Journal Papers by Ramin Rahmani
A tribochemical analysis of lubricants from field trials of hydrogen assisted dual fuel internal combustion engines
Journal of Tribology, 2026
The paper reports on the tribological performance of lubricant samples siphoned from hydrogen-ass... more The paper reports on the tribological performance of lubricant samples siphoned from hydrogen-assisted diesel engines of trucks participating in a real-world field trial. Lubricant rheology, as well as the ability of the residual additive package in forming tribo-films generated through tribometry are studied. Marginal changes in lubricant viscosity are noted with the use of hydrogen-assisted combustion due to the relatively short trial period. However, there are differences in lubricant chemistry and frictional behaviour under boundary regime of lubrication ascertained using X-ray Photoelectron Spectroscopy (XPS). The tests show that friction increases as boundary interactions become dominant and higher coefficients of friction are consistently observed at lower applied loads under mixed and boundary lubrication conditions. The differences in tribological performance are more noticeable at higher contact pressures. The impact of hydrogen-assisted combustion on the performance of lubricants in formation of tribofilms under laboratory conditions is discussed. There are differences in frictional characteristics with the introduction of hydrogen even at the rather low hydrogen content in the dual-fuel intake. There are particular noticeable differences in mixed and boundary regimes of lubrication, owing to the variations in the surface topography and tribofilm compositions. The surface topography of samples and presence of anti-wear additive constituents on the surfaces post-tribometric tests vary with the sliding speed. Gradual use of hydrogen as an assistive fuel in existing heavy-duty vehicle fleets offers a pragmatic transitioning pathway towards hydrogen-powered engines for haulage and off-highway applications.
Applied Sciences, 2026
The European Union's regulatory mandate requirements for vehicular components include the integri... more The European Union's regulatory mandate requirements for vehicular components include the integrity of sealing performance, mitigating leaks from fuel tanks and transmission systems in order to guard against environmental pollution. Non-compliance can result in significant costs for the OEM and their supplier base. The majority of the reported research regarding leakage from radial lip seals focuses on static analysis of leakage under a given set of laboratory conditions. However, in practice, seal conjunctions are often subjected to significant excitations due to vehicular vibration. In the current study, the case of a front-wheel drive vehicle, equipped with three-axle accelerometers and subjected to a comprehensive road test, is used as the basis for the development of a realistic representative test rig. The test rig is developed using bespoke components from the vehicle under investigation to assess the impact of the encountered natural frequencies on sealing performance in controlled laboratory conditions, when the system is subjected to controlled excitation. Experiments are conducted to evaluate leakage at the transmission interface, focusing specifically on the sealing system's performance. The influence of driveshaft manufacturing processes using corundum grinding and subsequent surface topography upon leakage performance are also considered. Identified modal response frequencies are imposed upon the test rig using a shaker, whilst the seal leakage is measured. The importance of shaft roughness characteristics, such as topographical skewness upon seal leakage rate under various resonant conditions, are ascertained. The results indicate potentially significant leakage rates under excitation conditions, with a non-optimised shaft roughness profile.
Tribology International, Nov 29, 2025
High performance constant velocity joints (CVJs) operate under extremely harsh contact conditions... more High performance constant velocity joints (CVJs) operate under extremely harsh contact conditions, including high contact pressures, temperatures, and complex kinematics involving a significant degree of sliding. Comprehensive models capturing the complex interactions between component multibody dynamics, tribology, contact mechanics, and subsurface stress evolution have not been presented to date. This study introduces a novel, validated multiphysics framework that integrates tribological modelling with multibody dynamics and 3D subsurface stress analysis. The results reveal that under low friction conditions dynamic behaviour is largely invariant to in-cycle frictional variations. However, under boundary lubrication regime, friction significantly alters dynamics force distribution, leading to amplification of subsurface stresses. The subsurface stress model quantifies the impact of these, showing that increased friction can indirectly raise subsurface shear stress by up to 30 %, drastically reducing fatigue life. The results demonstrate that minimising friction within the joint is crucial for reducing subsurface stresses and extending its operational lifespan. This work provides the first fully coupled tribo-multibody-dynamic and subsurface stress analysis of high-performance CVJs. The framework lays the foundation for development of physics-based digital twins in tribodynamic systems.
Mechanical Systems and Signal Processing, 2025
This study focuses on gear whine noise of electric drive units (EDUs) with helical gear transmiss... more This study focuses on gear whine noise of electric drive units (EDUs) with helical gear transmission. A combined analytical and experimental approach is adopted to better understand dynamic transmission error (DTE) as the root cause of emitted gear whine noise. A fully analytical gear model based on potential energy method is developed to calculate time-varying meshing stiffness (TVMS). A lubricated contact model based on identification of regimes of lubrication is applied to precisely account for contact stiffness and damping variations. It is observed that local variations in lubricated contact generate undulations in DTE and dynamic load signatures. The frequency of the undulations, as observed from the comparison between the measured acceleration and sound pressure signals and the predicted DTEs, is determined by the resonance between the meshing frequency (including its multiples) and the harmonic frequencies of the gear pair. Greater amplitudes of undulations in lubricated contact stiffness are observed, followed by excessive vibrations in DTE, when multiples of meshing frequency coincide with gear pair harmonics and super-harmonics simultaneously. The superimposition effect exacerbates the vibrations indicating possibility of higher gear whine noise. Lower gearbox and meshing orders propagate as airborne noise, whilst higher frequency multiples are more noticeable as structural vibrations. Such comprehensive analysis linking the emitted gear whine noise and vibrations to the changes in the regime of lubrication in gear contacts is novel contribution of this research and has not hitherto been reported for helical gear whine noise and EDU NVH analyses.
Proc IMechE Part J: J Engineering Tribology, 2025
This study investigates the tribological performance of lubricants used in hydrogen-assisted dies... more This study investigates the tribological performance of lubricants used in hydrogen-assisted diesel engines with specific focus on their rheology, additives and the formation of tribo-films. While there are concerns about reduced lubricant viscosity and increased wear due to hydrogen combustion, the impact of hydrogen on lubricant quality and tribology in Internal Combustion Engines (ICEs) is a less-explored topic. As an initial step, this study provides an in-depth analysis of lubricant performance in diesel engines, both with and without hydrogen assistance. The study includes chemical evaluations, and controlled tribometric performance analysis of lubricants from refuse trucks after an operation interval equivalent to 2000 km. The results show marginal changes in the lubricant rheology with the use of hydrogen, but there are differences in frictional behaviour under mixed and boundary lubrication regimes. In addition, X-ray Photoelectron Spectroscopy (XPS) analysis shows variations in anti-wear additive adsorption on surfaces, with a noticeable difference between fresh and used oil samples, aligning with the lubricant analysis data. Despite a noticeable difference in measured friction in mixed and boundary regimes of lubrication, the chemical analysis and XPS results show only marginal differences in the anti-wear additive content and the associated tribofilms. The study demonstrates that changes in the chemical and rheological states of the lubricant affect frictional performance and interaction of additives with surfaces in mixed and boundary regimes of lubrication. Understanding these changes is crucial for the future development of suitable formulations for use in dual-fuel hydrogen-assisted ICEs.
Energy Conversion and Management, 2025
This comprehensive study provides a detailed Well to Wheels (WTW) Life Cycle Assessment (LCA) of ... more This comprehensive study provides a detailed Well to Wheels (WTW) Life Cycle Assessment (LCA) of various Heavy-duty Vehicles (HDVs) including a Long-Haul Truck (LHT), Intercity Bus (ICB), and Refuse Truck (RT) powered by different energy sources and fuels including electricity, hydrogen, methanol, Liquified Natural Gas (LNG), and Low Sulphur (LS) diesel for benchmarking. The findings show that Hydrogen from renewable sources offers the lowest WTW CO2, CH4, and NOx emissions, though its production is energy intensive. Methanol and hydrogen from Natural Gas (NG) exhibit the highest emissions due to high fuel consumption and energy intensive production processes. LNG shows lower CO2 and NOx emissions compared to LS diesel but higher CH4 emissions, necessitating improvements in LNG production. Electrically powered HDVs, despite reducing NOx emissions, produce comparable CO2 and higher CH4 emissions due to the current global electricity mix. Amongst the studied HDV types, RTs exhibit the highest WTW CO2 and energy consumption due to frequent stops and idling, while LHTs show the lowest emissions and energy consumption. LNG-fuelled RT and LHT reduce WTW CO2 emissions by 8% and 5.6%, and NOx emissions by around 31% and 33%, respectively, compared to LS diesel. The study underscores the need for tailored solutions based on HDV type, advancements in renewable energy infrastructure, and supportive policies to facilitate the transition to sustainable fuel technologies. Focus on developing infrastructure for production of hydrogen from renewable sources, supporting innovations in energy efficient fuel production technologies, and the need for enhancing energy efficiency of vehicular powertrain to achieve a sustainable HDV sector are also highlighted.
Journal of Tribology, 2025
Surfaces of air foil thrust and journal bearings in high-speed turbomachinery are coated to impro... more Surfaces of air foil thrust and journal bearings in high-speed turbomachinery are coated to improve their operational integrity, particularly when the aerodynamic load-carrying capacity is reduced during instances of startup and shutdown. Surface coatings, as protective barriers in air foil bearings, can mitigate the adverse effects of direct surface interactions on such occasions. This article provides an in-depth review of the body of important research conducted for the study of coated air foil thrust and journal bearings, highlighting the state of the art in coating technology. The review features the role of composite coatings, designed to provide favorable thermal, mechanical, and frictional characteristics. This article also highlights the trends in the selection of coatings for air foil bearings, pertinent to desired thermo-mechanical performance.
Tribology International, 2024
An analytical EHL model suitable for highly loaded point contacts operating in mixed and boundary... more An analytical EHL model suitable for highly loaded point contacts operating in mixed and boundary regimes of lubrication is presented. The results are compared and validated with the experimental measurements of friction coefficient from an MTM tribometer. Measurements are carried out upon a PAO40 oil at medium to high temperatures. In general, a good agreement in trend and magnitude over a wide range of operating conditions is observed. Both the experimental findings and the model predictions show an inverse relationship between friction and contact load in mixed and boundary regimes of lubrication, an effect not hitherto reported in literature.
Lubricants, 2024
This paper presents an investigation of the frictional behaviour of three-piece piston oil contro... more This paper presents an investigation of the frictional behaviour of three-piece piston oil control rings. A bespoke tribometer replicates the kinematics of the contact between a full oil control ring and the cylinder liner. The three-piece oil control ring is composed of two segments, separated by a waveform-type expander. The experimental results indicate the dominance of a mixed regime of lubrication throughout the stroke. This is particularly the case when the experiments are conducted at 80 ◦C, a typical engine sump temperature, when compared with those at 20 ◦C (a typical engine startup temperature in the UK in the summer). A mixed hydrodynamic analytical model of the oil control ring–cylinder liner tribological interface is employed to apportion frictional contributions with
their physical underlying mechanisms. Therefore, combined numerical and experimental investigations are extended to lubricant contamination/dilution by ethanol-based fuels. This study shows that the transition from E10 to E85 would have an insignificant effect on the friction generated in the oil control ring conjunction. This holistic approach, using a detailed predictive l mixed regime of lubrication model and a representative bespoke tribometry, has not hitherto been reported in the open literature.
Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 2023
The paper reports on the use of a developed novel and specific test-rig for the assessment of per... more The paper reports on the use of a developed novel and specific test-rig for the assessment of performance of radial lip seals of automotive transmissions, with particular emphasis on determination of thermal performance. The test-rig can accommodate original components from a donor vehicle, thus accurately represents vehicular conditions. The specific aim is to experimentally determine the effect of shaft surface roughness parameters on the generated contact temperature. Excessive exposure to high temperatures can accelerate degradation and aging of elastomeric seals, resulting in increased wear. Corundum and CBN (Cubic Boron Nitride) ground shafts are used to ascertain the effect of surface roughness parameters in the shaft seal conjunction. An analytical thermal model is also developed in combination with temperature measurements to determine the exact temperature in the lip seal-shaft contact. It is shown that depending on the type of shaft surface grinding process some potential correlation exists between roughness parameters and generated contact temperatures. It is also shown that the contact temperature can be several degrees Celsius higher than those measured in the proximity of the seal contact.
Tribology International, 2023
An integrated gear tribodynamics model is proposed for the study of EV powertrains' performance. ... more An integrated gear tribodynamics model is proposed for the study of EV powertrains' performance. The model considers the transient effects of lubrication regimes, non-Newtonian shear thinning, inlet shear heating, deformation states of asperities in mixed regime of lubrication and contact temperature using a set of analytical routines, which are computationally efficient. The proposed gear tribodynamics model provides a breakdown of the interdependency of these attributes and studies their impact on the performance of gear contacts. The results indicate that up to 30% of the contact load can be carried by asperities, of which 80% undergo elastoplastic deformation. In addition, the contribution of lubricant to contact stiffness can be greater than that of surface asperities by an order of magnitude.
Lubricants, 2023
The paper provides a commentary on the theme of “Current and Future Trends in Tribological Resear... more The paper provides a commentary on the theme of “Current and Future Trends in Tribological Research: Fundamentals and Applications”, which is a special feature issue commemorating the 10th anniversary of the journal, Lubricants. A historical discourse is provided regarding various aspects of tribology as a multi-disciplinary subject that interacts in an inter-disciplinary manner with many other subjects: multi-body dynamics, thermofluids and heat transfer, contact mechanics, surface science, chemistry, rheology, data science, and biology, to name but a few. Such interactions lead to many important topics including propulsion with different sources of energy, mitigating emissions, palliation of friction, enhancing durability and sustainability, optimization through detailed analysis, and the use of artificial intelligence. Additionally, issues concerning kinetics at various physical scales (from macroscale to
microscale onto mesoscale and nanoscale) affecting the kinematics of contacts are discussed. The broad range of considered applications includes vehicular powertrains, rotor bearings, electrical machines, mammalian endo-articular joints, nanobiological attachment/detachment, and locomotion. Current state-of-the-art tribological research is highlighted within a multi-physics, multi-scale framework, an approach not hitherto reported in the open literature.
Tribology International, 2023
A barrier to the adoption of battery electric vehicles is consumer range anxiety. This paper inve... more A barrier to the adoption of battery electric vehicles is consumer range anxiety. This paper investigates the driving range conceded as a result of the parasitic frictional losses incurred at the gear mesh in a two-stage battery electric vehicle transmission. Amelioration of the parasitic losses through changes to surface roughness and application of thin wear resistant coatings such as DLC are considered. The tribo-dynamics model of the gear frictional losses includes topographical structure of the respective surfaces characterised through surface specific fractal parameters. Analysis shows that the use of the investigated amorphous hydrogenated undoped DLC coating could potentially save 1.1 km of vehicle range during extra urban driving and 0.9 km under urban driving over a full battery discharge cycle.
Tribology International, 2023
The trend towards electrification presents new challenges in bearing design. One such considerati... more The trend towards electrification presents new challenges in bearing design. One such consideration is the occurrence of electrical discharge contact pathways, which can lead to surface damage. The current study presents a novel comprehensive multiphysics model, incorporating bearing dynamics, mechanics of lubricated rolling element-to-races contacts and electrical contact model for both DC and AC voltages. The model also includes both electrical resistance and capacitance effects in the bearing contacts. Key bearing vibration frequencies such as cage frequency and the bearing base natural frequency along with the voltage supply frequency are observed as influential in the electric current discharge. The developed model enables the prediction of rhythmic fluting patterns commonly observed in the failed bearing applications subject to electrical discharge.
International Journal of Engine Research, 2023
In the quest for decarbonisation, alternative clean fuels for propulsion systems are sought. Ther... more In the quest for decarbonisation, alternative clean fuels for propulsion systems are sought. There is definite advantage in retaining the well-established principles of operation of combustion engines at the core of future developments with hydrogen as a fuel. Hydrogen is envisaged as a clean source of energy for propulsion of heavy and off-road vehicles, as well as in marine and construction sectors. A source of concern is the unexplored effect of hydrogen combustion on dilution and degradation of engine lubricants and their additives, and consequently upon tribology of engine contact conjunctions. These potential problems can adversely affect engine efficiency, durability, and operational integrity. Use of different fuels and their method of delivery, produces distinctive combustion characteristics that can affect the energy losses associated with in-cylinder components and their durability. Therefore, detailed predictive analysis should support the developments of such new generation of eco-friendly engines. Different fundamental physics underpin the various aspects of a pertinent detailed analysis. These include thermodynamics of combustion, in-cylinder tribological interactions of contacting surfaces, and blowby of generated gasses. This paper presents such an integrated multi-physics analysis of internal combustion engines with focus on hydrogen as the fuel. Such an in-depth and computationally efficient analysis has not hitherto been reported in the literature. The results show implications for lubricant degradation due to the use of hydrogen in the performance of in-cylinder components and the underlying physical principles.
The Proceedings of the Institution of Mechanical Engineers, Part K: Journal of Multi-body Dynamics, 2023
Since Euler's original gyro-dynamic analysis nearly two and a half centuries ago, the use of mult... more Since Euler's original gyro-dynamic analysis nearly two and a half centuries ago, the use of multi-body dynamics (MBD) has spread widely in application scope from large displacement rigid body dynamics to infinitesimal amplitude elastodynamics. In some cases, MBD has become a multi-physics multi-scale analysis, comprising contact mechanics, tribo-dynamics, terramechanics, thermodynamics, biomechanics, etc. It is an essential part of all analyses in many engineering disciplines, including vehicle engineering. This paper provides an overview of historical developments with emphasis on vehicle development and investigation of observed phenomena, including noise, vibration and harshness. The approach undertaken is comprehensive and provides a uniquely focused perspective, one which has not hitherto been reported in the literature.
Mechanism and Machine Theory, 2023
Accurate prediction of constant velocity joint transient contact loading and complex kinematics i... more Accurate prediction of constant velocity joint transient contact loading and complex kinematics is necessary to improve joint design and prevent incurring failure from rolling contact fatigue. A detailed multibody dynamic model is presented for cross groove constant velocity joints used in high performance automotive racing applications, hitherto not reported in the open literature. The developed model includes detailed contact mechanics and friction models for all contacts made between the components. A novel semi-Hertzian contact model is adopted for the cage-race conjunction. Local contact kinematics is evaluated based on a methodology used in tribological analysis. The proposed model is validated against available computational models provided in literature for cross groove designs. Using the model, contact pressures occurring in the ball-race and ball-cage contacts are shown to reach levels as high as 4 and 5 GPa, respectively. The assumed value of friction coefficient is shown to have a strong influence on the predicted contact forces. Local contact motions are presented for the ball-race and ball-cage contacts, demonstrating complex motion which varies between pure rolling and pure sliding within a single cycle.
Tribology International, 2023
It is generally established that use of well-designed artificially enhanced surfaces in certain l... more It is generally established that use of well-designed artificially enhanced surfaces in certain lubricated conjunctions can potentially improve the tribological performance under specific operating conditions. However, there is a considerable lack of understanding of the effect of texture features, which are not exposed to conditions in the contact footprint, to the overall tribological performance. Three discs have been manufactured, one without texture features and two with texture features but in different locations with respect to contact footprint. The discs were tested in a ball on disc tribometer under representative contact conditions from practical application cases as such contacts can experience a multitude of regimes of lubrication during the operation. The contacts were lubricated by grease. Texture features placed directly beneath the contact were shown to provide a negative effect by increasing the measured coefficient of friction relative to the untextured disc. Textures placed outside, but at very close proximity of the contact footprint track on the disc, were shown to reduce the measured friction coefficient and wear simultaneously.
Surface Topography: Metrology and Properties, 2023
This study aims at introducing the potential to utilise transfer learning methods in the training... more This study aims at introducing the potential to utilise transfer learning methods in the training of artificial neural networks for tribological applications. Artificially enhanced surfaces through surface texturing, as an example, are investigated under hydrodynamic regime of lubrication. The performance of these surface features is assessed in terms of load carrying capacity and friction. A large performance dataset including bearing load carrying capacity and friction is initially obtained for a specific category of textures with rectangular cross-sectional profile through analytical methods. The produced bearing performance are used to train a neural network. This neural network was then trained further by a minimal set of performance measure data from an intended category of textures with triangular cross-sectional profiles. It is shown that the resulting neural network performs with acceptable level of confidence for those intended texture profiles when trained with such relatively low number of performance data points. The results indicate that fast analytical methods can potentially produce a large volume of training datasets, which effectively allows for use of relatively lower number of training data sets from the intended category, where creating data for trainings can be more complex or time consuming. Use of transfer learning method in tribological applications and use of bearing performance parameters, as opposed to bearing design parameters, for training the neural networks are the major novel contributions of this study, which has not hitherto been reported elsewhere.
Proceedings of the Institution of Mechanical Engineers, Part K: Journal of Multi-body Dynamics, 2023
In this paper, a mathematical model of a rotor-bearing system is presented. The model includes mo... more In this paper, a mathematical model of a rotor-bearing system is presented. The model includes modal elastodynamics of a flexible rotor as well as the in-plane radial dynamics of the bearing with a flexible outer race. Elastodynamics of the flexible shaft utilises a solution based on Green's function to provide a computationally efficient approach. The flexible bearing outer race is modelled using Timoshenko beam theory. The system model also includes detailed lubricated contact mechanics of balls-to-races contacts with viscous friction. Therefore, the rotor-bearing analysis represents a detailed multi-physics tribodynamics and modal elastodynamic responses of the system which closely represents broad-band vibration response of such systems in practice, an approach not hitherto reported in the literature. It is also demonstrated that the outer race flexibility changes the location of the stability orbital centres, as well as the spread of limit cycle vibrations. Furthermore, it accentuates the occurrence of multiples of ball pass frequency. The importance of integrated system dynamics and lubricated contact mechanics is highlighted, showing that although the elastodynamic response of the rotor's flexible elements may not be clear in the acquired vibration signal, its effect on energy efficiency of the system can be quite important.
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Journal Papers by Ramin Rahmani
their physical underlying mechanisms. Therefore, combined numerical and experimental investigations are extended to lubricant contamination/dilution by ethanol-based fuels. This study shows that the transition from E10 to E85 would have an insignificant effect on the friction generated in the oil control ring conjunction. This holistic approach, using a detailed predictive l mixed regime of lubrication model and a representative bespoke tribometry, has not hitherto been reported in the open literature.
microscale onto mesoscale and nanoscale) affecting the kinematics of contacts are discussed. The broad range of considered applications includes vehicular powertrains, rotor bearings, electrical machines, mammalian endo-articular joints, nanobiological attachment/detachment, and locomotion. Current state-of-the-art tribological research is highlighted within a multi-physics, multi-scale framework, an approach not hitherto reported in the open literature.