Magnetic Continuously Variable Transmission

At the heart of many hybrid vehicle systems, such as the Synergy Drive found on the Toyota Prius, is an electronic continuously variable transmission (eCVT). An eCVT is a combination of a planetary gear and a motor/generator and is also found on other Toyota hybrids, including the Camry and Highlander and the luxury Lexus brand. A similar configuration is used by Ford in their Escape and C-Max Fusion Hybrid vehicles.

Magnomatics’ Magsplit, is an integrated magnetic equivalent of a planetary gear and a motor/generator, and just like the mechanical system, Magsplit’s gear ratio can be controlled over a broad range and is capable of fully disconnecting the output drive and of driving in reverse. However, it offers a 3 to 5 percent fuel economy increase in a more compact device than the mechanical system. Its inherent compliance, or elasticity, also enables the removal or a significant reduction in complexity, of both flywheel and torsional damper components.

Magsplit is currently being considered by a number of global car, bus and truck manufacturers. Ever more efficient hybrid powertrain configurations are essential if manufacturers are to meet regulatory targets and fulfill increasing customer demands for low carbon vehicles. But how does this magnetic continuously variable transmission work?

Magnetic Gear Operation – Magnomatics’ magnetic gear consists of two rings of permanent magnets with a ring of steel pole pieces in between. These steel pole pieces act as flux paths between each of the rings of magnets. This has the effect of creating harmonics in the fields produced by each ring of magnets. By careful selection of pole numbers one can couple to the harmonic field and this creates a gear ratio. For example, if there are 23 pole pairs of magnets on the outer array and 27 steel pole pieces then this will produce a dominant four pole field at the inner, high speed rotor. The gear ratio between the inner and outer magnet arrays will be 23/4 or 5.75:1.

The behavior of the magnetic gear is exactly the same as a mechanical epicyclical gear. One element of the gear is usually held static while the other two rotate with a fixed gear ratio. In many applications the outer ring of magnets is the static ring. A comparison between the magnetic and mechanical devices is shown in Figure 1. Magnomatics’ magnetic gears are currently in production for downhole pumping applications in the oil and gas sector.

Magsplit Operation – In the Magsplit device shown in Figure 2, a wound stator is introduced and the outer ring of magnets is free to rotate. Uniquely, this outer rotor now becomes the high speed rotor of the gear system (equivalent to the sun gear). This would be impossible with a mechanical gear. By controlling the rotational speed of this outer ring of magnets one can change the gear ratio between the inner magnetic rotor and the steel pole pieces. The ratio can be continuously adjusted and even set such that the output shaft is stationary or reverses, ideal for a hybrid vehicle. This is not a passive device since power flow is always required through the stator, either as a source or sink. Typically, the maximum power required in the control stator is 25 percent of the power being transfer through the powertrain.

Magsplit would typically be located between the engine output and final drive, as shown in Figure 3, and provides a first stage of gearing. The steel pole pieces and inner magnetic rotor are the primary means of transmitting torque into the differential and then the wheels. The gear ratio, and hence mechanical power delivered by the engine to the wheels through the mechanical path, is controlled by varying the electrical power flow through the control rotor. This control rotor can operate as both a motor, using power and as a generator, extracting power. This power flow into and out of the Magsplit, means that a second motor/generator must exist in the drive train to ensure that the power is circulated. Typically an energy storage system such as a battery is also used and this enables a pure electric mode.

The magnetic gear provides a wide range of gear ratios allowing the engine to operate over a narrow speed band equating to its peak efficiency while delivering a variable speed drive to the wheels. It can fully disconnect the output drive (de-clutching operation) and provide sufficient torque to crank the engine, when switching from all electric to hybrid modes for example. The inherent compliance of the magnetic gear can reduce engine torque pulsations down the drive line eliminating the torsional damper arrangement and contributing to reduced system cost.

Packaging benefits accrue because with a magnetic gear one can make the outer rotor the high speed shaft. This is impossible with a mechanical gear and as a result mechanical based systems need to control the speed of the inner rotor or sun gear to change gear ratio. Typically, this makes the eCVT quite a long device, as shown in Figure 4, with a hollow shaft permitting access to the inner sun gear to vary the gear ratio.

Magsplit Testing – Prototype Magsplits have been designed and fabricated by Magnomatics (Figure 5) with a rated through-power of 20 kW and torque of 100 Nm, aimed at small city car-type applications. The performance has been validated on a bespoke transmission dynamometer system at Magnomatics (Figure 6).

A larger model rated at 200 Nm is currently being prototyped and will be tested later this year. This unit is design to fit into a typical C Class passenger car. It is under 345 mm in diameter and less than 120 mm deep. It will be mounted directly onto the crankcase of the car’s engine. Following on from that, the Company intends to build a larger unit (~800 Nm) suitable for an urban delivery truck and city bus. This should be available by the middle of next year. Magnomatics has also been looking into the benefits Magsplit might bring to much larger vehicles, including HGVs and large off-highway vehicles, such as tractors, mining trucks and quarry vehicles. Initial simulations suggest that Magsplit could reduce fuel consumption in such vehicles by between 10 and 15 percent.

Future Developments – Magnomatics is now developing a second generation of its Magsplit product. Although technical details on the next generation are not being made available publically, this product will have fewer component parts and use substantially less magnet material to achieve the equivalent performance.

Magnomatics Limited is a high-technology spin-out company formed in 2006 from the internationally renowned research group in Electrical Machines and Drives at the University of Sheffield in the UK. The Company develops and delivers novel proprietary magnetic transmissions and ultra-compact and efficient motors and generators for some of the world’s largest multinationals and is active in a range of industries, including renewable energy, automotive, aerospace, marine, defence, oil and gas.

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