ON ANTIWEAR PROPERTIES FOR BLENDS OF JET FUEL WITH BIO-COMPONENTS DERIVED FROM RAPESEED OIL

A . I a k o v l i e v a Assisstant* E-mail: pinchuk_anya@ukr.net H . K u s z e w s k i PhD, associate professor** E-mail: hkuszews@prz.edu.pl O . V o v k Doctor of Sciences, associate professor* E-mail: o.a.vovk@mail.ru S . B o i c h e n k o Doctor of Technical Sciences, professor** E-mail: chemmotology@ukr.net K . L e j d a Doctor of Sciences, professor** E-mail: klejda@prz.edu.pl M . J a k u b o w s k i PhD, associate professor** E-mail: mjakubow@prz.edu.pl *Ecology department National aviation university Kosmonavta Komarova, 1, Kyiv, Ukraine, 03058 **Department of internal combustion engines and transport Rzeszow university of technology 8 Povstancov Warshavy ave., Rzeszow, Poland, 35-959 Досліджено протизносні властивості палива для ПРД, двох видів біокомпонентів, отриманих з ріпакової олії та їх сумішей. Встановлено, що змащувальна здатність біокомпонентів є вищою у порівнянні з нафтовим паливом для ПРД. Додавання біокомпонентів призводить до зміцнення граничної плівки та поліпшує протизносні властивості паливних сумішей. Встановлено, що модифікування біокомпонентів підвищує їх змащувальну здатність у порівнянні зі стандартними біокомпонентами Ключові слова: паливо для ПРД, альтернативне паливо, зношування, біокомпоненти, змащуюча здатність, навантаження до задирання, тертя


Introduction
Over the last years, increasing attention is paid to the question of traditional energy sources replacement with their alternative analogues [1,2].The question is particularly acute for transport sector [3][4][5].During last decade alternative fuels have been used not only motor transport, but for aviation as well.Today numbers of international organizations, such as ICAO (International Civil Aviation Organization), IATA (International Air Transport Association) pay much attention to the questions of making civil aviation "green".
One of the main tasks in the field of production and use of aviation fuels and lubricants is to expand the resource base and to develop progressive technologies for the production of aviation fuels.At the same time, alternative aviation fuels must meet a number of requirements related to efficiency, reliability and durability of aviation technics [2,3].

Literature overview and problem statement
Today, there is great variety of alternative jet fuels as well as technologies of their production.One of the most developed and widely used renewable jet fuels is hydrogenated synthetic paraffin kerosene that is well described in [4].Biofuel is made of plant and (or) animal fat-containing feedstock.Except that, there are some other renewable fuels, made of biomass via FT-synthesis, made of alcohols, various algae and even industrial and household waste.However, all of these biofuels can be added to conventional petroleum jet fuels in quantity up to 50 % [5][6][7][8][9].
Another possible way to replace conventional jet fuels with renewable fuels is use of methyl (ethyl) esters of plant oils or animal fats [6,7].Primarily these biofuels have become popular as a fuel substitute for motor transport, but lately they've been proposed to be used as components for jet fuel [9,10].This kind of biofuel is produced via esterification of plant oils with methyl or ethyl alcohols [3,7].According to the developed technology up to 30 % (vol.) of biofuel can be added to conventional jet fuel [10].
However, today properties of various biofuels are not studied completely.Special attention must be paid to research of exploitation properties of alternative fuels.Other words it is necessary to study the impact of such fuels on aircraft equipment and details during long-term use.
It is noted [3,11,12] that alternative jet fuels, obtained via FT-synthesis are characterized by low lubricity that in future may cause premature wear of engine details [3,8,9].At the same time, there is a lack of experimental data on lubricity properties of plant oils esters [12][13][14].There are only some information about esters application as a substitute of conventional diesel fuel [14][15][16][17].

Purpose and objectives of the study
The key purpose of this paper is to study the antiwear properties of blended jet fuels containing biocomponents made of rape oil.
In accordance with the set goal the following research objectives are identified: 1. Research of rapeseed oil fatty acids methyl esters influence on lubricating properties of jet fuels.
2. Studying the mechanism of biocomponents action as an improver of fuel lubricating properties.
3. Comparative analysis of lubricity of two kinds of investigated biocomponents.
4. Substantiation of possibility of biocomponents use as a partial substituent to conventional jet fuel.

1. Sample characterization
Lubricity of jet fuel, two kinds of biocomponents and their blends with jet fuel was investigated during the experiment.Jet fuel was presented by traditional oil-derived fuel for jet engines of grade Jet A-1 that meets the requirements of ASTM D1655.Biocomponents were presented by fatty acids methyl esters (FAME) of rape oil, meeting the requirements of EN 14214 and fatty acids methyl esters of rape oil that are specially modified for application as a component of jet fuel (FAME(M)).The modification was done by vacuum distillation according to the developed technology [10].The content of biocomponents in fuel blends was: 10-50 % (vol.).

2. Description of testing device and research methodology
Usually antiwear properties of jet fuels are estimated by the wear value of specific friction pair [18,19].Value of scuffing load may be also used to estimate this characteristic.Antiwear properties of fuel samples were investigated using a T-02U four-ball tester that consists of the testing device and a metering-control system.The mechanical part (testing device) includes body, power train, friction pair load bearing drive, ball cup assembly and a base.The friction pair (Fig. 1) consists of three lower balls fixed in a clamp and loaded with appropriate force applied by the top ball mounted on a spindle rotating with a determined speed [21].
The typical ball bearings of 1/2" diameter were used for the tests.The bearings are made of GCr15 bearing steel with hardness of 60÷65 HRC.The mechanical system allows linear increasing of the load applied to the friction pair during a test.The metering-control system consists of a dedicated microprocessor based controller, asynchronous motor controller and a computer with special control software [21].The tests were conducted under conditions of gradual load increase.The rotation speed during tests was 500 rpm, with the load increase rate of 409 N/s.The initial temperature of the fuel sample at the start of the test run was 60±1.0 °C.Within the scope of this test, by definition a friction pair seizure is said to have occurred once the boundary friction torque M T , of 10 Nm is exceeded.This value is determined by the durability of the top ball shank in the friction pair.The friction torque M T and the linearly increasing friction pair load P were recorded during testing.
The principle of the method is determining of the scuffing load applied to the friction pair [22].Load P, where the friction torque value starts to increase rapidly is called the scuffing load and denoted as P S .The values of P S were reading from files recorded during run test.The reading errors of P S have been estimated to be ±20 N. Thus, the fuel sample with the highest value of applied scuffing load is considered to have the best antiwear properties.Other words, it means that the boundary film of this fuel sample demonstrated the highest durability to breaking.

Investigation of scuffing load of fuel samples
The following Fig. 2-14 depict the friction torque, friction pair load and friction coefficient that were recorded during tests.The scuffing load describing the criteria for lubricity assessment in the chosen test methodology is marked.Thus, antiwear properties of jet fuel, two kinds of biocomponents derived from rapeseed oil and their mixtures were studied according to their scuffing load.The obtained results require further analysis and discussion.

Discussion of the results
In order to summarize the obtained results, the values of scuffing load P S for each fuel sample were recorded and shown in Fig. 15.
The presented results show that the conventional jet fuel of grade Jet A-1 has demonstrated the lowest value of scuffing load -510 N. At the same the highest values of scuffing load were observed for both types of biocomponents: 960 N -for rape oil FAME and 1150 N -for modified rape oil FAME.Taking conventional jet fuel as a reference sample we can easily conclude that use of rape oil esters positively influence the lubricity properties of jet fuel.Let us consider the mechanism of this effect.
Antiwear properties of jet fuels determine reliability and operational life of fuel system friction pairs in particular [18].Jet fuel cannot produce a continuous protecting film so aviation components normally operate in the boundary lubrication regime where the load is carried partly by the fluid film and partly by the contacting surfaces.The most likely explanation for wear in aviation systems is a simple corrosive wear process later followed by severe adhesive wear and scuffing as the component dimensions become reduced beyond tolerable limits [18,19].
Fuel lubricating properties depend on chemical composition, viscosity, thermal-oxidation stability, content of mechanical impurities, and presence of surfactants [18][19][20].In case of high unit loads the semi-liquid friction is usually observed -that is when wearing surfaces are not completely divided by the fuel.In these conditions the antiwear properties of jet fuels are determined by: 1) fuel viscosity that provides hydrodynamic effect of wearing surfaces division by liquid layer; 2) presence of surfactants in fuel that form high strength absorption layer at the wearing surface and divide wearing surfaces with decreasing of friction coefficient and details wearing.
Surfactants in fuel cause its surface activity -ability of fuel or its components to absorb at the surface of metal, form the boundary film and thus prevent dry friction of details.Solid bodies are characterized by high surface tension and their surface contacting to multicomponent liquid adsorbs substances that decrease this tension [23,24].Physical mechanism of adsorption at solid surface is explained by electrostatic interaction of polar molecules with electric field of particles that form solid surface, for example, metal ions [19,20].
Fuel hydrocarbons are non-polar and almost do not protect friction pairs from wear.Substances, which may be contained in jet fuel naturally and which are considered to be surface active, are gums, organic acids and other oxygen-containing compounds, sulfur-organic and nitrogen-organic compounds.At the same time presence of the mentioned substances is undesired, from the point of view of thermal stability and corrosion properties of fuel [25].
Biocomponents used in jet biofuel blends are the products of reaction between complex esters of glycerine and higher fatty acids (triglycerides) and simple alcohol (methanol or ethanol).Molecules of biocomponents are complex esters, which contain residuals of fatty acid and alcohol.Fatty acids esters have different number of carbon atoms in chain and different number of double bonds [26,27].General formula of FAME looks as following: where R -radical of fatty acid.Fatty acids radical are non-polar, but presence of carboxyl group in molecule stipulates strong polarity of complex esters.Such structure of biocomponents provides their surface activity and thus, ability to form boundary film.
Basing on these data we may assume that increasing of biocomponent content in blends causes increasing of surfactants quantity and as the result strengthening of boundary film between friction pair.
Another property that influences on details wear is fuel viscosity [20,25].Conventional jet fuels with lower viscosity are composed of compounds with smaller molar mass.Their molecules have smaller sizes and thus smaller dipole moment.As a result, the boundary film formed at the solid surface possesses less strength comparing to fuels with higher viscosity values.Except that, decreasing of molecules' size causes increase of average speed of their chaotic heat movement and promotes destruction (desorbtion) of the boundary film.
In order to verify theoretical data about viscosity influence on lubricating properties, the kinematic viscosity of investigated fuel blends was measured.Because the lubricity properties of jet fuel blends were tested at temperature 60 o C, the kinematic viscosity n 60 was measured at the same temperature regime.The measurement results are presented in Table 1.
From Table 1 we can see that kinematic viscosity of conventional oil-derived jet fuel is much lower comparing to viscosity of biocomponents.Such high values of FAME viscosity are explained by their chemical structure.Viscous characteristics of conventional jet fuel depends on their hydrocarbon composition: content of alkanes (paraffin's), cycloalkanes (naphthens), mono-and bicyclic arenes with average number of carbon atoms in molecule from 5 to 16.At the same time, hydrocarbon chain of ester molecules contain about 14-23 carbon atoms [26,27].This influences on molecules' size and thus decreases the speed of their chaotic movement.It is clearly seen from the table 1 that increasing of biocomponent content in blends causes increasing of their viscosity.Fig. 16 depicts the dependence of viscosity and scuffing load increment with increasing of biocomponents content in fuel blends.Fig. 16 shows the dependence between friction pair scuffing load and fuel viscosity.Increasing of fuel viscosity causes strengthening of boundary film at the surface of the friction pair; it results in increasing of scuffing load.Thus, the obtained results prove that antiwear properties of jet fuels directly are connected with fuel viscosity that provides wearing surfaces division by liquid layer; and presence of surfactants in fuel that form high strength absorption layer at the wearing surfaces that provides decreasing of friction coefficient and details wearing.

Conclusion
A four-ball tester with a facility for a continuous load increase was used for estimation lubricity properties of conventional jet fuel and its blends with rape oil derived biocomponents.The comparative tests of fuel blends indicated that rape oil FAME exhibit better permanent boundary film creation properties comparing to, conventional jet fuel within the scope of the tested friction pair.
This effect is explained by surface activity of FAME molecules and its high viscosity.The results of the tests have shown that increasing of biocomponent content in fuel blend caused strengthening of boundary film of friction pair surface.
It may be assumed that alternative jet fuels, which contain rape oil derived biocomponent are characterized by better antiwear properties.Other words, rape oil esters positively affect lubricity properties of conventional jet fuels and may be used as an improver of jet fuel antiwear properties.The use of rape oil esters may have a number of advantages.Among them: substitution of conventional oil with renewable resource, decreasing of exhaust gases emissions toxicity, improvement of safety parameters of fuel, decreasing of carbon dioxide emissions.

Table 1
Kinematic viscosity of tested fuel samples