## 4 Biodiesel Fuels
While the vibration acceleration decreased up to 2.79% (CaB40) with increasing the volume ratio of the biodiesel fuel blends, adding the hydrogen gases resulted in a  
\[a_{t}=\sqrt{a_{x}^{2}+a_{y}^{2}+a_{z}^{2}}. \tag{15}\]3 Total engine vibration acceleration (a\({}_{\text{total}}\)) vs. engine speed when the engine was fueled with low sulfur diesel and (**a**) sunflower oil; (**b**) canola oil; (**c**) corn oil biodiesel fuel blends [71]. Figures are reprinted with permission from publisher. further reduction in the vibration acceleration, up to 3.7% (SB20 + H6). These reductions were achieved mainly due to the combustion quality enhancement, ignition delay, and variation in peak pressure rise rate, after adding the hydrogen gases to the test fuels [75]. They also reported that although a higher engine speed generated more noise, substituting the conventional diesel fuel (D100) with the biodiesel fuels lowered the sound pressure level of the engine, up to 0.7 dB(A), when the hydrogen gas was mixed with the intake air. The reduction amount varied, as it is strongly dependent on combustion characteristics of each blend, such as ignition delay, peak pressure level, and pressure gradient inside the cylinders [76; 77; 78]. Compared to the diesel fuel (D100), the extra amount of oxygen in the biofuels enables a complete combustion, leading to a reduction in the CO emissions [75]. However, the improvement in the combustion process provides more available carbon atoms for formation of the carbon dioxide (CO\({}_{2}\)), causing an excessive increment in its emission value. This unfavorable product was fairly controlled by adding both H3 and H6 gases to the test fuels [79; 80; 81]. The other downsides with the extra oxygen content in the biodiesel in the presence of the hydrogen gas are the temperature rise during the combustion process and increase in the flame speed propagation [82]. These two factors accelerate the formation of NO\({}_{\mathrm{x}}\), the most detrimental pollutant at the combustion stage [83]. However, those researchers observed a reduction in the NO\({}_{\mathrm{x}}\) emissions upon addition of H3 and H6, since the atmospheric air was replaced with hydrogen gases, therefore, less nitrogen molecules flowed through the intake port (Figure 21). \[a_{t}=\sqrt{a_{x}^{2}+a_{y}^{2}+a_{z}^{2}}. \tag{15}\]4 The emission values of NO\({}_{\mathrm{x}}\): (**a**) without hydrogen; (**b**) with H3; and (**c**) with H6 addition [72]. The emission values increased at higher speeds in all test conditions. Figures are reprinted with permission from publisher. [PERSON] et al. [84] conducted a comparative study on an indirect diesel injection (IDI) engine fueled with the conventional pure diesel and mahua methyl ester (MME) oil biodiesel, blended with different alcohol-based additives. The engine contained dual combustion chambers, encouraging a lower exhaust emission and a clean combustion process. The objective of this study was to realize the efficacy of the biodiesel blends as alternatives to the conventional diesel fuel, and also investigate their impacts on the combustion pressure, heat release rate (HRR), and cylinder vibration of the four-stroke single-cylinder diesel engine. Five additives of methanol in MME were prepared with blending ratios of 1/99%, 2/98%, 3/97%, 4/96%, and 5/95%. The fuel blends were tested under five loading conditions: no load, 0.77, 1.54, 2.31, and 2.70 kW. To analyze the vibration of the cylinder head, FFT and time waveforms were measured and recorded at each load. It was shown that proper proportions of methanol additives could reduce the heterogeneity and rapidity of the combustion process, shifting a high frequency vibration to lower frequencies with higher amplitudes, which can be successfully damped before transmitting to the base. [PERSON] and [PERSON] [85] performed a time-frequency analysis on a 6-cylinder diesel engine fueled with different bioethanol-diesel blends (by volume), to realize the effect of the ethanol concentration on combustion, knocking, and vibration of the engine.