## 1 Introduction
The use of RME affected the dynamic response of the needle, especially at low injection pressures, becoming less important at high injection pressure, compared to diesel fuel. The higher viscosity of biodiesel slowed down the opening and closing of the injector needle, the flow regime, and the discharge coefficient of the nozzle's orifices. [PERSON] et al. [18] continued experiments with a one-dimensional model of a solenoid injector using the same fuels. They showed that the needle velocity during the opening is significantly lower for the biodiesel fuel; therefore, more time is needed to reach its maximum lift compared to diesel fuel. The needle lifts and the mass flow rates were also lowered for the RME, especially during the opening slope of the curve. This fact was attributed to the lower Reynold's number due to the higher viscosity of RME, which increases the friction force, affecting the needle dynamics. The dynamic delay was higher, and the start of combustion occurred about \(160~{}\upmu\)s later for biodiesel at \(40\) MPa but it decreased to \(15\) or \(20~{}\upmu\)s for higher injection pressures of \(80\)-\(180\) MPa under single-shot strategies. [PERSON] et al. [19] studied the spray characteristics of hydrotreated vegetable oil (HVO), and three vegetable oil methyl esters (FAME) using a direct photography technique to compare with those of reference diesel fuel. They found that high-density fuels such as FAMEs have longer spray penetrations than mineral diesel, whereas HVO with the lowest density achieves the shortest penetration distance (by \(5\%\)) and slightly higher cone angle. [PERSON] et al. [20] studied the effects of diesel fuel D100 and biodiesel-diesel blends BD25, BD45 and BD65 on the spray tip penetration (STP) and cone angle of the sprays injected by a CR system at the injection pressures of \(40\)-\(100\) MPa into the atmospheric chamber. They depicted that the STP grows more than two times quicker before break-up than after break-up time. Even though the initial STP of D100 is always longer, the higher viscosity and momentum of biodiesel assure a slightly longer STP length than that of diesel with a little narrower spray angle (BD65) at \(2.5~{}\upmu\)s after the start of injection. [PERSON] et al. [21] studied the impact of biodiesel properties on the injection characteristics of the common-rail system and discovered that fuel density is the main property that affects the mass amount of injected fuel and pressure wave in the CR system. They showed that pressure wave amplitude decreases with increasing fuel viscosity, bulk modulus, or density. It is interesting to notice that [PERSON] et al. [22], studying the influence of density and viscosity of nine fuel types (light and dense) on the pressure wave characteristics produced by a high-pressure CR diesel injector with three conical convergent oifices, declared similar findings. They showed that with the increase in viscosity from \(0.6\) to \(7.0~{}\mathrm{mm^{2}/s}\) the discharge coefficient decreases by \(10\%\) at the low injection pressure of \(25\) MPa but it does not have a significant effect on the discharge coefficient (less than \(1\%\)) at higher pressure differences of \(85\)-\(175\) MPa. Moreover, fuel density varying from \(683\) to \(876~{}\mathrm{kg/m^{3}}\) is identified as the only property driving the mass flow rate, the theoretical evolution of which versus the square root of fuel density represents the discontinuous line. The engine tests [23] showed that the higher flow pressure of viscous RO can damage microporous radial vee-shaped filtering cartridges. The flow capacity of the filter system can be enhanced by an arrangement of two or three identical filter units in parallel. A larger active surface area together with on-board heating of RO up to \(60~{}^{\circ}\)C would be the solution to reduce viscosity and differential pressure across the filters and thus achieve the needed oil flow capacity. Additionally, variation in the viscosity of the liquid often changes its surface tension as well [24].