Advances in bearing technology will help to deliver the electric drivetrain of the future, says Anthony Simonin, EV & HEV Competence Centre Manager at SKF
Gothenburg, Sweden, 6 April 2017: Electric vehicles (EVs) and hybrid cars are moving into the fast lane. While ‘conventional’ cars will continue to dominate the market, the need to reduce carbon dioxide and particle emissions will fuel the acceleration of these ‘alternative’ vehicles.
Where once they were little more than a sideshow, they are now undergoing a boom: industry analyst Frost & Sullivan recently estimated that EVs and hybrids could account for 10% of global car sales within five years. This means that further electrification of the drivetrain is inevitable. However, achieving this is no simple task. The drivetrain of a hybrid car, for instance, is far more complex than that of a conventional car – because of the need to switch between electric and conventional engines, or to use both at once. There are many ways to achieve this, and they are increasing all the time. The simplest is to integrate the electric motor into existing configurations, but in the next five or 10 years we could see more than 30 different designs of EV or hybrid drivetrain hit the market.
Designers of all these variants will be striving to make them as compact as possible, while seeking robustness, ease of assembly and – most importantly – greater energy efficiency. Their designs will be critical – as will the components that they choose to specify. Bearings are often a hidden component, but using them selectively will help to solve some of the problems affecting e-drivetrain design.
Energy recovery
A critical component in the powertrain of a hybrid car is the belt driven starter generator (BSG), which recovers braking energy and feeds it back into the engine to boost torque and engine performance.
SKF recently helped a Tier One supplier to improve the design of its BSG system, which formed part of a 48V powertrain system. For the latest version of the BSG, SKF developed a new design of rotor positioning bearing – which can be used either with synchronous or induction traction motors, used in electric and hybrid vehicles. Compact and light enough for use in this type of drivetrain, the rotor positioning bearing has been designed to have enhanced resistance to extreme conditions. It can withstand continuous temperatures up to 150°C, is unaffected by severe magnetic field disturbances or high levels of vibration, and can reduce torque ripple and electric noise.
Overall, the higher accuracy delivered by the device allows the electric motor to run more quietly, which delivers a smoother ride and a higher efficiency. As an added benefit, it is easy to incorporate into the BSG assembly.
Cutting friction
Despite the huge variations in design architectures, the challenges that underlie them remain the same. One of the most important is the automotive industry’s oldest enemy – friction.
The fight against friction takes place in just about every part of the car – from tyres to pistons. However, specific elements of electric and hybrid vehicles, most notably the higher power density and higher speed, accentuate the effects of friction. A way around this is to design-in components that can reduce friction. In a recent customer collaboration, SKF helped a Tier One supplier maintain the compactness of a 48V belt alternator starter (BAS) – at increased speed, load and temperature. This was achieved by fitting its eDrive ball bearings – in this case, deep groove ball bearings – which exhibit very low friction and are specifically designed for use in electric and hybrid drivetrains. They use a patented polymer bearing cage – as well as an optimised bearing raceway geometry and specially formulated grease -- to cut friction by up to 30%.
However, the process actually started earlier when SKF modelled the design of the part – using its simulation software – and tested it at a component level. Modelling components is usually the key starting point for getting them specified. In this example, the customer had originally been using standard bearings – but they were not up to the job.
Testing is also critical. In fact, SKF is already building a series of high speed test rigs that go beyond the current needs of the market – as it is likely that those needs will change in future.
Solving new problems
New technology can often be both a blessing and a curse. For every advantage it brings, there can also be a downside. A good example is electric inverters. For all their benefits – such as increased energy efficiency – their high-speed switching increases the signal frequency, causing them to leak current. This can harm components such as bearings, and reduce their working life.
A way round this is to use alternative parts such as hybrid bearings. These combine a stainless steel ring with a ceramic rolling element, which helps to isolate current leakage and protect components from stray current. The technology can cope with the high speed of electric drivetrains, as well as instances of poor lubrication. Electric and hybrid cars are also far quieter than most conventional cars. However, a slightly annoying consequence of this is that you can now hear all the squeaks and rattles that were once masked by engine noise.
Electric and hybrid cars are undoubtedly the way ahead for the automotive industry, with every major manufacturer working on its own range of vehicles. Design optimisation will be critical, and it seems that the humble bearing could – in many cases – have a massive effect on performance.