## 1 Introduction
As a result, the lowest quality of biogas (60% carbon dioxide) releases about 30% methane heat compared to 80% pure methane [44]. Oxygenated additives (papaya seed oil biodiesel blended with diglyme, butanol, and stone fruit biodiesel) can be a viable way to effectively use biodiesel blends in a diesel engine [45]. The engine's best features were stone biodiesel and papaya seed oil [46]. However, further research into tribological efficiency analysis is needed to make this ternary blend a future alternative energy source on a commercial scale [45]. This type of biodiesel (papaya-Carica papaya) complied with both ASTM ( American Society for Testing and Materials) and EN (European Union) biofuel standards [47]. However, before recommending commercially available papaya seed biodiesel as an alternative source of energy, additional research is needed on engine emissions, cylinder pressure, burning rate data, combustion analysis, and tribological performance analysis [48]. Similar trends have also been observed in the analysis of the use of biogas and diesel blends in internal combustion engines. Biogas and natural gas are predominantly made up of methane with its higher content in natural gas [5]. A homogeneous mixture of biogas and air significantly increases the pressure in the engine cylinders [49]. [PERSON] et al. [50] noticed that with the engine operating at low loads, biogas provided up to 90% of energy required for engine's operation. When increasing the compression ratio from 16.5 to 19.5, a biogas and diesel-fuelled engine ran smoother and emitted less exhaust gas. At the same time, adjusting the EGR valve reduced engine efficiency, which manifested at high engine loads in particular [51], because the increasing EGR flow slows down the combustion phase, prolonging the ignition time [52]. [PERSON] and [PERSON] [53] observed that when increasing the biogas content from 24% to 68%, the ignition delay decreased and the speed of combustion increased, thus, in order to stabilize the two indicators, advancing the start of diesel injection by three crank angle degrees (CADs) was proposed. The operation of dual fuel at the injection timing of 26 CAD BTDC (Before Top Dead Center) rendered a better overall result than other injection times [54]. Biogas was observed to be able to significantly reduce diesel consumption, thus, the diesel replacement ratio ranged from 15% to 88% in the present compression ignition engines. On the other hand, thermal engine efficiency is significantly affected by methane concentration and the flow of biogas, while the maximum thermal efficiency is achieved at the optimal biogas flow [55]. [PERSON] and [PERSON] [56] determined that the maximum threshold for the replacement of diesel with biogas was 92.49% and 97.55% depending on the load of a compression ignition engine. A further increase of the amount of biogas drastically reduced the engine efficiency coefficient. When comparing an engine fuelled on dual fuel (biogas and diesel fuelled compression ignition engine) and on biogas only, [PERSON] at al. [57] observed that the latter engine had a better thermal efficiency and a lower pollution. The use of biogas (i.e., having decreased methane concentration and increased CO\({}_{2}\) concentration compared to natural gas) allows increasing the Brake Mean Effective Pressure (BMEP) from 4 to 5 bars. Moreover, a lower CH\({}_{4}\) (i.e., an increased share of CO\({}_{2}\)) also allowed using delayed SOI for diesel, which led to reduced smoke emissions [58]. Unrefined biogas can also be used in biodiesel-powered diesel engines in dual fuel mode. However, reducing CO and HC (Hydrocarbons) emissions is also needed by using appropriate techniques [59]. Both natural gas and biogas and fuel blends with diesel allowed achieving better environmental indicators. The more homogeneous the blend is, the more the values of particulate matter/smoke drops.