Lubricity and tribological performance of 2-methylfuran and 2-methyltetrahydrofuran blended with diesel

Abstract

The implementation of biofuel additives for gasolinegasoline and diesel engines to improve their combustion properties has been investigated comprehensively. TheseThese blended fuels improve fuel performance by increasing oxygen content, enhancing octane number, and reducing greenhousegreenhouse gas emissions. This paper will study the lubricity and tribological properties of diesel blended with MF and MTHF (2,2-methylfuran / Diesel and 2,2-methyltetrahydrofuran / Diesel). The results will be assessed using a modified version of ASTM D6079, using a high-frequency reciprocating test rig. It's known that blending up to 30% of MF and MTHF with diesel improves engine efficiency. However, increasing the blend ratio to 30% doesdoes not resultresult in a significant enhancement of lubricity performance. It was found that 5% MF blends reduce the wear scar size by over 50% and significantly reduce the friction coefficient. Higher MF concentrations did not yield a noticeable improvement, so the optimal blend ratio is 5%. This was attributed to the planar aromatic molecular structure of MF.. In contrast, MTHF, an aliphatic molecule, adoptsadopts a boat conformation, which significantly increases steric hindrance in its blends, leadingleading to poor tribological properties. The wear mechanisms for MF differ at 5%-10%, 20%, and 30%. Further development of this research should gather information from surface analyses, such as surface topography, high-magnification SEM, and FTIR, to understand wear mechanisms and study the tribofilm properties resulting from the testing.

Introduction

The fuel performance of the internal combustion engine has been constantly refined and improved since its creation. Innovations enable cleaner, more efficient operation, with each engine generation better than the last. ¹ A key aspect of fuel performance is to improve the lubricity of fuels alongside the development of future engines, allowing vehicles burning this fuel to travel with higher efficiency over longer distances, with less engine wear, and with a longer engine lifespan. ² According to the literature referenced below, pure diesel has poor lubricity. Because it has poor lubricity, efforts have been made to improve diesel's lubricity in various ways, including adding additives and producing biodiesels. Lubricity of any fuel is defined by its ability to resist wear. Diesel engines use fuel-injection equipment, which is lubricated by the fuel itself rather than engine oil. ³ Pumps, injectors, pipes, and any parts that come into contact with the fuel rely on it for cooling and lubrication. Inadequate fuel lubricity, commonly caused by low-quality fuels, increases thermal pressures and moving parts are subjected to higher friction coefficients throughout the system. ⁴ This poor lubricity causes parasitic losses, stripping the engine of marginal gains in performance from other design improvements. Fuel injection systems are an expensive element of diesel engines, which are manufactured to very high tolerances. OEMs (original equipment manufacturers) that make hardware provide warranties to protect their brands. Extending the life of the fuel injection system through high-quality biofuels and avoiding the need to replace parts such as pumps and injectors early would reduce engine manufacturing costs and improve reliability. This, in turn, would continue to drive OEM investment and development in the rapidly growing field of tribology. However, if accelerated wear is attributed to poor-quality biofuels or biofuel additives in diesel, these manufacturers will then recommend not using these fuels with their equipment. Reducing reliance on fossil fuels and increasing investment in renewable, high-quality sources will help mitigate environmental damage caused by diesel pollution, which continues at an alarming rate. It is pivotal that any new fuel has good lubricity; this will sustain interest in its development and drive sorely needed change. The hydrotreatment of diesel to remove sulphur and the addition of kerosene to improve low-temperature performance both reduce diesel's lubricity. To combat this, additives such as fatty acids are blended with diesel to improve its lubricity. The quest for cleaner, more durable diesel engines has driven the development of biofuel additives that enhance performance and lubricity. Among these, 2-methylfuran (MF) and 2-methyltetrahydrofuran (MTHF) have emerged as promising candidates due to their renewable origins and favourable combustion properties. ⁵

Recent innovations have made MF and MTHF production from biomass highly efficient, allowing more sustainable integration into diesel blends. This paper will focus on two biofuel additives: 2-methyltetrahydrofuran (MTHF) and 2-methylfuran (MF). These chemicals are biomass-derived, with a promising environmental footprint, ⁶ and are produced from renewable sources. ⁷ Blending MF and MTHF with diesel has been shown to improve engine efficiency, reduce carbon monoxide and soot emissions, and enhance fuel thermal characteristics. There is increasing evidence that adding MF at optimal concentrations can dramatically reduce wear scar size and friction coefficients in engine components, surpassing the lubricity of conventional diesel or simple fatty acid additives. Mechanistic studies reveal that the planar, aromatic molecular structure of MF, in contrast to MTHF's aliphatic ‘boat’ conformation, facilitates more effective tribo-film formation and surface protection. ⁵ An improved method of MF production was discovered in 2009 by Dumesic, Román, and Zhao et al. The technique was refined, establishing a highly efficient system. Selective oxygen removal to produce MF and other renewable fuels, including DMF and ethanol from fructose, was achieved through this research.^8,9 MF was one of the main products of the stepped process, which initially removed three oxygen atoms via dehydration and then the final two via hydrogenolysis, as shown in Figure 1. MF has been proven to improve the engine thermal efficiency, fuel consumption, and emissions when blended with gasoline in a DISI engine. ⁷

Figure 1.

Path of dehydration & hydrogenolysis to create mf. ⁷

When MF is blended with diesel, it produces an improved biofuel with promising characteristics, as described in various papers.^10–13 It makes a longer ignition delay while reducing combustion duration. ¹¹ Compared to pure diesel, blends with MF result in higher brake thermal efficiency (BTE) and higher NOx emissions, which increase directly with MF percentage. Blends result in significantly lower soot emissions. ¹³ MTHF diesel blends show improved homogeneous low-temperature combustion; in addition, the blend produces nearly soot-free combustion.^13,14

Previous research has investigated lubricity and published interesting conclusions, such as confirming that increasing the chain length of fatty acids in various oxygenated compounds enhances lubricity.^3,15 Other research discovered small percentages of fatty acid methyl esters derived from rapeseed (RME) blended with an environmentally friendly Swedish diesel fuel, MK1, improved lubricity. ¹⁶ The use of 2-methylfuran (MF), a furan-derived ether-based additive, has been shown to enhance diesel engine combustion efficiency and substantially reduce pollutant emissions when blended at appropriate ratios, confirming its potential as a cleaner biofuel alternative. ¹⁷ The utilisation of additives in biodiesel blends has been demonstrated to enhance diesel engine performance and reduce pollutant emissions through optimised combustion, aligning with findings that bio-derived compounds can improve lubricity and emission characteristics in diesel blends. ¹⁸ Previous studies found that MF increases the lubricity when added to pure gasoline, including its functional properties and its resistance to wear (16).. ² At blends greater than 0%, MF improved the gas's lubricity. At 10%, the blend was considered, with any blend ratio above it regarded as saturated. With a higher blend ratio, the tribological properties changed little. Surface analysis revealed that MF formed a furan-based tribofilm (16). ²

This paper studies whether additives, MF and MTHF, blended with diesel, will improve the fuel's lubricity and reduce wear scars, friction coefficients, and other negative properties developed during combustion and throughout the system, including pumps and injectors within diesel engines. The aim of this paper is to investigate the tribological behaviour of MF, MTHF, and a diesel fuel as potential candidates for advanced propulsion systems. The test established the optimal blend percentages for the greatest reduction in friction and improvement in wear resistance, which will be implemented in diesel fuel to increase the lifetime of engines running the blend. The objective is to determine the optimal blend ratios that yield the best tribological properties when blended with diesel. The samples will also be investigated for irregularities to validate findings and support the theories of previous research. All the tests undertaken have been compared against commercial diesel fuel.

Materials and methodology

Methylfuran (MF) and Methyltetrahydrafuran (MTHF) (Figure 2), purchased from Sigma-Aldrich UK, were selected for this experiment due to their promising combustibility with gasoline and diesel blends. When blended as additives with these fuels, they’ve been proven to improve thermal efficiency and fuel consumption, and to reduce emissions. Shell Global Solutions UK supplied the European ultra-low-sulphur diesel (EN 590 specification). The characteristics of MF, MTHF, and Diesel are shown in Table 1 below.^3,19,20

Figure 2.

Molecular structure of MF (left) & MTHF (right).

Table 1.

Properties of diesel, mf and MTHF.

Properties	Diesel	MF	MTHF
Density (kg/m3))	875	913.2	854
Research Octane Number (RON)	-	103	86
Motor Octane Number (MON)	-	86	73
H/C ratio	2	1.2	2
O/C ratio	-	0.2	0.2
Lower Heating Value (MJ/kg)	47	31.2	32.8
Heat of Vaporization (kJ/kg)	189	358.4	375.3
Flash Point (°C)	60	−22	−11
Initial Boiling Point (°C)	180	64.7	78

The PCS Instruments ‘High Frequency Reciprocating Rig (HFRR), as seen in Figure 3, was used for the experiments. The HFRR was performed according to ASTM D6079. ²¹ Two specimens were used: 1) the upper specimen, a 6.0 mm diameter 5120 ball and 2) the lower specimen, a 10.0 mm diameter disk. The two specimens engage in a reciprocating motion over 1 millimetre. This is comparable to the operation of the fuel under compression-ignition engine conditions. High-speed operation of pumps and injection equipment is simulated using load-and-test operating speeds. The wear characteristics are deliberately amplified throughout the test to yield results indicative of wear in a compression-ignition engine. Specifically, in its working internals over a period of years. HFRR is the industry's preferred method for diesel lubricity testing and is calibrated against at least 7standards. ²²

Figure 3.

Schematic of the high-frequency reciprocating rig (HFRR). ²²

A single new, lower specimen and an upper specimen were placed into a clean beaker containing enough acetone to cover both specimens. This beaker was then placed in an ultrasonic cleaner for ten minutes. To remove any deposits and soot from previous tests. The upper specimen mounting arm was also cleaned with acetone, along with the lower specimen's bath and everything that came into contact with the blends. This was repeated for every specimen before each test run. Cleanliness was paramount, and cleaning protocols were adhered to. Clean forceps and gloves were used for each procedure to avoid cross-contamination and ensure accurate results. The lower specimen was inserted into the reservoir, secured, and then the reservoir was bolted to the HFRR apparatus. A temperature probe was then inserted into the side of the reservoir. The upper specimen was inserted into and secured in the upper specimen holder for reciprocation during the tests. 2 mL ± 0.2 mL of test fuel was placed into the reservoir. The gasoline PTFE conversion kit was used to prevent excessive evaporation and contamination of the fuel. ³ A 15 mL sample of fuel was used throughout the tests to minimise uncertainty from potential fuel evaporation. The testing parameters were set on the HFRR computer program for a frequency of 50 Hz and a duration of 75 min (28). The arm was lowered, and a 200 g ± 1 g weight was suspended. After these settings were completed and checked, the test was run. After each test, the weight was removed, the arm lifted, and the upper holder removed. The upper specimen (while still in the holder) was rinsed with acetone and allowed to dry. An electronic microscope was then used to capture an image of the wear scar, and the HFRR computer program enabled it to be measured accurately. The fuel was removed and disposed of. The wear scar was captured at 100x magnification using a spotlight, with light intensity adjusted to ensure a clear image and keep the scar edges in focus. The wear scar diameter measured in the software was recorded.

The lubricity of each blend was determined from the friction coefficient and wear scar diameter; the HFRR software automatically generated the friction coefficient. An electron microscope enabled the wear scar to be measured with 0.01 mm accuracy. The maximum and minimum edges of the wear scar in both the X and Y planes were selected on the computer, and the diameter was automatically calculated. The lower specimen was investigated using the TM3000 Tabletop Scanning Electron Microscope (SEM) (Hitachi High-Technologies Corporation, Tokyo, Japan). This allowed for detailed visible inspection and wear analyses. Energy-dispersive X-ray spectroscopy was also performed on the same instrument.

Results and discussion

The results show that MF had an apparent positive effect on lubricity when blended with diesel, improving wear scar morphology and reducing the friction coefficient. In contrast, MTHF appeared to reduce the friction coefficient and had little effect on the wear scar. 10% MF blended with diesel had the most promising results. This was the lowest blend concentration that was initially chosen. It was decided, having seen the first promising results, to also test 5% MF. The film developed on the surface of 10% MF, between the two specimens is the most stable compared to any other blends, both MF, MTHF and even pure diesel as seen in Figure Contrary to any prediction that could have been made from previous results, the 5% blend test never developed a film with thickness greater than 51% however the average friction coefficient was lower than that of any other test. As seen, the blend was the least volatile, with troughs and peaks of minimal size compared to other blend ratios. This shows that its lubricity is lower than that of any other blend; however, on inspection of the surface, it is suspected that the wear mechanism differs. This will be addressed later. As the MF blend ratio increased, film stability worsened. At various points during the 20% & 30% MF-Diesel blends, the film deteriorated. The film was either completely or partially destroyed; however, after a few moments, it always started to rebuild. This is probably because of MF's volatility. MTHF film thickness results are not included in Figure 4, as they did not demonstrate stable or representative film formation behaviour. In addition, no clear relationship was observed between film thickness evolution and lubricity performance, with all MTHF blends performing comparably to or worse than the control (neat diesel). Therefore, their inclusion was not considered informative for comparative analysis.

Figure 4.

Evolution of lubrication film thickness for MF-diesel blends (5–30%) and neat diesel (ASTM D6079, 60°C for 75 min).

The friction coefficient results are shown in Figure 5; increasing film thickness does not reduce the friction coefficient. The friction coefficient is no better than that of the control, pure diesel. It is found that MF aids lubrication; not only do blends with diesel decrease the friction coefficient by over 20% (see Figure 5), but they also significantly reduce wear scars across all blend ratios (see Figure 6).

Figure 5.

Friction coefficient for MF and MTHF blended with diesel.

Figure 6.

Mean wear scar diameter results for mf and MTHF blended with diesel.

The clear difference in the coefficient of friction between MF and MTHF blends theoretically could be due to their molecular structure. An MF blends surface free energy is different to that of MTHF because the bonding structure of MF and MTHF is very different. MF has a conjugated Pi system formed of the two carbon-to-carbon double bonds and one lone pair of electrons from the oxygen atom. It is therefore a planar, aromatic molecule. This orbital structure is shown in Figure 7. The increase in surface free energy from the conformation results in a decrease in the activation energy for vacancy formation. The number of vacancies increases, thereby decreasing the coefficient of friction. ²³

Figure 7.

Orbital structure of furan and boat conformation.^24,25

MTHF is formed only of Sigma bonds and adopts the boat conformation shown in Figure 7; it is therefore an aliphatic molecule. This minimises electron–electron repulsion caused by bonds, so the molecule is in its lowest-energy conformation. This indicates that the molecular skeletal shape mainly influences its poor lubricious properties.

Figure 7 is the molecular conformation of MF. This shows clearly what cannot be seen in the skeletal formula in Figure 2. The molecule is planar due to the delocalised pi system. Figure 8 shows the conformation of MTHF. In contrast to the structure of MF, the MTHF molecule is non-planar. The ring system adopts a boat conformation with pseudo-equatorial and pseudo-axial hydrogens. The tribological behaviour seen in the results is strongly influenced by the additive's chemical composition, structure and surface morphology.^26–28 The interfacial transfer layer ²⁹ dictates lubricity. This layer is developed by changing the contacting layer of the film to a material of low shear strength.^29,30

Figure 8.

Structure of 2-mf- left and structure of 2-MTHF – right. ²⁵

This boat conformation creates steric hindrance, far larger than that of MF, leading to a higher energy demand for any process and a slower reaction rate. This is a strong indication of why it has poor tribological properties. Much like the dry-lubricity properties of graphite, it is possible that the flatter MF molecular arrangement is responsible for a reduction in the friction coefficient. MTHF's apparent rough configuration could explain its lack of lubricity benefits and reduction in the friction coefficient at all blend ratios. This, however, does not explain the consistency of the wear scar across all tested MF blend ratios.

In line with the theory developed here, MTHF lacks lubricating properties. Fatemimoughari et al. ²⁹ have also suggested that steric hindrance from the methyl group prevents good surface coverage. ³¹ This, however, is not the case for MF. The explanation of MF's lubricious properties could also be due to the molecule's polarity. If the lubricant molecules adsorb to the surfaces, as they can if they are polar, the protective layer will then help optimise lubricity. The creation of this protective layer is a complex process. Many features of the tribological system, such as the structure of the lubricants and surface molecules, influence it.^32,33

The methyl group of furan donates electron density to the aromatic ring, increasing its negative charge. This makes the molecule more polar. No such effect is observed for MTHF because it is an aliphatic molecule. Hsu^{32, 33} defines this “polarity” as a function of several molecular properties, such as the reactivity of the functional group and the molecule's shape. The molecule's polarity will lead to MF adsorption onto a surface. This is shown in Figure 9, with Figure 10 demonstrating the perpendicular alignment the MF molecules would take along the surface of the material. This, in turn, will increase the material's protection. As the surface protection doesn’t change, it is possible that the scar cannot change without considerable change/damage to the adsorbed polar molecule protective layer.

Figure 9.

Protective layer of adsorbed polar molecules (re-drawn from Ref ²⁹ ).

Figure 10.

Alignment of adsorbed polar molecules.

The results show that MTHF does not aid lubrication. The black deposit seen in the pure diesel tests on the lower specimens does not decrease in size or intensity. In contrast, MF significantly reduces the deposit on the lower specimens as the MF percentage in the blend increases. The observed improvement in lubricity and reduction in wear scar size for engine fuels blended with 2-methylfuran (MF) parallels the fundamental lubricity mechanism established for 2,5-dimethylfuran, where the additive provided enhanced frictional properties and wear resistance compared to conventional fuels and other bio-based alternatives. ³⁴

The benchmark pure diesel test left a clear, black, soot-like deposit at both extremes of movement as seen by the concentrations in Figure 11. In addition, a characteristic auxiliary deposit with a flow-induced pattern was observed on the specimen surface, likely associated with the formation and redistribution of tribofilm material under reciprocating motion. For MF with increasing additive concentration in the blend, the deposit size and intensity decrease significantly, approaching zero, as seen in Figure 12 at 30%. In contrast, MTHF does not exhibit this behaviour, resulting in practically no change in the auxiliary deposit, and the deposits at either end of the wear scar did not change in size or intensity.

Figure 11.

Wear scar for benchmark pure diesel lower specimen at ×30 & ×100 magnification.

Figure 12.

Wear scar on the lower specimen ×30 magnification.

This can be seen in Figure 13 above, where the wear scar has been magnified x100. EDS was performed on the TM3000; the predominant detected compounds were iron, carbon, and oxygen. However, these results were not relied upon due to inconsistencies in testing. The EDS results appeared ordinary. Measurements were taken from the lower specimen on the wear scar surface and in the black mass at the ends to discover their constituents. The results confirm expectations: the black mass is a carbon- and oxygen-rich deposit. This deposit suggests a reaction caused by the heat generated during the testing. ³ This reinforces the theory that a tribofilm forms on the interface between the two specimens in the MF blends. However, the issue with the test lies in its method. Using this device allows the molecules to be identified, but does not provide any molecular information, including details about bonding.

Figure 13.

Wear scar on the lower specimen ×100 magnification.

Visual inspection reveals a different type of wear scar on the upper specimens at 5%, compared with the others, as shown in Figure 14. A smoother wear scar width and a different shape. As shown, this suggests a different wear mechanism. This could be due to a friction-induced chemical reaction that formed a tribofilm, altering the wear mechanism; however, this needs further investigation.

Figure 14.

Wear scars on upper specimens for all blends.

The lubricious properties of the MF, MTHF, and diesel blends result from complex tribological processes. The development of this research, along with studies conducted before and those to be conducted in the future, enables the identification of the best additives to blend with diesel. By increasing engine lifetime through a trend toward biofuel additives, OEMs will support rapid progress toward cleaner, longer-lasting engines. The reactions of the MF through the frictional process could not be evaluated on the HFRR, which will be essential to the validity of this additive, and therefore, research needs to be further developed in this area.

Conclusions

The objective of this paper was to determine the optimal blend ratios that yield the greatest improvement in tribological properties when used as a diesel additive. From testing using the HFRR and surface analysis with SEM and EDS, it was found that 2-Methyltetrahydrofuran has poor lubricating abilities. 5% MF/Diesel showed the lowest friction coefficient and wear scar, but left a deposit, unlike higher MF concentrations. This might be due to the change in the wear mechanism. The following conclusions can be drawn from this study:

5% MF blended with 95% Diesel showed the best frictional properties; however, it did not have the thickest film thickness. At all blends greater than 5%, MF increased the lubricity of diesel.

The boat conformation of the methyl group prevents good surface coverage, leading to far greater steric hindrance than with MF and requiring higher energy for any process. This meant its lubricious properties were poor, there was minimal change to the friction coefficient and MWSD of the control and a negative effect on the film thickness.

The planar MF molecular arrangement was responsible for the reduction in the friction coefficient compared to the control; however, the wear mechanism also contributed to the tribological properties.

The friction coefficient was not a function of consistent testing parameters; rather, it was determined by two conflicting factors. It is suspected that the inconsistent mechanism of wear influenced the frictional properties in an inverse relationship to the changing blend ratio, allowing for consistent results to be found, while the causes of the results varied.

Further research to obtain more information from surface analyses, such as surface topology, high-magnification SEM, and FTIR, would reveal in depth why the results appear the same and study the surface wear mechanism. Using the analysis technologies discussed above would also enable defining a relationship between the biofuel additive ratio and the black and auxiliary deposits observed. To investigate the different modes of wear, it is important to understand what mechanism of wear is developing and why. This research has found the lubricity of diesel with MF and MTHF at the tested blend ratios; however, a deeper understanding of tribological mechanisms is needed to develop a more robust hypothesis on the ideal blend ratios and the consequences of different modes of wear. The results showed that MF testing should be conducted with percentages ranging from 0 to 10 for diesel blends. EDS revealed what was present on the surface; the expected results were predominantly iron from the specimen, along with carbon and oxygen. To measure the tribofilm, XPS is recommended. It is a surface-sensitive, quantitative spectroscopic technique that determines the elemental composition, empirical formula, chemical state, and electronic state of the elements that form on the lower specimen during HFRR testing. This will provide spectrographic information and insight into the bonding types and reactions which lead to the development of a tribo-film on the surface of the lower specimen.