**1. Introduction**

Nowadays, fossil fuels such as coal, oil, and natural gas represent approximately 80% of the world energy consumption. Furthermore, it is expected that the industrialization, together with the growing population will increase energy demand in the next years, with the consequent negative effects on global climate [1]. Thus, it is mandatory the use of renewable energy sources, such as solar, wind, and biofuels, to avoid damaging the environment [2]. In this scenario, the production of fuels from biomass has become as one of the best options to provide renewable liquid fuels for transportation. On one hand, until today, biodiesel has been the mostly employed biofuel as substitute of fossil diesel. Biodiesel is produced through transesterification of triglycerides from vegetable oils (palm, corn, soybean, sunflower, etc.) with methanol or ethanol, being a sustainable and environmentally friendly alternative. Furthermore, it has excellent properties operating in the current diesel engines, not only

by itself but also in blends with fossil diesel. The main drawback in the biodiesel production is the generation of glycerol as by-product in around 10 wt. % of the total biodiesel, which makes the biodiesel production on an industrial scale economically unfeasible. Hence, the competitive commercialization of biodiesel is still a challenge [3].

On the other hand, the use of SVOs as fuel was already initiated by Rudolph Diesel, when he made the first demonstration of his compression ignition engine at the 1900 Paris Exposition with peanut oil, and their usage continued until the 1920s, when diesel engine was adjusted for running with a fraction of fossil petroleum (currently known as No. 2 diesel fuel) [4]. Nowadays, SVOs are considered a very attractive alternative to fossil fuels because of their renewable nature and their high availability. Moreover, some vegetable oils exhibit physicochemical properties analogous to conventional diesel, excluding their high kinematic viscosity that lead to engine problems (poor fuel atomization and coke formation). That is the reason why the use of vegetable oils as drop-in biofuels requires previous treatments (i.e., transesterification reaction to obtain biodiesel) that reduce their viscosity values.

Recently, it has emerged a new methodology, to the direct usage of SVOs as biofuels in C.I. engines, consisting of blending vegetable oil with a lower viscosity solvent (LVS), instead of undergoing them chemical process. This approach has the advantage of avoiding the energetic and economic costs associated to the transesterification process for the biodiesel production. Thereby, the high viscosity of oils can be reduced to limits imposed by EN 590 standard to operate in the present diesel engines. In this respect, an intensive investigation about di fferent LVSs used as fuels has been reported in literature, including di fferent alcohols (methanol, ethanol, and n-butanol) [5–8] and light vegetable oils (eucalyptus, camphor, orange, and pine oils) [9–12]. Generally, these compounds have been designated as LVLC (low viscous low cetane), since they exhibit low viscosity and energy density values and sometimes, low cetane number, as compared to conventional diesel [13].

Following this strategy, the low viscosity of gasoline has been harnessed to use it as LVS, in mixtures with SVOs from several seeds (canola, sunflower, camelina, and carinata). In this way, SVO/gasoline blends, with a content of gasoline ranging between 10 and 30% in volume, have allowed a higher level of fossil fuel substitution than their counterparts blends using fossil diesel. Aside from adequate density and viscosity values for being successfully employed in a diesel engine, the thermal e fficiency of these blends was close to conventional diesel, although the biofuel consumption was approximately 10% higher than that of diesel fossil [14]. Gasoline/SVO blending methodology has been more recently applied even with castor oil, which has a high viscosity value of 226.2 cSt. Furthermore, this is non-edible oil, avoiding ethical problems [15]. Thus, it has been reported that diesel/gasoline/castor oil triple blends generate similar perform to fossil diesel, allowing a substitution of fossil fuel up to 24%. For its part, the same triple blend using sunflower oil provided 36% of fossil fuel substitution. Other advantages obtained with these diesel/gasoline/SVO blends were a considerably reduction in smoke opacity as well as a significative improvement of flow properties at cold climates [15].

In addition to gasoline, diethyl ether (DEE) [16] and acetone (ACE) [17] have been employed as effective LVS of CO and SO, in triple blends with diesel. Both additives can be obtained from ethanol which, in turn, can be obtained from biomass, making the process more sustainable. Both of these oxygenated additives were able to lower soot emissions considerably, maintaining a good engine performance. In spite of favorable fuel properties of DEE and ACE [16,17], the low calorific power was revealed as the limiting factor to incorporate percentages of biofuel LVS/SVO higher than 40%. Several studies have also been published fueling a diesel engine with diesel/DEE/SVOs triple blends. Among the oils employed, it can be highlighted cashew nut shell oil [18], bael oil [19], as well as sunflower and castor oil [16].

Additionally, ethyl acetate has been evaluated as a potential oxygenated biofuel since can be readily produced from renewable feedstocks by several processes such as the direct esterification of ethanol with acetic acid [20], through dehydrogenative dimerization of bioethanol with liberation of molecular hydrogen [21], or by a biochemical process that includes acidogenic fermentation of agricultural wastes materials [22]. EA is an environmentally friendly compound that exhibit very low kinematic viscosity, high miscibility with VOs and fossil diesel, high oxygen content, high auto-ignition temperature which makes fuel safer for handling and transportation, and good cold flow properties (very low PP and CP values) to improve winter engine performance. Hence, the application of EA as solvent of second-generation oils, i.e., castor oil or waste cooking oils, could be an inexpensive and valuable alternative to accomplish the replacement of fossil fuel, which is mandatory to realize now with the operating car fleet.

Prominently improvements have been reported in brake thermal e fficiency (BTE) but with a slightly higher brake specific fuel consumption (BSFC) using ethyl acetate as fuel or additive for C.I. engines [23]. In addition, exhaust temperature as well as CO, HC, and NOx emissions have been diminished with this fuel [22–24]. Similarly, the addition of 10% EA to diesel has showed similar brake specific fuel consumption respect to diesel, but with a notable reduction of soot emissions [25]. On the other hand, the addition of ethyl acetate to some bio-oils has provided important advantages to enhance the storage stability, thanks to its non-hygroscopic nature [26]. This compound has been also applied in blends with gasoline, leading to decrease in CO and HC emissions by around 50% due to high oxygen content of ethyl acetate [27]. Moreover, EA has been also used as a surfactant to emulsify a 50% diesel/50% ethanol mixture, resulting in an increase in BTE and a decrease in BSFC, smoke density, particulate matter, exhaust gas temperature, HCs and CO [28].

In this research, the possibility of replacing fossil diesel by renewable biofuels obtained by blending EA with SVOs, either sunflower or castor oil, has been evaluated. First, the optimum EA/SVO blends that meet with viscosity requirements according to European normative, were chosen to be subsequently mixed with diesel fossil. Then, the di fferent EA/SVO double and D/EA/SVO triple blends have been tested in a C.I. engine. Several parameters were measured, i.e., power output, fuel consumption and soot emissions. Moreover, some of the most crucial fuel properties, e.g., calorific value, cetane number, cloud point, pour point, and kinematic viscosity were determined to ascertain the suitability of the mixtures as (bio)fuels. In addition, all these measurements were performed with fossil diesel for comparison purposes.
