DuPont “Science of Friction” : how managing friction can boost auto efficiency and reduce fuel consumption.
It has been calculated that six of every seven liters of fuel put into a vehicle are lost to inefficiencies. Connecting inefficiencies to friction, studies have found that somewhere around 10 percent of this lost energy is attributed to friction between moving parts. This is equivalent to several million barrels of oil a day, and presents a big opportunity for anyone who can improve control of friction.
Friction has long been known to be a significant power thief, but only in the past few years has a real sense of urgency surrounded the issue. This urgency focused the DuPont global technical and Research and Development network on science-based solutions, and on developing new products to help control friction - exemplified by DuPont™ Vespel® SP-2515, the latest material-solution to fight the effects of friction. Using a basic thrust washer test, DuPont demonstrated a 45 percent to 55 percent reduction in friction by using Vespel® SP-2515 instead of PEEK, even at cold temperatures.
The key findings of the company's research into friction were presented by Volker Plehn, Director Development and Sales for DuPont™ Kalrez® and Vespel® in Transportation, North America, at the International CTI Symposium + Exhibition "Automotive Transmissions and Drivetrains North America" in Michigan, USA, on June 9, 2010.
Entitled "The Science of Friction - High Performance, Cost Effective and Sustainable Solutions for the Transportation Industry", his talk presented highlights of how DuPont's "Science of Friction" program has been able to create new solutions to help control friction and get more power to the ground.Following is a summary of the presentation. For a copy of the full presentation, go here .
Two needs are driving the transportation industry today: First, the vital global need to reduce consumption of fossil fuels, and second, the urgent need to reduce greenhouse gas emissions.These needs are translated by our customers into active innovation programs in the drive for fuel efficiency, emissions control and enhanced performance, and result in new technologies for variable valve timing and lift, and increased use of turbochargers, for example. New propulsion concepts result in new 6, 7 or 8 speed transmissions, CVT’s, or electric/hybrid transmissions. Increased use of electronic controls drive changes in actuators, DC drives, stability systems and electric pumps.
All result in new wear and friction demands - thus the need for new materials, and the engagement of a science company like DuPont to innovate and provide new materials and solutions.
Many industry experts and customers confirm that only about 15 percent of the energy in a liter of fuel makes it to the drive wheels - i.e. 6 of every 7 liters of fuel are lost to inefficiencies. About 10 percent of this lost energy is attributed to friction between moving parts, equivalent to several million barrels of oil a day.It is clear that the ability to control friction would be extremely important for both existing applications and for the unprecedented number of new powertrain technologies being brought to market today.
DuPont has a long history of providing resins and additives to help control dry friction. However, one of the increasing market needs is the ability to have this same control over friction in WET systems - systems that are lubricated with oil, transmission fluid or grease. Simply immersing rotating parts in oil is not good enough today - systems need to be optimized to succeed in the competitive world of fuel mileage targets and emission limits.
So what does "Science of Friction" mean? Typically for our experiments we use a steel or aluminum disc sliding against a polymer disc or ring, with a lubricant in between. As rotating velocity and loads change, there is an optimum region where friction is at its lowest. Alternatively, friction can be increased to avoid slippage or power loss. Our aim is to hit the “sweet spot” where friction is lowest.
Three variables will be discussed - material selection, design practices, and temperature - with highlights of experimental results we have obtained for our customers, based on seal rings and thrust washers - common driveline and transmission components.
First, how does material optimization improve friction? Here are some of the test results. By using a basic thrust washer test we were able to demonstrate a 45 to 55 percent reduction in friction by using new Vespel® SP-2515 - a graphite filled polyimide - instead of PEEK (polyether ether ketone), even at cold temperatures (see Figure 1.). This cold temperature improvement could be a significant benefit in cold start or cold environment fuel economy tests (test results, data sheets and a technical paper on Vespel® SP-2515 can be found at www.tryvespel.dupont.com ).
Figure 1. Improving component efficiency, controlling friction, through material optimization. |
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Material optimization in combination with design - i.e. careful attention to component design and taking advantage of a material's properties can improve friction.We observed the results of designing a seal ring to have less contact area with the mating metal surface. By changing only the material to Vespel® results in about a 13 percent improvement in torque loss per seal ring. Then utilizing the high pressure and velocity limits of Vespel®, we designed a "T-shaped" cross section which allowed a smaller contact area, resulting in a 34 percent improvement in efficiency. Multiply this by the typical 5 to 6 rings used in a modern transmission, and the savings can become significant.
In a separate experiment we evaluated the effect of small oil grooves on the face of the seal ring, comparing the same two materials, PEEK and Vespel®. By using Vespel® we were able to demonstrate a 50 percent improvement in the measured torque loss with optimum groove design and material selection.
Based on such results, Vespel® SP-2515 has recently been specified in a new 6 speed automatic transmission for a global automotive OEM.
A very interesting relationship between temperature and friction was noticed in developing Vespel® SP-2515. When comparing several materials used in friction control applications, we observed bronze and PEEK reaching temperatures high enough to activate our safety interlock system which protects the oil from reaching dangerous temperatures.However, Vespel® SP-2515 not only had the lowest coefficient of friction, it also lowered observed temperatures by more than 50°C (see Figure 2.). This is consistent with the observation that SP-2515 has four times higher thermal conductivity than many conventional polymers, allowing more heat to be conducted out of the contact area.
Figure 2. In thrust washer evaluations, Vespel® SP-2515 exhibited the lowest coefficient of friction, and lowered temperatures by more than 50°C. |
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This testing supported our belief that both reducing friction and being able to conduct heat away from the bearing surface allows the system to operate in the lower friction "sweet spot".
An example of the work we are doing with our automotive customers is a DuPont™ Vespel® SP-21 thrust washer for the planetary gearset of a Magna Powertrain transfer case (see figure 3.). The final customers are Land Rover for the Discovery, and Range Rover and Mercedes Benz for their ML and GL products.
Figure 3. Thrust washers of Vespel® SP-21 for a Magna Powertrain transfer case. |
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The planetary gearset is supported by a set of DuPont™ Vespel® thrust washers, chosen for their outstanding wear and friction performance, stability in transmission oils, and ability to withstand extreme temperature up to 320°C continuously, with peak temperatures up to 550°C.
The continuously variable transmission for the MINI uses two Vespel® split rings of rectangular cross-section (see Figure 4.) to provide a dynamic high-pressure seal at differential pressures up to 800 psi (55 bar), and rotational speed differences up to 12 000 rpm.
Figure 4. Continuously variable transmission for the MINI uses two Vespel® split rings of rectangular cross-section. |
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In this application, Vespel® SP-21 meets the need for low wear and friction at high PV conditions, hot oil resistance, thermal stability and mechanical strength. It also saves on assembly cost, weighs about 1/5 as much as cast-iron parts, and offers maintenance-free operation.
We have observed a unique property of Vespel® that can give longer life to metal components. Microphotographs of contamination particles show how they interact with a wear surface (see Figure 5.).
Figure 5. Wear or contamination particles embed into Vespel® parts, remaining below the wear surface to protect mating parts. |
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In the top photo, a contamination particle remains on top of the metal wear surface and results in groove formation and damage to the surface of the aluminum part. In the lower photo, the contamination particle embeds into the Vespel® part and remains below the wear surface where it is no longer able to cause additional damage.
Figure 6. Test demonstrates the superior wear resistance of Vespel® SP-2515 seal rings in contaminated oil. |
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To demonstrate this behavior, seal rings were tested against an aluminum shaft using both clean oil, and oil with intentionally introduced contamination particles. Vespel® demonstrated 65 percent less wear vs PEEK (see Figure 6.).
In emission systems, customers have been successful in extending the life of bearing and wear surfaces by using Vespel® parts in EGR and bypass valves, and in associated components. These are typically dry applications which require not only low friction and high heat resistance, but also consistent friction over the life of the part (see Figure 7.).
Figure 7. Vespel® has been successfully used in numerous emission system components. |
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Turbocharger systems involve combinations of wet or dry friction situations, both of which operate at high temperatures in the presence of contamination. Here again, our customers have used Vespel® successfully in improving the life and consistent operation of control arms and linkages. We also see opportunities to improve friction and efficiency inside the turbocharger, in typically lubricated applications like shaft seal rings or shaft spacers (see Figure 8.).
Figure 8. Vespel® has been successfully used in turbo charger components. |
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Another advantage of new Vespel® SP-2515 is that it has a thermal expansion rate designed to be very close to aluminum and stainless steel — important in maintaining tight tolerances in components which must survive both very cold and very hot temperatures (see Figure 9.).
Figure 9. The coefficient of thermal expansion of Vespel® SP-2515 is within the target range of aluminum and stainless steel. |
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There are at least four key benefits from controlling friction:
- Direct efficiency improvements — energy efficiency is improved by reducing friction.
- Indirect benefit by reducing leak rates and rotating friction, especially in fluid control systems, smaller oil pumps can be used - resulting in lower parasitic energy losses.
- The ability of Vespel® parts to run directly against aluminum shafts and housings helps component engineers reduce system mass - especially important for light weighting vehicles, and in stop/start applications.
- Vespel® can provide lower total costs by eliminating the need for steel inserts, and by simplifying assembly operations. Customers have achieved cost savings by extending the life of components and by reducing or eliminating routine maintenance of parts subject to wear and friction.
As a science company, DuPont can support you with high performance, cost effective and sustainable solutions. As a solution provider to the transportation industry for over 40 years, DuPont offers the "Science of Friction" as a tool to understanding how we can control friction, up or down.Material science is progressing to meet your needs of increasing temperatures, higher pressures and velocities, higher torques, smaller and lighter transmissions, and extended component life. Find out how Vespel® high performance parts can provide solutions to many of those challenges by visiting www.vespel.dupont.com , or for specific details of new Vespel® SP-2515, go to www.tryvespel.dupont.com .
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