## 3 Results
### Analysis of Energy Indicators  
The reduction in exhaust temperature with delayed IVC suggests a shift in the combustion process and heat distribution. This is confirmed by the ROHR intensity, temperature rise, and temperature in the cylinder determined during the combustion process analysis. When applying delayed IVC, combustion occurs earlier, more intensively, and in a smaller volume, and the peak combustion temperatures inside the cylinder are higher (Figure 7) and sufficient to drive nitrous oxide formation. The subsequent expansion and cooling of the gases lower the exhaust temperature (Figure 8), but the NO\({}_{\rm x}\) formed during the high-temperature combustion phase remains present. Through delayed IVC, the Atkinson cycle is imitated, and this leads to a more complete expansion of combustion gases within the cylinder, extracting more work and reducing the energy (and thus temperature) of the exhaust gases.  
The BSFC is consistently lower for natural gas compared to gasoline (Figure 9) due to the \(\sim\)17.8% higher LHV of NG (Table 3). This is also influenced by the higher BTE of NG compared to G. Although the intake valve closing time does not have a significant impact on these indicators, it is worth noting that when evaluating the average of the data across the entire IVC range under different engine loads, BSFC decreases by \(\sim\)18% at 2000 rpm when using NG. At the increased engine speed of 3000 rpm, the BSFC change is even more significant at \(\sim\)20%. Other studies showed a \(\sim\)16% and \(\sim\)15% reduction in BSFC at different throttle positions and a decrease by \(\sim\)22% in BSFC when using CNG compared to G, explaining that natural gas has a higher LHV and burns more slowly than G [50; 51]. Due to the lower combustion temperature of NG compared to G, the heat energy losses through the cylinder head and liner to the cooling system are reduced. The lower exhaust gas temperature of NG also indicates lower energy losses through the exhaust system. All of this complexly increases BTE and decreases BSFC.  
Figure 8: The relationship between exhaust gas temperature (T\({}_{\rm EX}\)) and IVC timing for G and NG.  
Figure 7: The in\(-\)cylinder temperature and temperature rise while adjusting the IVC timing at 2000 rpm and 3000 rpm.  
Figure 9: The relationship between BSFC and IVC timing for G and NG.  
BTE is higher by \(\sim\)3.2% at 0.39 MPa engine load, \(\sim\)3.5% at 0.55 MPa, and \(\sim\)3.3% at 0.79 MPa when using NG compared to G; meanwhile, at 3000 rpm, it increases by 6.3%, 5.2%, and 5.7%, respectively (Figure 10). Comparing IVC timings of 24\({}^{\circ}\) and 59\({}^{\circ}\) at 2000 rpm and 0.31 MPa load, running on NG, BTE increased from 0.281 to 0.290 (3.02%). At 0.55 MPa load, BTE increased from 0.335 to 0.347 (1.05%) and 0.359 to 0.371 (0.46%) at 0.79 MPa. In the case of gasoline, the positive influence of LIVC on increasing BTE is less noticeable at the increased engine speed of 3000 rpm.  
The ITE can be increased by \(\sim\)2% at different operation conditions with the optimization of LIVC [52]. Other authors found that increasing the compression ratio and delaying the intake valve closing can also enhance efficiency by 2.04% under high-load conditions. These changes demonstrate that the LIVC strategy helps in improving thermal efficiency by reducing the effective compression ratio, thus reducing the compression temperature and mitigating knocking. Therefore, the Atkinson cycle can significantly improve fuel consumption and efficiency [23,53].