MotiveMagazine - Motice Tech: The Rotary Engine Explained

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Chances are that if your knowledge of rotary engines is limited to what someone's told you, you've heard one of the following statements:

"Oh man, they're so smooth, so quick to rev, and so lightweight. Sure, they're not impressive off the line, but they're great on the highway."

Or,

"Dude, they're garbage. No torque, crap mileage, and they burn oil. Garbage, I'm telling you."

Nothing in between. There's no middle ground with these odd little engines, no, "Yeah, rotaries are all right." People either love 'em or hate 'em. Maybe that's why 51 years after the Wankel (as the rotary is also known) engine had its first bench test — a timeline that would see the engine both sink an entire brand and become the heart of a legend — it still lives on the fringe. Only Mazda's low-volume RX-8 carries the torch. But let's back up a few thoughts — where did "Wankel" come from, anyway?



"Felix Wankel, a 70yo German inventor, fantasized of driving a car of his own creation, one that he told friends was "half - turbine, half - reciprocating".

Felix Wankel was a German scientist who, as a tale tells, had a dream in 1919. The seventeen-year-old inventor fantasized of driving a car of his own creation, one that he told friends was "half-turbine, half-reciprocating." Interrupted by his work for the German war effort, it wasn't until 1954 that Wankel first tested his motor. Nine years later, German manufacturer NSU revealed the Wankel Spider with a production version of the all-new engine, a 50-hp rotary capable of pushing the car to a top speed of 93 mph.

NSU's next move thrust the Wankel out into the mainstream, which in turn pushed NSU toward obsolescence. The Ro80 was a sleek sedan powered by a 115-hp twin-rotor engine that made its debut in 1967 and quickly won an International "Car of the Year" award. That was before the public started driving them, which is where the Ro80's problems started. Many early models suffered from excessive wear and some engines needed replacing within the first 30,000 miles. NSU didn't outlive the poor reputation that resulted from the problems, and Audi absorbed the failed automaker in the 1970s.



One of NSU's biggest problems became known internally at Mazda as the "nail marks of the Devil." These "nail marks" were actually chatter scratches on the rotor housing caused by vibration in the rotor's apex seals. That problem, along with excessive amounts of oil leaking into the combustion chamber, were overcome by Mazda through the development of better seals, and Mazda launched the Cosmo Sport in 1967, the same year that NSU's Ro80 came out. The Mazdas, however, didn't break — and 41 years later the company is still producing Wankel's childhood dream.

The rotary might seem complicated and alien to those who've worked with pistons their whole lives, but Wankel's design is actually much simpler than even a low-tech inline-four. There are no pistons, no connecting rods, no camshafts, no valves, no lifters, and no timing belts to break. That translates to an engine that's both smaller and lighter than a piston engine with a comparable output. Mazda's RX-8, for example, makes 232 hp and 159 lb-ft of torque from a dual-rotor engine displacing just 1.3 liters.

Crack that block open and you'll see an oval-shaped housing that holds each rotor, with the longer sides of the oval smashed in so that they're flat, creating a shape known as an epitrochoid. (An investigation at the U.S. Patent Office website reveals that Wankel tried various geometries for the engine, including an odd three-lobed housing with a square-shaped rotor.) On one of those flat sides live the intake and exhaust ports. The triangle-shaped rotor spins, beginning its four-stroke combustion cycle by drawing air and fuel in through the intake port.

From there, the rotor continues to spin until the side of the triangle is smashed against the opposite long wall of the housing. At this point, the air and fuel mixture is fully compressed and two sparkplugs in the wall ignite, spinning the rotor through the power stroke and into the exhaust cycle. Ignited fuel moves back out of the housing through an exhaust port in the same wall as the intake port.

All the while, the spinning rotor orbits a center shaft, which in exchange sends movement through the transmission and driveshaft to the wheels. Visualize the process and you'll realize that more is happening than first thought. The rotor's three-sided shape means that for each full rotation of one rotor tip, three rotations of the inner shaft have taken place.



What all that geeky talk means to you the driver can be broken down into two main advantages: The first is that the rotary engine does more with less. Because the outside of the rotor spins once for every three times the output shaft spins, the engine provides a higher wheel-speed-to-engine-speed ratio, which also helps rotaries last longer. The second is smoothness. Unlike the stop-and-go, up-and-down movement of a piston engine, a rotary only spins in one direction. As a result, engineers don't have to fight against a chassis that wants to shake back and forth at idle. Nor do they have to fight against a droning, coarse engine at a constant high speed, as rotaries are glass-smooth in highway driving. Without all the additional hardware of a reciprocating engine, rotaries can also spin faster — a manual-transmission-equipped Mazda RX-8 redlines at an impressive 9000 rpm.

But then there are the drawbacks. Back in the piston world, a 1.3-liter engine would almost certainly equate to Prius-challenging fuel economy in a lightweight sports car like the RX-8. But the rotary's combustion cycle is intrinsically less efficient than a piston engine's, and compression ratios are lower (the RX-8 runs at 10.0:1), meaning that fuel economy and emissions output both suffer. The RX-8 achieves an EPA rating of just 16/22 mpg.

If you're wondering why these engines have found more success in sports cars than SUVs, it's because rotaries lack low-end torque. So while they're a hoot on a racetrack, a Wankel doesn't do so well with heavy passenger loads or while proclaiming alpha-male status at stoplights. In addition, rotor apex seals are still prone to leaking more oil than traditional piston rings, and many owners complain about starting problems in below-freezing temperatures.

Still, Mazda hasn't given up on Wankel's idea and continues looking toward the future of the rotary in a world growing more concerned about efficiency and emissions. The simplest way to solve those two issues is to do away with gasoline altogether, and that's what Mazda has done. In Japan, Mazda has leased a small test fleet of hydrogen RX-8s to customers for purposes of real-world evaluation. The current model produces only 107 hp and has a range of just 60 miles on a tank of hydrogen, but it can be converted back to gasoline by a simple flip of a switch. Only time will tell whether the Wankel — or hydrogen for that matter — has a place in the automotive future, but it's been one long, crazy trip so far. We don't see the rotary disappearing without a fight.


Source: MotiveMagazine.com - Motive Tech: The Rotary Engine Explained