Bridging the gap

A new continuously variable transmission system that allows an engine to operate at its most efficient speed is the first to bridge the gap between hybrid and nonhybrid vehicles, its UK developer claims.

Created by London engineering start-up Nexxtdrive, the DualDrive system takes power from a vehicle’s engine and splits it between a mechanical drive path through an epicyclic gear train, and an electrical path through two generator/motors. The gear train is then used to combine the motion from the engine and power from electric motors.

While the majority of power is transferred through the gearbox, the electrical power path is used to precisely control the speed and torque of the transmission’s output shaft. This according, to the systems’ developer, makes it extremely efficient.

Movable arm Epicyclic, or planetary, gear trains typically consist of a number of outer ‘planet’ gears that rotate about a central ‘sun gear’. The gears are mounted on a movable arm, which can also rotate relative to the sun gear. This means that epicyclic gear trains are very useful for combining power from different sources and splitting power from different outputs.

Nexxtdrive’s chief engineer and inventor of the system, Frank Moeller, explained that when applied to a conventional drive train, one of the two electrical devices will act as a generator and the other as a motor. But the system also lends itself to use in a hybrid drivetrain, in which a battery can be added to the system and both devices can run as motors.

Chairman Rod Keech said that this makes it particularly attractive to manufacturers, as they would be able to use the same transmission for hybrid and non-hybrid vehicles.

‘The transmission will fit into the existing design envelope of an automatic transmission. It would not require the redesign of a whole vehicle as was the case with the Toyota Prius,’ claimed Keech.

While there have been a number of attempts to commercialise new transmissions in recent years — perhaps most notably the systems developed by Torotrak and Antonov — Keech claimed that DualDrive is totally different.

‘Ours is a power-combining system using electrical and mechanical transmission that lends itself to use in a hybrid drive system. We don’t see ourselves in the same space as them,’ he claimed.

Moeller added that the system has much more in common with the Powersplit device used on the Prius — with perhaps one crucial difference. He explained that while Toyota’s system uses an epicyclic gear train to combine the outputs of an engine and two motor/generators, the main electric motor is coupled directly to the vehicle’s wheels. This means that the generator rotates in synchrony with the wheels and is not free to choose its own speed.

With the DualDrive transmission, neither of the motor/generators are coupled directly to the input or output. Moeller said for this reason the torque requirements on the electric machines are lower than on the Prius. So, while in the Prius around 25 per cent of the power is transferred electrically, in the DualDrive systems it is only 10 per cent. This, claimed Moeller, makes the DualDrive Transmission more efficient than that used on the Prius. 

 With no physical prototypes built or tested, the technology is still very much at the design stage. But Keech is hopeful that ongoing discussions with OEMs will see the construction of a prototype system some time this year.

Meanwhile, the principles of the technology are already present in another of the company’s products — a supercharger known as Super- Gen2. This is a compressor that will be used to boost engine performance by increasing atmospheric pressure in the engine. It takes electrical energy from the vehicle generator to power an advanced gearing system that can accurately vary the speed of the turbo compressor from zero up to 100 times crank speed.

Keech said that he has recently signed an option agreement with with a major global tier one supplier.