Homer Rahnejat

Homer Rahnejat
University of Central Lancashire | UCLAN · School of Engineering

BSc, MSc, DIC, PhD

About

450
Publications
205,949
Reads
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7,101
Citations
Introduction
I've been researching in the area of tribo-dynamics of various systems since 1978, both in the academe and in industry. Tribo-dynamics deals with the interactions of system dynamics and tribological contact mechanics. In this perspective the overall system performance is viewed as an optimisation between energy efficiency (reduced friction), and noise, vibration and harshness (NVH) refinement. These issues also affect the environment and are multi-scale and multi-physics in nature.
Additional affiliations
August 2000 - December 2019
Loughborough University
Position
  • Professor
April 1994 - July 2000
University of Bradford
Position
  • Lecturer
Education
October 1979 - May 1984
Imperial College London
Field of study
  • Elastohydrodynamics (Tribology)
September 1977 - July 1979
Imperial College London
Field of study
  • Advanced Applied Mechanics
September 1977 - March 1979
Imperial College London
Field of study
  • Mechanical Engineering

Publications

Publications (450)
Article
Full-text available
This paper commemorates Ramsey Gohar by acknowledging his contributions to the fields of contact mechanics and elastohydrodynamic lubrication (EHL) within the context of the developments of these subjects. A historical discourse is provided on elastohydrodynamics, from its inception in the 1940s to present. We demonstrate that Ramsey Gohar was not...
Article
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...
Article
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A new method for the simultaneous in situ measurement and characterisation of molybdenum-based tribofilms is presented, based on lateral force microscopy. The simultaneity of measurements is crucial for a fundamental understanding of the tribochemical phenomena. A new analytical method is also presented, which combines a BET multi-layer adsorption/...
Article
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To extend drain intervals and improve efficiency new engine oils with increased dispersant concentration and reduced viscosity are required. Low viscosity engine oils can increase the prevalence of boundary friction at low temperature and increase its severity at higher temperatures. As a result, combinations of organic and inorganic friction modif...
Article
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Several approaches have been developed to measure instantaneous friction between the piston assembly and cylinder in internal combustion (IC) engines, such as floating-liner, reciprocating liner, instantaneous mean effective pressure (IMEP), fixed sleeve, and (P-ω) method and tribological bench tests. However, the “floating-liner method” and the “(...
Article
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A novel integrated multi-physics assessment of the piston top compression ring of an internal combustion engine under normal operation mode, as well as subjected to cylinder deactivation is carried out. The methodology comprises ring-liner thermo-mixed hydrodynamics, elastodynamics of ring, as well as combustion gas blow-by and emissions. Therefore...
Article
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In applications requiring high load carrying capacity, conforming contacting pairs with a relatively large contact footprint are used. These include circular arc, Novikov, and Wildhaber gears found, for example, in helicopter rotors. Closely conforming contacts also occur in many natural endo-articular joints, such as hips, or their replacement art...
Article
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Ball bearings are an integral part of many machines and mechanisms and often determine their performance limits. Vibration, friction and power loss are some of the key measures of bearing performance. Therefore, there have been many predictive analyses of bearing performance with emphasis on various aspects. The current study presents a mathematica...
Article
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This paper presents a brief review of elastohydrodynamic analysis in commemoration of the immense contributions of Duncan Dowson. This paper also presents an elastohydrodynamic analysis of the elliptical point contact problem under steady state as well as transient conditions. The overall methodology is validated against numerical predictions and e...
Article
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It is with great sadness that we note the passing of Professor Duncan Dowson on 6th January 2020. Duncan was an esteemed member of the Editorial Board of this journal. He will be remembered as one of the founding fathers of tribology and as a true gentleman. He was the last living member of the Jost Committee, set up by the UK Government (1964–1966...
Article
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A new finite element-based contact mechanics analysis of layered viscoelastic solids of low elastic modulus is presented. The methodology is based on the Maxwell viscoelastic element, with stress relaxation taken into account by the Prony series’ representation of the bulk and shear material moduli. Simultaneous solutions for deviatoric and volumet...
Article
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Energy efficiency and functional reliability are the two key requirements in the design of high-performance transmissions. Therefore, a representative analysis replicating real operating conditions is essential. This paper presents the thermoelastohydrodynamic lubrication (TEHL) of meshing spur gear teeth of high-performance racing transmission sys...
Article
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The viscosity and tribological behavior of nanofluids formed by dispersed nano-diamond particles within Poly-Alpha-Olefin (PAO6) lubricant is studied at different concentrations. The variation of coefficient of friction with nanoparticle concentration is measured using pin-on-disc tribometry under boundary, mixed, and hydrodynamic regimes of lubric...
Article
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Parasitic frictional losses in internal combustion engines of race vehicles adversely affect their performance. A significant proportion of these losses occur within the piston-cylinder system. This paper presents a study of the compatibility of cylinder bore surface materials with typical lubricant base constituent stock (poly alpha olefin and pol...
Article
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Tribofilms are activated using precision sliding strip microscale tribometry with a base and a fully formulated lubricant with a ZDDP anti-wear additive. The employed tribometry uses combined pressure, shear and temperature activation. The chemical compositions of the formed tribofilms are ascertained through use of Photo-electron X-ray Spectroscop...
Article
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IC engines contribute to global warming through extensive use of fossil fuel energy and emission of combustion by-products. Innovative technologies such as cylinder de-activation (CDA), after-exhaust heat treatment, surface texturing and coatings are proposed to improve fuel economy and reduce emissions of the vehicle fleet. Therefore, study of coa...
Article
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Optimum operation of clutch systems is dictated by their dynamic as well as thermal performance. Both of these aspects are closely related to the interfacial frictional characteristics of the clutch lining material, which also affects the noise, vibration and harshness response of the entire vehicular powertrain system. Severe operating conditions...
Article
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A multi-physics integrated analysis of piston top compression ring of a high-performance internal combustion engines is presented. The effects of transient ring elastodynamics, thermal gas flow through piston crevices upon chamber leakage pressure and parasitic frictional losses are investigated. The multi-physics analysis comprises integrated flex...
Article
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The parasitic drag losses incurred by wet clutches, used in transmission systems, can significantly affect vehicular powertrain efficiency. This paper presents a novel implicit solution for hydrodynamic parasitic drag losses of disengaged clutches. These are generated by conjunctional friction, taking into account lubricant film separation during c...
Article
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Multi-disc wet brake packs are commonly used for off-highway applications. Unfortunately, when disengaged, these components can generate significant levels of unwanted parasitic losses. This article presents a novel, validated model to predict the parasitic losses in disengaged multi-disc wet brake packs. The model employs pertinent boundary condit...
Article
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Thin curved rings used mostly as seals, including in internal combustion engines undergo complex elastodynamic behavior when subjected to a combination of normal radial loading and tangential shear with friction. In turn, their complex modal behavior often results in loss of sealing, increased friction, and power loss. This paper presents a new fin...
Article
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Fluid media such as water and ethylene glycol are usually quite poor conductors of heat. Nanoparticles can improve the thermal properties of fluids in a remarkable manner. Despite a plethora of experimental and theoretical studies, the underlying physics of heat transport in nanofluids is not yet well understood. Furthermore, the link between nanos...
Article
Gears are key components to the operation of many machines and mechanisms. However, their presence often affects system efficiency and can lead to noise, vibration and harshness (NVH) issues. Analyses described in open literature study tooth contact neglecting the effect of lubrication. In reality, contact mechanics and lubrication are closely inte...
Article
Planetary systems offer the advantage of desired speed-torque variation with a lighter, compact and coaxial construction than the traditional gear trains. Frictional losses and noise, vibration and harshness (NVH) refinement are the main concerns. Modification of gear teeth geometry to reduce friction between the mating teeth flanks of vehicular pl...
Article
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his paper investigates the frictional performance of a PECVD hydrogenated undoped DLC coating and the alternative super-finished gear steel surface pairs. A tribometer replicates the conditions found in the conjunction of a performance racing transmission spur gear pair. In mixed and boundary regimes of lubrication the DLC contact is shown to have...
Article
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Reduction of leakage from the shaft–radial lip seal conjunction is critical in ensuring enduring performance of entire lubrication system. This paper investigates leakage from three types of shaft surfaces, finished using different manufacturing processes. The measurement of surface topography is conducted in order to determine the pertinent roughn...
Article
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For future engines to become more efficient, with enhanced fuel economy and increased power output, accurate prediction of new designs is required over the full lifetime of an engine. The analysis uses of a local pressure coefficient of boundary shear strength of asperities value, taking into account the localised effects of surface texture, coatin...
Article
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A systematic approach is required in order to determine the frictional characteristics of a contacting pair in the presence of a tribofilm. Despite the clear benefits in functionality and in reducing wear, the generation of ZDDP-based tribofilms often lead to increased frictional losses. Such an increase is also observed in the tribometric tests re...
Article
Full-text available
The study of textured surface performance is one of the highly researched topics in recent times. This is mainly due to the advantages that such surfaces can potentially provide in practice, in mitigating adverse tribological conditions, such as friction and wear. However, considering the complexities found in practice, a methodological analysis an...
Conference Paper
Full-text available
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...
Conference Paper
Full-text available
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 e...
Conference Paper
Full-text available
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. Th...
Conference Paper
Full-text available
Friction modifiers are one of the 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 d...
Article
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Accurate simulation performs a crucial role in the design and development of new modern internal combustion engines. In the case of piston rings, simulations are used to effectively predict generated friction and power loss of proposed designs. These are consequences of viscous shear of a thin lubricant film, likewise boundary friction caused by di...
Article
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The lubricant-surface system is complex in nature and can significantly affect the frictional performance of high-performance transmission systems. The complexity stems from the coupled mechanical and chemical phenomena that occur at the interfacial tooth conjunctions. A combined analytical and precision experimental approach is presented to analys...
Article
Fuel efficiency is one of the main concerns in the optimisation of modern racing transmissions. The dry sump transmissions are the preferred choice for high performance racing applications. While it provides adequate lubricant for gear contacts, it minimises the system churning losses, and therefore enhances the system efficiency. An important aspe...
Article
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The dynamics of a nonlinear vibration energy harvester for rotating systems is investigated analytically through harmonic balance, as well as by numerical analysis. The electromagnetic harvester is attached to a spinning shaft at constant speed. Magnetic levitation is used as the system nonlinear restoring force for broadening the resonant range of...
Conference Paper
Full-text available
Choosing in-cylinder surfaces is complex. A well-chosen surface has low friction and wear. Conversely, oversight often leads to premature failure. Identification of appropriate surfaces is typically conducted in an empirical manner or through mimicry of tried and tested exemplars found in current engineering practice. This paper demonstrates the us...
Article
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In order to accurately predict the lubricant film thickness and generated friction in any tribological contact, it is important to determine appropriate boundary conditions, taking into account the oil availability and extent of starvation. This paper presents a two-dimensional hydrodynamic model of a piston ring pack for prediction of lubricant fi...
Article
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Dynamic behaviour of automotive dry clutches depends on the frictional characteristics of the contact between the friction lining material, the flywheel, and the pressure plate during the clutch engagement process. During engagement due to high interfacial slip and relatively high contact pressures, generated friction gives rise to contact heat, wh...
Article
Full-text available
Engine downsizing is desired for modern heavy-duty vehicles to enhance fuel economy and reduce emissions. However, the smaller engines usually cannot overcome the parasitic loads during engine start-up. A new clutch system is designed to disconnect the downsized engine from the parasitic losses prior to the idling speed. A multi-scale, multi-physic...
Article
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Key objectives in off-highway vehicular powertrain development are fuel efficiency and environmental protection. As a result, palliative measures are made to reduce parasitic frictional losses while sustaining machine operational performance and reliability. A potential key contributor to the overall power loss is the rotation of disengaged wet mul...
Article
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Improved fuel efficiency and reduced emissions are key drivers for modern drivetrain systems. Therefore, in recent years, dry sumps with air-oil mist lubrication have been used for efficient transmission design in order to reduce the churning losses. With dry sumps appropriate cooling measures should be implemented to dissipate the generated contac...
Article
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This paper investigates the effect of surface texturing in a partial pad journal bearing through a series of controlled experiments at operating conditions, promoting mixed or boundary regimes of lubrication. Improvements to load carrying capacity are observed under certain operating conditions. A comprehensive computational finite volume multiphas...
Article
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Thermo-mixed-hydrodynamics of compression rings and big-end bearings are presented. Frictional losses under normal engine operating conditions for a gasoline engine and those with cylinder deactivation (CDA) are predicted. With CDA, the combustion chamber pressure increases in the active cylinders, whilst some residual pressure remains in the deact...
Article
Full-text available
Planetary hub systems offer desired speed and torque variation with a lighter, compact and coaxial construction than the traditional gear trains. Generated friction between the mating teeth flanks of vehicular planetary hubs under varying load-speed conditions is one of the main sources of power loss. Modification of gear tooth geometry as well as...