## 5 Conclusions  
This review paper aimed to provide a comprehensive overview and sound summary of all the work undertaken recently, addressing the variety of methods and techniques developed for mitigating or reducing vibration, noise, and emissions (e.g., CO, CO\({}_{2}\), NO\({}_{\rm x}\)), mainly in diesel and direct injection engines. The conventional statistical tools and mathematical approaches that are commonly used in vibration diagnosis of the ICEs, such as root mean square (RMS), fast Fourier transform (FFT), short-time Fourier transform (STFT), spectral kurtosis (SK), are reviewed thoroughly. In particular, the paper elaborated on the efficient modifications that can be applied to the engine or potential use of innovative biofuels that can be used to substitute diesel in ICEs. The engine modifications were applied through synthesizing novel mechanisms and changing the design or geometry of crucial engine components. On the other hand, adding a wide range of additives to the conventional diesel fuel, or altering the properties of current biodiesel-diesel fuel blends, such as cetane number (CN), lower heating value (LHV), density, viscosity, etc., enables commercializing more reliable and environmentally friendly fuels with lower noise and chemical pollutions. The fuel additives discussed in this article were vegetable oils (sunflower, canola, corn), alcohols (methanol, ethanol), hydrogen gas, zinc oxide (ZnO) nanoparticles, and ethylene glycol monomethyl ether. The most important conclusions reached from the literature review are:  
* Introducing new balancing algorithms accompanied by time- and frequency-domain analyses enables reduction of the torsional vibration in rotary components of ICEs operating under high dynamic loading conditions. * By proper use of engine mounting system (EMS), when a high transmission ratio is selected, the vibration and noise can be remarkably isolated before they are transferred to chassis and body of a vehicle. * Accurate prediction of time and frequencies of the piston slap events during the engine cycle can be achieved by taking into account the impact of elastohydrodynamic lubrication (EHL) and piston transient dynamics. * The torsional vibration of ICEs can be significantly mitigated by optimizing the geometrical parameters of the engine's crank-slider mechanism and designing a novel kinematically driven flywheel (KDF). * To minimize the transmitting vibrations, the stiffness of the engine block can be enhanced by applying structural modifications such as thickening the crankcase walls, front gear cover, back flywheel cover, as well as adding ribs to its driveshaft. * The resistive torque fluctuations of an engine camshaft can be passively compensated by carefully designing a balancing cam mechanism. * RMS, kurtosis, STFT, FFT, and Morlet wavelet are among the most common statistical tools that are used for vibration and noise analyses in ICEs. * A biodiesel fuel (or biofuel) offers better physicochemical properties (i.e., density, viscosity, distillation, bulk modulus, surface tension, etc. ), to control the injection process. * Canola and corn vegetable oil fuel additives can avoid an abrupt combustion process due to increasing the oxygen content in the biodiesel fuels. * When hydrogen gas is added to the fuel blend, it improves the combustion quality, ignition delay, and variation in peak pressure rise rate, resulting in a reduction of the noise and total vibration of the engine. * The vibration performance of ICEs can be significantly enhanced by adding ZnO nanoparticles to biofuels, as they have a higher heat release rate (HRR), metal vaporization and oxidation, catalytic behavior, and combustion rate. * Finally, fatty acid methyl ether (FAME) biofuels containing ether group alcohols can reduce the ignition delay time and NO\({}_{\rm x}\) pollutants, leading to better biodiesel fuel combustion. **Author Contributions:** All authors contributed equally. **Funding:** This research received no external funding.