Sedan Ramblings - Reviews, comparisons and all things sedans
Willy Wonka and the Rotary
Engine
The story of the rotary, its inventor and the sedans that use it
Felix Wankel received his first patent for a rotary piston engine in 1929, five years after he first hatched the idea and 27 years after his birth in Lahr, Germany. Over the next decade and a half and during WW2, Wankel was involved in the development of a rotary disc type valve for use in aircraft and torpedo engines, thereby shelving the rotary engine project, and he held jobs at BMW, DVL, Daimler-Benz, Lilienthal and Junker Aircraft. Because of his military involvement, following the invasion of Germany in 1945 he was imprisoned until 1946, and his workshops and research were destroyed by French troops, further delaying his rotary research until 1951, when the German company NSU became interested in the project.
With the help of Walter Froede, head of NSU's motorcycle racing program (at the time, NSU was the largest builder of motorcycles in the world), in 1957 Wankel developed the first rotary engine that actually rotated, and developed 15hp at 9000rpm. A little fiddling by Froede improved the design and created the KKM Wankel Rotary Combustion engine, the grand daddy of all rotary engines.
Um...ok, so how's it all work? Well, see here, it's like this dude: The heart of a rotary engine is the rotor, which is a trochoid (roughly triangular) shaped component whose convex faces each act as pistons. There are usually a pair of them per engine, 180 degrees out of phase and slightly offset from the centre of rotation. Each rotor is housed in a rotor housing (!), which are epitrochoid (roughly oval) shaped and the two rotor housings, one for each rotor, are joined at the hip and capped on the ends by the side housings, thereby creating nice, cosy homes for the rotors. Running straight through the middle of this mess is the eccentric (or output) shaft, which has two smooth round lobes called journals mounted eccentrically, which mate with the bearings of their respective rotors. These lobes act somewhat like crankshafts in a piston engine, since any force that the rotor applies to the lobes creates torque in the shaft, causing it to spin. Attached to each end side housing is a stationary gear that is...em, stationary. Their outward pointing teeth mesh with inward pointing ones of the rotor gear, for the rotor contains both a smooth bearing and an internal gear, and keep everything in sync.
The rotor moves inside its housing in a really weird kaleidoscope kind of way. Its motion is the combination of the rotation about the shaft journal and the orbit of the eccentrically situated journal. These motions, synchronised, create the desired planetary motion, which also happens to look very pretty. In this way, the rotor's vertices always maintain contact and a seal with the rotor housing, thereby creating three gas chambers inside the housing, one for each rotor face. Here are some wonderful animations from rotaryengineillustrated.com to clarify things: the first (399 kb) shows the mesh between the stationary and rotor gear and the second (397 kb) shows the rotor's path. Note the eccentric shaft in blue/white and the journal in light grey.
In its rotation, a chamber starts out really small and expands, as it passes over the intake port and takes in a big gulp of the air/fuel mixture. As the chamber subsequently gets cut down to size, the mixture is compressed and ignited by the spark plugs near top dead centre, where the chamber has reached its smallest capacity. The combustion pushes the rotor, and therefore the journal and eccentric shaft through the power stroke, before the gasses are released as the chamber passes over the exhaust port, before restarting its cycle.
Well, that's all nice and good, and if you understood all that mumbo jumbo you're probably ahead of the game already, but what's the point? Well, see here yo, it's like this: First of all, a rotary engine is about 3 times lighter and more compact than a reciprocating engine (that's the kind you find in your Explorer, which goes up and down not round and round), utilising about 48% fewer parts and containing all of 3 moving components (compare that to 40 in a conventional engine)! By doing away with the reciprocating motion of a reciprocating engine, a rotary unit is also much smoother and can spin much faster, however the lack of torque at low speeds partially offsets this advantage by leading to greater fuel consumption. Wankel's long and narrow combustion chamber has a high surface to volume ratio, which makes it less thermodynamically efficient, but the cooler combustion leads to fewer nitrogen oxides emissions.
Ok, so now that you've absorbed enough for your mechanical engineering degree, it's nice to now that the rotary engine has actually had practical uses in 4-door sedans. NSU were the first to put their mouth where their money was, or, um something, and in 1967 the rotary-engined Ro80 hit the showrooms, in all its non-conforming glory. At the time NSU sold more cars than BMW, but they had stayed on the small car bandwagon too long, and with Germany finally experiencing an economic boom, sales were starting to slide and a fat cat executive cruiser was needed. Not only did the Ro80 receive flattering reviews and the Car of the Year award, but other manufacturers jumped into the fire as well: following a handsome payoff for Wankel and NSU, soon Citroen, Mercedes and Mazda became the proud parents of their own rotary-engined stepchild.
To provide the perfect backdrop for the 995cc twin-rotor engine, NSU also rigged the Ro80 with such niceties as front-wheel drive, independent suspension, all-wheel disc brakes, ABS, power steering and a 3-speed semi-automatic transmission. It also came with a host of safety features and amenities, plus the build quality matched anything the rest of Germany had to offer. Not only that, but it was looker. The Ro80's simple, elegant lines would have looked modern in the 1980s, and how much more so in 1967, while the resulting 0.355 coefficient of drag was like icing on the pie and eating it too. Despite all this, by the end of the Ro80's production run in 1977, some 37,398 cars later, NSU would no longer exist.
What the devil happened? you may courteously enquire. The rotary engine, the technological masterpiece and sensational highlight of the car, turned and stabbed the Ro80 in the back, that's what. An insatiable appetite for spark plugs and an unquenchable thirst for oil and gas (about 15L/100km or 16mpg on average) didn't help matters; that the engine expired after 60-80,000 km nailed the Ro80's coffin. And although its impressive 115 horsepower at 5500 rpm had only 2667lb of car to haul around, the 3-speed granny tranny limited the Ro's performance potential: 0-62 mph came and went in 12.8 seconds with a 112 mph top end. Felix Wankel would later say that the gearbox was the starting point of critical drivetrain problems. Others would say that the road to hell is paved with good intentions.
But that car's like way slow, bro'! you say. I thought these were supposed to be the world's fastest sedans. Well, I think it's hard not to be an admirer of the Ro80, but I see your point, hence the Mazda RX-8. A little creative interpretation of the RX-8's rear suicide doors is required to qualify it as a sedan, but there's no ambiguity about its speed. This car is fast! Not in-yo'-mamma's-face fast, but fast enough.
The RX-8's twin-rotor 1308cc Renesis engine is about as powerful as the RX-7's previous generation turbo motor. The secret lies in the relocation of the exhaust ports from their traditional location in the rotor housing to the side housing, where they can keep their intake cousins company, thereby eliminating overlap and providing lower fuel consumption and cleaner emissions. Larger ports, a fancy intake system and an electronic throttle all join the fray, and the result is 247 horsepower at 8500 rpm and 164 lb-ft of torque at 7500 rpm. Goose it and the almost indiscernible vibration at the controls grows to a subtle trembling as the Renesis approaches its 9000 rpm redline. A 6.3 second 0-60 mph blast and a 150 mph top end accompany an angry drone at the power peak.
The RX-8's engine is mounted up front, but it powers the rear wheels through a slick 6-speed manual transmission. Other goodies include big wheels and tires, Bosch stability control and Tokico monotube shocks. At the track it feels light and nimble, in tune with its 4-door sports car image. No surprises there: at a lean and well-distributed 3000lb, it's obvious the rotary diet has worked well for this handsome Mazda, despite being burdened (and reinforced) with beams, braces and buttresses every which way, in the doors, under the floor, in the front and in the back and around the passenger compartment.
Starting out at about 27-large, the RX-8's value hints at the old days of the RX-7. As for reliability, Mazda solved those little niggles when it first got its hands on rotaries, about 35 years ago. Good ol' Felix Wankel, the brilliant engineer, would be proud.
© Traian Popescu , World's Fastest Sedans (http://www.fantasycars.com/sedans/), 2002.
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