## 3 Results
### Analysis of the Combustion Process  
In the combustion process analysis, IVC timings were chosen at 24\({}^{\circ}\), 38\({}^{\circ}\), 52\({}^{\circ}\), and 59\({}^{\circ}\) aBDC and two engine loads (BMEP = 0.31 MPa, BMEP = 0.79 MPa) using both liquid and gaseous fuels. The pressure in the cylinder was evaluated during the experimental tests using a piezoelectric sensor installed in the spark plug, and the resulting values were used as one of the input data in the AVL BOOST(tm) software to calculate ROHR, MFB, temperature in the cylinder, and temperature rise. The in-cylinder pressure variation trends using different fuels are presented (Figure 4). It is noteworthy that the pressure at the end of the compression stroke before the combustion starts is higher when the intake valve is closed earlier (at 24\({}^{\circ}\) IVC) at different engine operating modes. The effective compression ratio approaches the geometric compression ratio, and this means the charge is compressed to a greater extent, leading to increased pressure and temperature upon the completion of compression. The pressure at the end of compression increases additionally, as the gaseous fuel displaces a portion of the in-cylinder volume when using NG. The air displaced by the gas is compensated for by opening the throttle more. When ensuring a stoichiometric mixture, natural gas requires even more air due to the higher H/C ratio in the gas (Table 3), which in turn determines a higher compression pressure. Figure 4: The in–cylinder pressure while adjusting the IVC timing at 2000 rpm and 3000 rpm. When IVC is being delayed, the pressure maximum in the cylinder increases and approaches TDC, although the effective compression ratio decreases. The highest pressure peak in the power stroke is seen when IVC is maximally delayed (to 59\({}^{\circ}\) IVC) when using gasoline, while the lowest pressure noticed for NG. An exceptional pressure trace behavior observed at 3000 rpm and 0.31 MPa engine load likely results from a complex interplay of factors. The combination of the high engine speed with the low manifold absolute pressure resulting from significant throttling required for this low load creates conditions where key engine phenomena become highly coupled and exceptionally sensitive. Consequently, the influence of intake and exhaust wave dynamics, combustion stability, and in-cylinder flow patterns becomes particularly reactive to variations in IVC timing and fuel type, leading to the observed deviations from broader trends. The coefficient of variation of indicated mean effective pressure (COVIMEP) under the considered load (BMEP = 0.31 MPa) is lower when the engine is operating on natural gas (COVIMEP = 0.0215), and when the load increases (BMEP = 0.79 MPa), the COVIMEP reaches a lower value when using gasoline (COVIMEP = 0.0143) (Figure 5). This indicates that NG ignites and burns more stably at lower loads, pressures, and temperatures, and gasoline burns more stably as the engine load increases. With the delay of IVC, the combustion stability decreases, likely due to the less stable turbulence of the compressed combustible mixture at a lower actual compression ratio, and in the case of low loads, when IVC = 59\({}^{\circ}\) aBDC, the COVIMEP increases most significantly (up to 0.035) when the engine is operating on gasoline. Other researchers noticed that engines running on NG tend to have higher variations in in-cylinder pressure compared to gasoline, especially at low loads and high engine speeds, which they attributed to increased turbulence and unstable combustion [48; 49]. Generally, maximally delayed IVC timing (to 59\({}^{\circ}\)) tends to result in a more concentrated and higher ROHR peak, suggesting faster and more intense combustion (Figure 6). However, this trend can vary depending on engine speed and load. Earlier IVC timing (24\({}^{\circ}\)) often leads to a lower ROHR peak, indicating that while a lower effective compression ratio is achieved, the combustion intensity can be influenced by in-cylinder flow and A/F mixing.