## 4 Biodiesel Fuels  
Biodiesel is an environmentally friendly, renewable fuel which can be used in any compression ignition engine to substitute diesel fuel. Chemically, biodiesel contains a mixture of methyl esters with long-chain fatty acids, and is typically derived from nontoxic biological resources, such as vegetable oils [35; 36; 37; 38; 39], animal fats [40; 41; 42; 43; 44; 45; 46], or even cooking oils [47; 48; 49; 50]. While many recent works analyzed the effects of biodiesel on the injection process, spray formation, and combustion [51; 52; 53; 54; 55; 56], this section will emphasize the potential of different biofuel blends in reducing the noise and chemical pollutions of ICEs. [PERSON] et al. [57] investigated the effect of different diesel-biodiesel blends on performance and vibration characteristic of a 6-cylinder diesel engine running at 1000-2200 rpm. Nine fuel blends were studied: B0D100 (pure petrodiesel), B5D95 (5% biodiesel + 95% diesel), B10D90, B15D85, B20D80, B30D70, B40D60, B50D50, and B100D0 (pure biodiesel). The biodiesel fuels were prepared from vegetable oil, animal fat, and waste oil, according to ASTM D6751-09 standard. The combination of the fuel blends and engine speeds generated several experiments. Time- and frequency-domain vibration analyses were conducted at various engine speeds. The RMS of acceleration for each experiment was calculated:  
\[a_{RMS}=\sqrt{\frac{\sum_{k=1}^{N}a_{k}^{2}}{N}}, \tag{14}\]  
where \(a_{RMS}\) is the root mean square of acceleration signal \(a_{k}\), \(k\) is the acceleration value in the time-domain signal, and \(N\) is the total number of acceleration data (\(N=\) 80,000), obtained for a duration of 1.6 s. For a multiple comparison, differences between the mean values of the experiments were compared, using the balanced 2-factorial analysis of variance (ANOVA) and [PERSON]'s multiple range test (DMRT). Also, the total acceleration (\(a_{t}\)) value was calculated from vertical (\(z\)), lateral (\(y\)), and longitudinal (\(x\)) components of the acceleration signal:  
\[a_{t}=\sqrt{a_{x}^{2}+a_{y}^{2}+a_{z}^{2}}. \tag{15}\]  
The combustion of different fuel blends does not follow a systematic trend. Although an increase in the fraction of biodiesel in the fuel blend is expected to enhance fuel viscosity and reduce the engine vibration due to incomplete combustion (low heating value and power) [52], the noise and engine vibration depend on many other factors, such as lubrication properties, cetane number (CN), flash point, thermal/physical properties, chemical/molecular structure of the fuel blends, oxygen level during combustion, and injection/spraying of the fuel. The cetane number and viscosity increases with biodiesel addition to the fuel blend, while the cylinder peak pressure drops [58; 59]. As illustrated in Figure 18, before the service engine, B40 and B20 biofuel blends had the lowest mean \(a_{t}\) values. These results, together with analysis of the vibration signal, suggested that among the other biodiesel blends,  
Figure 17: FBD of the proposed balancing mechanism [34]. The mechanism was able to cancel the resistive torque generated by the valve mechanism. Figure is reprinted with permission from publisher. B40 and B20 had a minimal effect on engine knocking, due to production of small and smooth gas pressure changes (fluctuations) within the cylinder chamber upon, and during, the combustion process. A diesel engine process includes fuel injection, spraying, mixing, ignition, and combustion. Among these stages, the fuel injection and fuel spray are highly affected by the fuel quality. The fuel spray strongly depends on the injection delay, injection pressure, dynamic injection timing (DIT), injection duration, and injection nozzle configuration [58]. Compared to diesel, biodiesel fuel offers better physicochemical properties (i.e., density, viscosity, distillation, bulk modulus, surface tension, and others), to control the injection process [52; 58].