Engines Exposed: How the Rotary Changed How We Think of Combustion
Internal combustion engines are modern marvels. The fact that we don’t notice the thousands of precisely controlled explosions happening every minute we drive is nothing short of astonishing. And just think, all that incredible engineering that goes into even the cheapest and buzziest of engines just so that they don’t disturb you and your girlfriend’s late-night pumpkin-spice latte run (Public Service Announcement: Uggs are bad for heel-and-toe downshifts).
Perhaps even more impressive is that the pistons in an engine are flying back and forth at such an unbelievable rate. For the flat-plane crankshaft Voodoo V8 that we’ve previously discussed, the average piston speed at redline is almost 60 miles per hour. As you can imagine, this creates a fair amount of resistance and vibration the engine has to compensate for as its internals are hurled back and forth. With all of our modern technology, surely there must be a different way to approach complex engine building, right? Well, as it turns out, there is…
Reciprocating internal combustion engines take a liner motion (the piston traveling back and forth in the cylinder) and converts it to rotary motion via the turning of the crankshaft. But way back in 1929, a German engineer named Felix Wankel came up with a novel concept: Why not create an engine that develops a rotary motion to begin with, which reduces the complexity of converting the power via a crankshaft? Twenty years later, his idea came to fruition. The first engine based on Wankel’s design was called the DKM 54, and it was introduced in 1951. While these engines have come to be known as Wankel engines inside engineering classrooms, thanks to Mazda and its successful RX-7 – which stands for Rotary eXperimental – they’re much more commonly known as Rotary engines.
There’s no easy way to explain how a Wankel engine works, so I’m going to turn to the animation above to help. As you can see, the operation is completely different from a reciprocating engine. This has some very distinct advantages.
First, these engines are able to rev to incredibly high speeds. The Mazda Renesis engine, which was introduced in 2003 for the RX-8 sports car, had a redline of 9,000 Revolutions Per Minute (or RPMs). Today, when V8s can now rev to 8, 9, or 10,000 RPMs, this no longer seems that impressive; but at the time, it was quite the engineering feat. Second, Wankel engines have high power to weight ratios, which makes them appealing for performance applications. The Renesis engine only weighed 250 pounds when dry. In comparison, Honda’s contemporary B20B, a dual-overhead camshaft inline-four, weighed 326 pounds. Third, Wankels are extremely smooth during their operation and have exceptionally flat torque curves. Finally, they’re much simpler, thanks to fewer moving parts.
With a cylinder reciprocating engine, the crankshaft must rotate twice for one combustion cycle to occur. For a rotary engine, the driveshaft only needs to rotate once to complete a combustion cycle, effectively doubling the number of power impulses per rotation. Furthermore, rotary engines can be built with multiple rotors: A common shaft connects them, and it’s analogous to adding additional cylinders to a reciprocating engine. The Renesis, for instance, was a bi-rotor engine. When it was being judged for various awards, it was actually considered as a 2.6 liter engine, despite the fact that its displacement is only 1.3 liters due to the number of combustions per rotation of the driveshaft.
I think we can agree that those all sound like great things so the question is: Why didn’t they ever become the engine of choice for all manufacturers?
The short answer is that there are distinct challenges to taming the Wankel engine. Fuel economy and emissions of these engines have never been great. With a Wankel, the combustion phase occurs in a “chamber” that is constantly changing shape, which makes it difficult for the flame to consume all of the mixture. Also, creating a seal between the rotor and the housing is very difficult; the apex seals, which are located at the corners of the “triangular” rotor, need to manage this job under changing conditions.
Unlike a reciprocating engine where the cylinder is heated by combustion and then cooled by drawing in the air/fuel mixture, the housing of a Wankel engine is stationary and the combustion part of the process always occurs at the same spot, which leads to localized high-temperature zones. The seals need to have very tight tolerances, but that creates wear and requires introduction of a lubricant into the combustion chamber. Similar to a two-stroke engine, oil is introduced into the fuel mixture, which helps to keep everything running smoothly. Then when the engine isn’t properly lubricated, very bad things tend to happen.
So is there a future for Wankel engines?
Despite the challenges of Wankel engine technology and its lack of mainstream success, the engine isn’t dead. At the Tokyo Motor Show, Mazda introduced the RX-Vision, a front-engine, rear-wheel drive sports car powered by the next-generation SKYACTIV-R rotary engine. When asked about the challenges of Wankel engines, the company stated:
Mazda has never stopped research and development efforts towards the rotary engine. The next rotary engine has been named SKYACTIV-R, expressing the company’s determination to take on challenges with convention-defying aspirations and the latest technology, just as it did when developing SKAYCTIV TECHNOLOGY.
With more than 60 years of evolution in Wankel technology, there’s no doubt that these engines are extremely sophisticated. Wankel would likely be proud of the work that has been done. Whether or not Mazda will succeed in capturing the hearts of the public with the new RX Vision remains to be seen, but I, like many other gearheads, am keeping my fingers crossed that the Wankel finally gets its day in the sun.
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