Overall weight reduction is key to optimizing the energy consumption of electric vehicles and to increased autonomy. Several projects have led to a reduction in structure, body and trim mass, but few have targeted powertrain mass.
At ARRK, we have worked on reducing powertrain weight. We looked at optimizing the weight of the typical two-stage electric transmission by substituting the aluminum housing material with materials of lower density. When doing so we had to consider all requirements to be satisfied. Carbon fiber reinforced thermoplastic materials provide a good combination of mechanical properties and fast manufacturing processes, but attaining the stiffness needed to reach the durability and acoustics expectations for the gearbox - especially at temperatures above 100°C - is challenging for this group of materials.
The functional complexity of electrical gearboxes is lower than conventional gearboxes because the number of gear ratios is much lower today. Conventional vehicles have up to seven gear ratios and electrical vehicles have only one or two.
What is most challenging for an electrical gearbox is the acoustics. Where you used to have a combustion engine that would drown the operational noise of the gearbox, you now have an electrical engine that is silent. This makes the gearbox acoustics dominant, especially at low speed. The sound of a transmission itself, usually a whining sound, is annoying for the occupant and must be managed.
Acoustic performance is also related to the production of thermal energy, or energy loss. Imperfect acoustic behavior generates thermal energy. In an ideal gearbox, there would be no deformation of the housing caused by the transmitted loads. But in real life, there is displacement of bearings of the shafts that leads to a change in position of the gears running on that shaft. The challenge is to achieve the same or better stiffness values with the new housing material as for the original aluminum gearbox. Axis deviation and axis inclination error of the intermediate and output shafts have to be on the same level or below the values for the aluminum housing.
In our gearbox project, we replaced the aluminum housing material of an electric transmission with fiber reinforced thermoplastic material. Our aim was to replace the housing material only – reusing all interior parts.
In the design we developed, an organo-sheet (carbon fibers fabrics with multi orientation, and polyamide thermoplastic matrix) is over-molded by a short fiber reinforced thermoplastic material (PPA with short glass fibers). Aluminum inserts, as bearing seats, ensure transmission of the bearing loads into the organo sheet and reduce the deviation of the gear mesh. Additional injection molding ribs and unidirectional tapes ensure the stiffness requirements.
ARRK P+Z (German entity) and ARRK Shapers (French entity) worked together to show the ability of the ARRK product development group to develop the parts, to build the tools, to set up the production process and to produce in a small series production size. ARRK P+Z was responsible for the engineering and ARRK Shapers set up the production process and built the tools and the prototype.
The engineering approach was simulation driven. Different types of simulations - finite element method, molding simulation, stamping simulation, optimization - were used. ESI provided simulation support with ESI’s composites manufacturing solution. Ultimately, the gearbox with the new material weighed 4kg, compared to 5.8kg with aluminum, so we achieved almost 30% mass saving. We demonstrated the capability of ARRK to design and prototype composites gearboxes, which outperform a standard aluminum gearbox.
Image courtesy of ARRK