In the current paper it is shown that, when other additives commonly used in engine oils are added to ZDDP solutions, quite smooth ZDDP reaction films can result. Despite this, the ZDDP still produces a marked increase in friction in mixed lubrication conditions, which suggests that surface roughening is not the main origin of friction enhancement by ZDDP reaction films. In a companion paper, Part II, it is shown that ZDDP reaction films, whether rough or smooth, enhance friction by inhibiting the entrainment of liquid lubricant into rubbing contacts, thereby reducing the elastohydrodynamic oil film thickness (13)
Boosting the Friction Performance of Amine Friction Modifiers with MoDTC
AbstractFor years amine surfactants, such as primary amines, ethoxylated amines and polyamines, have been used as friction modifiers in lubricating oils in order to improve fuel economy. This paper describes how the friction performance of amine containing lubricating oils can be improved with the addition of a small amount of molybdenum dithiocarbamate (MoDTC).Three fatty amines, tallow amine (Armeen® T), tallow propanediamine (Duomeen® T) and tallow dipropylenetriamine (Triameen® T), have been tested with Zinc Dialkyldithiophosphate (ZDDP) and with and without MoDTC in the Minitraction machine (MTM). It is shown that MoDTC improves the friction of Duomeen T and Triameen T while not for Armeen T. It is argued that the packing of Armeen T does not allow MoDTC to reach the surface and to create molybdenum disulphide (MoS2) sheets. Duomeen T and Triameen T have more nitrogen atoms and cannot pack as closely at the surface as Armeen T which allow MoS2 sheets to form. To validate the theory, Scanning Electron Microscopy combined with Energy Dispersive Xray Analysis (SEM-EDX) has been used to determine the amount of sulphur in the tribofilm. The SEM-EDX data show more sulphur in the tribofilm of Duomeen T compared to Armeen T which could be due to the formation of more MoS2.
Influence of gasoline engine lubricant on tribological performance, fuel economy and emissions
The requirement for increased performance, improved fuel economy and reduced emissions is constantly sustaining the demand for research into combustion, fuels and lubricants. Due to the nature of the operation of an engine and the current market climate the lubricant not only has to respond to these requirements, but also to changes in engine design, fuelling methods and fuel types, increased power densities and developments in emissions formation and after-treatment. This paper will describe advances made at the authors’ institution to elucidate the influence of gasoline engine lubricant on tribological performance, fuel economy and emissions, giving examples of work undertaken and then look to future possible lubricant demands.
Novel Test Methods to Optimize Fuel Economy in Passenger Cars Using Organic Friction Modifiers
Fuel economy is overall a very high priority worldwide, but the value of fuel economy can vary to different stakeholders. Many governments set minimum fuel economy standards that force OEMs to make significant investment to achieve. Engine oils and friction modifiers can provide a small but cost-effective contribution to overall fuel economy in cars and trucks. Bench tests that evaluate frictional characteristics often do not correlate well to actual fuel economy benefit. Optimizing a friction modifier to maximize the benefit requires testing the same way that OEMs are required to test fuel economy. Using vehicles on chassis dynamometers, the Highway Test Procedure (HwFET) of the FTP 75 requirement which OEMs follow to quantify their Corporate Average Fuel Economy (CAFE) requirements in North America was modified and used to evaluate individual friction modifier (FM) components and their effects on fuel economy. Since OEM value propositions involve entire fleets, the effect of friction modifiers on fuel economy was tested using several types of vehicles. A new FM additive is shown to demonstrate more than twice the fuel economy benefit compared to glycerol monooleate (GMO), a standard FM used in many engine oils, even at half the treat rate. A Chevrolet 5.7 liter engine on a test stand was also adapted to screen individual components in engine oil and their effects on fuel economy. Results are shown for various components, including an apparent increase in fuel consumption due to the antiwear (AW) component ZDDP.
Study of Zinc Dialkydithiophosphate Antiwear Film Formation and Removal Processes, Part I: Experimental
Two recent trends in engine oil formulation are a progressive reduction in phosphorus concentration so as to reduce its impact on the de-NO x catalyst, and an increase in dispersant concentration to control the level of lubricant viscosity increase over extended drain intervals. Unfortunately, both of these trends make it more difficult to generate and retain effective antiwear films on lubricated surfaces and both thus strengthen the need to understand the processes by which antiwear films are formed, and removed, during rubbing. This article and its companion outline a study of the kinetics of antiwear film growth and removal. In Part I, a test method for monitoring antiwear film thickness during rolling/sliding is described and employed to explore how various factors, including operating temperature, antiwear additive type and concentration, and the presence of dispersant, influence both the formation and removal of the tribofilms formed by the antiwear additive zinc dialkyldithiophosphate (ZDDP). Part II then analyzes the obtained results to derive a kinetic model of ZDDP film formation and removal (Fujita, et al. . Presented at the STLE Annual Meeting in Toronto, Ontario, Canada May 17-20, 2005 Review led by Elaine Yamaguchi
Tribological Bench and Engine Dynamometer Tests of a Low Viscosity SAE 0W-16 Engine Oil Using a Combination of Ionic Liquid and ZDDP as Anti-Wear Additives
We have previously reported an oil-miscible phosphonium-organophosphate ionic liquid (IL) with effective anti-wear functionality when added to a base oil by itself or combined with a conventional zinc dialkyldithiophosphate (ZDDP) for a synergistic effect. In this research, we investigated whether this synergy manifests in formulated engine oils. An experimental SAE 0W-16 engine oil was generated using a combination of IL and ZDDP with equal phosphorus contribution. The prototype engine oil was first evaluated using tribological bench tests: anti-wear performance in boundary lubrication and friction behavior (Stribeck curves) in elastohydrodynamic, mixed, and boundary lubrication. The forthcoming standard Sequence VIE engine dynamometer test was then conducted to demonstrate improved fuel economy. Results were benchmarked against those of another experimental engine oil with almost the same formulation except using ZDDP only without the IL (similar total phosphorus content) and a baseline SAE 20W-30 engine oil. The IL-ZDDP formulation consistently outperformed the ZDDP-only formulation and the results from the bench and engine tests are well correlated.
Effect of Fluid Flow through Clutch Material on Torque Fluctuations in Clutches
Improving vehicle fuel efficiency is a key market driver in the automotive industry. Typically lubricant chemists focus on reducing viscosity and friction to reduce parasitic energy losses in order to improve automotive fuel efficiency. However, in a transmission other factors may be more important. If an engine can operate at high torque levels the conversion of chemical energy in the fuel to mechanical energy is dramatically increased. However high torque levels in transmissions may cause NVH to occur. The proper combination of friction material and fluid can be used to address this issue. Friction in clutches is controlled by asperity friction and hydrodynamic friction. Asperity friction can be controlled with friction modifiers in the ATF. Hydrodynamic friction control is more complex because it involves the flow characteristics of friction materials and complex viscosity properties of the fluid. This paper shows how NVH and torque capacity can be controlled by optimizing the flow characteristics of friction materials and the complex viscosity of fluids.
Contribution of Oil Traction to Diesel Engine Cam Galling
Heavy Duty diesel engines typically use roller followers in contact with the cam to reduce friction and accommodate high Hertzian stresses. When the rolling contact slips into sliding, cam galling can occur that may lead to major cam failures. Oil traction has been identified as a possible source to cause slipping. In this study, oil traction was first measured in a Mini Traction Machine (MTM). The results were then validated by a series of engine tests to show that the measured oil traction correlated with the occurrence of cam galling. Finally, the MTM was used to evaluate various engine oil formulations. It is concluded that some advanced base oils, if not properly compensated by the additive package, exhibit dangerously low oil traction. Oil traction needs to be part of the oil formulation considerations.
Tribological Film Formation on Hydrogenated DLC/Steel Contacts From Fully Formulated Automotive Lubricants
Tribological film formation from fully-formulated synthetic automotive lubricants on hydrogenated DLC/steel contacts was compared with that on steel/steel contacts. Four DLC coatings containing various metal dopants (W and Si) were evaluated. Lubricants formulated with organic friction modifiers performed better on DLC/steel than on steel/steel in both sliding and rolling/sliding contacts (lower friction and wear). In contrast, lubricants formulated with MoDTC friction modifiers had higher sliding contact friction on DLC/steel than on steel/steel. No evidence was observed for tribological film formation on the DLC surfaces.
Tribological investigations of the piston assembly and liner of a gasoline engine
The automotive industry is being forced towards greater efficiency, increased engine power from smaller engines and lower environmental impact by both governmental legislation and public opinion. Oil drain intervals are increasing whilst emissions legislation limit the use of current wear protection and antioxidant additives containing elements such as phosphorus and sulphur. To address these demands and challenges an increased understanding of the link between lubricant degradation, its transport and residence time, and the effect on piston assembly tribology is required. The aim of the work reported in this paper was to further develop the understanding of the effect degraded lubricants have on piston assembly tribology. The small oil volumes and environmental conditions in the piston assembly make the affective lubrication and protection of components in this region one of the most challenging areas of tribology. This was carried out through an extensive experimental programme using a research engine, tribometer testing and chemical and rheological analysis of lubricant samples.
Nanoparticle-doped lubricants : Potential of Inorganic Fullerene-like (IF-) molybdenum disulfide for automotive applications
The growing environmental concerns, along with the continuous increase in the price of fossil fuels, have highly motivated car manufacturers worldwide to improve the efficiency of their vehicles. The tribological properties of engine and gearbox lubricants have a significant impact on the global efficiency of vehicles, as they contribute to reducing friction in many contacts and allow the downsizing of various components by providing their surfaces with anti-wear protection. The recent breakthroughs in nanoparticle synthesis have opened new prospects in terms of lubricant additivation, with the discovery of the excellent friction and wear reducing properties of nanoparticles such as Inorganic Fullerene-like (IF-) molybdenum or tungsten disulfides. The tribological potential of IF-MoS2 for automobile applications was investigated in this work. The respective influences of nanoparticle size and structure were first of all studied, revealing that poorly crystalline nanoparticles were more efficient in maintaining low-friction tribofilms on steel substrates in severe boundary lubrication regimes regardless of size (for the range studied). All the nanoparticles tested however showed similar performances when proper oil recirculation was ensured, providing a continuous feeding of the contact in nanoparticles. The IF-MoS2 nanoparticles lost their lubricating abilities when added to fully-formulated lubricants. This behavior was attributed to the presence of dispersants in the oil, which dispersed the nanoparticles effectively but prevented them from forming tribofilms on the rubbing surfaces. The well-dispersed IF-MoS2 were shown to enter the contact and exfoliate, but an excessive adsorption of the dispersants on the released MoS2 platelets and/or the steel surfaces is thought to prevent tribofilm adhesion. A balance between nanoparticle dispersion and tribological performance was then found, by using very low concentrations of dispersants. The behavior of nanoparticle-doped oils in various scenarios related to automobile applications was finally explored. The IF-MoS2 provided significant friction and wear reduction at ambient temperature and in milder rolling/sliding test conditions, for smooth and rough surfaces. The risks related to the presence of nanoparticles in the oil in full-film lubrication regimes were partially lifted, with no significant influence on friction witnessed for all the test conditions considered. The ability of IF-MoS2 nanoparticles to protect steel surfaces from surface-initiated Rolling Contact Fatigue was finally shown.
Tribological properties of diamond-like carbon coatings in lubricated automotive applications
Abstract, High-performance coatings are currently used in automotive racing engine applications and are being considered for commercial fleet applications. Because crankcase lubricants were designed for steel surfaces, the change to these new materials may result in different boundary lubrication mechanisms. The aim of the present study is to investigate the friction and wear properties of a series of commercial coatings using simple component-blend lubricants as well as commercial lubricant formulations. The tribological testing revealed that some of the commercial coatings possess advantageous friction and wear properties when compared with steel. The study also stresses that in many cases the use of these coatings results in increasing wear on the steel counterpart.
Tribology and energy efficiency: from molecules to lubricated contacts to complete machines
The impact of lubricants on energy efficiency is considered. Molecular details of base oils used in lubricants can have a great impact on the lubricant’s physical properties which will affect the energy efficiency performance of a lubricant. In addition, molecular details of lubricant additives can result in significant differences in measured friction coefficients for machine elements operating in the mixed/boundary lubrication regime. In single machine elements, these differences will result in lower friction losses, and for complete systems (such as cars, trucks, hydraulic circuits, industThe impact of lubricants on energy efficiency is considered. Molecular details of base oils used in lubricants can have a great impact on the lubricant’s physical properties which will affect the energy efficiency performance of a lubricant. In addition, molecular details of lubricant additives can result in significant differences in measured friction coefficients for machine elements operating in the mixed/boundary lubrication regime. In single machine elements, these differences will result in lower friction losses, and for complete systems (such as cars, trucks, hydraulic circuits, industrial gearboxes etc.) lower fuel consumption or lower electricity consumption can result.rial gearboxes etc.) lower fuel consumption or lower electricity consumption can result.
Tribological Properties of Sulphur-Free Antiwear Additives Zinc Dialkylphosphates (ZDPs)
The film-forming, friction, and antiwear properties of zinc dialkylphosphates (ZDPs) were investigated and compared with the corresponding zinc dialkyldithiophosphates (ZDDPs). The primary ZDPs generally show similar friction and antiwear performance to the primary ZDDPs, although some differences are seen between them in film-forming properties. For the secondary ZDP and ZDDP, there are some clear differences in their tribological properties. This indicates that the properties of the primary ZDPs and ZDDPs may be controlled predominantly by adsorbed films consisting the intact additives and/or their decomposition compounds, and that the properties of the secondary ones may be controlled by glassy reaction films consisting zinc/iron polyphosphates.
Effect of Friction Material on the Relative Contribution of Thin-Film Friction to Overall Friction in Clutches
In order to prevent shudder in automatic transmissions, friction must decrease as the sliding speed between the friction plates in clutches decreases. Theoretical studies have shown that friction in wet clutches is a combination of boundary friction and the friction due to flow of fluid through the friction materials (thin-film friction). Therefore, these physical properties of oils should control the anti-shudder performance of automatic transmission fluids. Recently, we demonstrated that boundary and thin-film friction contribute to friction measured at low speeds in JASO SAE No.2 and LVFA tests. Two different friction materials are used in these tests and the relative effect of thin-film friction on low speed friction is greater in the JASO SAE No. 2 test than in the JASO LVFA test. This difference in the relative effect of thin-film friction on overall friction could be due to the different speed conditions used to measure low speed friction in the SAE and LVFA tests or could be due to the difference in the friction materials used in these tests.Therefore, we have expanded our research to determine the effect of sliding speed and friction material on the relative effect of thin-film and boundary friction on overall friction measured in friction material-steel contacts. As sliding speed increases, the relative contribution of thin-film friction to overall friction increases. The relative contribution of thin-film friction to overall friction at each speed depends upon the friction material used. The relative effect of thin-film friction to overall friction is greater for friction materials in which the rate of fluid flow into the material is slow. The relative effect of thin-film friction on overall friction is also greater for friction materials with lower surface roughnesses.
Polyalkylmethacrylates (PAMAs) are widely used as both viscosity index improvers and dispersant boosters in engine, hydraulic, and transmission oils. Since they are employed in a relatively high concentration in these roles, it is desirable that they be able to enhance other characteristics of a lubricant and, in particular, its boundary lubricating properties. A series of functionalized PAMAs have been synthesized that can adsorb from oil solution onto rubbing surfaces to produce thick boundary films. These films enhance lubricant film formation in slow speed and high temperature conditions and thus make a significant contribution to their lubricating ability. The current paper describes a systematic study of the influence of functionalized PAMAs on boundary lubrication performance. The high frequency reciprocating test rig (HFRR) was applied to investigate friction and wear under pure sliding. A new test method has been developed which allows wear to be monitored in a rolling and sliding contact based on the mini traction machine (MTM). This, in combination with other tests, is employed to investigate the influence of polymer architecture, functionality, concentration and molecular weight on friction and wear in a range of lubricant formulations. This enables the tailored design of polymers which offer low friction and wear properties.
The effect of a ZnDTP anti-wear additive on the micropitting resistance of carburised steel rollers
Improved Processing of High Alloy Steels for Wear Components in Energy Generation Systems, Transportation and Manufacturing Systems
Oak Ridge National Laboratory (ORNL), in Partnership with Avure Technologies Inc. and Carpenter
Technology Corporation, explored methods to improve durability of steels considered essential to
bearings in wind turbine applications by using hot isostatic pressing of steel alloy powders as a
The objective of the project was to evaluate the wear mechanism of selected high alloy tool steels,
440C, 440XH, and M62, fabricated by powder metallurgy (PM) techniques via the hot isostatic press
(HIP) process, and compare these results to two conventionally produced wrought alloys, 52100 and
M50. Samples from the five candidate metals were characterized microstructurally via optical and
scanning microscopy methods. A micro-pit wear testing machine was purchased and used to simulate
bearing loads in a wind turbine gear box The candidate samples were subsequently heat treated in
accordance with industrial protocols and machined into wear test specimens suitable for the micro-pit
wear testing machine. . Wear testing produced surfaces that were subsequently analyzed for wear
and damage using both optical microscopy and scanning electron microscopy methods.
Optimized rolling-sliding experiments with a relatively small slip ratio (5%) and higher load (650N)
exhibited signs of abrasive wear on all specimens, but not all specimens had the same degree of
micro-pitting damage. The conventional wrought 52100 steel had the most severe micro-pitting,
consisting of distorted craters, many of which were connected by micro-cracks. Cross-sectional
examination showed multiple micro-cracks extending into the alloy microstructure from the same pit.
The depth of the cracks was several times the depth of the micro-pit. Type 440XH had the least
amount of micro-pitting damage under similar test conditions. Micro-pits on 440HX were more
localized and the micro-cracks associated with them tended to avoid carbides during propagation.
Abrasive processes were observed on all the test specimens and were produced by debris from micopitting.
The combination of micro-pitting in conjunction with abrasive processes could work together
to worsen the surface damage, diminishing bearing service life.
The results suggest that there is a potential advantage of PM consolidated high alloy tool steels, such
as 440XH produced via hot isostatic pressing to better resist micro-pitting when compared to other
alloy types and wrought processing methods.
Experience with a Disc Rig Micropitting Test
The experimental work carried out was aimed at developing a test method that was able to consistently
produce micropitting damage and could discriminate between a good oil (i.e., one that rarely produces
micropitting in service) and a poor oil (i.e., one that does produce micropitting in service). The small–scale
3–Disc test rig that was used for this work employs 3 discs to apply the test load to a 12mm–diameter test
roller. This test geometry allows a large number of stress cycles (typically 600,000 to 800,000 cycles/h) to be
generated at the contact track on the roller.
The disc rig control system allows test parameters such as entrainment velocity, contact stress and slide/roll
ratio at the disc/roller contacts to be accurately and independently controlled. This enables the effect of key
parameters to be studied in isolation, which is something that cannot be easily achieved using conventional
gear test rigs.
The early work carried out using the disc rig was aimed at producingmicropitting damage by operating the rig
at contact conditions similar to those used in the FZG micropitting gear test method. These early tests
confirmed that the damage produced to the roller track exhibits characteristics that are typical of micropitting
damage, and showed that the severity of the micropitting produced was affected by the amount of running–in
carried out on the roller prior to applying the full test load.
A test procedure has been developed which provides a good level of repeatability and which allows
discrimination between oils which producemicropitting in service and those which do not. In addition, a study
of the effect of slide/roll ratio (SRR) has shown that the severity of micropitting damage increases as SRR
increased, whereas at 0% SRR nomicropitting occurred and, at negative SRRs,microcracking occurred but
not micropitting. This is the way that SRR seems to affect micropitting in gears.
Effect of lubricants on micropitting and wear
Micropitting was studied using a three-contact disc machine having a central roller in contact with
three harder, annular counter-discs (‘‘rings’’) of precisely controlled roughness. Roughness, running
conditions, base stock and additive concentration were varied. The response of the same lubricants in a
reciprocating sliding wear test operating in the boundary regime was also studied.
Results of experimental studies of the rolling contact behaviour of carburised steel rollers are
reported. All the tests with the additive present led to micropitting. However, severe micropitting wear
was only observed when the calculated film thickness exceeded 12% of the centre-line average
roughness of the rings.
It was found that there was an approximately inverse correlation between the micropitting damage
in the disc machine test and the mild wear in the reciprocating sliding test. This was attributed to the
tendency of anti-wear additives to prevent running-in of the rough surface.
A viscometer for testing the viscosity of a fluid, comprising a rotor that engages with a stator, the rotor being coupled to a drive means capable of rotating the rotor with respect to the stator, there been a separation gap between the stator and the rotor for the accommodation of fluid to be tested, and measurement means for measuring the characteristics of the rotor and/or stator such that the viscosity of the fluid can be determined. The drive means accelerates the rotor from rest or from a low rotational speed which does not significantly affect the specified test temperature of the rotor, stator and test fluid, to the required angular velocity, and the measurement of the required characteristics are taken, all within a time period sufficiently short such that no temperature change significant to the calculation of the viscosity occurs. The required angular velocity is such that rotating the rotor at or near this velocity for longer than the said time period would cause a temperature change significant to the calculation of the viscosity to occur.
Development and Assessment of Traction Fluids for Use in Toroidal (IVT) Transmissions
The attributes of a traction fluid are fundamental to the successful operation of a traction drive transmission. The fluid must lubricate and protect the components against wear and corrosion, whilst simultaneously providing high traction to transmit power efficiently.
A selection of commercial and candidate fluids have been assessed with both a bench-test and a novel traction rig. The principal objective has been to achieve a balance between the conflicting requirements of low temperature viscometrics and high temperature traction.
Fluid performance is found to vary according to the rig employed underlining the need to test under prevailing conditions. Data from the traction rig is validated against a variator module.
Friction-Enhancing Properties of ZDDP Antiwear Additive: Part I—Friction and Morphology of ZDDP Reaction Films
Many phosphorus-based antiwear films, including those formed by zinc dialkyl dithiphosphates (ZDDP), cause a significant increase in friction in thin film, high-pressure, lubricated contacts. This can have a deleterious effect on engine oil fuel efficiency. Previous work has shown that friction is increased not under boundary, but under mixed lubrication conditions and it has been suggested that this phenomenon results from an effective roughening of the rubbing surfaces by the formation of unevenly-distributed reaction films.
Effect of EHL Contact Conditions on the Behavior of Traction Fluids
New infinitely variable transmission (IVT) systems are under development for the automotive industry as a means to achieving significant fuel economy benefits. These systems rely on the lubricating fluid to transmit the drive train loads across the interface of the transmission components. This requires the development of new fluids that exhibit high traction properties under elastohydrodynamic lubrication (EHL) conditions. However, it has been reported recently that the traction performance of some fluids can reduce dramatically as temperature is reduced. This may place severe operational limits on IVT systems and suggests that the low-temperature traction properties of fluids for these systems should be studied in order to understand the mechanism for the observed reduction in traction.
The work reported here is an experimental study aimed at identifying whether low temperature traction reduction is related to a fundamental change in rheological behavior specific to the fluids tested or to more generic changes in the EHL contact conditions. A series of model experiments were performed using a mini traction machine (MTM) on three high-viscosity polybutene samples. The results have been mapped against previously reported non-dimensional parameters used to identify different EHL regimes. The results show that dramatic reductions in traction occur when the contact transitions from the rigid piezo-viscous (RP) toward the rigid iso-viscous (RI) region. Similar results were also found for two other high-viscosity fluids of different molecular structure and lower traction properties. The results support the hypothesis that the reduction in traction observed at low temperature is due to a change in EHL contact conditions rather than being solely due to a change in the rheological performance of the test fluids.
Evaluation of EHD films by electrical capacitance
The elastohydrodynamic (EHD) lubrication regime occurs in many machine elements where a combination of hydrodynamic effect, elastic deformation of the loaded surfaces and increase in the viscosity of the lubricant with pressure ensures the formation of a very thin, but continuous film of lubricant separating the contacting surfaces. Electrical methods to determine this film’s thickness have preceded optical methods, which are widely used today. Although they generally give more qualitative thickness information, electrical methods have the main advantage that they can be applied to metallic contacts in machines, which makes them useful tools in the study of elastohydrodynamically lubricated contacts. This paper is part of a larger study on the application of electrical capacitance for the evaluation of film formation in EHD contacts. The main focus is on the quantitative measurements of film thickness using electrical capacitance. A new approach allowing the lubricant film thickness to be extracted from the measured capacitance is developed using a chromium-coated glass disc and subsequently applied to a steel-on-steel contact. The results show good agreement with optical measurements and theoretical models over a range of film thickness.
The Tribological and Chemical Analysis of Top Ring Zone Samples of Fully Formulated Oil Taken From a Four Stroke Gasoline Engine
With increasing pressure on engine oil manufacturers to extend oil drain intervals and reduce fuel consumption, whilst changing the composition of fully formulated oils to meet the new CEC, ILSAC and OEM specifications, there is an ever increasing need to understand the effect of oil degradation on the operating conditions and tribological performance of engines . This work samples oil from the rear of the top piston ring of an engine during the first 15 minutes from cold start and operating at steady state under three different loads. These samples, used 40 hour sump oil and fresh oil have been subjected to tribological tests and chemical analysis.
Lubricant film thickness and friction force measurements in a laser surface textured reciprocating line contact simulating the piston ring–liner pairing
Applying surface texture to piston liners may provide an effective means of controlling friction and hence improving engine efficiency. However, little is understood about the mechanisms by which pockets affect friction, primarily because of a lack of reliable experimental measurements. To address this, the influence of surface texture on film thickness and friction force was measured simultaneously in a convergent-divergent bearing, under conditions that closely replicate an automotive piston ring-liner conjunction. Film thicknesses were measured using a modified version of the ultra-thin film optical interferometry approach, enabling film thicknesses <50 nm to be measured under transient, mixed lubrication conditions. This involved using the out-of-contact curvature of the specimens in place of a spacer layer and analysing multiple interference fringes to avoid fringe ambiguity. Tests were performed on both a textured sample (with features oriented normal to the direction of sliding) and a non-textured reference sample, while angular velocity, applied normal load and lubricant temperature were controlled in order to study the effect of varying lubrication regime (as typically occurs in service). Results showed that the presence of surface pockets consistently enhances fluid film thickness in the mixed lubrication regime by approximately 20 nm. Although this is only a modest increase, the effect on friction is pronounced (up to 41% under these conditions), due to the strong dependence of friction on film thickness in the mixed regime. Conversely, in the full film regime, texture caused a reduction in film thickness and hence increased friction force, compared with the non-textured reference. Both textured and non-textured friction values show nearly identical dependence on film thickness, (showing that, under these conditions, texture-induced friction reduction results entirely from the change in film thickness). These results are important in providing film thickness data to validate piston-ring lubrication models and also in helping to understand the effect of surface roughness on texture performance
Effect of oil rheology and chemistry on journal-bearing friction and wear
Legislation and market pressures are calling for increased engine power, reduced engine size, and improved fuel consumption. The use of low-viscosity lubricants is considered as a means to enhance fuel economy by reducing viscous friction, particularly in engine bearings. Journal bearings mostly operate under hydrodynamic lubrication with a thin film of oil separating the journal and bearing shell. There are, however, certain conditions, especially under high load or low speed, when the film thickness will be low enough to allow boundary lubrication to occur. In this study, the effect of lubricants with different viscosities, different types of viscosity modifiers, different additives, different types of dispersants, and different lubricant formulations have been studied under hydrodynamic and boundary lubrication regimes. For hydrodynamic conditions, a high-temperature high-shear viscometer, meeting the requirements of ASTM D4741 was used to measure viscosity at 106 s−1. In addition, a new ultra high-shear viscometer, from PCS Instruments, was used to measure viscosity at shear rates near to 107 s−1. Bearing weight loss and load bearing capacity were measured on a rig developed in-house using a specially designed half-bearing shell loaded against a rotating journal. A PCS journal-bearing rig was used to measure the bearing friction under transient load.
Gear Oil Viscosity Modifiers and Their Impact on Efficiency
The essential functions of an automotive gear lubricant viscosity modifier (VM) are to maintain fluid film protection of gears and bearings as the lubricant warms to operating temperature, to improve cold temperature flow for efficient lubrication in winter and to minimise viscosity loss in a high shear, high load environment. Although a number of different VM technologies can be considered appropriately resistant to permanent shear for automotive gear oils use, their effect on fluid efficiency can vary widely. This paper outlines the study of a series of different VM technologies assessing relationship of operating temperature, operating viscosity and axle efficiency under different load and speed regimes. The fluids presented were formulated to equal kinematic viscosity at 100 °C but vary widely in viscosity index (VI), elastohydrodynamic (EHD) traction and EHD film thickness. The differences observed during efficiency testing were qualitatively related to the rheological properties of the VM technology present and further related to the operating temperature and operating viscosity.
Lubricating oil composition
The efficiency of a hypoid axle—a thermally coupled lubrication model
The final drive unit in road vehicles, such as medium and heavy trucks, and four-wheel-drive and rear-wheel-drive passenger cars, usually consists of a hypoid or spiral bevel geared transmission and differential, housed in a self-contained, dip-lubricated axle. Such units are subjected to very variable duty—including extreme combinations of speed, gradient, applied torque and external temperature—and are typically cooled by natural and forced convection on the exterior surface. On the other hand, there are appreciable internal power losses due to gear friction and churning and to bearing and seal losses. These losses are highly dependent upon the lubrication regime of the internal components and hence to the thermal behaviour of the entire axle.
In the present paper, we describe a thermally coupled model of axle lubrication. The torque and speed demand is first found from a specified duty (“drive cycle”) which includes terrain as well as speed-versus-time and external temperature data. The evolution of sump oil and component temperatures is followed, and increments of energy loss evaluated in each time-step. Elastohydrodynamic film thickness is determined for the hypoid gear set, using a development of Buckingham’s method, and friction losses calculated using a simple oil rheological model based on tribometer (MTM) testing. Churning, seal and bearing (speed-dependent) losses are found using empirical algorithms. Energy losses over complete drive cycles for different lubricants are derived, enabling the relative fuel economy for different oils to be evaluated.
Results show that (i) the bulk temperature rise of the axle is highly dependent on the specified vehicle duty and (ii) the efficiency can be strongly influenced by choices available to the lubricant formulator. Taken together, these findings suggest that specialist axle lubricant formulations for particular vehicle types and applications will be attractive as a route to optimum fuel economy.
The Rolling Contact Fatigue Behaviour of Motorcycle Lubricants
Rolling contact fatigue is a particular type of fatigue that occurs in heavily loaded, non-conformal contacts, such as gears and rolling element bearings. It is primarily a failure mode associated with repeated cyclic loading that generates high local Hertzian pressures, leading to local plastic deformation and substantial surface or sub surface stress. This in turn leads to crack formation and propagation. In some instances this results in sudden and often critical mechanical failure of contacting parts. This failure mode can, to a certain degree, be controlled by the appropriate choice of lubricant; in terms of both the physical and chemical properties of the films formed at the surface.
A three contact disc machine has been used to examine the rolling contact fatigue of motorcycle lubricants in such heavily loaded contacts. Three counterface test rings of equal diameter (54mm) are mounted 120° apart with a smaller (12mm diameter) test roller in the centre. Using this configuration, a large number of contact cycles are possible in a short period of time (up to one million per hour), which greatly accelerates the testing test.
Using appropriate test conditions, the rig is able to replicate the tribological conditions typically found in both gears and bearings, giving the macropitting failure mode prevalent in such components. The magnitude of the applied stress may well be below the yield strength of the material, but it is the repetitive application of load which is sufficient to cause fatigue failure. By its very nature, any form of fatigue is a statistically random process and any finite collection of similar specimens, run under a fixed set of conditions (load, speed and lubricant) will belong to a population defined by the manner in which the fatigue lives of individual members are distributed. The results are presented in the form of a single mode Weibull distribution.
In this paper it is shown that lubricants within the same viscosity grade can give statistically significant differences in rolling contact fatigue resistance. This is shown to depend primarily on lubricant composition (additive/base oil combination), which give quite different friction performance, particularly in the mixed lubrication regime. The paper will conclude with a discussion of the statistical significance of the fatigue results and its applicability to real engines.
Comparison of frictional properties of gear oils in boundary and mixed lubricated rolling–sliding and pure sliding contacts
The friction responses of five fully formulated gear oils including mineral and synthetic oils were studied. This article examines the impact of contact motion types (rolling–sliding and pure sliding) and contact pressure on boundary and mixed friction properties of the selected gear oils in MTM (minitraction machine) and SRV (Schwing-Reib-Verschleiss tribometer). Mineral oils are found to be less affected by contact pressure compared to synthetic oils. Gear oils that show adsorption appear to be less sensitive to contact motion type in mixed lubrication while behave much more sensitive in boundary lubrication regimes. The ranking of gear oils for mixed friction was similar regardless of contact motion types at low contact pressures while differ at high contact pressures
The Effect of Poly(alkyl methacrylate) Film-Formers on Pitting Performance of Driveline Fluids
Driven by diminishing fossil fuel resources, global warming and subsequently rigid legislation on CO2 emission, fuel economy is a major challenge for the automotive industry. Each element of the powertrain has been optimized or newly designed to increase efficiency. In this optimization process the engine oils and transmission fluids are important design elements and their contribution to improved efficiency is significant. Polyalkylmethacrylates (PAMAs) are widely used as viscosity index improvers in engine, transmission and hydraulic oils. They have been shown to adsorb from oil solution onto metal surfaces to produce thick and viscous boundary films. These films are maintained even in low speed and high temperature conditions and thus produce a reduction of friction and wear. It was found that specifically designed film-forming PAMAs can improve pitting performance of lubricant formulations. The paper describes the impact of tailor-made functionalized PAMAs on boundary film formation and explores their ability to increase the fatigue life of lubricants.
Thermal Behaviour of a Slipping Wet Clutch Contact
Wet clutches are used in automatic transmissions to enable gear changes and also to reduce energy loss in the torque converter. These friction devices are susceptible to stick–slip effects, which result in the vehicle giving an unsteady ride. Stick–slip effects can be avoided by ensuring the wet clutch and lubricant combination produces a friction coefficient that increases with sliding speed. Although wet clutches have been studied throughout the industry for many decades, the mechanism of the generated friction is still not fully understood. It is known that, because of the fibrous nature and thus very large roughness of friction material, the overall contact between clutch plates actually consists of many small, independent, contact units, which are the sites of the generated friction. Some authors have suggested that a temperature rise due to friction either at these contact units or of the overall clutch plate may be important in controlling friction behaviour. In this study, the flash temperatures at the contact units formed in the wet clutch contact have been measured using an infrared camera. Three friction materials have been tested. It was found that measured flash temperature in a pure sliding system similar to that present in a slipping clutch remain well below 5 °C, and are therefore unlikely to play a major role in the observed friction-speed dependency of slipping wet clutches at speeds below 2 m/s.
Contact properties of a wet clutch friction material
Wet clutches are required to transmit torque and also prevent motion in automatic transmissions. Their performance is critically dependent on a friction material which comprises one of the contacting surfaces. Friction materials are usually a composite of fibres, naturally occurring minerals and particles of silicon and graphite, which are all bonded together with a resin. The material formed has very rough surfaces with much steeper slopes than normally-finished steel surfaces. When the friction material is loaded against a relatively flat counterface the real area of contact is only a small percentage of the nominal area and consists of many small, independent “contact units”. It is important to know the conditions present in the contact units (spatial dimensions and pressure) in order to understand and model wet clutch lubrication.
In this study, the contact units formed between a paper based friction material and a glass counterface have been investigated under different pressures and during rubbing. A contact visualisation technique is used to directly view and capture images of the contact. The real area of contact and the number of individual units is subsequently determined by image analysis. It is found that the real area of contact increases approximately linearly with applied load, and increases rapidly with rubbing, due to wear. As the load is increased, the number of individual contact units increases up to a critical pressure, suggesting more parts of the material support the load. Above the critical pressure the contact units may be deforming elastically and/or plastically to form larger units. After rubbing, large contact units are formed by flat areas on the tops of the contacting fibres, which are formed during wear. The topography of individual fibres is studied before and after the wearing process using atomic force microscopy, and the results support the truncating wear mechanism.
Frictional Properties of Automatic Transmission Fluids: Part II
Origins of Friction–Sliding Speed Behavior
A wet clutch in an automatic transmission has very specific and unusual friction characteristics. For the clutch to operate efficiently and without stick–slip, friction must both increase with sliding speed and be high over the whole sliding speed range. This is achieved by the use of a very rough friction material, which inhibits fluid film formation, combined with sophisticated design of the automatic transmission fluid, with separate additives to reduce friction at low speed and increase friction at high speed. There has been much debate in the literature as to the underlying mechanism that allows friction to increase with sliding speed. This article critically discusses the main models that have been suggested, which range from a highly viscous, and thus high friction, hydrodynamic film forming at high sliding speed, to fluid drag supplementing friction as sliding speed increases. The authors then propose an entirely new model for wet clutch friction. This is based on the two principles: (1) the wet clutch morphology ensures that the contact remains in the boundary lubrication regime over the whole sliding speed range and (2) organic friction modifiers form adsorbed boundary films whose friction increases with the logarithm of the sliding speed. The latter behavior has been clearly demonstrated in a number of previous studies and ascribed to an activated shear process. The combination of these two principles means that when an effective organic friction modifier is present, wet clutch friction increases monotonically up to high sliding speeds. Based on friction–sliding speed measurements described in a companion article, Part I (Ingram, et al. (1)) it is then suggested that additives that increase friction do so by partial disruption of the organic friction modifier film, which allows more interpenetration of the films on the opposing surfaces and thus higher friction over the whole speed range. Toward the end of the article, the possible origins of the increase of friction at low sliding speeds observed for base oils and blends without effective organic friction modifiers is discussed.
Effect of EHL Contact Conditions on the Behavior of Traction Fluids
The Influence of Engine Oils on Friction Reductions
The borderline of Elastohydrodynamic and Boundary Lubrication
There is a growing trend for lubricated systems to operate for much of their operating life with very thin lubricating films. This paper reviews our current understanding of such films, at the borderline between elastohydrodynamic and boundary lubrication. The nature and properties of these films are very complex, since the proximity of the solid surfaces influences the structure and rheology of thin liquid layers, while boundary films can, themselves, possess rheological characteristics that vary with thickness.
Novel experimental tools, such as atomic force microscopy and ultra-thin-film interferometry have greatly accelerated our understanding of this area in the last few years and it has recently become possible to map lubricant film thickness within rough surface contacts. These tools are beginning to provide the level of understanding of thin-film rough surface behaviour required to develop accurate numerical and simulation models. The next 5 years should see a very rapid progression of our understanding of this important regime.