70, 90, 110, and 130 \({}^{\circ}\)C. CA10, CA50, and CA90 advance with increasing cylinder block temperature. At the cylinder block temperature of 130 \({}^{\circ}\)C, the advancement of CA10 is more pronounced, and CA50 moves forward to the point adjacent to the TDC; thus, the heat release time seriously deviates from the thermodynamically ideal value. The unreasonable heat release time leads to a reduction in thermal efficiency. In addition, the cyclic variations in CA10, CA50, and CA90 tend to decrease with increasing cylinder block temperature.