Brewing Hybrid Battle: Is One Electric Motor Better Than Two?

By John Voelcker

3,868 views Aug 24, 2011

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GM electric motor production

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The word "hybrid" means many things.

It can be interpreted as "high gas mileage," or "annoying leftie smugmobile," or--by engineers--an automotive powertrain that uses two different sources to generate the torque that moves the car.

But among "full hybrids"--those powertrains that can move the car under solely electric power under limited circumstances--there's an emerging divide between two fundamental ways of organizing the components.

On one side are the makers--led by hybrid pioneer Toyota and its Prius--that use a pair of electric motor-generators. This setup is more complex and potentially more costly, but gives the hybrid system more flexibility.

On the other is a growing group of manufacturers who are newer to hybrid design, all of which have chosen to use only a single electric motor.

Each side will argue for the advantages of its own design, and each setup has advantages and disadvantages.

From talking to many auto engineers over the years, here's our roundup of each design, and its pros and cons.

2010 Toyota Prius transaxle, at right, with larger, heavier transaxle from 2009 Prius at left

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TWIN-MOTOR HYBRID SYSTEM

How it works: Inside a transaxle, one or more planetary gear sets redistribute input torque, from the engine, a pair of motor-generators, and the road wheels, allocating it among the wheels and a motor acting as a generator that recharges the battery pack.

Components: Two electric motor-generators; up to two clutches; up to three planetary gear sets.

Pros:

• Infinitely variable ratios allow engine to be powered up or down to operate at maximal efficiency without affecting road speed
• Operation has no "shifting gears" feel to driver
• One motor can power the car while the other recharges the battery

Cons:

• More complexity and lack of standard transmission means more expense
• Control software is probably more complex
• Driving experience may seem foreign if engine noise isn't well concealed

Who uses it: Toyota (Hybrid Synergy Drive), Ford, General Motors (Two-Mode Hybrid), and soon Honda

2010 mercedes benz s400 hybrid motorauthority 001

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SINGLE-MOTOR HYBRID SYSTEM

How it works: An electric motor is sandwiched between an engine and a modified conventional automatic transmission. Clutches on either side let the motor propel the car (decoupled from the engine), add torque to the engine (both clutches engaged), or recharge the battery (decoupled from the transmission).

Components: Electric motor-generator, two clutches.

Pros:

• Fewer components and adapted conventional transmission make it cheaper to build
• Single electric motor is easier to package with the space of a standard engine and transmission, making this system easier to adapt to gasoline models
• Shifting patterns of automatic transmission feel "normal" to new hybrid drivers

Cons:

• System may be slightly less fuel efficient than twin-motor system
• Electric output power is limited by maximum motor diameter imposed by bellhousing size
• Lack of continuously variable ratios means braking can feel non-linear as transmission downshifts under regeneration

Who uses it: VW Group (Volkswagen, Audi, Porsche), Hyundai (and Kia), Nissan (and Infiniti), and a joint venture between BMW and Mercedes-Benz

Both types of systems will be used in new hybrid vehicles to be launched this decade, and it's highly unclear whether one will emerge victorious or both will coexist.

But with the exception of Honda, we're not aware of any makers now planning to launch new two-motor systems.

And already BMW and Mercedes-Benz, having experimented with and then walked away from the (highly complex, very expensive) GM Two-Mode Hybrid system, are working hard to evolve their joint single-motor system for a variety of future vehicles.

So if we had to lay odds, we'd probably bet on more makers opting for single-motor systems--which may be "good enough" to get the job done at a lower cost than the complex two-motor alternatives.

But we wouldn't put a whole lot of money on that position. It's too early to tell how, or whether, the field will shake out.

Meanwhile, the proliferation of hybrid-electric vehicles is saving us gas, improving fuel efficiency, and driving down costs. Which we think is all to the good.

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