Steinbeis testing system helps companies optimize energy efficiency
Funded by the German federal government, the PROMETHEUS project aims to reduce friction caused by tribological contact in engines. Experts at the Steinbeis Innovation Center for Materials and Surface Technology in Friedrichshafen have taken up this challenge by developing an innovative testing system based on a physical model of a oscillating long stroke tribometer. A number of leading manufacturers and automotive suppliers stand to benefit from the results by improving the energy efficiency of their products and systems.
Climate protection targets set by policymakers should result in long-term reductions in greenhouse gas emissions of 80 to 95%. Cars and commercial vehicles cause a significant share of these emissions, but they also account for a major proportion of the economy, so they represent an important point of leverage when it comes to implementing measures aimed at improving energy efficiency. Since it will still be some time before all vehicles are converted to electric engines, further development work needs to be carried out on existing systems to improve energy efficiency. Given the design of combustion engines, both transmission systems and bearings play a particularly important role when it comes to environmental and financial factors. In specific terms, optimizing friction within piston assemblies not only makes it possible to reduce direct consumption; it also allows improvements to be made when it comes to thermal loss and wear.
The goal of PROMETHEUS: reducing friction
These challenges are now the focal point of the PROMETHEUS project funded by the German Federal Ministry for Economic Affairs and Energy (BMWi). The aim of this project is to optimize friction in engines by making use of triboactive high-performance carbon and iron-based coatings and lubricants. The project will be carried out by a consortium, pooling the comprehensive know-how of OEMs, suppliers, and related institutes in order to arrive at a holistic solution to the project brief.
The overarching aim of PROMETHEUS is to reduce the friction caused by tribological contacts, especially on piston rings and cylinder walls. The idea is to cut CO2 emissions and energy costs by significantly improving friction properties. Another priority will be to use wear-reducing coatings to extend the service life of components, which will also make it possible to maximize energy efficiency across entire systems.
The model oscillating long stroke tribometer – an innovative testing system
The Steinbeis experts from Friedrichshafen are supporting the goals of the PROMETHEUS project by developing an innovative testing system. Their model of a oscillating long stroke tribometer is being combined with novel evaluation methods aimed at characterizing tribological properties on a test specimen level. Their work has shown that conventional testing based on short-stroke oscillation or rotation tribometers only provides limited insights into systems. To conduct comprehensive assessments of tribological systems in combustion engines, fundamental development work is still required on the testing systems used to investigate individual specimens.
To determine relevant parameters, among other things, combustion chamber pressure, piston speeds, lubricant properties, and the condition and morphology of surfaces are being looked at. Because pressure inside the combustion chamber changes over time, as does the piston speed, tribological behavior between the piston assembly and the cylinder wall is transient. To single out optimized tribological systems, it is important to ensure they have no negative impacts on other system components, particularly when it comes to wear. To check this, selected tribosystems are being examined for friction and wear properties.
The drive system being used for the oscillating long stroke tribometer is based on the conventional crankshaft drive of a serial engine with a stroke of 73 mm. The drive is powered by an electric motor, and a coupling system is being used to reduce torsional vibration. The cylinder heads are removed and a linear bearing system is fitted horizontally onto the crankshaft drive with the lubrication circuit left intact on the crankshaft drive. The axial force exerted within the measurement system is gauged by using two force transducers. The position of the piston ring in relation to the cylinder liner is determined by an inductive speed sensor installed on the crankshaft. To measure friction force, a piezoelectric sensor is used. In addition, a piezoelectric accelerometer is positioned to detect inertial forces.
To lubricate the test assembly, it is fitted with a separate, heated oil supply unit, which has a nozzle aimed dead-center at the bottom of the cylinder liner. To ensure lubrication is uniform, a twin-substance nozzle is used. Being able to adjust the lubricant flow precisely as required makes it possible to apply lubricant at a rate ranging from 10 ml/min down to a minimum of 1 μl/min. To prevent the temperature from dipping, the lubricant and compressed air are heated. The heating system, which is controlled by thermocouples, regulates the temperature of the entire system up to 130°C.
The potential offered by the Steinbeis testing system
The innovative oscillating long stroke tribometer makes it possible to individually characterizing serially produced components or materials such as piston rings, cylinder liners, and engine oils. Combining the oscillating motion kinematics of a crank mechanism with a long stroke makes it possible to assess operating factors with a bearing on energy issues within the tribological system of the piston group. The PROMETHEUS project can thus investigate the friction coefficients and wear potential of newly developed piston ring coatings. Examples of the results are shown on the graph.
The design of the test rig allows friction values to be displayed as a function of the crank angle. This also allows friction values to be assigned to individual functional areas of the components. For example, the top and bottom dead centers have higher friction values than the areas of maximum piston speed.
The results from the model tribometer lay a foundation for, among other things, pre-selecting cost- and energy-intensive single-cylinder and full-engine tests. Furthermore, correlations in terms of transferability to other areas can be investigated, combined with the simulated mapping of different abstraction levels (tribometer/single cylinder test/full engine test). Being able to transfer findings to other areas could prove to be of benefit when it comes to future optimizations made to tribological systems. In principle, it is also conceivable that testing could be carried out on other tribological systems subject to rapid oscillating movements.
The end users who will benefit from this include the BMW Group, MAN Truck & Bus, and Rolls-Royce Power Systems. This has been made possible thanks to close collaboration between the component and system suppliers Fuchs Lubricants, Schaeffler Technologies, Tenneco, and the equipment manufacturer VTD Vacuum Technology. The team led by Steinbeis experts Prof. Dr. Reinhold Holbein and Benjamin Kröger is acting as a research and development partner alongside AVL Deutschland, Fraunhofer IWS, Fraunhofer IWM, Fraunhofer LBF, and TU Dresden, thus pooling expertise in tribological fundamentals, coating development, component testing, and simulation.