## 3 Results and Discussion
### Combustion Indicators  
Combustion analysis was performed using the AVL BOOST software utility tool BURN. For the analysis, the in-cylinder pressure (Figure 8), fuel, air consumption and other parameters were used. The CO\({}_{2}\) concentration and the ST influence on the combustion parameters were visible through the change of the in-cylinder pressure. When the engine ST was constant (26 CAD BTDC), the in-cylinder pressure dropped by 9.6%, 30.3% and 40.7% using BG20, BG40 and BG50, respectively. Biogas with a higher concentration of CO\({}_{2}\) has a higher combustion delay and lower combustion speed [54]; therefore, the peak of the in-cylinder pressure shifted further from TDC (3, 3 and 8 CAD) and the maximum in-cylinder pressure occurred at a higher volume. Accessing the biogas combustion delay, a low combustion speed and selecting optimal ST increased the in-cylinder pressure by 9.9%, 20.9% and 27.4%, respectively, as compared with the constant ST of 26 CAD BTDC. The peak value of the in-cylinder pressure occurs in a range from 12-14 CAD ATDC when ST was optimized. A higher concentration of CO\({}_{2}\) (BG40 and BG50) reduced the in-cylinder pressure by 9.4% (ST 35 CAD BTDC) and 13.3% (ST 40 CAD BTDC) compared to BG0, because of the prolonged combustion process and the decreased lower heat value of the fuels [55]. Figure 8: Change of the in-cylinder pressure depending on the volumetric fraction of CO\({}_{2}\). (**A**) Constant ST, (**B**) Optimal ST.  
Figure 7: Change of the carbon monoxide and HC concentration in exhaust gas depending on the volumetric fraction of CO\({}_{2}\). From the combustion parameters (Table 5), it is visible that CO\({}_{2}\) increased the combustion delay. When comparing BG0 with BG20, BG40 and BG50, the start of the combustion delays is, on average, 8.2%, 9.7% and 18.6%, respectively. The CO\({}_{2}\) effect is also visible in the change of the combustion duration. The time of 10% mass burned fraction (MBF) increased by 0.54%, 11.4% and 22.3% when comparing BG0 to BG20, BG40 and BG50, respectively, with a constant ST of 26 CAD BTDC. In this stage, the optimal ST prolonged the CD, and 10% MBF time extends by 9.2% and 3.3% using BG40 and BG50, respectively. The increase of the CD is visible, because BG started to burn at a higher volume, and the distance from air and fuel molecules are bigger; therefore, combustion speed decreased. In case of 90% MBF, combustion duration increased by 0.44, 31.3 and 32.2 when comparing BG0 with BG20, BG40 and BG50 at a constant ST of 26 CAD BTDC. Optimized ST reduced the combustion duration by 8.4% and 10.6% (BG40 and BG50), because the main fraction of combustion occurred closer to TDC and fuel burned at a lower volume. In the case of spark timing, 26 CAD BTDC burned a larger amount of fuel in a large volume (piston moving away from the TDC), which resulted in an increase in the exhaust gas temperature. The change of the in-cylinder temperature rise (TR) can be seen in Figure 9. The maximum increase is tied to BG0. Increasing the CO\({}_{2}\) concentration of BG without an optimal ST shifted the peak of the in-cylinder temperature rise further from TDC (BG20-4 CAD, BG40-5 CAD and BG50-14 CAD) in comparison to BG0. This led to a higher exhaust gas temperature and increased the exhaust energy losses. Optimal ST reduced the peak temperature rise drift up to \(\pm\) 3 CAD and TR increased by 10.0%, 37.0% and 37.8% compared to BG20, BG40 and BG50, respectively, at a constant ST. The change in ST reduced the afterburning phase by 20.0%, 32.4% and 40.0% for BG20, BG40 and BG50, respectively, compared to a constant ST for 26 CAD BTDC. Therefore, exhaust losses decreased (Figure 4).