Mazda Rotary Turbos: Were All Those RX Motors Just Upside Down?

The engine that goes hmmm has always been a favorite of hardcore enthusiasts given to wringing engines out to redline before every shift. Its incredible power density, low mass, and ability to mount lower and farther aft in a car than piston engines of comparable output did much to improve the basic handling of every vehicle it was installed in. But the design has always suffered from inherent shortcomings.

Oil consumption and apex-seal wear plagued the earliest examples. New seal materials and revisions to the exhaust porting helped solve these problems, but then as emissions regulations phased in, the little hummer required elaborate thermal reactors and the like to achieve compliance, but Mazda engineers prevailed. Fuel economy has never been a terribly strong suit, which partly explains why the company eventually resorted to the boing-boing piston engines to power its mainstream vehicles, leaving the rotary to do sports car duty, where buyers are less concerned about economy. The most recent version, the 13B-MSP Renesis, produced from 2003 to 2011, featured side porting for both intake and exhaust to boost fuel economy and to clear the U.S. emissions hurdles, plus twin sequential turbocharging. But even this engine suffered reliability woes in its early years (cold-temperature wear due to lubrication troubles, flooding when cold, etc., which prompted a warranty extension) and the engine was withdrawn from Europe in 2010 for lack of emissions compliance.

Mazda’s RX-Vision concept unveiled last year in Tokyo was said to have been designed around a new Skyactiv-R rotary engine, and a Mazda patent application recently released gives us some clues as to how that engine might improve on the state of Mazda’s little hummer. The biggest change described in the patent’s text is that it has been turned upside down. Now the side exhaust ports exit upward, and the side intake ports are located near the bottom of the engine. How might this cure the Wankel engine’s persistent emissions and fuel economy bugaboos?

Well, packaging turbochargers and catalytic converters as close as possible to the point where the hot gasses exit the combustion chamber always improves an engine’s emissions output. When the engine is mounted as low as desired for crankshaft-to-driveshaft orientation and low center of gravity, there’s not much room for all that gear down low, so moving it up to the top of the engine should allow much closer coupling of everything, and all the various “hot-V” engines in production today have proven that hood paintjobs can be sufficiently insulated from the intense heat produced by a high-mounted turbocharger. It’s also hard to envision anything about this new design that would add substantial cost to the previous intake-above/exhaust-below design, but then neither does anything appear to be reducing costs relative to similar conventional engines.

Moving the intake down low pays another dividend mentioned in the patent, which is that the longer intake runner can provide a resonance supercharging effect at lower, more normal engine speeds. (This is the effect where a sonic pressure wave generated when an intake port opens to the runner travels back up the intake runner, gets reflected by the open volume of the intake manifold, and rockets back down toward the intake port, helping cram a little more air in when the port happens to be open at the right moment.) If that sweet-spot speed happened to line up with a typical highway cruising speed, it might improve fuel economy at that speed, but only very slightly, so don’t expect to find the Skyactiv-R powering Mazda sedans and hatches anytime soon.

I see nothing in this patent application to suggest any improvements in reliability, but another five years of progress in materials science and metallurgy—not to mention Wankel know-how on the part of the design’s champions at Mazda R & D—give ample reason for optimism that a new generation of drivers may soon enjoy the unique aural and visceral thrill that only a Wankel turbo can provide.

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