Ramin Rahmani | Loughborough University (original) (raw)

Journal Papers by Ramin Rahmani

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.

Tribology International, 2022

Predictions are provided for mixed elastohydrodynamic conditions of meshing hyoid gear teeth pair... more Predictions are provided for mixed elastohydrodynamic conditions of meshing hyoid gear teeth pairs of light truck differentials. Under transient conditions, pertaining to vehicle cruising or in urban driving, the lubricant is subjected to non-Newtonian traction. The meshing teeth are subjected to complex kinematics, comprising rolling, sliding and squeeze film motions. Furthermore, instantaneously varying angled lubricant flow into the contact constitutes a precession of contact footprint, further complicating the prevailing conditions. The inclusion of all these interacting kinematics shows that the lubricant film thickness, shear characteristics and transmission efficiency are significantly affected by the ensuing transience. This cannot be adequately represented by the usual quasi-steady analyses. The in-depth detailed analysis of the hypoid gear pairs of vehicular differentials constitutes the main contribution of the paper.

Lubrication science, 2022

Dust stop seals are widely used in powder and rubber mixing industries. Design of the sealing sys... more Dust stop seals are widely used in powder and rubber mixing industries. Design of the sealing system requires a continuous supply of pressurised lubricant, which is not recycled because of the risk of contamination. There is also the potential of large volume leakage of oil due to poor sealing, increasing operational costs and necessitating remedial measures to avoid environmental protection. Furthermore, the seal faces are prone to failure in relatively short periods of time due to reduced gap and lubricant leakage. The paper presents an analytical method and numerical predictions based on Reynolds equation under combined hydrodynamic and hydrostatic conditions with the entrant lubricant through hydraulically loaded feedholes. The validity of these methods is ascertained through comparison with a more complex but timeconsuming solution of Navier-Stokes equations. The numerical predictions allow for determining the prevailing tribological contact conditions and assessing its suitability for evaluating the sealing performance of mixing machinery.

The Proceedings of the Institution of Mechanical Engineers, Part K: Journal of Multi-body Dynamics, 2022

Constant Velocity Joint (CVJ) mechanisms enable torque transmission between two shafts at a fixed... more Constant Velocity Joint (CVJ) mechanisms enable torque transmission between two shafts at a fixed or variable angle. Cross groove CVJs are typically used in high performance automotive applications due to their versatility and light weight. Critical failure modes, such as pitting or abrasive wear, occur due to the harsh tribological conditions at ball reversals. In this research, an existing mathematical model is developed further for the case of cross groove CVJs including an accurate contact mechanics model. The developed model is validated against a published data set from literature. Surface topography of worn raceways are experimentally measured and the results from the developed model are corroborated with the measured surface parameters. This improved model shows the correlations between predicted contact force variation and wear scar depths during ball reversals, hitherto not reported in the literature.

proceedings of the institution of mechanical engineers, part D: journal of automobile engineering sjr, 2022

There is an ever-increasing stringent directive for environmental protection, even for the previo... more There is an ever-increasing stringent directive for environmental protection, even for the previously rather negligible lubricant leakage from vehicular drivetrains. Leakage from transmissions and differential units are no longer permitted and can lead to excessive warranty costs. Additionally, the trend in component downsizing, including for sealing systems has indirectly altered the leakage behaviour of transmissions. To guarantee leakage-free operation, it is important to establish standardised methods of manufacturing processes, particularly for surface topography of shafts. The paper investigates the leakage from two different shaft machining processes: corundum and cubic boron nitride (CBN) grinding. The performance of passenger car transmission sealing system is ascertained with the development and use of a novel detailed representative fully instrumented test rig. A test protocol is set up to assess the performance of shafts during break-in period and correlate shaft topographic data with its measured performance. The results show that the measured oil leakage correlates well with the shaft surface skewness whilst no particular trend is observed with Ra, Rz and shaft lead angle values, which form the basis of most current standards used in industry. Overall, it was observed that shafts with negative skewness in their roughness profile produced lesser leakage. A basic analytical predictive method is also developed which corroborates with the measured data.

Nonlinear Dynamics, 2022

Noise, vibration, and harshness (NVH) issues pose considerable challenges for electric vehicle po... more Noise, vibration, and harshness (NVH) issues pose considerable challenges for electric vehicle powertrain engineers. Gear vibrations generate an intrusive gear whine noise, with significant impact on the sound quality of electric powertrains. Dynamic transmission error (DTE) is the most quantitative indicator for gear NVH. Backlash, time variable meshing stiffness and damping contribute to DTE. Hence, a better understanding of these excitation sources is essential. A gear tribodynamics model is developed using potential energy method to estimate time variable meshing stiffness (TVMS). A fully analytical time-efficient model is proposed for lubricated contact stiffness based on transitions in the regimes of lubrication. The model accounts for the combined effects of surface elasticity and lubricant stiffness. Film thickness and damping coefficients are transiently updated at each instant during meshing cycle. The predictions from this model are compared with measured results from the literature and predicted results from Hertz contact model. The lubricated contact model successfully shows the contribution of the lubricant stiffness to TVMS and its variations with elasticity and viscosity parameters during meshing cycle. Gear harmonic and super-harmonic resonances are accurately estimated in terms of amplitude, frequencies and stiffness softening nonlinearities. Time history responses and phase-displacement diagrams show good agreement with the gear dynamics response at the main harmonic and second superharmonic frequencies. The proposed model has a reasonable accuracy, significantly better than those from Hertzian contact models, and is considerably time efficient in comparison to numerical EHL solvers.

Energy Conversion and Management, 2025

Journal of Tribology, 2025

Tribology International, 2024

Lubricants, 2024

Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 2023

Tribology International, 2023

Lubricants, 2023

Tribology International, 2023

International Journal of Engine Research, 2023

The Proceedings of the Institution of Mechanical Engineers, Part K: Journal of Multi-body Dynamics, 2023

Mechanism and Machine Theory, 2023

Tribology International, 2023

Surface Topography: Metrology and Properties, 2023

Proceedings of the Institution of Mechanical Engineers, Part K: Journal of Multi-body Dynamics, 2023

Tribology International, 2022

Lubrication science, 2022

The Proceedings of the Institution of Mechanical Engineers, Part K: Journal of Multi-body Dynamics, 2022

proceedings of the institution of mechanical engineers, part D: journal of automobile engineering sjr, 2022

Nonlinear Dynamics, 2022

Lubrication, Maintenance and Tribotechnology (LUBMAT 2023), 17-19th July, Preston, UK, 9 pp., 2023

A common strategy to reduce engine parasitic power losses is to decrease pumping and viscous fric... more A common strategy to reduce engine parasitic power losses is to decrease pumping and viscous friction losses through use of a low viscosity engine oil. However, reducing lubricant viscosity can also decrease the contact load carrying capacity, thus exacerbating direct interaction of contacting surfaces. This leads to boundary frictional losses in contacts prone to mixed regime lubrication. As a result, detailed experimental and modelling studies of engine component frictional behaviour is required to ensure the engine level trade-offs. This paper presents a combined experimental and numerical investigation of frictional behaviour of three-piece piston oil control rings. A bespoke tribometer replicates 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 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 the case of 20 °C (a typical engine start-up temperature in the UK in the Spring). A mixed hydrodynamic numerical model of the oil control ring-cylinder liner tribological interface is employed to apportion frictional contributions with their physical underlying mechanisms. The combined experimental-predictive approach provides key information for engine designers when considering the efficiency trade-offs.

SAE 2020-01-1508, 2020

Automotive clutches are prone to rigid body torsional vibrations during engagement, a phenomenon ... more Automotive clutches are prone to rigid body torsional vibrations during engagement, a phenomenon referred to as take-up judder. This is also accompanied by fore and aft vehicle motions. Aside from driver behaviour in sudden release of clutch pedal (resulting in loss of clamp load), and type and state of friction lining material, the interfacial slip speed and contact temperature can significantly affect the propensity of clutch to judder. The ability to accurately predict the judder phenomenon relies significantly on the determination of operational frictional characteristics of the clutch lining material. This is dependent upon contact pressure, temperature and interfacial slip speed. The current study investigates the ability to predict clutch judder vibration with the degree of complexity of the torsional dynamics model. For this purpose, the results from a four and nine degrees of freedom dynamics models are compared and discussed. Subsequently, the predictions are compared with the acquired data from an automotive driveline test rig. It is shown that the complexity of the dynamic model, intended for the study of a clutch system, can be as important as the thermo-mechanical and frictional properties of the clutch friction lining material.

SAE 2020-01-1504, 2020

Turbochargers are progressively used in modern automotive engines to enhance engine performance a... more Turbochargers are progressively used in modern automotive engines to enhance engine performance and reduce energy loss and adverse emissions. Use of turbochargers along with other modern technologies has enabled development of significantly downsized internal combustion engines. However, turbochargers are major sources of acoustic emissions in modern automobiles. Their acoustics has a distinctive signature, originating from fluid-structure interactions. The bearing systems of turbochargers also constitute an important noise source. In this case, the acoustic emissions can mainly be attributed to hydrodynamic pressure fluctuations of the lubricant film. The developed analytical model determines the lubricant pressure distribution in the floating journal bearings used mainly in the modern turbocharges. This allows for an estimation of acoustic emissions. The use of such an analytical approach is computationally efficient when compared with full numerical analysis approaches, whilst also providing reliable predictions. The results from the developed analytical model are used to determine the power loss as well as sound pressure levels generated in the turbocharger bearings due to oil flow which can be correlated with the acoustic emissions of turbochargers.

ASME 2013 International Mechanical Engineering Congress and Exposition, 2014

The main function of piston compression ring is to seal the space between the piston and the line... more The main function of piston compression ring is to seal the space between the piston and the liner, acting as slider bearing, subjected to reciprocating motion. The compression ring-cylinder liner conjunction has been extensively studied and it is responsible for a significant part of the total frictional parasitic power losses of an engine. Paradoxically, the required sealing function of the compressions ring can result in increased friction. Therefore, in order to improve engine efficiency, it is important to fundamentally understand and subsequently palliate some of these losses. Another problem in any slider bearing-type contact is lubricant film rupture and cavitation in the conjunctional outlet zone, reducing load carrying capacity and potentially leading to erosion damage. A cavitation model presented in two-phase flow CFD analysis of the ring-bore contact under isothermal conditions. Liquid flow is modelled as a continuous phase and a dispersed phase, representing cavitation bubbles. Many of the fundamental physical processes assumed to take place in cavitating flows are incorporated into the model.

45th Leeds-Lyon Symposium on Tribology: Smart Tribology Systems, 2018

Friction modifiers are one of most important additives in formulated lubricants for reducing para... more Friction modifiers are one of most important additives in formulated lubricants for reducing parasitic frictional losses in boundary and mixed regimes of lubrication. A significant amount of energy is lost due to generated friction in internal combustion engines. These losses account for 15% of energy losses, which can rise to 20-30% in urban driving cycles, of which 40-55% are due to piston-cylinder system [1], where 30% of all these losses have been shown to occur during piston reversal at the TDC in transition from compression to power stroke, where the regime of lubrication is predominantly mixed or boundary [2].

45th Leeds-Lyon Symposium on Tribology: Smart Tribology Systems, 2018

The tribological behaviour of gear teeth contacts in high performance transmissions is viewed as ... more The tribological behaviour of gear teeth contacts in high performance transmissions is viewed as a lubricant-surface system. Physiochemical and bulk rheological behaviour of the lubricant and mechanical and topographical variations of adjacent surfaces means that the frictional performance of the system as a whole is subject to continual change. Therefore, a detailed experimental methodology is required to accurately measure and characterise lubricant-surface system evolution and determine the frictional characteristics. A multitude of analysis techniques are used. The paper presents analysis of a high performance lubricant-surface system for multiscale frictional performance, wear and chemical interactions.

4th Biennial International Conference on Powertrain Modelling and Control (PMC 2018), 2018

Downsizing of transmission boxes and economic pressures in manufacture has gradually led to chang... more Downsizing of transmission boxes and economic pressures in manufacture has gradually led to changed sealing conditions in passenger cars. Radial shaft seals are widely used in power transmission systems to prevent oil leakage from the system. Leaking transmission systems can lead to both environmental pollution and excessive warranty costs. It is essential to develop a dedicated experimental rig to simulate the transmission system operation under controlled environment, providing access for various sensors and data acquisition systems. A pre-requisite for representative rig design is to determine the in situ vehicle conditions which must be replicated in such a design. The paper reports on the evaluation of these in situ conditions, particularly the imposed vehicular vibration conditions.

4th Biennial International Conference on Powertrain Modelling and Control (PMC 2018), 2018

Prediction of frictional losses in elastohydrodynamic lubricated contacts is of particular import... more Prediction of frictional losses in elastohydrodynamic lubricated contacts is of particular importance from the viewpoint of energy efficiency, thus reduced levels of emissions. There are increasingly stringent government regulations. Thus, the industry strives to identify potential losses and improve energy efficiency. Most losses occur in a large number of load bearing conjunctions in all forms of mechanisms and machines. These losses are affected by the operating conditions, such as applied load, contact kinematics and generated temperature. Prediction of prevailing conditions, such as generated pressures, film thickness is the prelude to evaluation of frictional power losses. Many contact conjunctions in vehicular drivetrains are subjected to elastohydrodynamic conditions, the fundamental aspects of which are still evolving. In particular, effective prediction of performance of elastohydrodynamic lubricated (EHL) contacts is subject to inclusion of realistic contact conditions, particularly with respect to inlet and outlet boundary conditions as well as kinematics of contact. This paper demonstrates the importance of boundary conditions on predictions of prevailing situations.

3rd International Brazilian Conference on Tribology – TriBoBR 2018, 2018

The primary function of the piston compression ring of an internal combustion engine is to seal t... more The primary function of the piston compression ring of an internal combustion engine is to seal the combustion chamber. This is achieved through the conformity of the piston, ring and liner. The conformity between the liner and ring is improved by the radial force produced by the gas pressure and ring tension. The radial loading however creates parasitic frictional losses, particularly in the vicinity of piston reversal where lubricant entrainment reduces. The frictional losses created in this conjunction account for up to 7-8% of total engine frictional losses. During reversal, the lubrication condition becomes more severe as the entrainment speed decreases the formation of a tribofilm becomes critical for reduction of wear and friction. There are a wide range of lubricant additives, however those of particular interest in the current work are dispersants [4], organic friction modifiers, inorganic friction modifiers [7] and anti-wear additives.

SAE 2018-01-1475, 2018

Lightly damped non-linear systems such as vehicular drivelines undergo a plethora of Noise, Vibra... more Lightly damped non-linear systems such as vehicular drivelines undergo a plethora of Noise, Vibration and Harshness (NVH) problems. The clonk phenomenon is one concern which occurs as the result of impulsive torque input in the form of sudden clutch actuation or throttle tip-in and back-out. The resulting impact of meshing gear pairs propagate structural waves down the driveline. With lightly damped thin-walled tubes having high modal density, elasto-acoustic coupling occurs. High frequency noise emission is of metallic nature and quite disconcerting to vehicle occupants as well as passers-by. It is perceived as structural failure and/or poor-quality build. Therefore, the occurrence of the phenomenon is a concern to vehicle manufacturers and progressively constitutes a warranty concern. This paper investigates the clonk phenomenon through use of a long-wheel base rear drive light truck test rig. The investigation uses psychoacoustic metrics to establish the severity of clonk noise in different maneuvers. This is an attempt to quantify the effect of transient clonk event which is usually ascertained through subjective evaluation ratings in a customer/jury clinic in industry. Alternatively, detailed numerical analysis is carried out with parametric studies to quantify different clonk events, a very time consuming approach which is not usually correlated with occupants' perception of the event. Therefore, the experimental NVH monitoring-psycho-acoustic approach is new for the case of clonk and not hitherto reported in literature. The study corroborates the results and conclusions of previous work, pointing to the loudness and sharpness of high frequency short-lived "metallic" response, which is attuned to human aural perception. The psychoacoustic analysis has shown this to be as the result of short duration hard impact (accelerative period). This hard impact duration accounts for the bulk loudness and sharpness of the overall event. Therefore, effective palliation should focus on the attenuation of particular frequency bands, which carry the main contribution to loudness and sharpness.

Clutches are commonly utilised in passenger type and off-road heavy-duty vehicles to disconnect t... more Clutches are commonly utilised in passenger type and off-road
heavy-duty vehicles to disconnect the engine from the driveline and
other parasitic loads. In off-road heavy-duty vehicles, along with fuel
efficiency start-up functionality at extended ambient conditions, such
as low temperature and intake absolute pressure are crucial. Off-road
vehicle manufacturers can overcome the parasitic loads in these
conditions by oversizing the engine. Caterpillar Inc. as the pioneer in
off-road technology has developed a novel clutch design to allow for
engine downsizing while vehicle’s performance is not affected. The
tribological behaviour of the clutch will be crucial to start
engagement promptly and reach the maximum clutch capacity in the
shortest possible time and smoothest way in terms of dynamics. A
multi-body dynamics model of the clutch system is developed in
MSC ADAMS. The flywheel is introducing the same speed and
torque as the engine (represents the engine input to the clutch). The
hydraulic pressure is applied behind the piston to initiate the
engagement. The angular motion of the plates is supported by friction
torque between the plates and friction linings. The conjunctions
between paper-based linings and steel plates are designed to be dry.
Friction (the most significant tribological feature of the linings in
torque transmission) is measured in a pin-on-disc tribometer and
mapped into the dynamics model in MSC ADAMS. The pin-on-disc
tribometer is able to capture the variation of friction coefficient with
contact pressure and sliding velocity. The surface topography is
obtained experimentally to examine the consistency of surface
properties. The normal pressure and tribology of the contacting
components determines the engagement time, clutch capacity and
dynamic behaviour of the clutch.

Sixth World Tribology Congress 2017 (WTC), Beijing, China, 17-22nd September, 2017

Compact light weight and dry sump (significantly reduced volume of lubricant) are the desired att... more Compact light weight and dry sump (significantly reduced volume of lubricant) are the desired attributes for high performance racing transmissions, whilst improving upon efficiency and reliability remain paramount objectives. The complex multi-objective nature of this task points to an integrated approach to lubricant-mechanical system optimisation. The extreme operating conditions in racing transmissions, such as contact kinematics and thermal loading present significant tribological challenges. Thin lubricant films in non-Newtonian shear are subjected to mixed thermoelastohydrodynamic regime of lubrication. Under these conditions boundary active lubricant species often determine the contact tribological performance rather than the bulk rheological properties of the lubricant itself. Therefore, the interaction of lubricant additive package with the contacting solid surfaces is the key to an optimised solution. The paper investigates the lubricant-surface interfacial effect upon frictional characteristics in contact conditions which are representative of gear teeth meshing conditions in high performance transmissions. The study uses pin-on-disc tribometry. As the contact conditions are mainly governed by the formation of surface-adhered tribo-films, their effect upon frictional characteristics is further investigated through use of atomic force microscopy (AFM) in lateral force mode (LFM). A test procedure is presented to benchmark lubricant additive package-surface combinations for improved tribological performance. The investigation takes into account surface material, surface topography and lubricant additive package, all of which affect the tribo-chemical absorption or bonding of a thin film to the contacting surfaces. The test protocol also includes surface chemical spectrometry and Scanning Electron Microscopy (SEM). The presented methodology has not hitherto been reported in literature.

Sixth World Tribology Congress 2017 (WTC), Beijing, China, 17-22nd September, 2017

Improving fuel efficiency of automotive engines and reducing emissions are key development target... more Improving fuel efficiency of automotive engines and reducing emissions are key development targets. In a typical light duty automotive engine, there is 50-60% thermodynamic heat loss from the cooling system and exhaust gasses [1]. The frictional losses accounts for almost 15-20% of all engine losses [2]. This can increase up to 20-30% in urban driving cycle [3], of which 45% is due to the frictional loss of the piston assembly [1]. The piston-ring-liner interface in a typical internal combustion engine results in 7-8% of energy loss [4,5]. Therefore, this provides an opportunity for efficiency enhancement. It is essential to design systems to minimise frictional losses in piston-cylinder system [6].

CITATIONS 0 READS 53 6 authors, including:

A tribo-dynamics model of a differential hypoid gear pair is presented, integrated with lubricate... more A tribo-dynamics model of a differential hypoid gear pair is presented, integrated with lubricated contact of meshing teeth pair with lubricants of varying rheological properties. Particular attention is paid to the effect of lubricant formulation and gear geometry on the system efficiency. The influence of gear torsional dynamic response is taken into account in a 4-Degree of Freedom (DoF) model. The contact geometry and kinematics of the hypoid gear pair are estimated, using Tooth Contact Analysis (TCA). Two fully formulated gear lubricants of the same viscosity grade (SAE 75W-90) blended with the same additive pack, but with different types and concentration of viscosity modifier (VM) are considered. Conjunctional friction is predicted for viscous shear of fully characterised lubricants as well as boundary interaction of rough surfaces. The results show loss of friction resulting in resonant response of the gear pair with impact of meshing teeth exhibiting non-linear jump phenomenon. The predictions also show that lubricants with higher pressure – viscosity (PV) coefficients tend to exhibit increased power loss.

The piston ring pack accounts for a disproportionate amount of the total engine frictional losses... more The piston ring pack accounts for a disproportionate amount of the total engine frictional losses. The frictional behaviour of piston rings is significantly affected and governed by its flexible dynamics. The dynamically changing shape of the ring determines its contact geometry with the cylinder liner and hence affects the frictional losses. The compression ring undergoes a multitude of complex motions during the engine cycle prescribed by the gas pressure, contact reaction, ring tension, friction between the ring and its groove and inertial forces that excite a plethora of the ring’s modal responses. This adversely compromises the functionality of the ring through a number of undesired phenomena such as ring flutter, twist, rotation and jump. Therefore, a prerequisite for improving the prediction of tribological conditions is an accurate determination of the ring’s elastodynamic response. This paper presents a methodology to directly solve the governing differential equations of motion for different forms of beam cross-section, where the shear and mass centres are not coincident, typical of the complex cross-sections of a variety of different piston compression rings. Combined numerical and experimental investigations are undertaken to determine the dynamic behaviour of the compression ring.

Increased machine performance through reduction of drivetrain power losses is an important goal i... more Increased machine performance through reduction of drivetrain power losses is an important goal in powertrain engineering. One key area of power loss in the driven axles of heavy on-road vehicles and off-highway vehicles is the disengaged wet brake conjunctions. The resultant power loss, particularly under cold start conditions, can be quite significant. The addition of patterned grooves into the brake friction linings assists lubricant flow to dissipate heat during contact, which complicates the prediction of performance, making design improvement a multi-variate problem. A Reynolds-based numerical model with the inclusion of lubricant inertial terms is developed, allowing time efficient prediction of the conjunctional torsional viscous losses. The numerical model is validated with CFD as well as experimental measurements, using a developed component based test rig. Good agreement is found for predictions against measurements for lower viscosity lubricant flow at higher bulk oil temperatures. The results show deviations at lower temperatures promoting higher viscosity inlet starvation, which is not taken into account with the assumed fully flooded inlet.

The frictional performance of the clutch mating surfaces varies transiently during clutch engagem... more The frictional performance of the clutch mating surfaces varies transiently during clutch engagement. The friction characteristics depend on the clamp load, relative slip speed of the mating surfaces and the generated contact temperature. These operating conditions affect the characteristics and lead to clutch take-up judder during engagement. To study this phenomenon as well as issues relating to clutch wear, it is essential to evaluate the friction lining characteristics. This paper replicates the clutch conditions through careful experimentation with a fully instrumented pin-on-disc tribometer. It is shown that the kinetic coefficient of friction for the clutch friction lining material in contact with the sinusoidal wavy and rough surface of pressure plate decreases with rising contact temperature and increasing slip speed.

Poor lubrication can occur under reciprocating motion where the momentary cessation of sliding le... more Poor lubrication can occur under reciprocating motion where the momentary cessation of sliding leads to a lack of lubricant entrainment into the contact. This condition occurs in many applications including piston-cylinder systems at and near the top and bottom reversal points. However such surfaces contain larger-scale features than the typical surface roughness features mainly produced through honing or cross-hatching of the cylinder liner. A development in modelling large scale roughness and surface features could lead to improvements in engineering the contacting surfaces. A combined numerical and experimental approach for the solution of a 2D average flow model, originally proposed by Patir and Cheng [1-2], is presented in the current study. The numerical approach is aimed at predicting the lubrication in an in-house reciprocal sliding test rig. The test rig is designed to simulate reciprocal motion of the piston compression ring in an internal combustion engine near the reversals. The study considers the effects of created large scale surface features, such as cross-hatching and honing marks, on the flow factors which represent the surfaces in real engine applications. To obtain the problem-specific flow factors, which take into account the larger surface features, a larger set of areas are chosen for statistical analysis. Surfaces representing typical real engine components were created and topographically measured in detail. The flow factors and the associated numerical approach were then verified through comparison with the friction results from the sliding test rig. The proposed approach indicates a better agreement with the experimental results compared with the cases in which the conventional flow factor proposed by Patir and Cheng were used [1-2]. References: [1] Patir, N. and Cheng, H.S., 1978, " An average flow model for determining effects of three-dimensional roughness on partial hydrodynamic lubrication " , J.

A fully coupled tribo-dynamic model, capable of predicting the inefficiency and dynamic response ... more A fully coupled tribo-dynamic model, capable of predicting the inefficiency and dynamic response of automotive differential hypoid gear pairs, is presented in this study. A gear dynamics solver is coupled with an analytical friction solver, which calculates the viscous shear, as well as the boundary conjunctional friction force. The time varying geometry and contact characteristics of the hypoid gear pair are taken into account by using realistic data available in the literature. The rheological models employed cover a range of two different behaviors: Newtonian and non-Newtonian Eyring (shear thinning). The Chittenden-Dowson equation is used to calculate the central film thickness of the elasto-hydrodynamic teeth conjunctions. The boundary friction force is calculated using the Greenwood & Tripp model. Finally, the actual surface topography of a run-in hypoid gear is obtained using a stylus profilometer. The results indicate an overestimation of the viscous friction by the Newtonian model, as opposed with the non-Newtonian model, mainly due to shear thinning effects. Comparative studies are performed for different operating conditions, namely near or away from resonance, as well as for conditions corresponding to a non-linear sub-harmonic resonance. The frictional damping effect on the dynamic transmission error, which is an indication of the NVH response of the gear pair, is also examined.

Proceedings of the UNIfied Conference of DAMAS, IncoME and TEPEN Conferences (UNIfied 2023). TEPEN IncoME-V DAMAS 2023 2023 2023. Mechanisms and Machine Science, vol 152. SpringerCh, am., 2024

The treatment of tissue damage caused by pressure ulcers (also known as bed sores) and urinary tr... more The treatment of tissue damage caused by pressure ulcers (also known as bed sores) and urinary tract infections costs the global medical industry more than £100 million every day. This research investigates the hypothesis that terminal tissue damage is caused by excessive shear stress rather than normal contact pressure. Therefore, this study considers the effect of normal and shear stress components on cell health through a combined experimental, analytical, and numerical modelling approach. Finite Element Analysis has been employed to understand the effect of normal and shear (tangential) forces on a cell structure with a 100-micrometre diameter. The structure of the cell was represented by the six major cell components: membrane, actin cortex, cytoplasm, microtubules, intermediate filaments, and nucleus. The initial model predictions show that the cell membrane and actin cortex are penetrated by the microtubules at a cell deformation of 16 micrometres. A Von Mises analytical approach was used to determine the yield stress at cell death. The study concludes that the application of shear force during compression increases the rate of cell death.

Fundamental of Tribology, 2018

Along with the pursuit of coatings to enhance tribological performance and development of formula... more Along with the pursuit of coatings to enhance tribological performance and development of formulated lubricants with appropriate additive packages, surface texturing techniques have also been developed to guard against direct interaction of contacting surfaces. We have already shown (chapter 1) that mixed and boundary regimes of lubrication occur as the result of any direct interaction of rough surfaces. This increases the generated friction and reduces the system efficiency, as well as giving rise to wear and other forms of surface damage. The surface textures act as small reservoirs of lubricant, under conditions which retention of a coherent film of lubricant would not be expected, such as in cessation of rolling and/or sliding motion or in the presence of high contact loads. In chapter 12 we show the role of surface textures in piston-cylinder system and its beneficial effect in terms of reduced friction, thus power loss. In chapter 12 we provided results of some numerical analysis for textured surfaces as well as some experimental benefit of their effect in terms of reduction of power loss. However, the numerical predictive approach is quite complex and leads to long computation times. Furthermore, not everyone has access to such complex numerical models. Therefore, here we provide some representative analytical methods.

Tribology and Dynamics of Engine and Powertrain: Fundamentals, Applications and Future Trends, 2010

The application of textured surfaces in tribology has recently gained a huge momentum. In this ch... more The application of textured surfaces in tribology has recently gained a huge momentum. In this chapter, a systematic approach to investigate the maximum outcomes from employing such surfaces is introduced with an insight into their application in internal combustion engines. A combination of various affecting parameters on the tribological performance of such surfaces is studied and the optimum results are introduced. The effect of employing such optimised textures in enhancing the lubrication condition in piston ring/cylinder liner contact is also studied.

Advanced Computational Methods in Heat Transfer, 2006

A numerical study of flow field and heat transfer rate in external flow of a novel heat exchanger... more A numerical study of flow field and heat transfer rate in external flow of a novel heat exchanger (Anglia Ruskin University, 2001) was conducted. The design comprises of confined impinging jets from a staggered bundle of tubes in which the fluid flows in an opposite staggered arrangement array after impingement. The RNG k-ε model and enhanced wall treatment near wall turbulence modelling was applied to model a three-dimensional computational domain. The accuracy of the model was validated in two- and three-dimensional cases for single impinging jets with available experimental results. The arrangement of the staggered array was fixed Sn/D=2.1 and Sp/D=1.6 where Sn and Sp are the distances between tubes transverse and parallel to fluid flow respectively and D was the hydraulic diameter of tubes. The dimensionless tubes to impinging surface distance (H/D) were in the range of 0.2, 0.5, 1.0, and 2.0 and the Reynolds number based on the tubes’ hydraulic diameter and average fluid velocity at the exit of tubes in the range of 1000, 5000, and 20000 were studied. The global heat transfer rate on both impinging and confinement plates increased with decreasing of H/D and increasing of Reynolds number; however, the slope of increasing Reynolds number was sharper in low H/Ds. The friction factor increased with a decreasing H/D and an increasing of the Reynolds number. The local Nusselt number was studied on both impinging and confinement plates. The temperature contours and velocity vectors are also presented.

PhD Thesis, Anglia Ruskin University, 2009

SAE Technical Paper Series, Sep 30, 2020

Clutch take-up judder is a torsional vibration phenomenon induced by stick-slip oscillations at t... more Clutch take-up judder is a torsional vibration phenomenon induced by stick-slip oscillations at the friction lining interfaces between the clutch friction disc and the flywheel and pressure plate surfaces during the process of clutch engagement. This is short-lived transient phenomenon affected by the clutch lining material’s friction characteristics, topography of the mating-sliding surfaces and operational conditions during engagement such as contact pressure , interfacial slip speed and contact temperature. The phenomenon leads to driver discomfort as well as gradual wear of contacting surfaces, even if short-lived. Its frequency is is usually reported to be in the range 5-20Hz depending on the vehicle type. The friction characteristics alter in a transient manner during engagement and it should be measured under controlled conditions. In this paper the measured interfacial friction characteristics together with clamp load variation (contact pressure) under different surface temperatures are included in a multi-degree of freedom dynamic analysis to obtain torsional vibrations of the system, pertaining to take up judder conditions. Such an in-depth investigation has not hitherto been reported in literature. The paper shows that take up stick-slip judder is omnipresent under all clutch engagement conditions, but its poignancy is most evident at cold surface temperatures. It is also shown that the transient judder response has a broader spectral content that is generally understood.

Proceedings Of The Institution Of Mechanical Engineers, Part K: Journal Of Multi-body Dynamics, May 22, 2017

Tribology International, Sep 1, 2017

SAE Technical Paper Series, Jun 13, 2018

Lubrication science, Nov 17, 2022

Tribology Letters, Jun 4, 2013

Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, Jan 5, 2012

Tribology International, 2018

SAE Technical Paper Series, Jun 30, 2014

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A study into the effects of cylinder de-activation on the tribological performance of piston ring... more A study into the effects of cylinder de-activation on the tribological performance of piston ring-liner conjunction

Applied sciences, Oct 12, 2019

Tribology transactions, Jul 11, 2016

Proceedings Of The Institution Of Mechanical Engineers, Part K: Journal Of Multi-body Dynamics, Feb 19, 2014

International Journal of Thermal Sciences, Dec 1, 2016

Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, May 26, 2015