A droplet may be prevented from wetting a solid surface by the existence of a lubricating film of air, driven by theromcapillary convection, between liquid and solid surfaces. The noncontact nature and the load-carrying capability of a nonwetting droplet lead to potential engineering applications, e.g., low-friction bearings. The present research consists of two thrusts. The first is aimed at quantifying nonwetting-system failures (film and pinning) triggered by application of a mechanical load, gaining insights to failure mechanisms. Experimental results show that film failure occurs over a wide range of droplet volumes when the temperature difference between the droplet and the plate, the driving potential of the free-surface motion, is small. Interferometric observations reveal flow instability just prior to film failure, with the growth of a nonaxisymmetric disturbance on a free surface (m = 1). Pinning failure becomes more prevalent as the temperature difference is increased, stabilizing the film flow. As part of the present investigation, a system was devised, allowing an oscillating free-surface to be reconstructed from a series of interferograms. The dynamic responses of the free surface reveal mode coupling, with harmonics of the input frequency excited through nonlinearity. The second thrust of the research succeeded in levitating and translating a droplet using the mechanism of permanent nonwetting. In this scheme, the droplet is heated by a CO2 laser and is placed above a cooled glass surface in order to drive the lubricating film that supports the weight of the drop. Furthermore, the position of the droplet can be controlled by moving the heating location, which leads to an asymmetry of the flow fields, driving air from the cooler-end of the droplet and propelling it towards the heat source. These demonstrations suggest the techniques potential use as a liquid-delivery scheme in a Lab-On-a-Chip system. Modeling is carried out to estimate propulsive forces on the droplet and to explain oscillatory behavior observed when excessive heating is applied on the drop. The concept to sandwich a droplet between two plates, a necessary configuration for levitating smaller droplets (less than mm-scale), is also discussed.

In this study a fiber optic interferometer (FOI) was designed and constructed to measure micron level clearances occurring in piston cylinder arrangements. A Cartesian model of the piston cylinder assembly is manufactured and lateral motion and vertical displacement are generated via a step motor, and micrometers, respectively. Clearance measurements are conducted in air and also in a lubricant. The range of vertical displacements is kept between 0-50 &amp / #956 / m, and the lateral motion is 13.5 mm. The effect of the step motor and lateral motion carriage on distance measurement is determined and this is used to correct displacement measurements.

Excavators which used as earthmoving machinery are exposed to heavy loads and operate long hours repeatedly. The forces that are transmitted trough pin bearings are observed to cause failure due to wear. Therefore, durability of bearings is crucial for excavators. The aim of this study is to perform wear analysis of excavator bearings running in boundary lubrication regime. A hydraulically powered test rig, which simulates the operating conditions of excavator bearings is designed and constructed. Wear is obtained applying both force and relative motion between pin and bearing on the test rig. Several tests are performed in this test bench. Force and material type are varied as the factors influencing wear. Results are compared and the effects of the factors on wear are determined.

Electromagnetic launchers are currently being developed for their use as military weapons. These devices launch a projectile to extremely high speeds using very large electric currents. One obstacle facing the development of electromagnetic launchers is damage to the rails and armature during launch. The damage occurs due to current arcing in the armature-rail interface and is denoted as a transition. One solution is to use a lubricant injection system contained inside the armature to provide a conductive lubricant to the interface. The lubricant will ensure good electrical contact, prevent solid-solid contact, and cool the interface to prevent a launch from transitioning. Various different armature designs are currently under development. Each design must be analyzed through armature-rail interface modeling in order to predict the physical behavior and identify causes of transitions. There have been many studies on the physical behavior of sliding contacts. Some of which are directly applied to electromagnetic launch. In particular the magneto-elastothermohydrodynamic model is the most comprehensive model found for use in simulating electromagnetic launch. It includes calculation of the electromagnetic field, elastic deformation of the armature, calculation of the armature temperature history, and a hydrodynamic study of the lubricant both in the injection system and the armature-rail interface. The magneto-elastothermohydrodynamic model has been applied to only one armature design with limited success due to the assumptions used. The magneto-elastothermohydrodynamic model is applied to six different armature designs each requiring modifications to be made in order to predict the distinct behavior of each launcher. Modifications to the model include consideration of turbulent flow in the injection conduit, unique injection configurations, dry-out of the armature-rail interface, two dimensional pressure fields, and analyses of cylindrical bore launcher designs. The results show the model is effective in predicting when a transition will occur and what physical event leads to a transition when compared to experimental launch data. Additionally, experimental observations are used to affirm the simulation of other physical characteristics. It is found by the simulation that the base case armature is successful in preventing a transition of the shot, which is consistent with the experimental results. The simulation of NRL shot 223 reveals that such a small amount of lubricant is supplied by the reservoirs that the armature-rail interface partially dries out making a transition likely at a time of 4.7 ms; agreeing with the experimentally observed transition at a time of 4.5 ms. It is determined that the transition of NRL shot 406 is not due to a lack of lubricant inside the interface and that the amount of lubricant which leaks from the joint is negligible. IAP shot 7 did not transition in the experiment, however, after a time of about 3.5 ms the muzzle voltage began to rise. The simulation presents a possible explanation, showing that the armature-rail interface is beginning to empty out after 4.2 ms. The simulation of the GTL-2-4C armature shows that the experimentally observed transition is caused by the reservoirs emptying out at about 2.1 ms. The exploratory simulation of a modified GTL-2-4C armature determines that the absence of the slit in the armature trailing edges will not prevent the transition nor extend the successful portion of the shot.

<p>In recent decades much effort has gone into reducing particle emissions in the exhaust gases of heavy-duty diesel engines. Engine development has now reached the stage where it is worth to put heavy focus on the contribution of lubricating oil to particulate emissions in order to further reduce these emissions.</p><p> </p><p>A literature study demonstrates that the cylinder system is usually the largest source of oil-related particles. Oil consumption in the cylinder can be divided into <em>throw-off</em> effects when inertia forces act on the piston, piston rings and oil; <em>evaporation</em> from hot surfaces; <em>reverse blow-by</em> when gas pressure drives the oil consumption; and <em>top land scraping </em>when oil is scraped off the cylinder liner.</p><p> </p><p>The pressure between the compression rings strongly affects the stability and position of the upper compression ring as well as the oil consumption caused by the reverse blow-by. A method to measure the inter-ring pressure was developed and evaluated. The measurements showed that cycle-to-cycle variations were small, but that the inter-ring pressure varied over time. Calculations with AVL Excite Piston and Rings confirmed that ring gap positions can have a major influence on the inter-ring pressure.</p><p> </p><p>The measured particle size and number distributions at motoring conditions show interesting and unexpected results. The high number of particles with a diameter of around 100 nm was greatly reduced when the temperature in the diluter was increased. The mean number particle diameter decreased until 10 nm and then became stable independent of further temperature increase. Other authors have found that the small particles (nucleation mode) are reduced and the larger particles (accumulation mode) are more or less unaffected when exhaust gases are heated up and diluted.  </p><p> </p>

Habilitation à diriger des recherches : Génie mécanique : Villeurbanne, INSA : 1999. / Document rédigé également en version française. Titre provenant de l'écran-titre. Bibliogr. p. 135-155.

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Engine development is now driven by cost, performance, governmental regulations and customer demands. Several of the requirements have tribological associations. Tribological improvements which consist in lowering friction and improving wear resistance in engines, will play a major role to increase reliability and life cycle. The components studied here are parts of the valvetrain mechanism of heavy-duty Diesel engines. The injection cam is one of the most problematic parts of the camshaft, as it is subjected to high pressures from the fuel injector. Lubrication is of significant importance in the prevention of cam failure caused by wear. However, the satisfactory lubrication of the cam and roller contact has proved to be one of the most difficult tribological design challenges to take up. For a lubricated contact, the degree of separation between surfaces has a very strong influence on the type and amount of wear. This degree of separation is termed as specific film thickness ; its value provides a measure of the severity of asperities interaction in the lubricated contact. In this report, attention is drawn on the evaluation of oil film thickness in the cam-roller contact, in order to predict regimes of lubrication and thus to identify the probable wear zones of the injection cam. Then, confrontation with experimental results is performed(observation of worn cam surfaces). Future work to achieve is to discover the influence of the different parameters on oil film thickness, by performing a multivariate analysis. The next step will focus on modelling the wear of injection cams, and finally establishing quantified correlations between wear and specific film thickness.

This thesis is devoted to the study of spinning contacts located in bearing between the roller-end and the ring flange. The main direction of the lubricant flow may change when the contact is subjected to skew. This complex kinematics influences the contact behaviour. A dual experimental-numerical approach is proposed to study this problem. The Tribogyr test-rig allows the experimentation of the contact at the 1:1 scale. A film thickness measurement method, based on white light interferometry, was developed on Tribogyr. This method allows the measurement of film thickness between 0 and 800 nm with an accuracy of a few nanometres. The measurement of forces in the main flow direction shows similarities with classical rolling-sliding contacts. However, the friction coefficient is globally lower as soon as spin is involved. Transverse forces are of the same order of magnitude as the longitudinal forces. This is due to transverse shearing caused by the spin. A numerical model has been developed for the simulation of these spinning contacts. The finite element model, which is based on a fully-coupled solving strategy, takes into account the temperature calculation and the lubricant non-Newtonian rheology. Its validation with Tribogyr experimental results in terms of film thickness and friction has been conducted. Spin and skew effects induce high shear-thinning and thermal-thinning of the lubricant that lead to a decrease of the film thickness. Under high spinning condition, the lubricant exiting the contact may be re-injected to the contact inlet. Consequently, the heat transfers between the lubricant and the solids in contact are modified. In contact subjected to high skew, a local increase (dimple) of the film thickness may occur. Important skew may also lead to starvation conditions. Many experimental campaigns, coupled with an intensive use of the numerical model, allowed to understand the physical phenomena involved as well as to predict the efficiency, in terms of power losses, of the spinning contacts.

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