## 3 Results
### Analysis of Ecological Indicators  
It is noticeable that CO\({}_{2}\) emissions (Figure 11) are significantly lower when switching from G to NG in the entire range of IVC, engine speed, and load, with reductions ranging from \(\sim\)19.8% to \(\sim\)25.5%. When evaluating the impact of IVC on CO\({}_{2}\) emissions, it can be seen that there is no significant change in the range from 24\({}^{\circ}\) to 52\({}^{\circ}\). However, for gasoline operation at 2000 rpm, delaying IVC to 59\({}^{\circ}\) results in a CO\({}_{2}\) reduction ranging from \(\sim\)3.7% at 0.31 MPa to \(\sim\)5.7% at 0.79 MPa, while a CO\({}_{2}\) reduction is noticed from \(\sim\)4.4% to \(\sim\)5.0% across the BMEP range at 3000 rpm. For NG operation at 59\({}^{\circ}\) IVC, the CO\({}_{2}\) decreases are more pronounced, respectively, from \(\sim\)5.6% to \(\sim\)6.3% and \(\sim\)3.9% to \(\sim\)7.1%. Figure 11: The relationship between CO\({}_{2}\) emissions and IVC timing for gasoline and natural gas. Figure 10: The relationship between BTE and IVC timing for G and NG. Such a change can be explained due to different fuel properties and the port injection system in the intake manifold. The ~2.1 times higher H/C ratio (Table 3) of NG compared to G, reduced fuel mass consumption, and increased BTE (Figure 10) results in less carbon dioxide formation during combustion. Additionally, partial air space is occupied by compressed natural gas, thus reducing cylinder filling with air, which is compensated for by opening the throttle more and maintaining a stoichiometric mixture throughout the entire IVC range. However, LIVC lowers the effective compression ratio while preserving the expansion ratio and improving thermal efficiency, which can be evaluated as lower fuel consumption and CO\({}_{2}\) emissions. Other researchers reported similar results with a delayed IVC in a turbocharged SI engine while demonstrating that natural gas exhibits higher efficiency compared to gasoline, indicating better energy utilization and lower CO\({}_{2}\) emissions as well [54; 55]. Carbon monoxide (CO) emissions (Figure 12) are lower when using natural gas. The change in CO pollutants has a correlation relationship with CO\({}_{2}\) emissions with different IVC timings between 24\({}^{\circ}\) and 52\({}^{\circ}\). Although CO is not as stable in this range as CO\({}_{2}\), it is worth emphasizing the more significant formation of CO when IVC is maximally delayed. Comparing the change in emissions when IVC is 24\({}^{\circ}\) and 59\({}^{\circ}\), it is observed that the increase in CO is up to ~31.9% when using G at 2000 rpm and different BMEP levels, while it increases up to ~33.2% with NG. After increasing the engine speed to 3000 rpm, the change remains close enough, up to ~27.1% with G and ~27.7% with NG, respectively. The NG used in the experimental research consists of ~94.3% methane and generally tends to burn cleanly compared to gasoline. The simpler hydrocarbon structure promotes more complete combustion and results in fewer CO emissions. When IVC is delayed to 59\({}^{\circ}\), CO emissions increase due to uneven mixing at a lower effective compression ratio. LIVC disrupts the air and fuel mixture homogeneity, creating localized rich zones with insufficient oxygen for complete combustion. The complex interaction between IVC and the in-cylinder flow patterns can lead to variations in the cylinder's A/F distribution. It was observed that CO formation was favored, leading to higher overall CO emissions despite the earlier SOC, indicating the general improvement in combustion efficiency due to more intense heat release and higher combustion temperature (Figure 6) in the presence of LIVC. Lower CO emissions are observed in a similar study when operating a bi-fuel engine (natural gas and gasoline) and attributed to the cleaner combustion characteristics of NG [50], while a substantial decrease in CO emissions is reported when using NG as an alternative fuel in an SI engine [56].