Knowledge

A wheel/rail friction coefficient that is too low can result in damage to the wheel and rail due to slips and slides, delays and safety concerns. A friction coefficient that is too high can result in excessive wear, noise and rolling contact fatigue. Changing contact and environmental conditions cause variations in wheel/rail friction, so friction management products, applied via wayside or onboard applicators, are used to either increase or decrease the friction coefficient so that an improved level is reached. They can be split into three classes; traction enhancers, lubricants and top-of-rail products (including water-based, oil/grease-based and hybrid products). This paper focuses on top-of-rail products and describes the different apparatus, contact conditions, product application methods and result interpretation that have been used to test these products and highlights the requirement for a more standardised test method. A proposed test method is outlined, which uses a twin disc test rig to collect “effective level of friction” and “retentivity” data to assess product effectiveness. More comparable and standardised data will ensure that maximum benefit is obtained from each set of results and help both product development and the approvals process.

Climate change demands urgent actions towards CO2 emission reduction. Through their effect on friction losses, new engine lubricants play a key role in reducing fuel consumption and, consequently, CO2 emissions. Besides oil viscosity optimization, friction contributions are primarily dependent on friction reducer (FR) chemistry, although secondary impacts exist for detergent, dispersant, and antiwear additives. Eni has been working for several years in the development of innovative friction reducer additives as well as in the definition of testing methods for evaluating the performances of a large number of molecules and selecting the most promising ones for engine or vehicle tests. According to this approach, a tribological method has been firstly set up by using the Mini Traction Machine (MTM); this equipment allows to measure friction coefficient under various operating conditions and can also reproduce the Stribeck curve, which embraces all the lubrication regimes, thus qualitatively predicting friction behaviour of a lubricant. The performances of a large quantity of candidate additives were evaluated, both as fresh and after appropriate aging. Among these, a very promising metal –free additive, derived from renewable sources, was selected and then put in low viscosity engine oils for the engine and vehicle tests evaluation; standard engine tests, like Sequence VIE and JASO M366 Fuel Economy, as well as chassis-dyno tests were carried out, obtaining results that meet API SP/ILSAC GF-6 and JASO GLV-1 limits. The same additive was also evaluated as fuel-borne FR in chassis-dyno tests based on an in-house procedure composed by a 48h running at low oil and coolant temperature, aiming at transferring the friction reducer additive into the oil, followed by different WLTCs for CO2 measurement and fuel consumption calculation. The additive in 95 RON gasoline was compared with the same fuel without additives. The promising behavior of friction reducer additives at the different scales is the subject of this paper aimed to give a valid support in the roadmap towards CO2 reduction.

Wear tests involving thousands of operating cycles can be impractical and costly. Therefore, the use of numerical methods to predict surface evolution due to wear can be a good alternative. A generalised three-dimensional finite element (FE) method is presented here to determine the wear of mechanical components. The method is tested and validated against the results obtained from the ball on disc sliding tests performed on different materials and in different conditions, with the aim to predict the wear profiles of the ball and disc. The wear simulation procedure is, for simplicity, based on Archard’s wear law and is implemented in a commercial FE package, ABAQUS. The developed methods are appropriate for 3D surfaces and the wear profiles are based on the contact pressures calculated on both surfaces. The surface evolutions of both contacting bodies are calculated simultaneously. The generation of artificial roughening of the worn surfaces due to numerical restrictions of element sizes is minimised using a local smoothening procedure. The developed methods are compared with the results obtained for reciprocating sliding tests, which result in wear of few microns wear depths, and continuous sliding tests, which result in tens of microns wear depths. The results were generally in good agreement with the test, with the maximum discrepancy of ~10% in all indicators used considering the statistical error margin of the tests. The developed method is versatile and applicable to a wide range of sliding wear tests. The model can be updated to consider both more complex material descriptions and to incorporate tribofilm or oxide layer growth or removal.

Mechanochemistry is known to play a key role in the function of some lubricant additives, such as the tribofilm growth of zinc dialkyldithiophosphate (ZDDP). This raises the intriguing possibility of tailoring the mechanochemical response of additives by modifying their alkyl substituents. Here, we study the tribofilm formation rate of ZDDPs containing several different alkyl groups on steel surfaces from a high-friction base oil. We use macroscale tribometer experiments under full-film elastohydrodynamic lubrication conditions to enable careful control of the temperature and stress during tribofilm growth. We show how the chain length and the presence of branches or bulky cycloaliphatic groups can lead to large differences in the temperature- and stress-dependencies of the tribofilm formation rate, which can be explained through variations in packing density, steric hindrance, and stress transmission efficiency. Our rate data are successfully fitted using the Bell model; a simple modification of the Arrhenius equation that is commonly employed to model the kinetics of mechanochemical processes. Using this model, we observe large differences in the activation energy, pre-exponential factor, and activation volume for the various ZDDPs. Our findings show how structure–performance relationships can be identified for lubricant additives, which may be useful to optimise their molecular structure.

Oral tribology receives growing attention in the field of food sciences as it offers great opportunities to establish correlations between physical parameters, such as the coefficient of friction, and sensory effects when interacting with components of the human mouth. One important aspect covers the astringency produced by wine, which can be described as the sensation of dryness and puckering in the mouth, specifically occurring between the tongue and the palate after swallowing. Therefore, this article aims at shedding some light on recent trends to correlate physical measures, such as the coefficient of friction derived by oral tribology, with prevailing theories on underlying physiological causes for sensory perception of wines. Some successful cases reported the potential of correlating wine astringency perception with the coefficient of friction in tribological experiments. Our critical assessment demonstrates that the findings are still contradictory, which urgently asks for more systematic studies. Therefore, we summarize the current challenges and hypothesize on future research directions with a particular emphasis on the comparability, reproducibility and transferability of studies using different experimental test-rigs and procedures.

Dry and lubricated highly deformed contacts appear in engineering, physics, and bio-(fluid)mechanics, with increasingly many applications where the soft materials can exhibit viscoelastic behaviour. In this work, the viscoelastic solid lubricated point contact problem has been studied theoretically and experimentally. An efficient visco-elastohydrodynamic lubrication (VEHL) numerical algorithm has been developed by implementing a novel viscoelastic deformation equation. The relevant dimensionless parameters of the VEHL problem are identified and a parameter study is presented showing the effect of solid viscoelasticy on the pressure and film thickness in the conjunction. In addition experimental results are presented for film thickness measurements in configurations of a PMMA ball rolling against a glass disc, and a steel ball rolling against a glass disc using optical interferometry. Finally the scaling of the pressure and film profiles in the inlet and outlet regions to the contact have been studied, investigating self similarity of the pressure and film solution in these regions. The results presented provide a good framework for the understanding and interpretation of viscoelastic solid effects on the film and pressure behavior in highly deformed (soft) lubricated contacts.

Polysaccharides are often used as rheology modifiers in multiphasic protein-rich food systems. Recently, proteinaceous microgels have garnered research attention as promising lubricating agents. However, whether proteinaceous microgels can be used to replace polysaccharides in a tribological context remains poorly understood. In this study we compared the flow and oral-tribological behaviour of Newtonian solutions of the polysaccharide dextran (D, 1–11 wt%) when combined with a dispersion of whey protein isolate (W, 1–13 wt%) or whey protein microgels (WPM, 41.7 vol%) and compared with microgels of D conjugated to W (Conj(D[11] + W[5])MG) or dispersions of WPM in W solutions. W and WPM alleviated frictional forces between elastomeric surfaces as well as biomimetic tongue-like surfaces in the boundary lubrication regime. Despite the negligible influence of D on the thin-film lubricity, its impact on viscous-facilitated lubricity was significant. The importance of measurements with the tongue-mimicked setup emerged where Conj(D[11] + W[5])MG did not show significant lubricity enhancement despite its outstanding performance with conventional tribo-testing setups. By optimising a combination of WPM and non-microgelled W, we demonstrate that a combined viscous and thin-film lubricity could be achieved through a single-protein-component without the need of polysaccharides. The dispersions of WPM (41.7 vol%) deliver the same flow and viscous-friction behaviour to that of 5 wt% D and excel in thin-film lubricity. These findings pave the way towards design of processed foods with clean labels, taking advantage of using a single proteinaceous moiety whilst delivering enhanced lubricity and viscosity modification without the need of any additional thickener.

Orange oil was extracted by steam distillation from the peels of oranges produced as waste in the orange juice factories. This raw orange oil, a potential source for biojet fuel, was analysed by FT-IR and GC-MS, and compared with distilled orange oil and pure d-limonene, which is its main chemical constituent. Both distilled orange oil and d-limonene were hydrogenated under reaction conditions (from 3 to 18 bar) which are mild enough to be industrially feasible, to improve its properties, especially to reduce their sooting tendency. Some important properties such as density, viscosity, heating values, lubricity, flash point, crystallization onset temperature, and smoke point were measured for hydrogenated orange oil and d-limonene (as a reference for comparison) at different conversions. These hydro-biofuels were blended with Jet A1 to check their suitability as biobased blending components for aviation. Based on the results obtained for the main aviation fuel properties, it is concluded that up to 15 vol% of partially hydrogenated orange oil could be blended with Jet A1 without any significant drawback for the performance of the actual airplanes. Flammability reduction systems would be needed to further increase the blend proportion of this drop-in biofuel in Jet A1.

The chemistry and structure of base oil and polymer additive molecules in lubricants directly affect key performance metrics such as viscosity index, thickening efficiency, and traction coefficient. However, the relationship between molecular properties and these metrics is still not fully understood, inhibiting the design of new fluids with potentially improved performance. This study used molecular dynamics simulations to identify structure–property-function relationships for model lubricants consisting of branched and linear polymers with chemistries consistent with commercially available products. First, fluids with similar kinematic viscosities at 100 °C were formulated with five different polymers. Then, the simulation-calculated Newtonian viscosities at 40 and 100 °C, viscosity index, thickening efficiency, and traction coefficient in full film lubrication at 40 °C were validated by direct comparison to experimental data. Next, the molecular origins of differences in the viscosity index, thickening efficiency, and traction coefficient between the fluids were investigated by calculating multiple structural properties of the simulated polymers. Finally, the simulations were used to develop simple empirical models using the best subset linear regression analysis to rapidly predict viscosity index, thickening efficiency, and traction coefficient. The atomistic simulations and empirical models developed in this work can ultimately guide the design of new lubricants or additives.