Rotary Revolutionary: The NSU Ro80

SIDEBAR: Inside the Rotary Engine

(Regular readers will note that this text is borrowed — more or less — from the sidebar in part 1 of the Mazda rotary article; we repeat it here for ease of reference.)

The rotary combustion engine — commonly known as the Wankel — is a type of four-stroke internal combustion engine in which the movement of a three-lobed (trochoidal) rotor within a peanut-shaped (epitrochoidal) housing completes the four stages of the combustion cycle. The rotor drives an eccentric shaft through cycloidal gears and the eccentric shaft in turn drives the output shaft, which rotates three times for each revolution of the rotor. The following animation illustrates the process:

Wankel Cycle anim_en copyright 2005 User:Y_tambe CC BY-SA 3.0 Unported
The basic combustion process of a rotary engine. Note that the hypothetical engine illustrated above has its intake and exhaust ports in the rotor housing (peripheral ports). The position of the ports has an effect comparable to the camshaft profile of a reciprocating engine; peripheral ports improve high-speed breathing and power at the expense of low-end torque while side ports (which in this view would be mounted ‘behind’ the rotor in the upper part of the chamber) have the opposite effect. Most NSU rotaries had peripheral ports, but Mazda opted for side intake and peripheral exhaust ports for better idle quality and low-speed response. Dual spark plugs were common but not universal on the production rotary engines. On Mazda rotaries, one plug fired 5 to 15 degrees after the other to promote more complete combustion. (Animation: “Wankel Cycle anim en” © 2005 User:Y_tambe; used under a Creative Commons Attribution-ShareAlike 3.0 Unported license)

As shown in the above animation, the rotary is dramatically simpler than a reciprocating engine. While the rotary does have counterweights at each end of the eccentric shaft to balance the wobble caused by the rotor’s eccentric motion, a Wankel engine has no connecting rods, no crankshaft, and no valves or valve gear. Intake and exhaust are through fixed ports, either in the side plates or in the rotor housing. (Of course, the rotary still requires the same accessories as a piston engine: water and oil pumps, alternator or generator, et al.)

Compared to piston engines, rotary engines have a number of advantages and several serious disadvantages:

PROS

  • Fewer parts: A rotary engine has fewer than half as many parts as a piston engine, which reduces manufacturing costs and (at least in theory) repair and overhaul costs.
  • Light weight and compactness: With no valvetrain, connecting rods, or bulky crankshaft, a rotary engine takes up less space than a comparable reciprocating engine and usually weighs less, benefiting packaging (and often performance and handling as well).
  • Smoothness: Unlike the pistons of a reciprocating engine, the rotors in a rotary engine never change direction and each power cycle has a longer duration than that of a reciprocating engine; both factors greatly reduce vibration. Because of the eccentric motion of the rotor, the rotary is not quite as smooth as is a turbine engine, but rotaries have little of the shake inherent to many piston engine configurations.
  • Rev potential: With excellent volumetric efficiency (which at some speeds can exceed 100%) and relatively low rotational inertia, a rotary engine can rev quickly and reach very high engine speeds.
  • High specific output: A rotary engine can produce more power than a reciprocating engine of the same swept volume (geometric displacement). With the advent of variable valve timing and fuel injection, the difference is no longer as great as it once was, but for many years a rotary engine was considered comparable to a piston engine of two times the rotary’s total geometric displacement. For example, the output of a 995 cc (61 cu. in.) rotary engine was roughly equivalent to that of a 1,990 cc (121 cu. in.) piston engine.
  • Modest octane requirements: Because of the size and shape of their combustion chambers, rotary engines generally have lower octane requirements than do piston engines.
  • Low NOx emissions: Rotary engines tend to have lower combustion temperatures than do piston engines, substantially reducing nitrogen oxide (NOx) emissions.

CONS

  • Ease of manufacture: A rotary engine may have fewer parts than a piston engine, but some components, such as the rotor housing, are complicated or difficult to produce, driving up manufacturing costs.
  • Difficult sealing: With their mathematically complex curves, adequate sealing is challenging and often problematic for rotary engines, from the corner and apex seals at the tips of the rotors to the O-rings between the rotor housings and side plates. Oil sealing is also more complicated than the piston rings of a reciprocating engine.
  • Fuel consumption: Compared to OHV or OHC piston engines, the thermal efficiency of a rotary engine is poor and a certain amount of fuel mixture clings to the chamber surfaces and rotor, where it is eventually forced out the exhaust ports without being burned. As a result, rotary engines tend to be thirsty for their size and output, with high specific fuel consumption (units of fuel burned per unit of power produced per hour). Based on the Society for Automotive Engineers’ thermal equivalency formula, a rotary engine has thermal efficiency (and thus fuel economy) comparable to a reciprocating engine of three times its geometric displacement. By that formula, for example, a 995 cc (61 cu. in.) rotary engine would be about as thermally efficient — and thus about as thirsty — as a 2,985 cc (182 cu. in.) piston engine!
  • Oil consumption: Even with effective oil sealing, rotary engines consume some oil for rotor lubrication, much like a two-stroke engine. Many production rotaries have used metering systems to inject small amounts of oil either into the carburetor or (in later engines) directly into the rotor chamber itself.
  • High HC and CO emissions: The same factors that cause the rotary’s high fuel consumption and low nitrogen oxide emissions contribute to higher levels of unburned hydrocarbon (HC) and carbon monoxide emissions.
  • Higher cooling requirements: The rotary’s low thermal efficiency means that more of the energy of combustion is lost as heat than in most modern reciprocating engines. That waste heat puts a heavier load on both the cooling and oil systems of a rotary engine than with a piston engine of comparable output, requiring greater radiator capacity and sometimes the use of an engine oil cooler.

48 Comments

Add a Comment
  1. I believe this is the most comprehensive NSU story ever published in America.
    The research for the article is top notch.

  2. When I moved to Boulder, Colorado, in January 1972, there was an NSU dealership downtown with a Ro 80 in the showroom. I had read some articles about the car and knew what it was, and I don’t know why I didn’t stop in for a closer look at this one. I remember it being burnt orange. I don’t know if it was an incoming ’72 or a leftover ’71.

    I think the dealership was still there when I left Boulder in August 1973, but I hadn’t really been keeping tabs on it.

    1. It’s possible — I wasn’t able to find any definitive answers on how long NSU had U.S. franchises or what happened to those places after NSU pulled out. Did they get Audi franchises? Did they just go away? If anyone has more specific information, I’d be very curious.

      As I said in the text, it’s hard for me to see NSU sticking around long after MY 1973. By that point, the Prinz and other models were out of production, and I don’t know of any attempts to make any of NSU’s cars (including the Ro 80) compatible with the U.S. bumper standards.

  3. Regarding Ro80 interior storage, there’s plenty. There’s a large “magazine” shelf below the 5 band radio and glove box.
    The front seat backs have two large pockets for those sitting in the rear seats.

    1. Fair enough. I should note that the complaints about interior storage came chiefly from British critics, who were accustomed to having a whole array of door bins and oddments trays on even little cars like the Mini. I think they looked at the Ro 80’s large and uncluttered interior as a missed opportunity in that respect. (Of course, for one accustomed to British luxury cars, the Ro 80 was a little plain inside, but large enough to provoke fits of agoraphobia…)

      1. I ran a ’74 model Ro80 in the mid eighties, bought when they were
        practically worthless and I have been a Wankel fan ever since. The Ro80 interior was then criticised for it’s “Germanic” plainness but not by me; I have never gone for the leather / wood / wool interiors that Brits. are supposed to like.
        I loved the space, the quiet, the view through those huge windows, the ride, the fine steering and superb brakes. And of course that fantastically smooth engine! My car had 47000 odd miles when I acquired it, definitely on the original engine. There were some starting issues at first but these were fixed with a change to NGK spark plugs. I think I got about 20 miles from a UK gallon, not good but the engine did encourage full throttle. Sadly it had to go in the end due to rust.

        1. Well, the Ro 80 may not have matched cars like the XJ6 for that British men’s club feel, but from a packaging standpoint it was really no contest. When [i]Car[/i] compared the Ro 80 to the BMW 525 and Jaguar XJ6L Series II in 1974, they made the usual complaints the NSU looking too stark, but they had to admit that Ro 80 was roomier than even the long-wheelbase Jag.

  4. I guess the story comes full circle with the Wankel-engined, hybrid concept, the Audi A1 e-tron.

    (By the way came across the site some weeks ago and I have to say the stories are excellent, and very informative, especially for European people)

  5. Aaron,

    GREAT article, as always. Love your research and your writing. Have a small correction to your apex seal diagram captions: The part labeled "sealing bolt" is actually a corner seal:

  6. Thank you again, do you read my mind? I have reflected upon the Ro80 many times with Mazda ceasing production of a rotary engined auto. I was so enthused by your article that I had to share it at work. I thought that my description of a rotary combustion engine would be perfectly understandable. This was not the case.I did, however, garner some understanding of my interest when I E-Mailed an image of the Ro80. Even a complete non car person offered a comment that it could not be that old.Indeed.

  7. I always thought the RO80 was one of the sharpest designs of the ’60’s and wanted one, despite the problems with the early Wankel. Still a stunning looker and a real pioneer!

  8. Absolutely terrific article!

    But one little “gotcha”….. You wrote “The rotor housings of the KKM 612 are aluminum castings, although the inner surfaces are plated with nickel silicone carbide.”

    Looks like you fell into the Silicon-Silicone trap! Silicone is a rubbery material familiar to most people as bathtub caulking. It is also what attaches the tiles to the space shuttle. There is no such thing as Silicone Carbide!

    The above quote should have read “silicon carbide”…. a very hard crystalline substance.

    But none of the above detracts from the excellent piece on the Ro80!

    1. That was a typographical error in the photo caption; I’m well aware of the difference between silicon and silicone, and you’ll note that it was spelled correctly in the first instance in the text.

  9. Thanks again Aaron, a lot of stuff I did not know about the RO80, I've only seen one.

  10. [quote=Administrator]It’s possible — I wasn’t able to find any definitive answers on how long NSU had U.S. franchises or what happened to those places after NSU pulled out. Did they get Audi franchises?[/quote]

    Just speculating, but on general principles, I wouldn’t think that NSU dealers would automatically be grandfathered in. I’d think that Audi would need to feel the need for another dealership in that area and would need to see the ex-NSU dealer as a worthy candidate for the franchise.

    You’ll notice that on the license plate frames of Jim’s car, it says Lake City. Lake City is a neighborhood in the northeast corner of the city of Seattle proper, and Lake City Way is a major arterial through the area.

    I moved to Seattle in 1981, and I have dim memories of seeing a billboard for an NSU dealership on Lake City Way in the early ’80s. It was a couple of miles south of the commercial strip in the heart of Lake City, where the dealership would presumably have. Of course, at this point the billboard didn’t necessarily mean that the dealership was still around.

    1. Well, my thought process was that while Audi might not have been keen to give out new franchises to ex-NSU dealers, NSU-Audi did have to deal with U.S. franchise laws, which tend to favor the franchise holder in such situations. If NSU dealers did NOT switch to Audi at some point, it’s hard to see how they would have stayed in business, since I’m pretty sure that by the start of the 1974 model year (fall 1973) there were no NSU cars still in production that met U.S. regulations

      1. All NSU dealers in America sold other imported cars. Most sold other German cars. The largest was Overseas Imports in Livonia, Mich. Their main line was Italian cars because the owners were Italian. I asked about the why they sold NSU’s and was told they respected the NSU marque and they liked to quick and nimble cars that were easy to work on… he said one can change a clutch in 30 minutes. Which is true.

        1. At least in the U.K., the “book” time for a clutch job was listed as 5.3 hours, although that was still not bad for a FWD car and a good deal shorter than a Citroën DS (which was 12 hours) or a Rover 2000/2200 (which was over 10 hours), both engine-out jobs.

          1. I think the 30-minute clutch change applied to the rear-engine air-cooled NSUs.

  11. Lake City Auto Imports was located at 91st and Lake City Way, which is in Seattle City limits. I bought my Ro80 from one of the owners who was going through a divorce and need to liquidate assets. I paid full market value at the time.
    After having three kids and having three citations in so many years I decided to retire the car till I wised up.
    The car set in my shop untouched for over 30 years. I went through the car and changed all fluids, wiped off 30 years of dust with a diaper to prepared it for Seattle’s huge 2010 Greenwood Car show. Out of 900 cars entered the Ro80 came in number 27 of 30 awards. My car won Best of Class. There were so many fine cars in the show I would have been happy to be in the top 100. Being a judge at a car show is a tough job.

  12. What an excellent piece of research here. I have an RO80 myself. It was apparently brought over by a US military officer. I have attended the Essen Techno Classica show a few times, and looked for an RO 80 in Germany, but they are hard to find even there, and then of course there is the rust issue. I was stationed in Wiesbaden in 1970-72, and I remember seeing them exit the autobahn, with trails of blue smoke following them as they de-accelerated. I have also found a few Birotors for sale in France – the factory tried to buy every single car back to destroy them, but a few slipped through their fingers. Now that would be a find!

  13. Hi ROers, I am sitting on 3 Ros in Stuart Florida.
    A 1976 in super condition silver. A 1971-2? in good condition orange, great black vynil interior.
    And a 1969 ceased engine, avacado green.
    Actually looking to sell the 71 & 69 if anyone is interested. Email me. Rust minimal in the two for sale.

    1. Hi,

      I just read your post from Jan, 2013. Do you still have the 2 RO80 for sale ?
      I am interested in getting one. I am in California.

      Rainer

  14. nice write up by Aaron ..
    Jim your car looks fantastic
    Too bad no company ever put Bentele’s direct injection technology from 1962 into action.
    [U.S. Patent No. 3,246,636]

    1. I regret that for space reasons I haven’t been able to get into Curtiss-Wright’s Wankel development, either here or in the Mazda article. C-W did a lot of work on the Wankel over the years.

      One thing that’s worth mentioning that I didn’t exactly spell out is that NSU’s license agreements gave NSU access to its licensees’ rotary development information. NSU didn’t always implement the ideas that its licensees came up with (NSU and Toyo Kogyo went their own ways on a number of points), but Fröde et al were definitely fully aware of those developments.

  15. The lights! The lights!

    Our family had RO80s from about 1975 to 1980, when I was 9 to 14.

    The things I remember as outstanding were brakes, headlights, bootspace, interior space and silence, and that they are quite roly-poly, the amount of fuel used, and the cost of spares.

    This was a Brit car with the sealed beam halogen lights.

    No one wants them in less than pristine condition, so our 3 cars (1 had last run in 1980, the others were non runners for rebuild and parts bought back in 1980), so all 3 were cut up and scrapped last year when we moved to a smaller house.

  16. Very nice article and it is difficult to find well researched information about NSUs in general.
    A couple of corrections:
    1) the figure for the number of Spiders produced is 2x – the correct number is ~2,475
    2) The Spider always had only a single spark plug in production. It did not use a coil, but had an early electronic ignition which utilized an electron tube to generate the necessary hot spark.
    2) The Ro80 never had 2 distributors (Maxda’s did, though) – the early 2 plug per rotor engine (I have one) had a single distributor with four plug wires and two for the coils. The distributor rotor has two contacts opposite each other to fire the 2 plugs

    1. JJ,

      Thanks for the corrections — I’ve amended the text.

  17. hey,i have read the comments on this site and find some quite interesting.the ro80 was and is a unique auto,i know i owned four or five and even drove one coast to coast;california to new jersey years ago. bought the first one from dec auto in boulder,co. and the last one was converted to a 12a mazda. still have a parts and repair manual in english along with usa headlite assembly and amber directional lens for usa. give me a call to chat about the car and experiences of owning and driving one.

    1. Thanks, Dan. I redacted your phone number so that random spammers don’t use it to bug you.

  18. I have been seeing an Ro80 on the road here in Barbados fairly frequently of late,and a couple of weeks ago I met the owner, whose name is John King, at a classic car show.
    He said an English expatriate imported the car around 2002 (and I do recall seeing it once or twice in the north of the island back then. The expat sold it to a local car collector, who drove it for about 75 miles before putting it in storage. John bought it in 2013 after seeing it at a classic car show and has been using it as his daily driver since then.
    This car is a 1974 model and uses the upgraded NSU engine. John says the engine works well and he has only changed the water pump on it. He noted that it was not the most fuel efficient car and required oil changes every 1000 miles or so, but he said the frequent oil changes were advantageous in that they ensured the engine was always properly lubricated and the oil was clean.
    Cosmetically, it was resprayed in the original light blue, and the cloth inserts in the upholstery were changed, but everything else was original and all the accessories, including the electric sunroof, were in good working order.
    John added that cricket legend and Barbadian National Hero, Sir Garfield Sobers, brought the first Ro80 into Barbados in the early 1970s when he was at the height of his cricketing career. It was a 1972 model, whichi he kept for a couple of years. Eventually it passed through the hands of several owners until, to the best of his knowledge, it was scrapped some time in the late ’70s or early ’80s.

    1. Interesting! It’s always neat to see the unexpected places cars end up. Thanks for the info.

    2. Please send me photo of the Ro80 in Barbados for our archives.

      BTW-My grandfather 6 generations ago (1767-1790) had business dealings in Barbados….. he was the bookkeeper for a shipping firm…. their ship was named Barbados.

  19. Cannot help but wonder whether all that investment towards the Rotary by NSU would have been better spent on developing say a Lancia-inspired 2.0 narrow-angle V4 Twin-Cam engine for the NSU Ro80.

    1. Or buying engines from Lancia, which could have used the income! The difficulty, of course, is that controlling the rights to the Wankel engine was also a big part of the reason NSU had the money to develop the Ro80; rotary engine royalties were a major source of income for what was after all a very small company. A V-4 would still have cost them a lot to develop and wouldn’t have offered the same income potential.

      1. Apparently SAAB was a more likely candidate for the Lancia V4 engines until they went for the same unrefined / underpowered Ford V4 in the NSU Ro80 conversions for the SAAB 96, a pity really that the 96 never utilized the Fulvia V4.

        Also the Fulvia V4 was said to be at the limit capacity wise though a shelved 1600cc Fulvia V4 project with variable valve timing for rallying was looked into prior to the Lancia Stratos project.

  20. How many cylinders would an Otto-cycle engine need to have in order to have the same number of power strokes per revolution as a two-rotor Wankel?

    1. No one told me there would be math…

    2. Four cylinders – one bang every 180 degrees of the flywheel.

  21. From asking around the Ro80 Clubs, it seems that some within NSU did indeed investigate the use of a conventional Flat-6 engine for the Ro80 until it was stopped by NSU officials who found it undesirable.

    Unfortunately no further details of the Flat-6 prototype engine are available.

    Though the following is baseless speculation is on my part, it makes one wonder whether closer cooperation between NSU and Citroen beyond the Comotor join-venture or an alternate non-Comotor arrangement was considered.

    Particularly if such cooperation between NSU and Citroen potentially connects back to the NSU Flat-6 prototype engine in the event the Flat-6 is somehow revealed to NOT be a unique in-house NSU design, but rather in essence a (possibly water-cooled) Flat-6 version of what became the air-cooled Citroen GS Flat-4 (via the earlier Citroen C-60 and F projects) or air-cooled 2-litre Panhard X4 prototype Flat-4 (later used on the Panhard AML) engines.

    Worth mentioning as well as that the water-cooled Volkswagen / NSU K70 engine was essentially a development of the air-cooled Inline-4 engine mounted in the rear of the NSU 1200, so it might be the case that water-cooling was considered for the NSU Flat-6 engine regardless of its true origin.

    1. Hard to say. One point worth considering is that, as Chevrolet found with the Corvair, air-cooled flat-fours and flat-sixes are to some extent different animals — at least in automotive applications, the cooling requirements are quite a bit different. With water-cooled engines, I imagine that developing a six from a four or vice versa would be somewhat easier.

      The upshot, I think, is that for NSU to develop a workable flat-six — assuming they did not have a production partner from which they could purchase or license an existing design — would have been a substantial additional investment in engineering and production resources, which wouldn’t necessarily have been any less risky than the Wankel. So, I can see why the NSU directors would be reluctant to go that way.

      1. Considering what happened to both NSU and Citroen, one cannot help but wonder whether they would have fared better had they not been fully committed to the Wankel Comotor joint-venture, but rather focused on more conventional engines.

        NSU had the 1.6-1.8 K70 engine that may or may not have been capable of being enlarged further to a 2-litre as well as the Flat-6 prototype engine.

        There was also the NSU K50 project aka Audi 50 / VW Polo that may have either used what became the VW EA111 engine from the outset or water-cooled developments of the air-cooled NSU Prinz/1200/etc engines, the latter assumes NSU remains independent from VW.

        Citroen meanwhile could have further enlarged its Flat-4 engine beyond 1299cc to around 1600cc or beyond, perhaps even find a use for the 2-litre Panhard Flat-4. Though there is also the fact that Citroen around that time owned Maserati as well.

        1. For Citroën, there’s a strong argument that they should have put more into modern four-cylinder engines, which ended up being a weak point in a couple of different areas. The 1.3-liter flat-four’s expansion potential is uncertain. People have taken them out to 1.6 liters, but as Julian Marsh’s excellent Citroënet site notes, Citroën was very wary of doing so for cooling reasons, which is fair. I don’t know that the Panhard engine was a reasonable modern alternative, but the reciprocating-engine GS could certainly have benefited from a bigger option, no argument there.

          As for NSU, they were dealing with the facts that (a) they were running out of money and (b) the Wankel was, while not quite their ONLY asset, the only one that was headline-worthy. They were a tiny company with something that, at least for a while, everybody was interested in. A me-too response to that class would have been technologically safer, but also sounds like a recipe for commercial oblivion.

          1. Agreed that Citroen should have developed modern water-cooled 4-cylinder engines, though they did look a transverse-mounted 1.6-litre water-cooled engine derived from the DS engine for Project F.

            Interested in reading up the Citroenet link on the uncertainty of further expanding the Flat-4 engine beyond 1.3-litres.

            Since the Citroen C-60 project did look at a 1.4-litre Flat-4 and assuming any relation exists between the Flat-4 and 2CV Flat-Twin engines, the Citroen Project F was to feature a 750cc Flat-Twin derived from the 2CV engine that doubled up as a Flat-4 creates a 1.5-litre Flat-4 (with some people even enlarging the 650cc 2CV Flat-Twin to as much as 800cc).

            Guess NSU had to take a risk with the Wankel if it was indeed in a dire position though was under the impression they did rather well in cars and motorcycles. Curious to know whether NSU did consider producing 4-door versions of their rear-engined NSU Prinz 1000 / NSU 1200 models.

          2. Regarding risk, the dilemma for small companies not exactly rolling in capital (or individuals in the same position) is that if you aren’t doing at least relatively well, you aren’t in a position to do anything financially risky. If you can’t pay your rent, you aren’t going to buy stock and you probably aren’t going to start a new business venture. Examining the history of the auto industry, innovation very often comes from companies that have had some success — enough to give them some money to play with — and that are looking to grow and bolster their position; that’s generally the most exciting and interesting phase of a company’s development. Established segment leaders don’t have much incentive to take risks and players who are struggling can’t afford to.

  22. Regarding the NSU Spider (and even the NSU Sport-Prinz), were non-Wankel versions considered such as regular and TT/TTS spec 996-1177cc NSU air-cooled OHC engines?

    1. Not so far as I know. That might have been a logical development if NSU had considered the Sport-Prinz or the Spider a commercial priority, which I’m not sure they did. I don’t know that the Sport-Prinz was ever a big seller, and the Spider was pretty clearly a short-term expedient: “How can we create a ‘new’ model to showcase our first production rotary engine without investing a lot of money in anything other than the engine, which we’re still trying to make work?” The fact that the Spider and Sport-Prinz were dropped about the time the Ro80 debuted is probably indicative in this regard. It’s possible (speaking strictly speculatively now) that NSU might eventually have developed replacements, offering both piston and rotary engines, but NSU just didn’t have a lot of money and the Ro80 and K70 were their principal priorities at that time.

Leave a Reply

Your email address will not be published. Required fields are marked *

Click here to read our comment policy. You must be at least 18 to comment. PLEASE DON'T POST COPYRIGHTED CONTENT YOU AREN'T AUTHORIZED TO USE!
Except as otherwise noted, all text and images are copyright © Aaron Severson dba Ate Up With Motor. (Terms of Use – Reprint/Reuse Policy) Trademarks referenced herein are the property of their respective owners and are used here for informational/nominative purposes.