Why aren’t AWD cars half electric?

by philg

We’re still shopping for a new car to supplement the (awesome) 2018 Honda Odyssey. As we mull the options (waiting to test-drive the Mazda CX-30, for example), one thing that jumps out at me is that AWD (four-wheel drive) continues to be implemented via mechanical driveshafts, differentials, gears, etc. We’re about 12 years into the modern electric car era (Tesla Roadster was launched in 2008). Given the volumes of all-electric cars currently being produced by multiple manufacturers, shouldn’t it be cheaper to implement AWD via low-horsepower electric motors at the back wheels to supplement a conventional FWD system?

A car at highway speeds uses only about 20 hp (source). Thus, even for providing a certain amount of improved handling in slippery conditions, wouldn’t a 10 hp motor on each rear wheel be sufficient for escaping an icy driveway and touching up handling dynamics on the road? Acura announced this for high-performance cars in 2011 with 27 hp per rear wheel (Car and Driver), but the typical consumer just wants to overcome fear of getting stuck (FOGS). It seems that Acura is currently shipping this system in their MDX vehicle, using 36 hp per rear wheel (source). The MDX weighs over 4,000 lbs. For a lighter vehicle with fewer aspirations to greatness, shouldn’t 10 hp per rear wheel be enough? Why isn’t that cheaper than all of the mechanical parts to transmit power from the engine to the back of the car and then distribute between the wheels? A bit of searching on Alibaba shows that a 10 hp electric motor costs about $150 in small quantities. Presumably a car manufacturer would pay much less.

Plainly it actually is cheaper to do it the 1970s mechanical way. But why?

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17 thoughts on “Why aren’t AWD cars half electric?”

  2. Yeah I was about to tell you to look at the rav4 hybrid. Supposedly Toyota is about to come out with a plug-in version which would be eligible for Tesla-style subsidies.

    • Dave: It is a beautiful system in which low-income people are taxed to subsidize wealthy buyers of new Teslas (and then, via their ongoing gas tax payments (funding road maintenance) from which Tesla owners are exempt, to subsidize the continued operation of said Teslas). When will our government realize that low-income people also need to pay taxes to subsidize the luxury jewelry purchases of the rich?

    • Phil:
      It’s even more beautiful if you live in far left states like Colorado (it’s not purple anymore). There are very generous state tax credits on top of federal credits.

    • Dave: The rooftops of the $1-2 million houses in our town are all sprouting solar panels, funded by transfers from the middle-income taxpayers of Massachusetts who struggle to pay rent.

    • > and then, via their ongoing gas tax payments (funding road maintenance) from which Tesla owners are exempt, to subsidize the continued operation of said Teslas

      As owner of two Teslas, I would love this, but in WA there is now a separate electric vehicle tax payable at tabs renewal time in lieu of gas tax :(, what’s worse, it’s not related to the miles driven. Apparently I’m missing on wealth transfer by not living in such progressive state as MA.

      Anyway, get Tesla Model 3 long range with Autopilot. In Boston traffic this automation will make driving many times easier. It works great in traffic jam or on highway, which is the most driving nowadays. I know it’s hard to believe, but it’s big quality of life difference.

    • I thought that was the purpose of govt was to tax the poor to give to the rich… trickle up economics

  4. Keep in mind the cost is not just the motors but the necessary batteries and the large generator used to keep the batteries charged and all of the electronics needed to coordinate the system.

    Drive shafts must be pretty cheap by comparison but nevertheless your approach is starting to take off.

    • ^ This.

      The Subaru “Symmetric AWD” concept based on their boxer engine dates from 1972, although it didn’t spread to the entire model line until 1996. The Audi Quattro system began in their Group B Rally cars in the early 1980s (which were so fearsome and powerful they were really dangerous, not just to the competition) so there’s a lot of inertia associated with the old gears, clutches, differentials and driveshafts. The early passenger car versions of the Quattro system all had mechanically lockable center and front differentials, by means of a vaccum plunger on the center console of the car beneath the radio. The later cars were considerably more advanced with all sorts of electronic oversight, but still basically mechanical.

      But Philip’s point is really what is driving the transformation: how many horsepower do you really need for extra stability, traction assist, and so forth in the average family sedan? Now that the electronics are capable of monitoring the car’s dynamics wheel-by-wheel, system-by-system millions of times a second, the all-electronic AWD systems make a lot more sense. If they can keep the reliability high and the unsprung weight low, that’s where it’s going.

      Still, the Audi Quattro Sport S1s were awesome cars that almost single-handedly legitimized AWD as a concept for passenger cars in the 1980s. It’s hard to believe anyone could drive anything this crazy, but they did:

  6. Why not? Auto mfg. R&D hasn’t been actually looking to innovate… rather, only been focused on integration, a/v systems, etc., while using component firms like Bosch, all adding their own margin into the cost.

  7. I think synchronizing the driving wheels is non trivial with independent motors. I think Toyota’s hybrids drive the same wheels because both gas & electric go through the same drive coupling.

  8. On the subsidy issue (above), a friend who lives in a 12,000 square-foot house just got some tax handouts extracted from middle income taxpayers to fund his new Tesla 3 Performance (around $60,000; delivered a few hours prior to 2020). This will supplement the handouts he gets every day for his solar panel array.

  9. I would expect the electric motor, alternator, regulators etc to cost more than the driveshaft and rear differential, if nothing else because of the volume produced. The real advantage would be in avoiding the need to free space for all those components, but that’s true only for totally new designs, not as a modification (unless you find something useful to do with the space, e.g. add batteries for a hybrid version).

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