RX-Rated: Mazda’s Early Rotary Cars, Part 1

SIDEBAR: Inside the Rotary Engine

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.

21 Comments

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  1. I’ve always loved Mazda’s rotary cars. Fantastic article, and I can’t wait for part 2…

  2. Great story looking forward to part 2. A friend in Tasmania had several of those bertone Luces nice cars the later models had the 1800 Capella engine.

  3. Thanks for the Mazda rotary article. I’m looking forward to reading Part 2. Despite growing up around Mazda rotary-powered cars, I learned quite a bit!

  4. It’s a real shame that no one can seem to lick the engine’s fuel and oil consumption problems. I have heard some discussion of Mazda using rotaries in hybrids, which makes some sense to me. Rotaries are so small and, on paper at least, elegantly designed.

    Man, that Luce coupe is a looker.

    1. I don’t know about hybrids, but Mazda has done quite a bit of development on a hydrogen-fueled rotary, which has been offered on a limited basis for fleet sales in some markets.

      If the next-generation 16X engine materializes, Mazda is hoping to reduce fuel consumption substantially, in part by adopting direct injection. Still, since piston engines keep improving in that regard, as well, I don’t know that the rotary will ever match the reciprocating engine in specific fuel consumption. Some things can be mitigated (like wall quench), but other factors, like the combustion chamber surface area to volume ratio, are sort of the nature of the beast.

      The Luce R130 is indeed a very nice-looking car. I’d never seen one before I started researching this story.

  5. Very interesting article, well, as usual, Aaron!
    The topic was somewhat forgotten in France after Citroën heavily invested in the technology, eventually failed to make it work and had to drop the project in the early 70’s. They had been so serious about it that the models developed in the late 60’s, the GS and the XM, were designed for a rotary. They had to hastily develop a reciprocating engine for the GS and make it fit in the engine bay that was not large enough.
    The XM eventually was painfully fitted with a Peugeot engine.
    Anyway Citroën was never able to design a good engine. This huge investment and its failure played an important role in the demise of the company.

    Nick

    1. “They had been so serious about it that the models developed in the late 60’s, the GS and the XM, were designed for a rotary.”
      You mean the SM, don’t you?

      1. I believe Nicolas was probably referring to the CX, which replaced the Citroën DS in 1974. I’ve never heard anything about the SM being intended for rotary power — of course the production cars had the Maserati V6 — but I think the CX was. The XM was the CX’s eventual successor, introduced in the late eighties.

  6. Right Aaron, my pen slipped, it was the CX.
    The XM was its successor.
    The SM, stangely enough, was fitted with the (in)famous Maserati V6 even though Citroën had such a faith in the future of the rotary as the ultimate replacement of the reciprocating.

    Nick

    1. Timing may have had something to do with it. Citroën didn’t build the first M35 single-rotor cars until the fall of 1969, and as I understand it, they were essentially evaluation models, not yet intended for large-scale production. The BiRotor wasn’t introduced until 1974, about four years after the SM debuted. Even if Citroën were keen to give the SM rotary power, it probably wouldn’t have been ready until a few years after launch, even in a best-case scenario.

      If things had worked out differently, I imagine Citroën might have added a rotary engine to the SM later, perhaps in a second-generation version for the mid-seventies. Of course, even if the Comotor engines had been more successful, the SM was not, and might have been dropped without ever getting a rotary engine.

  7. For them the rotary was the future type of engine for all applications, just as well as they were persuaded they had a market for the SM.
    With NSU, Mazda and others working on it it’s understandable.
    Your article is very interesting by showing how Mazda made a success of it, or at least could partly make a living with it, well… that’s a success, isn’t it?
    Strangely enough it didn’t catch on as an aviation engine either.
    Nick

  8. [quote=Administrator] Citroën didn’t build the first M35 single-rotor cars until the fall of 1969, and as I understand it, they were essentially evaluation models, not yet intended for large-scale production. The BiRotor wasn’t introduced until 1974, about four years after the SM debuted.[/quote]
    Starting in 69 a limited number M35, and in 73 GS Birotor, were sold to selected, faithful (and masochist) clients but the engine proved such a burden to maintain that Citroën offered to buy them back and scraped them. A few people only turned down the offer. The maintenance contracts were canceled for them. The few models still in existence are now very expensive collectors’ items, the day dream of all the GS enthusiasts.
    So there was actually a future for the rotary! ;-) As usual the car that nobody wanted became the car that nobody can afford.

    Nick

    1. The source I was looking at (John Hege’s [i]The Wankel Rotary Engine: A History[/i]) suggests that Citroën had basically intended to buy back the early evaluation engines from the outset, which would make a lot of sense.

      I don’t know about France, but in the U.S., automakers are legally obligated to provide parts support for production models for a specific period of time, typically 15 years — obviously not an appealing prospect for cars or engines that don’t end up in mass production! For that and other reasons, some automakers have tended to offer such evaluation vehicles only as a closed-end lease or other type of loan-out, with no option to actually purchase and keep the vehicle at the end; I assume that not actually selling it avoids triggering certain legal requirements.

  9. The Europeans have basically the same obligations as the Americans. As far as I understood, the deal was under specific conditions and since Citroën offered to buy them back it could cancel any support for those who rejecter the offer. It’s stupid it didn’t keep one example for history.

    Mazda is the only one who succeeded with a rotary over the years while all the others flopped.
    Well done!
    Nick

  10. This is an interesting article as usual, I’m waiting for the second part. While you’re at it, how about an article covering GM’s attempt to build a rotary engine?

    1. I thought about it, but in researching this article, I’m finding that detailed information about its development seems to be surprisingly scarce. While the development of the NSU, Mazda, and Curtiss-Wright engines is pretty well-documented, GM played it very close to the vest. To really do it justice would probably require talking to some of the engineers who worked on it, assuming that the people involved are still living, and willing (and able) to talk about the program.

  11. No need to mourn it’s passing. A technological dead end. I don’t miss the
    ffffttttt exhaust “note” of them at all.
    Used to be a few about Brisbane, Delighted to see and hear that rust and enlightenment of the owners has made them almost extinct.

    Good riddence. So it could rev to 5 digits.
    BFD.

  12. Wow, FANTASTIC article! Thanks for the great piece on Mazda, the detail and depths you go to are above and beyond. One of the best history-of-automaker stories that I’ve read. Thanks again!

  13. Another great article Aaron. Really appreciating your narrative drive and level of scholarship. I’m starting to believe the R100/1200 body was designed by Bertone as well, but can’t verify. Do you know of any text that addresses the connections between the Italian design houses and the Japanese manufacturers in depth?

    1. I so far haven’t found anything to suggest one way or another whether the first-generation Familia was done by Bertone, although it’s certainly plausible given that Bertone did the first Luce and the Luce Rotary Coupé in that period. Even if Stilo Bertone didn’t do the Familia or the first Capella, those designs have a definite Italian flavor, much more so than subsequent products of Toyo Kogyo’s in-house design studio, which feel more typically mid-seventies Japanese.

  14. I really like that little sidebar referring how to calculate the Wankel’s full displacement. I know Japan has different regulations than the U.S. and that Mazda had no choice to only count one chamber for each rotor (Geometric Displacement) due to extra taxes being placed on “bigger” cars. Either way, I really hope Mazda brings their Wankel rotaries back to the streets, because that awesome RX-Vision concept needs to be on the roads

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