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Possible Mazda RX-9 Patent Reveals Wild Rotary Hybrid With Capacitor Boost

Design incorporates in-wheel motors, a capacitor, and Mazda’s favorite spinning Dorito.

As recently reported, Mazda has a mid-size rear-wheel-drive car platform and Skyactiv-X inline-six in the works for the next Mazda 6 sedan. To amortize development costs, that new platform will have to underpin other vehicles, too, including the next CX-9 SUV. But the platform isn't Mazda's only trick up its sleeve. We know 48-volt mild hybrids and plug-ins are in the cards, and this latest patent filing shows Mazda is considering revolutionary in-wheel electric motors powered by a capacitor—and the high-tech hybrid setup could be destined for a future Mazda RX-9 coupe.

The patent, uncovered by Japanese Mazda blog T's Media, appears to be an update on an earlier patent filed last year. Both filings detail a front-engine, rear-drive car equipped with two in-wheel electric hub motors for the front axle, a third motor located at the back of the engine, a capacitor, and small lithium-ion battery. The first filing shows an inline four-cylinder engine, but things get much more interesting in the latest one published on April 9. In that one, a "V-type" engine of unknown cylinder count and a Wankel rotary engine are depicted.

The problem with in-wheel electric motors historically has been the extra unsprung weight they add, which can negatively affect ride and handling. Mazda's solution is a relatively small motor located at the front wheel hubs and driven by a double-layer capacitor in the engine bay (on top of the engine in the case of the rotary or nestled in between the cylinder banks for the V engine). A capacitor can rapidly discharge its energy and also recover energy from braking more efficiently than a battery. A speed sensor dictates when the front wheel motors can be used.

But how does Mazda plan to drive the wheels with such tiny motors? The in-wheel motors and capacitor operate at a higher voltage than the rest of the car (120 volts). Higher voltage means a motor should be able to make the same power as a lower-voltage motor at lower current, and lower current means wiring and windings can be smaller. According to the patent filing, this all contributes to a compact, lightweight e-AWD solution. But as our technical editor points out, an in-wheel motor won't have the same torque advantage as one located at the end of an axle—because leverage.

To accommodate the different system voltages, three inverters are used, including one attached to the main drive motor between the engine (which has no flywheel) and the driveshaft. That 25-kilowatt permanent magnet synchronous unit is powered by a 48-volt 3.5-kilowatt-hour lithium-ion battery pack (oddly placed in the driveshaft tunnel, running down the center of the vehicle) and can drive the rear wheels alongside or in place of the internal combustion engine. Crucially, this design takes up the space where a typical transmission might go—on the back of the engine—so the Mazda design calls for a rear transaxle.

One benefit of the rear transaxle touted in the patent filing is the front interior space gained by not having a transmission protruding into the floor (never mind that battery . . . ). That allows for a more central and symmetrical seating position, something Mazda is all about lately. Rear transaxles are also generally good for weight distribution in a front-engine car, which is why you typically see them in performance cars such as the Corvette.

So what we have here is a vastly complicated design for a front-engine, all-wheel-drive hybrid that could use a rotary engine. Though it's possible the system could be adapted for the upcoming RWD Mazda 6 and its inline-six engine, another patent filing suggests that that powertrain will have a traditional eight-speed transmission hanging off the back. The rear transaxle and practically everything else in this patent simply don't exist in Mazda's current arsenal, and we'd be surprised if the automaker employed anything like this in its mass-market cars anytime soon. But for a halo car like the on-again, off-again Mazda RX-9, developing a three-motor e-AWD rotary hybrid system seems less far-fetched. Besides, it'd be a shame to let a gorgeous design like the RX-Vision concept go to waste.

UPDATE: A Mazda spokesperson declined to comment on this story due to company policy regarding future product and media speculation.

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