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

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GM electric motor production

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

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

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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.


  • 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


  • 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

2010 mercedes benz s400 hybrid motorauthority 001

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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.


  • 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


  • 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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Comments (16)
  1. I'm leary of this article. I know there are no clutches in the current Prius Power-Split module (i.e. "transmission"), nor any belts like the photo shows. The Power Split device is MUCH simpler than a standard transmission. I think there are only 40 parts.

  2. @Charles: I had to make the two-motor description generic enough to include the GM Two-Mode design (2 motor-generators, two fixed gears, two clutches, three planetary gear sets).

  3. Thanks for the clarification. I didn't think the Toyota HSD had clutches.

  4. The Priuses dont use clutches, but the previous generation did use a flexible metallic belt/chain to drive the wheels. Incidently, the current generation of the Prius does not use any belts at all .. the water pump is driven by an electric motor and so is the air conditioner compressor.

  5. Some of the other hybrids made by Toyota do use clutches, Lexus uses (or used) a transmission that is similar to the GM 2 mode RWD unit. This is done to allow more flexibility.. more torque at higher or lower speeds etc. The system used in the Prius is more suited to a family car application, not a sports car.

  6. Thanks for the meaty article. I have always thought that it is strange that the Prius has two motor-generator sets. This article clears up why.
    The added simplicity of the single motor is attractive, but I put my money on Toyota's proven technology.

  7. Why have you totally forgotten about the SIMPLEST Hybrid available. A SERIAL Hybrid. In a serial hybrid, the electric motor powers the wheels and can act as a generator (for regenerative braking), while some other device (could be an ICE, fuel cell, whatever) generates the electricity used to power the electric motor. In a serial hybrid, there may or may not be a battery involved. Typically there is, otherwise regen braking would not work without some type of energy storage device - perhaps a (super)capacitor. But, no clutch necessary for this drive train. It is SIMPLE and direct. Other variations on this put electric motors at each of the drive wheels-i.e. independent motors, coupled by the electronic control system. 750 char limit

  8. But, thinking about this a bit more logically... This whole "hybrid" concept should eventually go the way of the Dodo. Once battery technology can hold a charge good enough for somewhere between 500-1000 miles, hybrids will be a thing of the past. Why? Because just look at all the maintenance and complexity you'll be doing away with. The Tesla Model S will have a 300 miles battery and it's due out next year. It is too expensive for ordinary people, but their base model will have a 160 mile battery. I am hoping to be an early adopter of one of these for several reasons. a) cost of gas isn't going down b) reduced maintenance (many fewer moving parts) and c) pure electric IS the "greenest" way to go.

  9. I agree with you, Bob, and we are talking about hybrids because General Motors wants to keep America on domestic and foreign oil. And GM said that an electric car with an electric motor and with its own battery charger connected to its axle would never work and we have had that set up for over 100 years and it has worked fine.

    In less than five years we will have a battery that will give a range that will put ICE to shame and the hybrid will go the way of the dodo and GM (because of their overproduced hybrid) will go bankrupt again.

  10. serial hybrids have been used since the 20's in locomotives.. today they drive the Chevy Volt, countless ships, submarines and the largest trucks used in mining. The crawlers used by NASA to transport rockets to the pad (in use since the 60s) are also serial hybrids.

    A new application will be the Fisker Karma, its a serial hybrid with a battery and a GM 2L Ecotec engine.. the ICE only drives a generator.

  11. Actually, Herm (and Bob), what you're referring to is called a SERIES hybrid (not "serial")

  12. Actually, Bob, what you're referring to is called a SERIES hybrid (not "serial")

  13. GM uses the BAS + system, stands for belt-alternator-starter and now called eAssist. This is a liquid cooled motor/alternator/starter combo that drives the engine using a belt.. its low cost, probably around $1500 to GM. Standard in the Buick LeSabre 4 cylinder.

    Toyota's (and Ford's) 2 motor system is very mechanically simple and rugged.. probably the most rugged transmission in the market now, as seen in the durability of many taxis that use it.. thru several million produced Toyota has driven the cost very low, probably around $1500 or so.. one major advantage over the single motor system is that it REPLACES the transmission (and alternator and starter etc).. modern ATs have gotten very delicate/expensive lately.. and no clutches.

  14. Perhaps I should have noted that the article above applies only to FULL hybrids, i.e. those that can move the vehicle solely on electricity at least for short distances.

    The GM Belt-Alternator-Starter (BAS) system you describe (known as eAssist in its second generation on Buicks) is a mild hybrid system, not a full hybrid: It cannot move the car solely on electric power at all.

    I'm curious about your comment that modern automatic transmissions have "gotten very delicate" lately. Data, please?

  15. Hyundai hybrid does have a second motor (Integrated Starter Generator).

  16. It is important to note that Single-Motor Hybrid System requires a separate gearbox for transmission.

    Twin-Motor Hybrid System like HSD does not have a separate transmission hardware. Torque multiplication is done with electricity (using a generator and a motor) rather than using mechanical gear ratios.

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