**1. Introduction**

Nowadays, fossil fuels constitute the main source of energy supply for transport. In fact, about 11 billion tons of fossil fuels are consumed each year worldwide [1]. Furthermore, it is expected that the fossil fuel demand will continue to rise, which will unavoidably lead to a scenario where fossil fuels run out [2]. In addition to the depletion of fossil fuels, as the demand for energy continues growing, the undesirable environmental effects linked to its production and consumption are becoming more evident by the day. In fact, emission of smoke, particulate matter (PM), carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx) and unburnt hydrocarbons (UBHC) from fossil fuel combustion are the primary causes of both atmospheric pollution and human health damage. For this reason, the generation of a safe, efficient and clean energy system is a priority

objective [3]. Then, there is an urgen<sup>t</sup> necessity for a transition from non-renewable and polluting energies, used up to now by society as a resource to guarantee their energy needs, to other renewable and environmentally sustainable alternatives [4]. However, in order to be competitive and viable, this energy transition model cannot ignore the actual vehicle fleet (more than one billion) operating with fossil fuels [5]. This is the reason why biofuels seem to be the right candidates to start the energetic transition abovementioned. The use of biofuels diminishes the fossil fuels depletion, minimizes the negative impact of Greenhouse Gases (GHG), and also allows using the current car fleet without any mechanic modification of compression-ignition (CI) or diesel engines [6]. In addition, biofuels can be easily integrated into the logistics of the global transport system, through the gradual replacement of fossil fuels by mixtures of diesel/biofuel. In this sense the EU stated that, in 2010, tra ffic fuels must contain at least 5.75% renewable bio-components, increasing this percentage up to 20% in 2020 and 30% in 2030. These measures foresee to achieve a reduction of 40% in the Greenhouse Gases emissions in comparison to those in the year 1990, with 27% of energy consumption from renewable sources and, at least, an increase of 27% in energy e fficiency [7]. Despite these objectives that are apparently not di fficult to achieve, replacement of fossil diesel with conventional biodiesel is still considered economically unfeasible, due to di fferent factors associated to the biodiesel purification process, e.g., long reaction times and high energy consumption. Furthermore, during biodiesel production, glycerol is obtained as a by-product, being approximately 10 wt % of the total biodiesel produced.

To solve this problem, selective transesterification of triglycerides with ethanol has been described to produce monoglycerides (MG) as soluble glycerol derivatives using di fferent lipases as catalysts [8]. Thus, through the partial transesterification of one mole of triglyceride (TG) with ethanol, two moles of ethyl esters (FAEE) and one mole of monoglyceride (MG) are generated, obtaining a biofuel called Ecodiesel that integrates glycerol in the form of a derivative soluble in the FAEE mixtures [8,9]. Recent studies stated that the presence of ethanol and other short-chain alcohols has a favorable e ffect on the emissions of the biofuels [10]. These mixtures improve the volatility of the fuel and constitute the so-called E-diesel, oxidiesel or oxygenated diesel, which in addition to reducing the emissions of the CI engines, improves the flow properties (viscosity) and the essential parameters that limit the application of diesel when operating at low temperatures, such as the "Cloud Point" (CP), "Pour Point" (PP), cold filter plugging point temperature (CFPP), point of occlusion of the cold filter (POFF), and emission levels of the motors without any significant negative e ffect in most of the parameters that define the quality of biodiesel [11].

In addition, the use of straight vegetable oils (SVO) in double blends with conventional diesel can be also considered as a potential option. All the relevant physicochemical properties of these blends are analogous to conventional diesel, except for the viscosity, which is much higher in oils than in diesel. Since fossil diesel has a much lower viscosity than oils, there will be a maximum percentage for each oil to be mixed with diesel in order to comply with regulations of the EN 14214 standard [12,13]. In this sense, it has been reported that blends with 10–20% of vegetable oil in diesel can be directly employed in diesel engines without any mechanic modification [14]. Following a strategy similar to that of E-diesel production, the incorporation of alcohols to form triple blends (diesel/biodiesel/alcohol) would further allow increasing the substitution of diesel. However, short-chain alcohols will have di fficulty if to be blended with SVO due the di fferent solubility, and also because a phase separation occurs after a short period of time, limiting the use of short-chain alcohols, mainly methanol and ethanol, with vegetable oils in the triple blends abovementioned. In addition, ethanol is corrosive and cannot be easily employed in today's engines or be shipped cheaply through current pipelines [15].

Nevertheless, there is a very interesting exception when castor oil is employed as an SVO due to the special structure of ricinoleic acid, which favors its solubility with alcohols, making possible a higher incorporation of them in triple blends. In fact, a diesel/castor oil/2-propanol triple blend in a proportion of 50/25/25 has been employed in conventional diesel engines, achieving very good results [16,17].

Regarding the use of vegetable oils, another strategy consisting the blending of them with less viscous and lower cetane (LVLC) has also been reported [18]. Pine oil (viscosity value of 1.3 cSt) has been employed in mixtures with castor oil to compensate for the high viscosity of it (226.2 cSt). The properties of castor oil and pine oil are mutually balanced causing a good balance of generated smokes [18], although the low cetane number of pine oil limits its amount in the blends (30% by volume) due to engine knocking problems. In addition to pine oil, eucalyptus, camphor and orange oils have been also considered as LVLCs [19]. Likewise, gasoline exhibits viscosity values sufficiently low to obtain important reductions in the viscosity of the oil/gasoline blends. Furthermore, the high energy density of gasoline, only slightly lower than diesel and of course higher than short chain alcohols, as well as its high availability, would allow its use as a blending agen<sup>t</sup> in a more advantageous way than with other renewable compounds already described.

With this in mind, in this study, gasoline has been evaluated, for the first time, as a blending agen<sup>t</sup> to produce a gasoline/oil mixture acting as a LVLC in blends with diesel. This is possible due to the high solubility of any type of vegetable oil with gasoline. The main goal of this research is to achieve a high substitution of fossil fuels, in a feasible way from a technical and economic point of view, and in a short period of time. To do so, two types of oils, which do not compete with food uses and which present high availability, have been chosen. On one hand, sunflower oil has been studied as a standard reference of waste cooking oils and, on the other hand, castor oil has been studied as a reference to oils which are not employed in food uses.

To obtain the optimum gasoline/oil mixture which can be blended with diesel, and maintaining the appropriate parameters of the EN 590 standard, the kinematic viscosity at 40 ◦C has been chosen as the most significant parameter, since this is the unique parameter that varies significantly with the proportions of the gasoline/oil blend. The effect of biofuel blends on the performance and emissions of internal combustion engines can be extremely complex to predict, because oils and gasoline show antithetic effects on engine performance in important parameters such as the cetane index (or energy density) and flash point, which promotes positive or negative interactions that are difficult to predict a priori. Furthermore, the cloud and pour point of the blends have been studied.

Once the adequate diesel/gasoline/oil blends were obtained, i.e., met the EN 590 standard parameters, they were tested in a conventional CI engine, operating as an electricity generator, as it is foreseeable that they exhibit different behaviors. The efficiency obtained is related to the effective electrical power, determined from the voltage and amperage generated by the engine. Furthermore, the contamination degree obtained from the opacity values of the generated smokes has been evaluated, as well as the fuel consumption of the different blends employed.

## **2. Materials and Methods**

#### *2.1. Double Blends of Gasoline*/*Oil and Triple Blends of Diesel*/*Gasoline*/*Oil*

Commercial sunflower oil (food quality), locally obtained, and castor oil (Panreac, Castellar Del Valles, Spain) were blended with gasoline in a first step to obtain the double blends. The double blends which met the requirements of the EN 14214 standard for being employed as biofuels were blended with fossil diesel (from a Repsol service station) in different proportions to obtain the triple blends.

#### *2.2. Characterization of the Biofuel Mixtures*

The rheological properties that influence most in the correct performance of biofuels are the kinematic viscosity, measured at 40 ◦C, and cold flow properties. The cold flow properties are determined by several parameters that define its behavior at low temperatures, such as Cloud Point, Pour Point, and point of obstruction of the filter at low temperatures. At low temperatures, the formation of nuclei of solid crystals occurs, increasing in size as the temperatures decrease. The temperature at which the crystals become visible (diameter ≥ 0.5 mm) is defined as the Cloud Point, because the crystals typically form a cloudy cloud or suspension. The Cloud Point usually occurs at a temperature higher than the Pour Point. Solids and crystals grow quickly and block the passage of fuel lines and filters causing operational problems [20].
