A scalable electric power transmission for all EV types
06 May 2015
Driving axles for larger electric vehicles (EVs) are invariably heavy, expensive and take up a lot of space.
Addressing this problem, Fraunhofer researchers have joined forces with industry partners to develop an optimised axle module design for both commercial vehicles and cars that is lightweight and compact without compromising on power, as well as being cost-effective to manufacture.
As far as transportation is concerned, the future is electric, and commercial vehicles are no exception. But despite the many attempts to develop a viable electric motor package for commercial vehicles, these have stalled at the prototype stage or have proved extremely expensive, the commercial vehicle motor costing some two and three times as much as their conventional equivalents. One reason for this is the lack of suitable technologies for series production.
This is where the Electric, Scalable Axle Module (ESKAM) project comes to the fore. This project is sponsored by the German government and is a collaboration of eleven partners, including the Fraunhofer Institute for Machine Tools and Forming Technology (IWU), based in Chemnitz.
The team is currently working on the development of an axle module for commercial vehicles, comprising motor, gearbox and power electronics, all contained within a common, compact housing that is assembled in the vehicle using a special frame construction also developed by the project team.
This new axle module offers a number of advantages, high power density and high torque among them, providing quick acceleration. Moreover, the ESKAM motor achieves speeds of 20,000 rpm, as Hans Bräunlich, project manager at IWU explains:
“When we started on the project three years ago, we were the only ones who could obtain such high speeds. In the meantime, others have been attempting similar high speeds, but our head-start has given us a technological edge, which we intend to exploit.”
Drive performance aside, the main breakthrough has been in the area of manufacturing. As well as designing the axle module, the project researchers and developers simultaneously worked on the series production technologies that would be needed to make the whole project economically viable. IWU had the lead role in this work as well as being the technological lead for the overall project. According to Dr Bräunlich, there is great flexibility when it comes to manufacturing the modules – for both small quantities and large batches alike. Series production brings economic advantages, with reductions in production costs of up to 20 percent, he claims.
Take the gearbox that forms part of the axle module as an example. Like most gearboxes, it consists of shafts and toothed wheels. Usually, shafts like these are manufactured from expensive cylinders or by means of deep-hole drilling. In both cases, the excess material is unused. By contrast, researchers at IWU have chosen new, short process chains together with methods that allow greater material efficiency.
One such method is spin extrusion, a technique developed by IWU. Although it also uses a block of material, here the blank is shorter than the finished shaft. “To help visualize the process, think of pottery,” explains Dr Bräunlich. “The material is extruded during the shaping process and pressed outward in a longitudinal direction. This allows us to use virtually all the material, cutting material costs by approximately 30 percent and reducing the overall weight of components.”
The toothed wheels are also made using a different process. Instead of milling them from the material, they are now manufactured using a special forming process called gear-rolling -0 another technique developed at IWU. This method does not produce any metal chips so, effectively, no material is lost.
The flexibility of the axle module is not limited to manufacturing batch sizes; it also extends to geometry. Because the module is scalable, it can be used in everything from small vans and municipal vehicles to buses and trucks. With a wheel hub motor, that would not be possible.
While wheel hub motors have definite advantages – such as a wider steering angle and greater responsiveness – they are not suitable for commercial vehicles, as they scarcely deliver more than 2,000 rpm. Since each wheel also requires its own power electronics, the costs are higher. “Both developed versions have their own clear raison d’être and should be chosen specifically for a planned vehicle type,” Dr Bräunlich adds.
A number of modules have been completed and are ready for testing, as are the new manufacturing techniques. For the next stage, the consortium is planning a demonstrator, and beyond that they hope to fit the axle module into a real vehicle for testing by the end of 2015.