For the automotive industry, plastics have long been a weight-saving material of choice, with a wide range of high-volume applications from body panels to interiors and underhood components—but transmission housings and gears are not among them.
Now that may change. Two European companies are collaborating in a study to achieve solutions that could herald a much wider role for plastic composites across transmission applications, and they are using electric vehicle (EV) research to help refine the technology.
The companies are U.K.-based Drive System Design (DSD), an engineering consultancy specializing in transmission design, development, and control, and Brussels-headquartered Solvay, an international chemicals group operating in sectors that include automotive, aerospace, energy, and the environment.
Based on their joint initiative to create a plastic transmission housing to improve NVH characteristics of a future pure EV, both companies are also exploring the possibility of using the material to improve the efficiency of meshing gears via tooth. In terms of noise, that would rule out using metals.
DSD Managing Director Mark Findlay explained: “There is an immediate weight saving from substituting plastic materials for conventional metal castings, but equally important is the potential for improved efficiency. The inherent damping provided by polymeric materials permits the use of much more efficient gears, such as reduced helix angles or spur gears, that would have unacceptable noise characteristics in a conventional casing. By using shorter teeth, typical tooth profiles for higher efficiency would have reduced sliding and increased rolling.”
He believes there is potential for shafts, casings, and hydraulic valve bodies to be made from plastic (suitably reinforced where appropriate), and states that full implementation could produce savings of up to 45% in casing weight for a typical passenger car transmission. With an NVH “skin” added, the saving would still reach 25%. A reduction in transmission losses would be “up to 0.5% per gear mesh.”
There is nothing new in wanting to extrapolate plastic’s roles into fundamental powertrain technology, but wanting and achieving are not the same things.
In the late 1960s, General Motors considered composite gearboxes and created prototypes. Formula One and aerospace industries have also embraced R&D programs that looked at possibilities.
In the 1980s, when such advances were seriously mooted, contemporary composites’ technology could not deliver radical powertrain application solutions such as casings and gear teeth for high-volume requirements; now it may be able to.
Findlay is pragmatic about these possible developments and stresses that it is in the premium EV category that the technology is likely to find its first application to help counter NVH: “The low cabin noise levels in a vehicle without an IC engine expose any NVH issues arising from the driveline, making the inherent damping of a plastic housing advantageous.”
Temperatures encountered in an EV are lower than an IC engine powertrain, so are more compatible with lower cost polymer temperature limits of around 120°C (248°F). An interesting point made by Findlay is that current production EV production volumes are hugely lower than those of conventional vehicles, making it easier for manufacturing technology eventually to migrate from prototype quantities to series production levels.
There are challenges, he said: “New and unfamiliar materials bring pitfalls for the unwary because of the subtleties of the mechanical properties, which can change by up to 50% over the operating temperature range due to non-linear behavior. Polymers soften above their glass transition temperature, which can significantly affect mechanical properties; even the moisture absorption of polymers can influence properties. It’s always good practice to work with a material supplier from the earliest stage of design but, when the material properties are as different as polymers and metals, it is absolutely essential.”
That is why DSD and Solvay are busy cooperating to meld their individual specialist capabilities.
For the plastic transmission study, low-cost composite technology is being incorporated from the outset to combine structural capability with volume-feasible manufacturing costs. Including the typical industry allowance for weight reduction at $10/kg saved, DSD believes composite transmission casings can be engineered to be competitive in price with existing aluminum products.
Solvay states that durability prediction has been greatly enhanced by effective finite element (FE) analysis, backed by proven data on mechanical properties and appreciation of the influence of parameters such as mold flow characteristics and fiber orientation (for composites). The recycling of plastic-only components is regarded as being straightforward, and research into composite recycling is ongoing; an issue that is common to all material manufacturers. All this is germane to the possible drivetrain developments.
Said Findlay: “Our preferred approach for a transmission casing is composite construction involving overmolding a polymer around a structural frame to provide a continuous barrier against any ingress of oil, which could otherwise infiltrate and weaken the bond between the inserts and the polymer.”
DSD and Solvay are currently discussing with vehicle manufacturers the areas within transmission and driveline systems that offer the best potential for material substitution in the future. Currently, the technology is in the development phase to optimize and prepare the most suitable materials and processes in a near-production-ready state.
DSD and Solvay anticipate a five- to 10-year timescale before the first applications come to market.
Solvay’s Global Automotive Marketing Manager, Mark Wright, underlines that it is important to approach potential customers with a range of alternative ideas: “Each customer has individual priorities, whether for weight reduction, NVH improvement, or increased efficiency. We have to reflect that by presenting the most appropriate options for their particular case.”
He explained that Solvay has taken part in “a number” of high-profile projects to demonstrate the potential of its materials: “We supply the Solar Impulse—an experimental zero-carbon, solar-powered aircraft attempting to fly around the world—with 15 different Solvay products, and also support the Polimotor 2 race engine program by providing several different thermoplastic materials.”
Polimotor 2 is composites intensive and will use Solvay’s advanced polymer technology to develop up to 10 engine parts, including a water pump, oil pump, water inlet/outlet, throttle body, fuel rail, and other high-performance components. Solvay materials targeted for use encompass Amodel polyphthalamide (PPA), AvaSpire polyaryletherketone (PAEK), Radel polyphenylsulfone (PPSU), Ryton polyphenylene sulfide (PPS), Torlon polyamide-imide (PAI), and Tecnoflon VPL fluoroelastomers.