lubricated with mineral oil and water, to determine the effect of load and speed on friction and wear performance. The results show that at a specific load both the friction coefficient and the wear rate increase to a level comparable with the data in unlubricated tests. The magnitude of the transition load was found to depend on the lubricating fluid and the test speed. It was hypothesized that the transition is related to the contact temperature or tribo-chemical reactions. These results strongly suggest that the performance tests for ceramics must be conducted at different loads to determine the transitional behavior. This is essential in determining whether the operating conditions for the intended application falls above or below the transition load, especially with ceramic materials which show an order of magnitude increase in friction coefficient and wear rate after transition.

Design Tool Development

The goal of this task area is to develop models and methodologies and to provide data that can be used for the design of advanced tribological components and systems. The ongoing and planned work is focused on two major areas; the development of a computerized tribology information system (ACTIS) which is both comprehensive and user-friendly, and the development of models for simulating and analyzing the performance and predicting the lifetimes of tribological components and systems.

The Department of Energy ECUT Tribology Program is playing a key role in the development of ACTIS. Research in tribology is attempting to reduce the estimated costs, $100 billion annually, that the U.S. economy incurs as a result of friction-and-wear processes. Approximately 40% of those losses might be saved by improved understanding and better technology transfer. The complex, interdisciplinary research required to achieve these savings involves scientists and engineers of rather diverse areas of expertise who report their research results in correspondingly diverse media. As a result, progress in this field is often hindered by the inaccesibility of information on critical efforts. An international consortium of distinguished scientists, under the overall guidance of NBS, is overcoming the most serious of the technological and organizational barriers to information dissemination through the creation of ACTIS.

The overall plan for ACTIS is a comprehensive design that will bring together in one reference system five database elements: numeric, design codes, bibliographic, research in progress and a newsletter. Initially the system will be contained in a PC-computer form addressable by an IBM/PC-compatible computer protocol. Eventually, an expanded version of the system will be available in a central host-facility gateway system that will enable ACTIS to access other international databases.

The development of ACTIS has progressed in several stages. A major milestone was reached in 1986 with the formal creation of ACTIS as an interagency government project. A government Steering

Committee (GSC), consisting of representatives from DOE-ECUT, NBS, DOD/Fort Belvoir, DOD/Wright Patterson Air Force Base and NSF, was established as the governing body.

The overall plan for a computerized system has been developed in four orderly stages. Component-design codes for various kinds of bearings, gears, seals, cams, etc., will be used in the first stage as a demonstration vehicle for feedback from industry, with shell modules for electronic mail, bibliographic and numeric data sources and directories and also the DOE/RECON Research-in-Progress database. At the same time, an initial prototype will be assembled on a central VAX system for callup by PCs. This second-stage system will include a minimal bibliographic thesaurus and the beginnings of a comprehensive tribological numeric-data collection, evaluation, updating and access system.

The ACTIS program is being sponsored by several government agencies and professional societies. In its initial year, primary support was provided by the DOE-ECUT Tribology Program together with significant funding contributions from NBS, ASLE and ASME. Continuing substantial, cost-sharing support for ACTIS over the next three years has been pledged from ECUT, NBS, the air force and the army. A proposal for cost-sharing by NSF is also under review.

ACTIS is currently designed as a stand-alone, PC-based system to contain the numeric and design databases. The software can be run on an IBM PC or equivalent system with 512K of random-access memory, monochrome monitor and one or two floppy-disk drives. The software is contained on floppy disks. The system architecture was written in C-Programming with menus and form-fill screens.

The numeric database consists of metal and ceramic properties, lubricant and additive properties, lubricated friction-and-wear, erosive wear, metal-cutting and machining and rolling-element bearings subject areas. Data for these areas are being assembled by a team of tribology experts. The design database is being assembled by a design task force composed of designers and code developers. This database will contain a component selector guide, four codes (fluid-film journal bearings, rolling-element bearings, spur-gear analysis and mechanical seals) and two design calculations (elastic-contact stress and elastohydrodynamic-film thickness).

In gasoline- and diesel-powered engines, the pistons and piston ring packs operate in cylinders distorted by the thermal environment and also by assembly and machining operations. The shapes of these distorted bores and the magnitude of the distortions vary not only from one engine design to another but also in engines of identical design produced on an assembly line. Compression rings are designed with a cam shape which cause pressure to be developed between the ring and cylinder wall when the rings are installed. This pressure, together with combustion-gas pressure, causes the rings to conform to the distorted bore if distortions are smooth and relatively small. In cylinder bores with large distortions, the rings are unable to conform to the bore, resulting in gas blowby and loss of oil.

Compu-Tec Engineering is developing a computersimulation model (the RING code) which will predict the gap area that exists between rings and out-of-round bores, and the friction forces and associated energy losses at any point in a four-stroke cycle. The RING code will analyze rings and liners of current production and also advanced low-heat-rejection engines. Loads, stresses and thermal distributions that ring and liner materials encounter will be determined so that researchers in these areas can select optimum metallic and/or non-metallic materials for specific applications. Allowable bore-distortion guidelines could be established and the effect of distorted bores on oil consumption, engine performance and emissions determined.

At each increment of crank angle the RING program computes gap area and frictional forces using databases of combustion pressures and bore distortions (thermal and mechanical). Included in the modeling are the effects of piston dynamics and inner ring gas pressure. The oil-film thickness, which varies circumferentially in an out-of-round bore, is the most important consideration in evaluating frictional forces. For a relatively large oil-film thickness there exists viscous friction due to oil-film shear. When the film thickness is small (same order as surface roughness) boundary friction occurs. This type of friction includes both viscous friction and friction caused by asperity contacts. The frictional forces acting over an incremental distance (related to an increment of crank angle) are summed to give an energy loss per cycle which is a measure of efficiency. The gap-area computation is also used to predict oil consumption. Ring-gap predictions of the model are currently being experimentally tested and confirmed by two different measurement techniques.

A current U.S.-production, six-cylinder engine was received from a manufacturer and tested for oil control at various speeds and loads. A radiometric technique was used to measure oil consumption. This engine was then disassembled and bore distortion measurements were made at six elevations in each of the six cylinders. These bore-distortion data were input to the RING code in order to predict the gap areas that exist between the rings and cylinders. Gap areas are constantly changing throughout a cycle of operation for a number of reasons. The distorted shape of the bore varies along the stroke. Combustion-gas pressures vary and so do the lateral forces acting on the rings due to piston dynamics. For these reasons, the ringsimulation program computes gap areas at a selected number of crank-angle increments and an average gap area is computed for the complete cycle. The location of the ring free-end also significantly affects the gap area occurring between the ring and bore. For this reason, gap areas were computed for various orientations of the ring free-end and ring orientations resulting in maximum and minimum gap areas were determined. The compression rings are currently being pinned in position to produce maximum gap area between the rings and bore. Another radiometric test of the engine will be conducted after the engine is reassembled. After the maximum-gap-area test is completed, the engine wilt once again be disassembled, the rings will be pinned to give minimum void area and a third radiometric test will be made.

Numerous interactions of Compu-Tec Engineering with the automobile industry (Chrysler, General Motors, etc.) and with automotive parts manufacturers (TRW-Ramsey Division*, etc.) have occurred in its work on modeling of ring-cylinder phenomena. These interactions have included loan of production engines for use in validation experiments and the provision of extensive data on engine characteristics and performance obtained by engine manufacturers.

Program Impact

The DOE-ECUT Tribology Program has already and will continue to have major, beneficial impacts on U.S. industrial energy and cost efficiency. In the lubrication area new test methods are being adopted by industry (e.g., Penzoil Products, Midwest Research Institute) and the basis for the next generation of advanced lubricants has begun to emerge. For the first time, the chemical kinetics of lubricant degradation can be studied in-situ utilizing a new, unique, time-resolved micro-Raman facility. The development of a method for effective vapor phase delivery of lubricants provides the most significant, near-term promise for effective wear and friction control at very high temperatures (e.g., in the upper cylinder-wall region of advanced low-heat-rejection engines). Industrial interest in this new vapor-phase lubrication technology has been expressed by Detroit Diesel (which is building an experimental engine vapor phase lubrication system), Cummins and Martin Marietta. Another major potential application for this technique is in near-net-shape forming of metals. Program work on friction and wear reduction in gears is producing a validated theoretical model that will enhance the capability for the design of more efficient and reliable power transmission systems.

The R&D efforts in surface modification and coatings has resulted in the development of hard, wear-resistant coatings which can be deposited on steel (by a low temperature PVD process) without affecting substrate properties. In an application to tool steel cutting inserts, these processes produced coated inserts with wear lifetimes 3 to 10 times greater than that of the uncoated tool steel. Ion beam enhanced deposition of solid lubricating coatings will produce low friction, self-lubricating surfaces, for applications such as the high-temperature upper-cylinder wall regions of proposed advanced heat engines for which no means of lubrication presently exist.

Friction and wear studies have produced the first comprehensive definition of wear test procedures for ceramics and have laid the essential methodological

'Now a division of Seal Power Corp.

and data basis for future research in the tribology of ceramics. The program has also defined the critical conditions for laboratory tests for the advanced heat engine program. Experimental results on improved strength and fracture toughness in SiC whisker/AI203 composites indicate a potential for high temperature improved wear-resistant materials in advanced heat engines.

The Design Tool Development area of the program has made significant progress in the establishment of a self-sustaining, tribology information system (ACTIS) containing "best-judgment" numeric data, design codes, bibliography, research in progress and newsletter components in a computerized format to enhance industrial capability for advanced energy conversion and utilization technology. This will improve prospects for wide-ranging interdisciplinary communication and increase the potential for breakthrough research and development in tribology. Work on the engine ring-cylinder interaction model (the RING model) will provide the means for accurately predicting the effect of various structural and operating parameters on friction, wear, oil consumption, engine efficiency, emissions, etc. This model can be utilized to optimize piston/liner/ring pack designs in conventional engines and in advanced heat engines.

Work supported by the Tribology Program, U.S. Department of Energy, Energy Conversion and Utilization Technologies.

0 0

Post a comment