## 3 Results and Discussions
### Intake Pressure  
Boosting the intake pressure can improve the cross-scavenged gas exchange process, helping to reduce the amount of residual exhaust gas in the cylinder and increase the intake charge. The combustion pressures and heat release rate curves averaged over 200 consecutive test cycles for the intake pressures (gauge pressure) of 0, 4, 8, and 12 kPa are given in Figure 9. The compression ratio was adjusted to 16.5, and the cylinder block  
Figure 8: CA10, CA50, and CA90 at different cylinder temperatures. temperature was controlled at 80 \({}^{\circ}\)C in the tests. In Figure 8(a), p\({}_{\text{max}}\) increases from 4.03 MPa to 4.31 MPa, and A\({}_{\text{pmax}}\) is advanced from 12.1 deg ATC to 10.1 deg ATC as the intake pressure gradually increases from 0 kPa to 12 kPa. In Figure 8(b), the low-temperature exothermic reaction time and the heat release time are advanced with increasing intake pressure, but the low-temperature exothermic reaction becomes weaker. This phenomenon is similar to that presented when the compression ratio is increased and demonstrates the closer correlation between the low-temperature exothermic reaction in the pre-combustion stage and the in-cylinder residual exhaust gases. As the intake pressure rises, the fresh intake charge can push more exhaust gas out of the cylinder during the cross-scavenged gas exchange process so that the amount of high-temperature residual exhaust gas in the cylinder will decrease. In the next cycle, the temperature of the pre-compressed air-fuel mixture that is mixed by the fresh intake charge and the residual exhaust gases in the cylinder will be reduced, making the low-temperature combustion reaction weaker. However, the reduction in residual exhaust gas also benefits the subsequent primary combustion process, resulting in an earlier combustion time and improved combustion stability. Figure 10 shows the cyclic variations in p\({}_{\text{mi}}\) over 200 consecutive test cycles for the intake pressures of 0, 4, 8, and 12 kPa. As the figure shows, the combustion stability can be improved with increasing intake pressure, and COV\({}_{\text{pmi}}\) decreases from 14.7% at 0 kPa to 7.7% at 12 kPa. The reduction in residual exhaust gas in the cylinder can explain the improvement in combustion stability. However, p\({}_{\text{mi}}\) does not change much, and its mean value remains around 0.3 MPa at the different intake pressures. The effect of intake pressure on in-cylinder combustion is less pronounced than that of compression ratio and cylinder block temperature. Figure 11 illustrates the trend of p\({}_{\text{mi}}\) with CA50 over 200 consecutive test cycles for the cylinder block temperatures of 80, 100, and 120 \({}^{\circ}\)C and different intake pressures of 4, 8, and 12 kPa. As seen, boosting intake pressure affects in-cylinder combustion differently at different cylinder block temperatures. In Figure 10(a), the cylinder block temperature is 80 \({}^{\circ}\)C. At the intake pressure of 4 kPa, many test cycles burn late with CA50 later than 20 deg ATC, present low p\({}_{\text{mi}}\), and partially exhibit misfire or partial burning. When the intake pressure is increased to 12 kPa, the number of test cycles with CA50 later than 20 deg ATC decreases, and combustion stability is improved. Notably, the increase in intake pressure does not contribute significantly to the rise in p\({}_{\text{mi}}\). In Figure 10(b), the cylinder block temperature is 100 \({}^{\circ}\)C. Boosting the intake pressure slightly affects combustion stability but does not result in a noticeable rise in p\({}_{\text{mi}}\). In Figure 10(c), the cylinder block temperature is 120 \({}^{\circ}\)C. As  
Figure 9: Combustion pressures and heat release rates at different intake pressures (under the cylinder block temperature of 80 \({}^{\circ}\)C): 1—0 kPa (black); 2—4 kPa (green); 3—8 kPa (yellow); 4—12 kPa (red). the intake pressure increases from 4 to 12 kPa, combustion stability is little affected, but Pmi rises from 0.205 to 0.249 MPa on average.