Why two-motor hybrid systems are better than those with just one

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2017 Toyota Prius Prime Premium

2017 Toyota Prius Prime Premium

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Six years ago, we wrote that a number of single-motor hybrid systems were about to enter the market.

They represented a new approach to a technology pioneered in 1997, when the first-generation Toyota Prius was launched in Japan.

Through 2010, all the full-hybrid vehicles introduced by Ford and General Motors used systems designed around a pair of motor-generators.

DON'T MISS: Brewing Hybrid Battle: Is One Electric Motor Better Than Two? (Aug 2011)

Since then, numerous hybrid vehicles using single-motor systems have come onto the market from BMW, Hyundai-Kia, Mercedes, Nissan, and VW Group.

They started as conventional hybrids, but with larger batteries and stronger single motors, several of them have morphed into plug-in hybrids that can travel 10 to 30 miles on battery power alone.

Ford, GM, and Toyota have also added plug-in hybrids based on two-motor systems as well, of course: two generations of Chevy Volt, Ford's pair of Energi models, and the latest Toyota Prius Prime that followed the first Prius Plug-In Hybrid.

2017 Kia Optima Plug-In Hybrid

2017 Kia Optima Plug-In Hybrid

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We've driven almost all of those cars, and spending five days and 330 miles with a Kia Optima Plug-In Hybrid a couple of weeks ago got us thinking about the nature of modern hybrids.

As we wrote last month, modern hybrids are vastly better vehicles than earlier generations were.

They drive better, they're far quieter inside, and many of them no longer have the disconnected "motorboating" effect that runs engine speed up to the max while the car slowly gathers speed.

READ THIS: Six things to know about new hybrid cars (if you haven't paid much attention lately)

We liked the plug-in Optima, and found it a good value among plug-in hybrid mid-size sedans.

That said, we've concluded that two-motor systems deliver a more pleasant experience to drivers and occupants than single-motor hybrids.

The challenge with a single-motor system is that one motor-generator can either provide torque to the drivetrain or act as a generator to recapture energy on engine overrun or regenerative braking. But it can't do both at the same time, while a two-motor system like Toyota's or Ford's can.

Ford Fusion Energi charging.

Ford Fusion Energi charging.

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That means the single-motor systems in Hyundai-Kia and various German systems are constantly switching their motors back and forth depending on speed, acceleration, and grade.

Added to that is the fact that a two-motor system provides widely varying output ratios, while the single-motor systems are paired to a conventional transmission, whether it's an automated direct-shift gearbox or a regular automatic transmission.

This means that even in all-electric mode, the one-motor systems upshift and downshift like a regular car.

CHECK OUT: Plug-in hybrids to catch on more quickly than electrics: Toyota

That's disconcerting to drivers with experience in electric cars or earlier hybrids, but that's not the problem. The challenge is that it adds an additional element to manage among all the components that must be coordinated by the system's control logic to provide a smooth driving experience.

We've concluded that single-motor systems simply get caught out more by sudden on-off acceleration: we haven't driven one that wasn't at least slightly lumpy or jerky at some point.

To be fair, those times were rare, and only occurred in specific sets of conditions. But they are nonetheless real.

2018 Chevrolet Volt

2018 Chevrolet Volt

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Take, for example, the circumstance in which a single-motor hybrid car is slowing down under light braking, just at the point that regenerative braking ebbs at lower speed and the friction brakes must be blended in.

If the driver accelerates suddenly, the system has to switch the electric motor from regenerative braking to delivering torque while the brakes are released and the transmission downshifts and the engine switches on.

That's a lot to coordinate, and no single-motor hybrid system we've yet driven pulls it off imperceptibly.

The message is not that these systems are bad, but that buyers should be aware of which kind of hybrid they're buying.

Not all hybrid systems are the same, and their driving behavior and power delivery reflect that.

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