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The strengths of the rotary/Wankel engine seem perfectly suited for the needs of a motorcycle (small, lightweight, smooth, high revving). The main disadvantage (lousy fuel economy) also tends to be less important on a motorcycle. I know there were a few produced back in the 70's, such as the Suzuki RE5.

Surely there's good reason why they faded away. Why didn't they catch on?

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    You're making an assumption that fuel economy is not important. If this is not true (and I strongly believe it is the case) then the premise on which this question is based falls apart – Zaid Jul 27 '16 at 18:13
  • I was comparing priorities of a motorcycle to automobiles. Since motorcycles use relatively small amounts of fuel compared to a car, and because they are more often luxury items, cost of operation is less important. – masospaghetti Jul 27 '16 at 18:27
  • Fair enough. I look forward to seeing what answers the community has for you – Zaid Jul 27 '16 at 18:29
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    @masospaghetti The other main disadvantages are sealing issues and they burn oil by design so you have to keep a constant supply, and they have terrible emissions. The thing with rotary engines is they're actually not that great. The major advantage is their mechanical simplicity, but there's too many disadvantages to make that worthwhile. – Jason C Jul 27 '16 at 19:40
  • google.com/amp/s/www.motorcyclenews.com/amp/news/2017/april/… Read this it's making a comeback – Clinton spratt Jun 6 at 11:17
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I don't think this question needs to be limited to motorcycles; in fact, when you consider the plight of the Wankel with respect to cars, it answers the question for other applications as well.

I believe the last great production rotary was in the Mazda RX-8, and stopped about 4 years ago.

Apex sealing has always been troublesome, especially on cold starts. Rotor lubrication was also always an issue, and the Suzuki RE-5 you mentioned had a system to pump oil into the trochoid chamber.

Heat has been a real issue since day one in the late 50s. The exhaust stays in the chamber for a long time, transferring the heat to the rotor and trochoid chambers. Because of the shape and geometry inherent with a Wankel, it's difficult to get cooling passages where they need to be. Cooling systems tend to be complex and problematic.

The ignition system is usually complex, consisting of a minimum of two spark plugs per rotor. The compressed area is a long chord, and because of its shape does not promote a single-point ignition with a resultant efficient flame front.

Emissions from a rotary are not easily controlled to today's exacting standards. This may not be so much of an issue for a motorcycle, but I believe it was a strong influence of the demise of the RX-8 in Europe.

The bottom line is something @TMN referred to: return on investment, and just a plain "Why?" Modern motorcycle engines are making in excess of 200hp/liter. With reciprocating 4-stroke pistons. Why would anyone even consider a Wankel-engined motorcycle, when 500cc and 750cc monster beasts, (quite tractable, meeting all emissions, excellent fuel economy, gobs of torque) ... are available at every Honda, Yamaha, and Kawasaki dealer around??

What seemingly made conceptual sense and promised 12A or 13B utopia in the seventies didn't come to fruition, whether for cars or motorcycles.

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    There is a nice video on YouTube that I thought of when I read this question, which supplements this answer well: youtube.com/watch?v=v3uGJGzUYCI (primary issues with the rotary engine being low thermal efficiency and low fuel economy due to incomplete combustion, sealing difficulties esp. due to sharp temperature differences in block, burning oil by design = constant oil supply needed, and consequential poor emissions). – Jason C Jul 27 '16 at 19:33
  • @Jason C awesome, the visuals really tie it together and make it obvious. – SteveRacer Jul 27 '16 at 19:47
  • Thanks for the in-depth explanation. I keep hoping the wankel will come back just because it's "cool" and different, but I certainly understand that it must be near impossible to find engineers and designers to build and refine Wankel engines whereas there's an essentially unlimited supply of talent for reciprocating engines. – masospaghetti Jul 28 '16 at 20:21
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    @masospaghetti Don't give up hope, Mazda has even recently claimed they are still working on a next-generation Wankel. Youtube also seems to have an unending stream of "revolutionary" rotary engine variants, some out of garages, some from major university projects. If good apex sealing without oil injection is ever achieved, it will come back.. Materials and tribology nanotechnology may make such things possible. – SteveRacer Jul 28 '16 at 20:49
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Not a definitive answer, but I would guess it was strongly influenced by the requirement to have engineers who could successfully design a motorcycle engine using a Wankel rotor. It's completely different from an Otto-cycle piston engine (as I'm sure you know), so you need to have engineers who know (or can learn) the various stresses and fluid flows and other design centers that engine development requires. Plus, unless the company is going to invest in this engine design in a big way, you're going to wind up with a lot of unique engine parts that can't be shared across other models, so their development cost can't be amortized over larger production runs. So you wind up paying for specialized engineers to produce smaller runs of specialized parts, with no clear indication that you'll get any competitive economic advantage from it. A new manufacturer that planned to commit to a rotary-engined bike might make a go of it, but I don't think there's any compelling reason for a mainstream manufacturer to make the heavy investment in R&D required to bring what would basically be a novelty bike to market.

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    The Wankel engine is still a four-stroke Otto cycle, just that it accomplishes them without a reciprocating piston. – Zaid Jul 28 '16 at 2:29
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Considering a few characteristics of the Wankel engine, durability has been its Achilles heel. There simply would be no market big enough to justify it. It would be commercially unviable to further develop the tech given the other inherent issues (fuel consumption and emissions to name the big ones), even with the positives taken into account (simpler design, power/weight etc). The simple fact that piston engines just work, are a reliable, proven, world-wide standard that continues do prove itself and evolve kind of makes other engine designs unfeasible (that and the fact that electric has already positioned itself as the future of propulsion).

  • If, as you say, piston engines just work, then why are there not more diesel engined motorcycles? and yes there are a paltry few... And are you sure it will be electric and there is nothing else possible? – Solar Mike Jun 7 at 16:03
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    @user241146 welcome to the site! Could you flesh out this answer a bit? I'd love to see some references for some of those items. – Cullub Jun 7 at 16:51

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