## 3 Results
### Analysis of the Combustion Process
Earlier IVC timings, while not exhibiting the highest ROHR peaks, lead to a quicker rise in the MFB curve, suggesting a faster initial burn rate. In contrast, maximally delayed IVC timings, which often show higher ROHR peaks, also demonstrate a rapid rise in MFB, indicating efficient overall combustion. Comparing fuels, gasoline tends to show a slightly faster rise in the MFB curves compared to natural gas, suggesting a faster burn rate. Increased engine load and speed also result in a quicker rise in the MFB curves, reflecting the more rapid and complete combustion under these conditions. Evaluating ignition delay (ID), it is noticeable that in all test conditions, the ignition delay (ID) is shorter when using natural gas, which at lower engine speeds (\(n\) = 2000 rpm) and at different loads lasts from 1.3% to 7.3% less (Table 5). This difference may be due to the different physical and chemical properties of gasoline and natural gas, as well as the peculiarities of the combustion process. Natural gas, being gaseous, can distribute earlier and more evenly in the cylinder than liquid gasoline, which advances the start of combustion. In addition, natural gas generally burns later than gasoline, but this later combustion can be compensated for by increased turbulence in the cylinder. Figure 5: The coefficient of variation of indicated mean effective pressure (COV\({}_{\text{IMEP}}\)) at 3000 rpm for 100 engine operating cycles. When the engine is operating at higher speeds, at the lowest load, and with the earliest intake valve closing (IVC = 24\({}^{\circ}\)), there is an approximately 33% reduction in ID when comparing natural gas to gasoline. This large difference can be explained by several factors. At 3000 rpm, the turbulence of the air-fuel mixture increases, and natural gas, distributing earlier in the cylinder, can burn earlier. At low loads, when the amount of fuel is small, even mixing is especially important, and the earlier distribution of natural gas can advance combustion. Figure 6: The ROHR and MFB while adjusting the IVC timing at 2000 rpm and 3000 rpm. \begin{table}
\begin{tabular}{l l c c c c c c c} \hline \hline \multirow{2}{*}{**Test Conditions**} & \multirow{2}{*}{**Combustion Phase**} & \multicolumn{2}{c}{**IVC\({}_{\text{r}}\)**} & \multicolumn{2}{c}{**IVC\({}_{\text{r}}\)**} & \multicolumn{2}{c}{**IVC\({}_{\text{r}}\)**} & \multicolumn{2}{c}{**IVC\({}_{\text{r}}\)**} \\  & & **24\({}^{\circ}\) aBDC** & **38\({}^{\circ}\) aBDC** & **52\({}^{\circ}\) aBDC** & **59\({}^{\circ}\) aBDC** \\ \cline{3-8} \cline{7-9}  & & **G** & **NG** & **G** & **NG** & **G** & **NG** & **G** & **NG** \\ \hline \multirow{3}{*}{\(n\) = 2000 rpm \(n\) = 0.31 MPa} & ID & 14.3 & 13.6 & 13.7 & 13.2 & 12.8 & 12.2 & 10.8 & 10.1 \\ \cline{2-8}  & MFB50 & 17.3 & 18.5 & 13.4 & 14.2 & 10.1 & 10.7 & 7.6 & 8.3 \\ \cline{2-8}  & MFB90 & 32.9 & 38.0 & 31.2 & 34.7 & 28.0 & 29.3 & 21.1 & 21.4 \\ \hline \multirow{3}{*}{\(n\) = 2000 rpm \(n\) = 0.79 MPa} & ID & 9.6 & 8.9 & 8.7 & 8.4 & 8.1 & 7.8 & 7.4 & 7.3 \\ \cline{2-8}  & MFB50 & 8.9 & 10.3 & 7.5 & 8.8 & 5.9 & 7.1 & 4.3 & 5.4 \\ \cline{2-8}  & MFB90 & 19.0 & 23.3 & 18.2 & 22.1 & 16.9 & 20.2 & 14.2 & 17.0 \\ \hline \multirow{3}{*}{\(n\) = 3000 rpm \(n\) = 0.31 MPa} & ID & 11.7 & 7.8 & 11.4 & 10.3 & 11.2 & 10.7 & 11.0 & 10.9 \\ \cline{2-8}  & MFB50 & 5.7 & 5.3 & 5.7 & 5.6 & 5.8 & 5.6 & 5.7 & 5.5 \\ \cline{2-8}  & MFB90 & 19.4 & 18.8 & 20.0 & 19.6 & 20.7 & 20.5 & 21.7 & 21.6 \\ \hline \multirow{3}{*}{\(n\) = 3000 rpm \(n\) = 0.79 MPa} & ID & 7.8 & 6.7 & 7.4 & 6.5 & 6.5 & 6.2 & 6.8 & 5.8 \\ \cline{2-8}  & MFB50 & 5.0 & 6.8 & 4.3 & 6.2 & 3.7 & 5.3 & 2.8 & 4.8 \\ \cline{1-1} \cline{2-8}  & MFB90 & 17.4 & 19.6 & 16.5 & 19.0 & 15.6 & 18.2 & 14.7 & 17.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The ignition delay (ID), 50% and 90% of the mass fraction burned (MFB50, MFB90). Also, ECU may optimize combustion differently for gasoline and natural gas under these conditions.