Rotary car revival: Mazda's RX-8 four-door sports car packs innovation, refinement and engineering daring into a very convincing package

Automotive Industries,  Feb, 2003  by Don Sherman

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Technology marches at such a forced pace that engines seldom get a second chance to prove their mettle. When Studebaker ceased flat-head engine production in 1960, they were gone for good. Straight eights and inline threes also went the way of the buggy whip. So the second coming of the rotary engine under the hood of Mazda's new RX-8 is truly remarkable. The combination of a unique engine with an unprecedented body style represents more creative thinking than the auto industry has seen since the Tucker Torpedo.

The rotary bounced back because a hard core of Mazda enthusiasts was convinced its merits deserve one more play on the automotive stage. To its everlasting credit, Felix Wankel's creation is fundamentally simpler, smoother and more compact than 4- or 6-cylinder engines with comparable power. But, as is the case with all mechanical devices, there are negatives. Past rotaries demonstrated questionable reliability, high fuel consumption and difficulty meeting emissions standards.

For the Mazda RX-8 to be successful, the rotary had to clean up its act AND compete favorably against some of the best piston powerhouses on the market Eight years ago, just as the third generation RX-7 began migrating into the history book, Mazda engineers presented a thoroughly reconfigured engine at the Tokyo Motor Show. Their major breakthrough was relocating the exhaust ports from a peripheral to a lateral position.

Forty years ago, when Mazda began developing the rotary under NSU license rights, exhaust outlets were oriented redially to avoid coking--burned-oil deposits in rotor sealing grooves. At the start of Mazda's rotary revival program, engineers solved that concern with improved seals and drastic reductions in the amount of oil injected to lubricate the seals. This paved the way to side exhaust ports, precisely the breakthrough the rotary needed to achieve major gains in power, fuel efficiency and emissions.

While piston engine designers struggle to pack in more valves and larger fluid passages, their rotary counterparts enjoy ample freedom determining the size and position of their induction and exhaust ports. To create a normally aspirated new-generation rotary that beat the outgoing turbo version on all counts, Mazda engineers doubled the size of the exhaust ports and increased intake area by nearly 30 percent Positioning the port array (two or three intakes and two exhausts per rotor) to eliminate overlap improved idle stability, dramatically reducing both emissions and fuel consumption. (As in a piston engine, overlap is the short circuit by which spent gasses invade the fuel-air charge.) To take maximum advantage of the new-found volumetric efficiency, the mass of the rotors was trimmed by 11 percent and the flywheel was lightened by 20 percent Elaborate resonance-tuned intake passages with computer-controlled shut-off valves were devised to force feed the rotating combustion chambers throughout their expan ded rpm range. On the exhaust side, piping was made as large and as straight as possible to minimize flow restriction. Secondary air delivered by an electric pump after a cold start was added to speed warm-up of the RX-8's catalytic converter.

While they were at it, Mazda engineers gave their RENESIS (the rotary engine's genesis) powerplant new fuel injectors providing better atomization, longer-lasting iridium-tipped spark plugs, a drive-by-wire throttle, a mass-air-flow sensor (replacing the previous speed-density means of fuel-air-ratio control), double-loop exhaust-oxygen sensing and a much smarter 32-bit power-train control computer.. The payout is a power peak raised by 2,000 rpm (to 250 hp at 8,500 rpm), a 90 percent reduction in exhaust emissions and EPA combined fuel economy figures over 20 mpg. Two engine configurations are offered. The "standard" edition, with two intake ports per rotor and a 7,500 rpm redline for use with a four-speed automatic, delivers 210 hp and 164 lbs-ft of torque at 5,000 rpm. The "highpower" version inhales through three intake ports per rotor, winds to 9,000 rpm through a 6-speed manual transmission and produces 250 hp at 8,500 rpm and 159 lbs-ft of torque at 5,500 rpm.

After such brilliant work by the engine department, RX-8 program manager Noboru Katabuchi was duly inspired to create an equally remarkable car to carry the rebom rotary engine. Recalling the origin of this new species, the 30-year Mazda veteran reports, "What we needed to succeed the RX-7 was one car suitable for three diverse markets--Japan, Europe, and the U.S. In 1998, we began considering various possibilities with ideas solicited from our advanced-engineering branches in those three locations. Our goal was a four-seat RX-7 with all the driving pleasure that concept implies. Within a year, our idea how to achieve this was accepted and the program was approved for production."

Asked how this radically different vehicle fits into the grand Ford Motor Co. scheme of things, Katabuchi-san is quick to point out, 'While Mazda's front-drive products are integrated with Ford's model plans, the new RX-S platform is exclusive to Mazda. We intend to use this foundation for other sporty cars in the future. The platform was engineered with broad capabilities to yield an RX-7 derivative, a convertible bodystyle, and, if necessary, cars powered by conventional [non-rotary] engines."