# A Comprehensive Review of the Application Characteristics of Biodiesel Blends in Diesel Engines
## 6 Noise and Vibration  
There are three main sources of engine noise: combustion noise, mechanical noise and aerodynamic noise. Combustion noise is caused by the cyclically sharp rise in pressure in the combustion chamber. Mechanical noise comes from the periodic mechanical movement of engine components. Aerodynamic noise generally comes from the intake and exhaust process and fan rotation [131]. Combustion characteristics such as in-cylinder pressure and heat release rate have a great influence on the generation of noise. Most researchers found that noise coming from the engine fueled by biodiesel is lower. [PERSON] et al. pointed out that both the combustion noise and engine noise of fuel with soybean biodiesel is lower than conventional diesel under almost all loads [132]. As shown in Figure 16, the engine external noise is always higher than the combustion noise, and the external noise of biodiesel blends is about 1-3 dB lower than that of diesel. [PERSON] et al. found that engine noise increases with speed, but combustion produces lower noise for most biodiesel blends [14; 133]. The engine noise level is often related to the cylinder pressure variation and the maximum heat release rate. The fuels that produced larger HRRmax generally showed a higher noise level. This may be because the higher cetane number of biodiesel shorten the ignition delay time, resulting in a lower pressure increase rate and HRRmax, which produces a lower noise level [132; 134; 135]. Meanwhile, the properties of biodiesel such as viscosity and volatility may lead to poor atomization and the appearance of large liquid beads. This is not conducive to the combustion of fuel in the premixed state, but to the reduction of the maximum in-cylinder pressure [16]. In addition, the higher bulk modulus of biodiesel will extend the injection timing and increase the noise radiation, but the impact is limited [134]. [PERSON] et al. also found that biodiesel blends improved noise levels while they also improved acoustic roughness [135]. Engine vibration is caused by the reciprocating and rotating mass of the engine, and the operation of other accessories, which can affect the smoothness and stability of the engine. Vibration in the vertical direction (z-axis) is particularly obvious among the three orthogonal directions due to the vertical movement of the piston caused by the pressure variation generated when the fuel burns. In addition, torsional vibration of the crankshaft, vibration of the valve train, and vibration of the oil pump and fan are also important components of engine vibration. [PERSON] et al. tested waste cooking oil biodiesel and conventional diesel at 1200, 1500, 1800, 2100, 2400 rpm and found that the vibration performance of biodiesel is better than conventional diesel at all speeds, as illustrated in Figure 17[136]. Studies [7; 14; 132; 137] also reported the great damping function of biodiesel. However, some researchers observed that the vibration damping effect of biodiesel is not absolute [138; 139]. [PERSON] et al. tested nine biodiesel-diesel blends and found that in most cases, the fuel with the lowest vibration is B20 or B40, while the largest appeared in B15, B30, and B50[138]. Vibration performance depends on the net effect of characteristics such as oxygen content, viscosity and calorific value of the fuel mixture. The lower heating value of biodiesel and the  
Figure 15: Polycyclic aromatic hydrocarbon (PAH) emissions in urban mode (**a**) and extra-urban mode (**b**) for rapeseed methyl esters fuels (adapted with permission from [13], Elsevier, 2010). undesirable atomization caused by the larger viscosity will reduce the power of the engine and alleviate vibration [138]. Meanwhile, the higher oxygen content of biodiesel also brings better combustion quality [14; 133]. The larger cetane number of biodiesel shortens the ignition delay time, resulting in a shorter mixing-controlled combustion phase, making the pressure peak point closer to the top dead center, which will help reduce vibration [136].