## 3 Results
The same tendency of late start of the ignition when the ST is set to 24 CAD BTDC is visible in the MBF data (Table 5). It took 3 CAD longer to burn 10% of the fuel, which reduced the pressure in the cylinders and reduced the NOx concentration. It is clear from the exhaust emission data that increasing the hydrogen concentration in the fuel mixture alters the hydrogen-carbon molecule ratio in the fuel mixture. Figure 5 shows the reduction in unburn fuel emissions when adding hydrogen. In the case of CNG ST, emissions of unburned hydrocarbon (HC) decreased by 3.60, 8.99, 21.08, and 40.36% when adding 14, 24, 33, and 43 vol% hydrogen. When adjusting ST, the HC reduction was 11.52, 15.16, 21.84, and 43.53% respectively. HC reduction was affected by the decreased amount of carbon molecules in the fuel mixture and the increased combustion and mixing of fuel and air [63]. ST influenced the increase in HC emissions when the engine ignition timing was advanced. Without hydrogen and the advanced ST by 8 CAD BTDC, the HC emissions increased by 12%. The increase in HC emissions is noticeable in all measured point. The advanced ST reduces the exhaust temperatures, which consequently, reduces the speed of the oxidation reaction of the unburned hydrocarbons. Figure 4: Change in exhaust gas temperature (Teah) and specific emission of nitrogen oxides (NOx) depending on the volumetric fraction of hydrogen and spark timing. A similar tendency is noticeable for CO\({}_{2}\) emissions. Increasing the hydrogen concentration in fuel causes a reduction in CO\({}_{2}\) emissions. Without ST adjustment, CO\({}_{2}\) emissions decreased by 3.81, 6.46, 7.05, and 8.36% compared to emissions when the engine is running on biogas. In the case of optimal ST for the mixture, CO\({}_{2}\) emissions decreased by 5.45, 5.83, 6.82, and 6.96% respectively. Increased concentration of hydrogen caused a reduction in fuel consumption and carbon molecules in fuel mixture, resulting in reduced CO\({}_{2}\) emissions [34]. With hydrogen concentrations of 24, 33 and 43 vol%, adjusting ST led to an increase in CO\({}_{2}\) emissions as compared with the case when ST was set to be 28 CAD BTDC. This increase was influenced by increased fuel consumption and the higher amount of carbon molecules filling the combustion chamber. The main combustion parameters such as rate of heat release (ROHR) and temperature rise were analyzed with utility BURN in AVL BOOST software. The parameters used for analysis include measured in-cylinder pressure (Figure 6), fuel consumption, air consumption, and engine data. The engine running on biogas with the addition of hydrogen and at constant ST of 28 CAD BTDC (Figure 6B) displayed increases of 14.3, 22.9, 31.1 and 38.2% in the cylinder pressure. Maximum pressure was obtained 1, 3, and 4 CAD earlier, while hydrogen volumetric concentration was 24, 33, and 43%. Hydrogen increased the burn speed of the fuel due to the better fuel mixing and higher flame speed of the added hydrogen [64; 65]. Adjusting ST enabled an increase in cylinder pressure  
Figure 5: Change in the specific emissions of unburned hydrocarbons (HC) and carbon dioxide (CO\({}_{2}\)) depending on the volumetric fraction of hydrogen and spark timing.