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  A Novel Magnetic-geared Electronic-continuously Variable Transmission
Propulsion System for Hybrid Electric Vehicles

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With ever increasing concern on energy efficiency and environmental protection, there is fast-growing interest in hybrid electric vehicles (HEVs). Differing from battery powered electric vehicles, HEVs involve two energy sources, namely the gasoline and the battery, as well as two propulsion devices, namely the engine and the motor. Hence, they take the definite advantages that they can offer a much longer driving range and be easily refueled at existing gas stations. Their key challenge is how to effectively combine the engine driving force and the motor driving force in such a way that the engine can always operate at its optimal efficiency and produce the minimum tailpipe exhaust. The existing HEVs essentially adopt the same technology, termed the planetary-geared electronic-continuously variable transmission (E-CVT) propulsion system, to combine the engine driving force and the motor driving force. However, this system suffers from the drawbacks of low power density, high transmission loss, wear-and-tear problem and annoying audible noise.

Rather than using physical contacts to perform force transmission as adopted by mechanical gears, magnetic gears can transmit the force using contactless magnetic attraction. The use of coaxial arrangement can further enable magnetic gears to simultaneously utilize all magnets for force transmission. By purposely designing the modulating ring of the magnetic gear to be rotatable, it can work as the planetary gear for power splitting. Namely, the engine power flow can be split into two paths – one path is via the outer rotor of the magnetic gear (shared with the rotor of the motor) while another path is via the inner rotor of the magnetic gear (shared with the rotor of the generator). The corresponding power flow is controlled by using two back
-to-back converters separately coupled to the stators of the motor and the generator. Increasingly, both the motor and the generator can be integrated into the magnetic gear to form a single machine unit. By artfully controlling the converters to perform power splitting, effective E-CVT propulsion can be achieved.


Principle of magnetic-geared electronic-continuously variable transmission propulsion system for hybrid electric vehicles




Research group

This magnetic-geared E-CVT propulsion system can fundamentally solve all key drawbacks of the existing E-CVT systems for HEVs, thus offering high power density, high transmission efficiency and quiet operation. By electronically controlling this system, the engine can always operate along its optimal operating line at which its fuel consumption is minimized throughout the whole operating range. Hence, the overall energy efficiency of HEVs can be improved while their tailpipe exhaust can be further reduced. Additionally, this system enables the use of one-side-in and one-side-out structure which can eliminate the bulky and lossy silent chain employed in the existing E-CVT systems.

Limited by the unsatisfactory performance of rechargeable batteries and the high cost of fuel cells, HEVs have been identified as the best choice to bridge the current situation and the demand for clean and energy efficient road transportation. The proposed E-CVT system possesses the definite merit of higher energy efficiency than the existing ones, which can further reduce the oil consumption and the exhaust emission due to road transportation. Moreover, its another definite merit of higher power density can facilitate the design of HEVs with smaller size and lighter weight, which are vitally important for extending their full-electric zero-emission driving range.

Prof K.T. Chau
Department of Electrical and Electronic
The University of Hong Kong