Magnet gear trains drive efficiency in hybrid vehicles

Hybridisation is being given increasing priority by the automotive industry as it strives to reduce emissions and ramp up the efficiency of vehicles.

However, this has put the many challenges of effective hybridisation firmly in the hands of engineers. Many concepts for hybrid drive transmissions have been proposed and while many seem elegant on paper, the devil is in the detail and practical implementation is rarely straightforward. The advantages and disadvantages of hybrid cars are by no means as simple as some would have you believe. While tailpipe emissions might be better, increased electrification of the overall powertrain relies on largely lithium batteries that are not the easiest components to dispose off or recycle at the end of life. In addition, while automotive engineers strive to lightweight, hybridisation adds weight as it essentially requires two different powertrains. The integration of an internal combustion engine with electric motor/generators also throws up some interesting mechanical issues in terms of gearing and power transmission. The ability to switch and split power between the two easily and smoothly, without too much energy loss and weight penalty is no small feat. Probably the best known hybrid car, the Toyota Prius, overcame this issue by developing its Hybrid Synergy Drive. At its heart, this is essentially a single planetary differential gear set that removes the need for a more traditional stepped gearbox and transmission components. Aptly named, Toyota's Power Split Device (PSD) is a continuously variable transmission (CVT) but with a fixed gear ratio between the internal combustion engine (ICE) and the two electric motor/generators. It is generally referred to as an eCVT due to its application in hybrid vehicles. As with all hybrids, however, there are drawbacks. First, it is complex. The number of moving parts in the design is far from ideal, with each requiring a bearing. This results in some associated loss of energy as well as a need for lubrication and maintenance. Also the eCVT is expensive, adding cost to the vehicle. And finally, due to the nature of its application, it is heavy, adding weight, which is a key area of reduction for today's automotive design engineers. The frustration of taking significant steps to reduce weight, only to be counteracted with the increasing introduction of hybrid technology is, however, being addressed, in part, by one Sheffield-based spin-out technology company. Magnomatics was established in 2006 to commercialise magnetic transmission systems. To date, it has been able to successfully demonstrate and prove the technology for a number of applications and has recently been invited to join The Proving Factory, a collaborative project set up by the automotive industry to industrialise key innovative technologies. Key to this development is the Magsplit, designed to improve hybrid powertrains in performance, weight and cost. The Magsplit essentially fulfils the same functions as the eCVT of the Pruis but has been shown to improve hybrid systems' fuel efficiency by 3% to 5%, as well as reducing complexity and weight. "The principle of a magnetic gear is to use an array of magnets to create a flux field at the centre of a rotating cylinder," says David Latimer, business development manager from Magnomatics, speaking at this year's Advanced Engineering Show. "We then insert steel pole pieces in to the flux field to give a preferred flux path leaving airgaps between them to act as a flux inhibitor. This modulates the flux field, creating a harmonic that rotates faster and in the opposite direction to the rotating cylinder. We then place another magnet array within the first cylinder to lock on to the harmonic and give a high speed output." The principle of the gearbox is analogous to an epicyclical gearbox with the inner array of magnets acting as the sun gear, the steel pole pieces as the planet carrier, and the outer array of magnets behaving like the ring gear. "But there are differences," says Latimer. "One of the interesting things with a planetary gear is you always have to have a high speed shaft on the inside as the sun gear is the smallest gear, and therefore has to be the fastest. As we are dealing with magnetic poles, we can actually move that high speed shaft to the outside." This allows Magnomatics to do a number of things. By using three rotors that are all free to rotate, it can use the inner array of magnets and the steel pole pieces as the primary input and output shafts, with the outside acting as the control mechanism. Then, by moving the control rotor in the same direction it can add RPM to both shafts, change the gear ratio or rotate the control rotor backward fast enough to actually stop the output rotor, equivalent to declutching or being in neutral. Magnomatics already has its first gear in operation driving a pump within an oil well. However the company is firmly setting its sights on the volume markets of mid-range automotive producers. Hybrid vehicles are likely to increase significantly over the next five years and its Magsplit could improve the existing eCVT systems currently being used on cars like the Prius and Ford Fusion. "Typically Magsplit can replace three items compared to an eCVT; the motor/generator, the planetary gearbox or power split device, and also they can remove the flywheel and even the torsional damper in some cases," says Latimer. "Also there is no contact, so no wear. And that means it is very efficient. We have measured efficiencies more than 99.5%." Further development of the technology has lead to the development of its latest iteration, the Magsplit 2. While this has all the same functionality as Magsplit 1, it uses fewer magnets and fewer parts. The design of the Magsplit 2 allows the transmission to be overloaded much more frequently, making it ideal for drive cycles that experience frequent peaks. "In Magsplit 1 the losses are mainly speed dependent, whereas in Magsplit 2 they are mainly torque dependent and the choice of which to use really depends on the drive cycle," says Latimer. "So, for a car where you need a lot of peak torques and powers Magsplit 2 is best. For a truck based power train Magsplit 1 is the optimum." Magnomatics claim that the Magsplit transmission results in better fuel economy compared to hybrid cars based on an eCVT power split device. The Magsplits are also lubricant-free, offer a shorter package length and can replace several components with a single one. The Magsplit transmissions are scalable, with studies, and interest, from motorbike to very large truck manufacturers. Studies have even shown applicability to quarry trucks. In addition both Ford and Volvo have expressed interest in developing the transmission for future hybrid vehicles, with trials and evaluation currently taking place.