Impact of N , N -Bis(2-ethoxyethyl) Fatty Acid Amides on the Lubrication Performance of Kerosene Fuel F-34 for Use in CI Engines

: In an attempt to avoid serious problems that can affect the efﬁciency of refueling ground-operated vehicles and aircraft during military operations, the Armed Forces of the North Atlantic Treaty Organization (NATO) are introducing the use of a unique fuel for both air and land use. The fuel that has been selected is the F-34, similar to Jet A-1, which is used in civil aviation, in order to replace diesel fuel in many applications. It has to be mentioned that tests performed with this fuel, which is kerosene type on the high frequency reciprocating rig (HFRR) have shown that such fuel is responsible for severe wear. This very high wear is related to the very low lubricity of aviation fuel. Having the idea to improve the lubricity of aviation fuel to the level of fuels used in compression ignition engines (diesel fuel), seven N , N -Bis(2-ethoxyethyl) fatty acid amides were formulated from various vegetable oils (sunﬂower oil, soybean oil, cottonseed oil, olive oil, tobacco seed oil, coconut oil, used frying oil), and they were evaluated as lubricity improvers of the aviation fuel. The required tribological measurements for lubricity rating were carried out by employing ISO 12156-1 test method on an HFRR instrument. The test conditions during the measurements were in the range of 55% to 58% for the relative humidity and 24 ◦ C for the temperature. The results from the tribological measurements showed that all N , N -Bis(2-ethoxyethyl) fatty acid amides used were rated as efﬁcient in order to provide an acceptable mean wear scar diameter (below 460 µ m) at concentrations from 150 to 300 ppm. Additive concentrations below 150 ppm did not improve the lubricity at the required level. The increase of N , N -Bis(2-ethoxyethyl) fatty acid amides at concentrations over 300 ppm did not have any signiﬁcant decrease in the wear scar diameter. A comparison between the N , N -Bis(2-ethoxyethyl) fatty acid amides showed that those formulated by non-polyunsaturated oils like olive oil and coconut oil seem to have better lubricity improver characteristics.


Introduction
The implementation of jet propulsion engines in the U.S. Air Force fleet was accompanied by the introduction of the first aviation fuel, which was a "wide-cut" fuel, which is a hydrocarbon mixture within the boiling range of heavy gasoline and kerosene. The need to proceed to the implementation of single fuel for military use, especially in the case of operations, became imperative mainly for reasons of simplification in the supply chain. This transition took place in the 1970s when NATO air forces agreed and virtually completed the process of converting from the F-40 wide-cut fuel (JP-4) to a more safe and less flammable F-34 fuel, which is mainly kerosene-type fuel (F-34 is also known as JP-8 or Fuels 2023, 4 20 AVTUR/FSII). This change was largely based on the experience of the United States from its engagement in the Vietnam War [1].
The requirements for the operation and maintenance of NATO's extensive and costly pipeline network in times of peace, crisis, and conflict have led NATO member states to proceed with the implementation of a unique fuel (JP-8) for all air and land military assets and equipment when used on the European battlefield. This idea has been called the Single Fuel Concept (SFC) [2,3]. The fuel that has been chosen to implement the SFC is the F-34 (the military version of the kerosene type JP-8 fuel), which is similar to the civil aviation Jet A-1 fuel but includes also anti-icing, antistatic, and lubricity-improving additives. It may also contain antioxidant and metal-deactivator additives [4]. However, the findings of the research and the sufficient experience gained in peacetime were subsequently tested on the battlefield. A typical example is the sharp increase in the occurrence of infusion pump failures, due to the increased use of equipment, observed in Operation Desert Shield/Storm in 1990-1991 in Saudi Arabia [5,6]. From the experience gained, NATO considered it necessary to carry out further studies. Thus, the exchange of ideas and experiences between the member states of the alliance would reveal the areas of development and activities required for a better understanding of the engine performance when F-34 (JP-8) fuel was used, in order to solve the identified operational problems and failures [7,8].
It should be noted that a key problem was mentioned during the implementation steps of the single-fuel concept. Generally, the F-34 or the similar fuel F-35 was found to have lower lubricity than commercially available distillates in the diesel fuel range. A possible explanation is that diesel fuel contains longer hydrocarbon chains, (higher boiling range) compared to kerosene and has a higher amount of natural lubricating agents [8].
The main object of this work was to investigate the lubricating characteristics of kerosene-type JP-8 fuel with seven added N,N-Bis(2-ethoxyethyl) fatty acid amides. The tests that were performed, provided data to look up for the minimum concentration of the fatty acid amides, capable to provide mean wear scar diameter below 460 µm. The value of 460 mµ, which is widely accepted by the industry as a minimum requirement for acceptable performance on the field, has been proposed by the European Committee for Standardization (CEN) in 1997 [9]. This limit is used in the specifications of automotive diesel fuel in the European Union [10].
Fatty acids and their derivatives are significant additives that reduce friction (and wear) as they are oxygen-containing compounds [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. The additives used as lubricity improvers contain a polar group that is attracted to the surfaces of the metal. This attraction has as result the formation of a thin surface film, which acts as a boundary lubricant between the metal surfaces. The addition of fatty acids [11,25], synthetic amide compounds [11], and biodiesel fuel [11,13,28] have been reported to enhance the lubricity of ultra-low sulfur diesel fuel as it is evaluated by the High Frequency Reciprocating Rig test. The improvement in fuel lubricity has been reported also for aviation turbine fuels when ester-type additives were used. Linear alkyl polar compounds have been proven effective when used as lubricity-improving additives in diesel fuels when they are used above a minimum level of 10-100 ppm [26,28]. Wei and Spikes have reported, that a significant improvement in lubricity was achieved by oxygen-containing compounds with phenolictype or carboxylic acid groups. This improvement was recorded at a concentration of just a few parts per million [29]. It has been reported that the oxygen-carbon ratio (O/C) of fuel reduces particulate matter emissions (reduction to below 0.5 on the Bosch scale) when the O/C ratio is more than 0.2 [30][31][32][33][34][35][36][37][38][39].
Acid esters and amides have been extensively studied as lubricity enhancers for automotive diesel, but their efficiency on the lubricity of lower viscosity aviation fuels in order to be used in compression ignition engines has been limited evaluated.

Materials
Sodium methoxide, N,N-Bis(2-ethoxyethyl) amine, and analytical reagents were supplied from Sigma Chemical Co.,(St. Louis, MS, USA) and they were of high grade. Sunflower oil, soybean oil, and olive oil were of market quality and they were purchased from a local grocery store. Cottonseed and coconut oil, non-commercially available, were obtained from Elin Verd Biofuels S.A. The waste frying oil was collected from local fast-food restaurants, and from the analysis, it was found to be a mixture of sunflower oil and olive oil. The tobacco seed oil, which was not commercially available was extracted from tobacco seeds on a laboratory scale. The procedure used for the extraction of tobacco seed oil was the following; the seeds were ground to a fine powder and then dried at 100 • C for 2 h. A Soxhlet extraction apparatus was used for the extraction from the seeds by hot hexane as extraction solvent, at a temperature of 65-70 • C. The extraction process lasted 4 h. At the end of the extraction process, the oil was separated from the hexane in a rotary evaporator under vacuum, and after that, it was dried at 60 • C and weighed. No further purification was used on the oils that were used. Table 1 gives their main quality characteristics. The kinematic viscosity of the vegetable oils ranges from 27 to 40 cSt. The oils that have the lowest value of kinematic viscosity at 40 • C were coconut oil and tobacco seed oil, while the oil with the highest value of kinematic viscosity at the same temperature is used frying oil.
The composition of fatty acids of the oils used in this study is given in Table 2. The fatty acid composition analysis showed that except for coconut oil, which contains a high amount of saturated fatty acids, the rest of the oils contain mainly (>70% m/m) unsaturated fatty acids. Olive oil and used frying oil contain high levels of mono-unsaturated fatty acids. For both oils, oleic acid is the major fatty acid, with concentrations of 75.0% m/m and 48.7% m/m, respectively. On the other hand, sunflower oil, soybean oil, cottonseed oil, and tobacco seed oil are clearly poly-unsaturated vegetable oils since they contain high amounts of linoleic acid. In addition, cottonseed oil had a significant amount of palmitic acid (22.2% m/m) compared to the other vegetable oils.
The scope of this study was to investigate the impact of the selected N,N-Bis(2ethoxyethyl) fatty acid amides on the lubrication efficiency of kerosene type fuels, aviation fuel was provided by Hellenic Petroleum Elefsis Refinery, and it was used as the base fuel for all the tribological measurements. The main quality characteristics of the fuel are given in Table 3, with the standard test methods that were used for the measurement of the relevant properties.

Synthesis of N,N-Bis(2-ethoxyethyl) Fatty Acid Amides
The reaction was conducted in a glass reaction flask with a round bottom, which was submerged in an oil bath. The reaction equipment included a mechanical stirrer, thermometer, and condenser, in order to ensure proper mixing, temperature control, and reflux. N,N-Bis(2-ethoxyethyl) amine reacted with the oils at a stoichiometric ratio of (N,N-Bis(2-ethoxyethyl) amine oil with sodium methoxide (1.5% by mass of N,N-Bis(2ethoxyethyl) amine and oil) as a catalyst. The reactions were conducted at 120 • C, and the formation of N,N-Bis(2-ethoxyethyl) fatty acid amides was checked by Thin Layer Chromatography. After the end of each reaction, the reaction mixture was left to cool at ambient temperature and later it was dissolved in diethyl ether in a separating funnel. The ether phase was washed to neutralize the alkalinity of the catalyst with 5% aqueous hydrochloric acid. The ether phase was separated, washed with water for neutralization, and dried passing over sodium sulfate. The ether was removed from the neutralized ether layer using a rotary evaporator. The reaction scheme is depicted in Figure 1.

Tribological Measurements
The tribological measurements were carried out following the ISO 12156-1 method on the HFRR apparatus. In order to run the tests, a fuel volume of 2 mL was used, and the test temperature was 60 • C. The relative humidity was in the range of 55% to 58%, and the mean ambient temperature was 24 • C. The lubricating characteristics of all samples were rated by measuring the average wear scar diameter (WSD) of the spherical steel specimen.
A photomicroscope was used for the measurement of the WSD. The efficiency of the fuels to provide lubricity was rated by the measurement of the average wear scar diameter (WSD) of the spherical specimen by using a photomicroscope. The measured values of wear scars were corrected to provide WS 1.4 values, according to the requirements of the test method. The following equation was used for the calculation of the repeatability of the results [40]: The seven N,N-Bis(2-ethoxyethyl) fatty acid amide mixtures were dissolved in the base fuel, at the same concentration levels, which were 50, 100, 150, 200, 250, 300, 350, and 400 ppm.

Results and Discussion
The initial step was the determination of the lubricity of the base fuel. The value of the corrected WSD for the base fuel, measured on the first day of its production, is given in Table 3. It is clear that the wear scar diameter of the fuel is much higher than 460 µm, which is the maximum acceptable limit for automotive fuel, and was therefore characterized as fuel with very poor lubricating characteristics. The repletion of the lubricity measurement on the next day gave an almost identical result and confirmed this remark. Due to the poor lubrication characteristics, this fuel was characterized as proper in the attempt to evaluate the lubricity effectiveness of the additives. The efficiency of soybean oil N,N-Bis(2-ethoxyethyl) amides on the lubricating characteristics of the base fuel is presented in Figure 3. From the analysis of the results, it is shown that when soybean oil N,N-Bis(2-ethoxyethyl) amides were added in concentrations between 50 and 250 ppm even though it improved the lubrication characteristics of the base fuel, it did not approach the required value of 460 µm. The lubricity improvement at an acceptable level took place when the concentration of soybean oil N,N-Bis(2ethoxyethyl) amides was increased from 250 to 300 ppm, where the maximum acceptable wear scar diameter of 460 µm measured by the HFRR, mandatory for commercial diesel fuels was achieved.         Table 2, is about 70%.   From all the above results, it is clear that all types N,N-Bis(2-ethoxyethyl) amides of fatty acids tested in this experimental procedure, had a positive impact on the lubrication characteristics of a low-viscosity fuel in the range of kerosene. Apart from this remark, an interesting conclusion has been derived from the comparison of the fatty acid composition of the various oils used for the production of the amides. From the various types of fatty acid N,N-Bis(2-ethoxyethyl) amides, those synthesized from oils with low content of polyunsaturated oils, like olive oil and coconut oil were found to have better lubrication efficiency. The better impact on lubrication characteristics can be correlated to the higher amount of saturated and mono-unsaturated N,N-Bis(2-ethoxyethyl) amides. In the case of saturated compounds, the molecules can be easily aligned in straight chains and are packed more closely on the metal surface, providing in this way the protective strong lubricating layer [41,42]. On the other hand, in the case of polyunsaturated molecules, the double bonds do not allow rotation and force the chains to bend. This bending makes more difficult the adjustment of the molecules close to each other, resulting in the formation of a thinner and weaker lubricating layer [43,44].

Conclusions
In this series of experiments, the main goal was the investigation of fatty acid N,N-Bis(2-ethoxyethyl) amides on the tribological characteristics of kerosene, in order to be used as fuel for compression ignition engines. For this reason, seven mixtures of N,N-Bis(2ethoxyethyl) amides were synthesized from various vegetable oils (olive oil, sunflower oil, soybean oil, cottonseed oil, tobacco seed oil, coconut oil, used frying oil) and they were added to a military aviation fuel JP-8. The main conclusions that can be extracted from this study are as follows: • Aviation fuel of kerosene type, when used in land equipment and vehicles of the army in the implementation of the single fuel concept, is a fuel with poor lubrication properties that does not provide the necessary lubricity and may cause serious damage in the fuel pumps of the engines.

•
The necessary dosage levels of the fatty acid N,N-Bis(2-ethoxyethyl) amides, in order to decrease the wear scar diameter below the maximum acceptable limit of 460 µm, were in the range of 150 to 300 ppm. Any further addition of amides did not provide any significant improvement in the lubricity of the base fuel.

•
Among the individual types of fatty acid N,N-Bis(2-ethoxyethyl) amides, those derived from non-polyunsaturated oils, such as olive oil and coconut oil appear to be better lubricants.