## 3 Results  
During all experiments, the engine was operated at 1500 rpm, _BMEP_ 0.42 MPa, and a stoichiometric mixture of air and fuel was determined. The obtained results were compared with the biogas results. The addition of hydrogen, at first, reduced brake-specific energy consumption (_BSEC_) in both ST situations (Figure 3). Adding 14 vol% hydrogen increased the mixture's _LHV_ by 9.8% and increased the combustion speed (the duration of the 50% mass burn fraction (_MBF_) was reduced by 21.7% when ST was set to 28 crank angle degrees before top-dead-center (CAD BTDC) and by 45.9% when ST was 36 CAD BTDC). Because of the slight increase in the _LHV_ of the mixture and the enhanced combustion, the _BSEC_ decreased by 2% with spark timing optimal for CNG (CNG ST) and by 0.7% with spark timing optimal for the mixture (mixture ST). A further increase in the hydrogen concentration caused the _BSEC_ to increase by 0.1, 3.0, and 4.1%, when setting ST to 28 CAD BTDC and by 2.3, 4.1, 6.7 with optimal mixture ST conditions. A further increase in hydrogen concentration caused an increase in the _LHV_ of the mixture and the amount of energy one engine cycle. Furthermore, the low density of hydrogen deteriorated engine volumetric efficiency; thus, fuel energy was used less efficiently, and this translates into an increase in BSEC. The engine running on biogas with adjusted ST increased the brake thermal efficiency by 4.3%, because biogas has a lower burning speed than CNG and needs advanced ignition timing. The addition of hydrogen increased the burning speed of the mixture, and only 14 vol% H\({}_{2}\) enhanced _BTE_ by 2.6% at CNG ST and by 0.6% at the mixture ST. A further increase in hydrogen reduced brake thermal efficiency because of the decreasing burning process stability and volumetric efficiency. The increase in 24 vol% of H\({}_{2}\) reduced _BTE_ by 0.7% at CNG ST and by 1.9% at the mixture ST; 33 vol% of H\({}_{2}\) reduced _BTE_ by 3.3% at CNG ST and 4.0% at the mixture ST, and 43 vol% of hydrogen reduced _BTE_ by 5.0% at CNG ST and by 6.4% at the mixture ST. The environmental parameters in Figure 4 show that the increase in hydrogen concentration leads to increased emissions of nitrogen oxides (NO\({}_{\text{x}}\)), because hydrogen improves the quality of the combustion process, as confirmed by other researchers [61]. Without ST adjustment, NO\({}_{\text{x}}\) emissions rose by 79.3, 113.8, 135.3 and 153.9% with each increase  
Figure 3: Change in the brake-specific energy consumption (BSEC) and brake thermal efficiency (BTE) depending on the volumetric fraction of hydrogen and spark timing. in hydrogen concentration. This is because of the increase in cylinder pressure and the temperature of the burning fuel mixture with higher _LHV_[62]. Figure 4 also confirms the increase in the in-cylinder temperature, because the addition of hydrogen increases the burn rate of fuel and less burning fuel passes through the exhaust valve. The exhaust gas temperature dropped by 5.9% when engine was running on the 57 BG43H2 (57% of biogas and 43 % of hydrogen) fuel mixture with an advanced ST from 24 to 28 CAD BTDC due to the longer time for combustion and the lower amount of burning fuel escaping through the exhaust valve. When adjusting ST for the mixture, the NOx emissions rose by 77.0, 88.3, 93.5 and 95.1% with each increase in hydrogen concentration. Comparing the two different types of ST adjustment, it becomes clear that the advanced ST causes an increase in NOx formation and a reduction in exhaust gas temperature. In all H2 concentrations, NOx formation is greater with the optimal ST for the mixture, excluding the last measured point of 43 vol% hydrogen. This time, ST was set for 24 CAD BTDC in the case of optimal ST and 28 CAD BTDC in the case of CNG ST. This led to an increase in NOx emissions in the case of the advanced ST, and it shows that 24 CAD BTDC is too late for optimal ignition of this mixture.