**4. Conclusions**

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The present study aims to make the process of replacing fossil diesel feasible, without making mechanical changes in the engines of the current car fleet. In this way, not only the pollution produced by petroleum is minimized, but also the dependence of the countries that produce the fuels is reduced. Given the relatively high economic cost of any additional chemical transformation of the vegetable oils, either in biodiesel or in other biofuels of appropriate viscosity values, a novel strategy employing green fuels derived from vegetable oils has been investigated. In this respect, to ge<sup>t</sup> high levels of fossil fuel substitution in a technically and economically feasible way, a renewable compound like DEE, which can be obtained from biomass, has been studied as an additive in triple blends with diesel and two differents SVOs on the engine performance and exhaust emissions. DEE has contributed to a reduction of viscosity values in SVOs through its very low viscosity, complying with European regulations to operate as biofuel in current diesel engines. Additionally, all blends comply with EN 590 specifications concerning the flash point.

According to the results obtained, the tests were carried out successfully up to 18% blending of DEE and 22% of SVO with diesel, this means that substitution of 40% of fossil diesel can be achieved by the use of diesel/DEE/SVO triple blends, using either sunflower or castor oil, performing similarly to diesel in terms of power. Furthermore, several advantages of using these blends were obtained. On one hand, DEE addition leads to an improvement in the combustion, since an important reduction of pollutant emissions occurred (up to 77% lower than fossil diesel), which is mainly due to the oxygen content of DEE. Likewise, an enhancement in the cold flow characteristics of the blend in respect to fossil diesel was also attained, which means the diesel engines can run at lower temperatures. On the other hand, the increment of the flash point in blends with respect to fossil diesel, makes these biofuels safer for handling, storage, and transportation. All the abovementioned shows the competitive value of diethyl ether as an oxygenated additive in comparison to other natural products studied of greater economic cost. Hence, the addition of this oxygenated additive up to 18% (by volume) is a promising way for using diesel/vegetable oil blends efficiently in diesel engines without any modifications in the engine. It can be concluded that this research proffers a practical and economically viable alternative to the chemical production of biofuels.

**Author Contributions:** This research article is part of the doctoral thesis of L.A.-D., directed by R.E. and F.M.B., who in general way conceived and designed the experiments. D.L., C.L., J.H.-C., A.P., and A.A.R., made substantive intellectual contributions to this study. L.A.-D. performed the experiments and wrote the paper. R.E., D.L. and F.M.B. supervised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors are grateful for the funding received from the Spanish MINECO through the project ENE2016−81013−R (AEI / FEDER, UE), the Junta de Andalucía and FEDER (P18−RT−4822).

**Conflicts of Interest:** The authors declare no conflict of interest.
