Fuel is a material whose burning is used to produce heat, energy, or light. Its importance is accompanied by the evolutionary process of man, the heat energy of the sun being the source that originated all the others. The production and consumption of fuels have increased in recent years associated with population, economic and technological growth. Petroleum still represents the most significant part in the amount of world fuel consumption, standing out with energy sources such as coal, natural gas, biofuels, and electricity . Among the commonly used vehicular fuels are gasoline, diesel, natural gas, ethanol, and biodiesel.
The petroleum fraction with the highest commercial value is gasoline, and the main processes used for its production are: distillation, cracking, hydrocracking, reforming, alkylation, polymerization, and isomerization. The gasoline composition is complex comprising hydrocarbons ranging from four to twelve carbon atoms, typically having hydrocarbons of five and eight carbon, and having boiling points between 38˚C and 205˚C  .
With the emergence of cheaper and sustainable fuels, industries started to add non-oil products to gasoline and diesel, such as ethanol in gasoline and biodiesel in diesel. With the ban on tetraethyl lead as an additive in gasoline due to its toxic and carcinogenic effects, oxygenated compounds have become an alternative to add positive environmental and performance aspects to gasoline. Currently, the most used oxygenated compounds correspond to aliphatic alcohols and ethers containing one to six carbon atoms  .
The mixture of fuels aims to improve different aspects from the environmental and economic points of view. For example, the common gasoline in Brazil (type C) is the fuel obtained from the mixture of gasoline and anhydrous ethyl alcohol, in the proportions defined by current legislation, currently 27%, v/v . The use of alcohol as fuel and in mixture with gasoline was stimulated in the 1970s with the petroleum crisis. Then, alcohol emerged as an alternative fuel to reduce dependence on fossil fuels. Its application is encouraged by presenting affordable price, be obtained from renewable sources and its burning emit less polluting compounds when compared to fossil fuels   . However, ethanol presents disadvantages such as low calorific value, high volatility, high solubility in water being conducive to contamination with the water content in the atmosphere. Rovai  reported from measures of lubricity in blends of ethanol and gasoline that the lubricity of the blend decreases with increasing alcohol content in the blend. The addition of ethanol to diesel shows a similar effect, which may reduce lubricity and create potential wear problems in fuel pumps as described by Li et al. .
The quality of gasoline is related to the use of additives, which are fundamental for its better use and commercialization. Still, with the advent of flex-fuel engines, the study of performance of fuels and mixtures becomes very relevant. Oxygenated compounds such as ethyl octanoate and ethyl oleate added to gasoline showed good fuel properties , as well as a promising prospect of improving the lubricity of gasoline, which influences the reduction of friction and wear of the engine . Thus, the objective of this study was to evaluate the hourly consumption and brake specific fuel consumption of formulations containing gasoline + oxygenated additives, in SI engine Otto cycle, considering as additives: ethanol, ethyl octanoate, and ethyl oleate added 15% v/v in the mixture, in order to compare the application of new oxygenated additives with ethanol that is widely used in gasoline blending in several countries.
2.1. Ester Synthesis and Chromatographic Analysis
The esters, ethyl octanoate and ethyl oleate, were obtained by esterification reaction. The reaction conditions were: 1:3 fatty acid and ethanol (Synth, lot 190231, P.A.) molar ratio, H2SO4 catalyst (Sigma Aldrich, lot SZBE2260V) with 1% of the fatty acid mass, temperature of 80˚C and time of 60 minutes. After the reaction, the product went through solvent extraction (water), chemically active extraction with 0.01M sodium bicarbonate solution (Êxodo Científica, lot BS8763KA), adsorption of traces of water by adding anhydrous sodium sulfate (Dinâmica Química Contemporânea, lot 55937) and filtration  .
The esters were evaluated by gas chromatography analysis (Shimadzu, model GC-2010) coupled to mass spectrometer (Shimadzu, model QP 2010). A 30 m long, 2.3 × 10−7 m thick CP8751 column with 2.5 × 10−4 m internal diameter was used. Column temperature was programmed from 150˚C to 250˚C. Helium was used as carrier gas at a constant pressure of 1.105 kPa and flow of 1.08 × 10−6 m3 per minute.
2.2. Fuel Consumption
The tests were done on a 4-stroke gasoline engine whose specifications, according to the instruction manual, are shown in Table 1.
Fuels analyzed: pure gasoline—type A (GAS A), common gasoline—type C (GAS C), gasoline type A + ethyl octanoate 15% v/v (OC 15), gasoline type A + ethyl oleate 15% v/v (OL 15).
The engine was coupled to an electric dynamometer (Kohlbach, 112MB) equipped with a load control cabinet. Tachometer (Instrutemp, ST-707) was used to check
Table 1. Engine specifications .
the engine speed. Fuel consumption was measured using burettes and a stopwatch. Before analyzing a new formulation, the engine ran long enough with an amount of the new formulation to ensure that the formulation analyzed earlier did not interfere with the next result  .
To perform the tests, the engine was maintained at full load as a function of the rotation. The rotation was conserved at 4400 rpm and power was varied from 3 kW to 12 kW, with intervals of 3 kW to obtain fuel consumption curves. Figure 1 shows a schematic drawing of the experimental apparatus used.
Initially, the motor was started with zero load for 20 minutes for stabilization. After this period the rotation was adjusted to 4400 rpm with the aid of a stroboscopic tachometer. With the load at 3 kW, the consumption analysis was started by measuring the time required for the engine to consume a quantity of 50 mL of the fuel, and the time was timed for every 5 mL of consumption. The consumption in the engine was also analyzed in loads of 6, 9 and 12 kW, for each formulation arranged in the burettes.
The fuel consumption was determined by measuring the amount of fuel consumed per unit of time, according to Equation (1) .
C: consumption (g/h);
ρi: fuel density (g/cm3);
v: volume of fuel consumed (mL);
t: fuel consumption time (s).
Figure 1. The schematic of experimental setup.
The brake specific fuel consumption was estimated through Equation (2)  being defined as the relation between fuel consumption and power.
BSFC: brake specific fuel consumption (g/kW·h);
C: consumption (g/h);
P: power (kW).
3. Analysis and Discussion of Results
3.1. Chromatographic Analysis of the Esters
The esters were evaluated by gas chromatography coupled to mass spectrometer. Figure 2 and Figure 3 show the chromatograms of ethyl octanoate and ethyl oleate, showing the different retention times that each compound elutes, where the esters were detected and identified.
Figure 2. Chromatogram of ethyl octanoate.
Figure 3. Chromatogram of ethyl oleate.
Figure 2 and Figure 3 evidence the good degree of purity of the esters, these esters being the results of the esterification reaction with conversion from 97% to ethyl octanoate and 98% to ethyl oleate. For esterification reactions following the methodology described, the yield of ethyl octanoate was 92% and the yield in ethyl oleate was 93%.
3.2. Consumption in SI Engine
Figure 4 shows the results of the analyzed for the consumption of the fuels in the engine.
It can be seen from Figure 4 that the consumption of the formulations increases as the power dissipated by the engine increases. Even esters having lower heat of combustion, hourly consumptions for the mixtures containing these additives were similar to pure gasoline (GAS A) and gasoline with ethanol (GAS C). Although the consumption of the same volume of each sample has been analyzed, the formulation with the highest density favors a lower consumption, since a unit of mass greater per volume enters the combustion chamber, as occurs in formulations containing the esters.
The brake specific fuel consumption of the formulations analyzed is shown in Figure 5.
As shown in Figure 5, it was verified that the brake specific fuel consumption decreases with the increase of power, for all formulations analyzed, in which the BSFC relates the mass flow of fuel that the motor consumes and the effective power, being a measure of the efficiency of the engine. Thus, at 12 kW power the engine requires a smaller amount of fuel to produce 1 KW than at lower power, as shown in Figure 5 in that by increasing power a better conversion of the mass of fuel burned in energy. Also, regarding the brake specific fuel consumption curves, it was noticed that as the power increases the difference of specific consumption between the formulations decreases.
Figure 4. Consumption for formulations containing gasoline and ester.
Figure 5. Brake specific fuel consumption for formulations containing gasoline and ester.
The results are very relevant and show the compatibility of ethyl octanoate and ethyl oleate as additive to gasoline in terms of engine performance. In addition to alcohol and ethers, esters have also been highlighted as additives. Wang et al.  reported that 3-Hydroxybutyrate methyl ester when mixed with gasoline, its performance was similar or superior to that of gasoline mixed with ethanol in terms of oxygen content, dynamic viscosity, flash point and boiling point. Jenkins et al.  evaluated mono and diesters for their potential as a substitute for aviation kerosene, mineral diesel or gasoline. Pelaez-Samaniego et al.  evaluated the use of gasoline and “bioflex” (a mixture of carboxylic esters from pyrolysis oil) in an Otto engine. Power and fuel consumption were statistically similar for type A gasoline, type C gasoline and mixtures of 10% by volume of bioflex with type C gasoline. As well as other oxygenates, ethyl octanoate and ethyl oleate, appear as an important alternative to be used as an additive.
This study evaluated the influence of two types of ethyl esters, ethyl octanoate and ethyl oleate, in the additive of gasoline based on fuel consumption analysis in Otto cycle engine. Even esters having lower heat of combustion, consumption was similar to pure gasoline and gasoline with ethanol. The brake specific fuel consumption also showed that the blends containing the esters showed efficiency similar to that of pure gasoline (GAS A) and gasoline with ethanol (GAS C).
Therefore, the consumption curves obtained in the Otto cycle engine showed that, regardless of the type of fuel, efficiency does not change, showing a promising result for the application of ethyl octanoate and ethyl oleate as a gasoline additive.
The authors are grateful to CNPQ and PPGEQ/DEQ/UFRN for their financial support.
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