Rotary Redux
HIROSHIMA, Japan When Mazda Motor Corp. launches the RX-8 sports car in spring 2003, consumers can thank a cross-functional team of powertrain and production engineers for making the car and its hallmark rotary engine an affordable reality. Mazda's previous-generation rotary powered RX-7 was last sold in the U.S. in 1996. It was an acclaimed enthusiasts' sports car, but its price ballooned such that
March 1, 2002
HIROSHIMA, Japan — When Mazda Motor Corp. launches the RX-8 sports car in spring 2003, consumers can thank a cross-functional team of powertrain and production engineers for making the car — and its hallmark rotary engine — an affordable reality.
Mazda's previous-generation rotary powered RX-7 was last sold in the U.S. in 1996. It was an acclaimed enthusiasts' sports car, but its price ballooned such that it became the poster child for Japanese auto makers' misguided market ambitions for once-elemental coupes like the RX-7 and the Nissan 300 ZX.
After more than a half-decade's absence, Mazda's rotary engine — and its equally famous sports car that surrounds it — is staged for yet another comeback (see sidebar, next page). Only this time, Mazda expects to produce the car's Renesis (name derived from “rotary” and “genesis”) rotary engine for around $2,000, a key to avoiding the escalating-cost mistakes that led to Mazda's withdrawal of the rotary from the U.S. market.
Predicated on monthly output of 5,000 units, the company says, amazingly, that Renesis' cost is slightly less than it would pay for a standard V-6 engine. By implication, Mazda plans to produce 60,000 RX-8s annually, though management declines to disclose sales targets.
Keys to holding down engine costs:
Engineers used the existing RX-7 rotary, the 13B, as the base engine for the Renesis. As a result, they expect to hold facility investment to around ¥2 billion ($15 million) by utilizing older tooling and equipment in most areas of production. Like the turbocharged 13B, the normally aspirated Renesis, first displayed at the 1995 Tokyo Motor Show (code name: MSP-RE) for the RX-01 concept car, will be produced at Mazda's Ujina engine plant here.
Design innovations made it possible to eliminate the twin turbochargers employed by the current 13B rotary engine. By moving the exhaust ports from the trochoidal housing to the side housing of the rotor chamber, the overlap between the exhaust and intake port openings was eliminated — a crucial emissions and power-generating factor. Meanwhile, reducing rotor weight contributes to higher output of about 250 hp and torque of 159 lb.-ft. (216 Nm) — figures that are roughly equivalent to the 280 hp and 152 lb.-ft. (206 Nm) of the twin-turbo 13B in the existing RX-7 still sold in Japan. Rotor weight was trimmed by 14%.
At a technical briefing last autumn, research officials explained that the Renesis was designed with two peripheral exhaust ports for each rotor (the 13B has one), doubling the exhaust port area. The results are improved exhaust flow, increased thermal efficiency and better fuel economy.
They added that Renesis also traps unburned hydrocarbons in the exhaust chamber and retains them for combustion in the next cycle, a process that emulates a piston engine's exhaust-gas recirculation (EGR) and which greatly reduces emissions. As a result, the new engine meets Europe's Stage IV emission standards, while achieving an estimated 23.5 mpg (10L/100 km) in city driving, up from RX-7 levels of just more than 18 mpg (13L/100 km). Mazda expects to raise fuel economy another 30%, to 30 mpg (7.8L/100 km), in 2005. And by expanding the intake port area, the engine runs smoothly at 9,000 rpm — already a rotary hallmark.
Other advanced features include a 3-stage induction system, twin electronically controlled throttles, a plastic intake manifold, “ultra-fine” fuel injectors, a “double-skin” exhaust manifold, a low-profile, wet-sump lubrication system and a hermetically improved rotor apex seal.
Mazda engineers note that the engine's variable induction system features three intake ports per rotor, with direct-current motors to “open and close intake port shutter valves to utilize the incoming air's dynamic charge effect and improve filling efficiency.”
Meanwhile, the oil pan for the Renesis' new wet-sump lubrication system is only about 1.5 ins. (4 cm) deep, half that of the 13B. This, they explain, contributes to a 3% weight reduction compared to the dry-sump system employed by the first generation of the Renesis displayed in 1995.
Coinciding with last year's Tokyo Motor Show unveiling of the final engineering version of the RX-8, Mazda unveiled a new high-pressure, 3-way catalyst combined with a 3-way hydrocarbon trap catalyst for the car. Mazda says the unit reduces hydrocarbon emissions by 50% during the first 20 seconds after engine startup.
The auto maker also claims that the new catalyst employs a new oxidizing agent that maintains efficiency even at high temperatures.
Water-cooled, the twin-rotor Renesis, like its predecessor, the 13B, has a displacement of 1,308 cc. Bore (chamber width) and stroke are 87.5 mm and 94.0 mm.
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