Rotary engine: Wankel’s impossibly logical design

Every few weeks I receive word at the office that some tinkering genius has revolutionized the internal-combustion engine. These notices arrive on pages still smelling of Damascus lightning, and promise not merely improved efficiency or reduced emissions but a kind of salvation -- an end to pollution, an end to foreign oil, an end to the petty tyranny of thermodynamics.

In Greek drama, such a device was called a deus ex machina.

The latest and strangest came a few weeks ago from inventor Ronald R. Meritt, who was tendering his International Pollution-Free Alternative Fuel Motor Conversion Kit on EBay. Meritt claims the technology will allow any internal-combustion engine to run on any gas or liquid, even water. Which prompts the question: What about Kool-Aid?

Undeniably, the four-stroke, reciprocating-piston gasoline engine is inefficient: About 65% of the energy in gasoline is lost as heat in a typical car engine. The design is also inelegant: 13% of fuel energy is consumed by internal friction and pumping losses due to the furious, almost comic flailing about of the internal parts.

No wonder so many starry-eyed grease monkeys have thought there must be a better way.

Felix Wankel was one of these. Wankel invented the rotary engine that has become Mazda’s signature technology. The new RX-8 sport coupe is the only car on the market to have this power plant (Mazda’s other products have piston engines).

But the rotary had to come a long way to be here. Its presence under the hood is a testament to Mazda’s strange faith in what was always a reluctant piece of engineering.

In the summer of 1919, in Lahr, in southwestern Germany, Wankel, then 17, had a dream about a car with a new type of engine, “half-turbine, half-reciprocating.” His dream engine would eliminate James Watt’s rod-and-crank connection between piston and wheel; it would eliminate the energy-sapping to-and-fro of the reciprocating pistons. Wankel’s engine would whirl like Ptolemy’s universe, each part harmonious with the next.

The design the young inventor eventually settled on featured a triangular rotor spinning inside a chamber shaped like a peanut (an “epitrochoid”). As the rotor turned like a spirograph stencil, the spaces inside the chamber would expand and compress, creating the pumping action needed for internal combustion. The fuel-air mixture would be sucked into the chamber by the spinning rotor; once inside, the fuel-air charge would be squeezed between the rotor and the chamber wall, where it would be ignited by spark plugs; the force of combustion would drive the rotor clockwise and the spent gases would be chased out through an exhaust port.

Such an engine would be lighter and more compact. It would be simpler, without the kinetic daisy chain of cams, pushrods, rocker arms, springs and valves; smoother and quieter, with neither the thudding pistons nor clattering valve train to create a stir under the hood; and more durable.

In 1933, when Wankel applied for his first patent, his design must have seemed every bit as weird and Salvationist as Meritt’s horsepower by hydrotherapy. But four decades after his midsummer dream, in 1959, Wankel -- with his industrial partner, the German motorcycle company NSU -- announced the commercialized version of the Wankel rotary engine. Wankel went on to become very wealthy. The self-taught engineer was given an honorary doctorate by Munich’s Technical University in 1969. Professor Wankel never held a driver’s license.

It took the Mazda Motor Corp. another eight years of frantic development before it began selling cars with its twin-rotor 10A engine. When the R100 came to America in 1970, the motor produced a stunning 110 hp out of a little less than 1,000 cc’s of displacement; only a few of the world’s best piston engines hit the 100 hp-per-liter mark today.

But then there was a problem: emissions. The conventional Wankel engine, like a two-stroke piston motor, is a pretty smoky device; in both engines the combustion charge -- the puff of gas-air fire -- is chased out before fuel has a chance to burn completely. To pass the emissions standards of the Muskie Act of 1970, Mazda retrofitted a “thermal reactor” to its R100 cars. This was rather like a glow plug in the exhaust manifold that helped ignite fugitive hydrocarbons.

The fact is the Wankel engine had any number of liabilities that might have proved fatal but for the warrior determination of Mazda’s engineering department. In the next decade, the company struggled to raise fuel economy and lower emissions. For a while the entire company hung on the outcome. Another problem was the engine’s relative lack of torque -- a function of displacement, regardless of engine design. The company’s experiment in rotary-powered pickups proved short-lived (1973-1977) and ill-advised.

In 1985, Mazda introduced the 1.3-liter 13B rotary engine, which used a passive supercharging system: the staccato pressure waves from the exhaust were diverted to help force-feed the engine intake. This was the motor in the beloved first-generation RX-7 (1978-1985), the car that put Mazda on the sports car map in the United States. Turbochargers were added to the next two generations of RX-7s. Yet, even boosted to the gills, the last generation of the RX-7 only put out 255 horsepower and 217 pound-feet of torque.

The positive attributes of these little motors -- low mass, high specific power (horsepower vs. size) -- aren’t necessarily a perfect fit for a road car. In a race car, however, rotary power is a beautiful thing. In its time, the RX-7 dominated the GTU class in International Motor Sports Assn. road racing. The 1980s-vintage RX-7s still win in Sports Car Club of America Improved Touring class, where the car’s light weight and freaky durability have made it a perennial favorite. Aging 13B motors, virtually maintenance-free, still power Formula Mazda open-wheel cars.

In 1991, after years of trial and disappointment, Mazda won the 24 Hours of Le Mans. In the midship of the 787B racer was a four-rotor, twin-turbocharged rotary engine that wouldn’t be denied.

The previous generation RX-7 (1998-2002) was one of my favorite sports cars, an asphalt jet ski, as glossy and sleek as Murano glass. Though it didn’t have the knockout torque of a Chevy Corvette or Toyota Supra Turbo, once the 2,800-pound coupe was on the balls of its feet, it danced like Ali. Few road cars ever had its effortless athleticism and grace.

But no amount of massaging would enable its twin-turbo 13B motor to pass rising emissions standards in California and Europe. The biggest problem dated back to Wankel’s time: The intake and exhaust ports were located along the perimeter of the housing, causing “overlap”; that is, the intake port was open before the exhaust port was fully closed. Some of the fuel-air mixture escaped, unburned, out the exhaust port.

The new Renesis engine relocates the ports in the sidewall of the chamber. These ports are enlarged for better airflow. Much as high-tech piston engines modulate engine breathing with variable-valve timing and lift, the Renesis employs a curious array of three-stage fuel injectors, four-stage sequential induction and other measures to improve pumping efficiency over a range of engine speeds.

Meanwhile, the rotary engine’s strained and blatty exhaust note -- which always sounded to me like a conch shell trumpet -- is transformed into a husky growl, thanks to various resonators and silencers.

The Renesis now meets California’s LEV II and Euro 4 standards. In the new RX-8, the high-power Renesis produces 237 hp at 8,500 rpm -- ultrasonic toothbrush range -- and 159 pound-feet of torque.

But these are not impressive numbers, and are easily superceded by piston engines only marginally larger and heavier. The Renesis output is actually lower than the engine it replaces. And although the project costs are impossible to know, the Renesis engine must have been horribly expensive to develop to its current state of tune.

Reasonable people may ask, why bother? Why would Mazda continue to exert its intellect on a troublesome engine that has to be dragged, kicking and screaming, into the 21st century?

It goes beyond torque curves and emissions. Engine designs have a gravity all their own for car companies, quite apart from the machinery. Chrysler has its Hemi, Chevy has its small-block V8, Porsche its flat-6. In an overcrowded, look-alike market, the rotary engine is where the soul of Mazda resides.

But I think it goes even beyond brand equity. For any engineer who has mused on the odd, duck-walking kinematics of a piston engine, the rotary must seem such an elegant solution, so narcotic in its simplicity, spiraling seductively like a hypnotist’s wheel.

Wankel was only the first to fall under its spell.