Farrukh Qureshi

The Generalized Newtonian Fluid Model and Elastohydrodynamic Film Thickness

The nature of real shear-thinning in elastohydrodynamic contacts is well-known from both experimental measurement and nonequilibrium molecular dynamics to follow a power-law. Shear-thinning will affect the film thickness when the Newtonian limit is low enough to occur in the inlet zone (less than about I MPa shear stress). Then kinetic theory tells us that film thinning should occur for molecular weight greater than 2000 kg/kmol. We present a review of generalized Newtonian models, flow curves for real lubricants and comparison of calculated and measured film thickness. The calculations utilize measurable liquid behavior, in contrast to most previous work.

Observations of Shear Localization in Liquid Lubricants Under Pressure

A High Pressure Flow Visualization Cell has been designed and constructed to perform a fundamental investigation of the deformation behavior of liquid lubricants under lubricated concentrated contact conditions. A pressure of 0.3 GPa and a shear stress between parallel plates of about 25 MPa has been demonstrated. Time averaged velocity profiles show no continuous slip either in the bulk or at walls. Localized slip at shear bands inclined to the walls was demonstrated to occur during nonlinear shear response. The number of shear bands increases with shear rate (and shear stress) from as few as one at the onset of non-Newtonian flow until the shear region is essentially filled with bands with a spatial periodicity of 7 μm

Accurate Measurements of Pressure-Viscosity Behavior in Lubricants

The piezoviscous effect is accurately characterized for a number of representative lubricating oil base stocks and a simple, well-defined hydrocarbon. Procedures for the generation of accurate pressure-viscosity coefficients are outlined. The reciprocal asymptotic isoviscous pressure may be measured to within perhaps 1%. The piezoviscous effect may be important to automotive fuel economy. The departure from exponential behavior at very low pressure becomes more pronounced at high temperature. Presented at the 57th Annual Meeting Houston, Texas May 19–23, 2002

The high pressure rheology of polymer-oil solutions

We present perhaps the most comprehensive work on the high-pressure shear-thinning of polymer-blended mineral oil. Three solutions of commercial polymer VI improver in mineral oil are characterized for the effect of pressure and shear rate on viscosity. We show that the film thinning measured in concentrated contact can be explained by ordinary shear-thinning of the power-law type. We present extended high-pressure flow curves for a model polybutene-mineral oil blend that has been previously studied and show the possibility of two shear-thinning transitions; one for the polymer and one for the base oil. Solutions of well-characterized, narrow distribution polyisobutylene in squalane were prepared. The high-pressure mixing rule was determined and Carreau parameters were extracted from flow curves and compared to existing models for calculating characteristic times from molecular weight and concentration.

The low-shear-stress rheology of a traction fluid and the influence on film thickness

Abstract A flow curve generated by pressurized Couette viscometry clearly shows the presence of polymer in a traction fluid used for traction simulations. This finding was supported by experimental film thickness comparisons with the base oil. Insights regarding the effect of polymer additives on film thickness were obtained from idealized calculations.

New EHL Modeling Data for the Reference Liquids Squalane and Squalane Plus Polyisoprene

ABSTRACT An important part of the new quantitative approach to elastohydrodynamic lubrication (EHL) is the use of reference liquids with well-characterized thermophysical properties. New measurements are reported for the thermal and rheological properties of squalane to high pressure and of high shear rate and high-frequency viscosity of squalane thickened with polyisoprene (SQL + PIP) at ambient pressure. The glass transition viscosity of squalane at ambient pressure was found from published viscosity measurements and new glass transition measurements by transient hot wire. The glass transition viscosity so determined was incorporated into the improved Yasutomi model and the calculated glass transition temperatures as a function of pressure are comparable to those directly measured, although the hybrid model yields better agreement. The glass transition viscosity of squalane by any definition must be substantially lower than the “universal value” of 1012 Pa·s. The second Newtonian inflection cannot be characterized in steady shear at ambient pressure for SQL + PIP due to cavitation; however, acoustic viscometry with matching layer does characterize the second Newtonian inflection. To form the analogy between steady and oscillatory shear requires that the steady shear rate be compared with the ordinary frequency rather than the angular frequency for SQL + PIP.

Time-Temperature-Pressure Superposition in Polymer Thickened Liquid Lubricants

Shear-dependent viscosities have been measured over a range of temperature and pressure for seven engine oils blended to have kinematic viscosity of 14 mm2/s at 100 °C with two base oils and four viscosity modifiers. Elevated pressure measurements were performed with a pressurized thin-film Couette viscometer and ambient pressure measurements were done with a PCS USV viscometer. These measurements were fitted to a generalized Newtonian model with the effective shear modulus specified by an empirical power-law shifting rule. The use of PAO-40 as a thickener delayed the shear-thinning to very high stress as compared with the polymers. The rate sensitivity of the oils thickened with nondispersant polymers was similar. Like the Tannas TBS viscometer, the PCS Instruments USV viscometer provides shear-dependent viscosity measurements, which can be essential for the most accurate time-temperature-pressure shifting. Viscosities measured at high viscous power in the ambient pressure viscometer, however, tend to be influenced by thermal softening and at high stress by shear cavitation.

Systematic formulation of efficient and durable axle lubricants for light trucks and sport utility vehicles

Consumer demand for size, weight and horsepower has dictated a prominent role for sport utility vehicles and light trucks in the product lines of major North American automobile manufacturers. Inherently less efficient than passenger cars, these vehicles will be facing more stringent light duty CAFE (Corporate Average Fuel Economy) standards beginning in 2005 when mileage targets will be elevated to 21 mpg; this figure will be further increased to 22.2 mpg by 2007. In order to accommodate both public demand and CAFE requirements, vehicle manufacturers are seeking ways to improve fuel economy through design and material modifications as well as through improvements in lubrication. The axle lubricant may have an important impact on fuel economy, and axle lubricants can be tailored to deliver higher levels of operating efficiency over a wide range of conditions. Improvements in city-highway axle efficiency can be gained through the lubricant when appropriate rheological properties are coupled with lighter (SAE 75W-90) viscosity grades to minimize frictional churning losses. Light trucks and sport utility vehicles (SUVs), often functioning under high duress and heavy loading, demand lubricants which are capable of controlling operating temperatures; historically, higher viscosities (SAE 75W-140) coupled with special rheological characteristics have served this purpose. The penalty for high load durability and longer vehicle life is often a loss in city-highway efficiency; at the same time, high city-highway efficiency ratings are not generally consistent with vehicle durability under highly stressful operation. The challenge is bridging the efficiency-durability gap. Laboratory test rigs which simulate the FTP-75 and various trailer-tow duty cycles, recently described by these authors, are valuable tools which enable the formulator to design versatile and balanced axle lubricating fluids. Both rigs operate with a high level of control over testing conditions and produce repeatable fluid rankings. Herein, we will describe a fluid development program utilizing key physical and chemical screening methods, efficiency and durability rig testing, bearing life testing, and field validation testing. Of particular interest, certain SAE 75W-140 grade candidate fluids emerging from this process have demonstrated traditional high torque durability (and bearing life) but also significantly improved operating efficiency.

Ordinary shear thinning behavior and its effect upon EHL film thickness

Measurements are presented of ordinary shear thinning behavior of liquid lubricants using laboratory rheometers. Flow curves are constructed and compared with theoretical models. The data can be fitted to the well-known Carreau equation. This model is used to calculate rolling/sliding film thickness under EHD line contact for conditions typical of tribological contacts. This work provides useful insights regarding the ordinary shear thinning behavior of liquids typical of lubricants, and its impact on EHD film forming behavior of lubricants.

Innovative technique of measuring follower rotation in real production engine using Gradiometer sensor and the effect of friction modifier

Purpose – The purpose of this paper is to understand the effect of engine operating conditions and lubricant friction modifier on direct acting tappet rotation. In this research work, novel method of measuring engine tappet rotation speed has been developed. The technique is so novel. It allows the measurement on real production engine with no modification to the engine tappet bore. Also, In this paper, the effect of engine operating conditions and the effectiveness of friction modifier on tappet rotation is reported. Design/methodology/approach – For the very first time, for the purpose of measuring follower rotation in a real production engine, a 4 × 6 mm2 electronic chip called Gradiometer is mounted outside the tappet housing, allowing the monitoring of tappet rotation speed without the need to machine a hole in the tappet bore. This novel technique is adopted on Mercedes Benz OM464 engine to study the effect of engine conditions and lubricant chemistry on tappet performance. Findings – The main outco...