Mazda Design Breakthrough May Give Rotary New Life

HIROSHIMA, Japan – The rotary is dead. Long live the rotary.

Despite plans to stop production of the RX-8 sports coupe in June and no commitment from management to replace it with another Wankel-powered car, Mazda may be on the verge of a major rotary breakthrough.

Mitsuo Hitomi, general manager-powertrain development, says the auto maker soon will complete development of an all-new rotary engine that meets future fuel-economy and emissions standards.

While Hitomi won’t confirm a completion date, his disclosure is evidence the rotary is alive and well at Mazda and that a new car powered by the engine could be on the way.

“We think we’ve found a way to improve the rotary’s fuel economy to be truly equal to that of conventional piston engines and, if so, we believe we can reintroduce the rotary to the market,” he tells WardsAuto in an interview.

Hitomi declines to pinpoint when that might happen, but suggests the timeframe roughly would be equivalent to any other engine plugged into one of Mazda’s vehicle programs. Best guess is two years, once the application has been determined.

Fuel economy and emissions were improved in the developmental engine “first, by changing the shape of the troichoid housing,” the Mazda executive says.

“The rotary has many seals, and ‘sealability,’ particularly at the apex, or tip, of the rotor has been a longstanding problem” dating back to the mid-1960s and the Cosmo Sport, Mazda’s first rotary car.

“Even with our current 1.3L Renesisrotary, gaps can develop between the apex seal and troichoid housing in light-load operation when imbalances in centrifugal force and gas pressure occur,” he says.

Specifically, centrifugal force pushes the seal onto the housing surface. In low engine-rpm ranges, or under low-load conditions, gas pressure in the combustion chamber can cause the seal to lift off the surface, resulting in combustion gas leaking into the next chamber.

By changing the shape of the troichoid housing, the seals remain flush to the housing, Hitomi says. “In addition to reducing emissions, better sealing improves fuel economy and overall performance.”

A second engineering enhancement focuses on ignition.

“I can’t specify how we plan to address this problem,” the Mazda engineer says, “but the rotary’s spark plug is in a recessed position (below the housing surface), compared to that of a piston engine.

“This causes ignitability problems and increases fuel consumption. We’ve found a way to make dramatic improvements,” he says.

In June, AutoCar reported Mazda would introduce a laser-injection system to replace conventional sparkplugs.

Hitomi counters popular perception that a rotary engine’s fuel economy is inherently poor.

“In the past, Mazda engineers made a conscious decision to focus on the rotary’s weaker points – low torque, for instance,” he says. “This led to adopting a complicated and large variable-intake system, as an example.

“Moving forward, we plan to put greater emphasis on the rotary’s strong points – low vibration, compact size and high power output. And in this way, we can improve fuel economy.”

One result of that: Mazda will simplify the intake system, which in turn will reduce engine size.

Hitomi believes the new engine eventually will be adopted for extended-range electric vehicles like the Chevrolet Volt. He doesn’t say so, but that could mean a future RX-8, RX-9 or even an RX-7 EREV.

Mazda isn’t alone in looking at EREV applications for rotary engines, which are drawing interest from others, including engineering experts AVL, for their compact size and light weight.

The new Mazda engine will not necessarily be the 16-X prototype displayed at the 2007 Tokyo Motor Show, which reportedly was targeted at 1.6L in displacement and would generate 350 hp.

Mazda’s Skyactiv fuel-efficiency technology could be applied to the new rotary, as well.

“I think that whatever engine we introduce – conventional gasoline and diesel or rotary – the technology is applicable,” Hitomi says.

At a press briefing in December, Hirotaka Kanazawa, senior managing executive officer in charge of Mazda’s technical operations, openly discussed three Skyactiv engine series: the gasoline Skyactiv-G now on the market, diesel Skyactiv-D and rotary Skyactiv-R.

Since 1967, when Mazda introduced Cosmo Sport, the auto maker has sold nearly 2 million rotary vehicles. In the engine’s 1970s heyday, four out of five Mazdas sold in the U.S. had rotaries.