## 4 Conclusions  
An experimental investigation and combustion process analysis were conducted using a Nissan HR16 DE 1.6 L internal combustion engine operating on gasoline and natural gas, employing late intake valve close ranging from 24\({}^{\circ}\) to 59\({}^{\circ}\) crank angle degrees after bottom dead center, at engine speeds of 2000 and 3000 rpm, and break mean effective pressures of 0.31, 0.55, and 0.79 MPa. Data obtained during experimental tests (in-cylinder pressure, fuel consumption, pollutant emissions) and the AVL BOOST(tm) software's BURN utility were used to evaluate the combustion process. The following conclusions can be drawn based on the results obtained:  
Natural gas consistently demonstrates a shorter ignition delay than gasoline, particularly at lower engine speeds (\(n\) = 2000 rpm), and decreases by 1.3% to 7.3%, likely due to improved mixing. At 3000 rpm and low load, ignition delay reduces by -33%, highlighting turbulence effects. While natural gas generally burns later, it burns earlier at 3000 rpm and low load, correlating with the shorter ignition delay. In this condition, 50% mass fraction burned occurs 1.8% to 7.5% earlier with natural gas, and 90% mass fraction burned is observed 0.5% to 3.2% earlier. The engine operating on natural gas demonstrated better energy efficiency: the average brake-specific fuel consumption decreased by -18-20%, and the average brake thermal efficiency increased by \(\sim\)3-6% compared to gasoline. This was attributed to the higher lower heating value of natural gas and a more efficient combustion process. Using natural gas resulted in an average reduction of \(\sim\)20-25% in carbon dioxide emissions. Lower carbon monoxide and nitrogen oxide emissions (-22-38% lower, depending on load and speed) were also observed. However, methane hydrocarbon emissions were higher, while propane hydrocarbon emissions were lower than when using gasoline. Delaying the intake valve closing to 59\({}^{\circ}\) after bottom dead center, especially when using natural gas, resulted in a slight increase in break thermal efficiency (up to \(\sim\)3%), consistent with Atkinson cycle principles. This also led to reduce carbon dioxide (-4-7%) compared to the earliest intake valve close (24\({}^{\circ}\) after bottom dead center). Late intake valve close (59\({}^{\circ}\) after bottom dead center) resulted in a more intense combustion process (higher rate of heat release peak, faster in-cylinder temperature rise), although the peak in-cylinder temperatures were slightly lower than with earlier intake valve close due to the reduced effective compression ratio. Delaying the intake valve close from 24\({}^{\circ}\) to 59\({}^{\circ}\) after bottom dead center caused a significant increase in carbon monoxide (up to \(\sim\)33%), methane hydrocarbons (up to \(\sim\)80% with gasoline, up to \(\sim\)30% with natural gas), propane hydrocarbons (up to \(\sim\)8%, up to \(\sim\)2.5 times, respectively), and nitrogen oxide emissions (up to \(\sim\)97% and up to \(\sim\)2.4 times, respectively). This is associated with poorer air-fuel mixture homogeneity at a lower effective compression ratio, incomplete combustion, and higher localized temperatures during the more intense combustion phase. Utilizing natural gas instead of gasoline significantly reduces carbon dioxide emissions and nitrogen oxide pollutants while improving the engine's energy efficiency. The late intake valve closing strategy can further enhance brake thermal efficiency and reduce carbon dioxide, but it adversely affects carbon monoxide, hydrocarbon, and nitrogen oxide emissions. Achieving optimal results requires a trade-off between increasing efficiency, reducing carbon dioxide, and controlling other pollutants, possibly by combining late intake valve close with other engine control strategies. To ensure compliance with European Union emissions standards, future research will investigate the combined supply of gasoline, natural gas, and hydrogen within internal combustion engines. Conceptualization, T.V. and A.R.