The Pros And Cons Of Rotary Engines
In a world dominated by pistons, the rotary engine was something different for motorists. It was the vision of German engineer Felix Wankel, built on the belief that the up-and-down motion of pistons in engines would be far less efficient than something rotating. The unorthodox design didn't catch the eyes of many, but Mazda fell in love with it, adding it in the RX series of coupes for decades and using it to power the Japanese brand to victory at Le Mans.
With the discontinuation of the RX-8 in 2012, we've gone over a decade without rotary power on the market. That doesn't mean it's been totally abandoned, as Mazda has been constantly experimenting with a potential return to Wankel power. The company put out a plethora of concepts over the past five years, including the RX-7-inspired SP and, most recently, a hybrid grand tourer in the form of the Vision X-Coupe.
Time and time again, we've been teased, but the folks at Mazda still fail to pull the trigger on an RX-9 — or any rotary-powered car, for that matter. But why not? What's kept this legendary engine down for so many years, and what could potentially see its return in the near future?
Rotary engine pros
So why go with the rotary? For starters, it's able to do a lot with a little. The RX-8's Renesis twin-rotor cranked out 238 horsepower despite only being 1.3 liters. Its horsepower-per-liter ratio of 183:1 clears Honda's S200, with that car's F20C engine making almost the exact same levels of horsepower at nearly twice the size. In modern comparisons, the RX-8's ratio is farther away from the Civic Type R's 157:1 and closer to the Ferrari F8 Tributo's 182:1. Being able to cram so much power into such a small size keeps the engine light and tiny, making it perfect for sports cars — on top of being easy to package inside crowded engine bays.
These engines aren't just small, but simple, too. Compared to piston and cylinder engines, there are far fewer moving parts inside of a running rotary. At full song, only three components in a two-rotor powerplant are actually moving: the eccentric shaft and the pair of rotors. Fewer things moving means fewer parts that could break, making diagnostics and maintenance incredibly straightforward.
In motion, the rotary has two main advantages: low vibration and high revving. By nature, the reciprocating mass of pistons causes engines to vibrate when running. The rotary, however, has far less vibration, since it's rotating and not going up and down like a piston. On top of making the engine incredibly smooth on power delivery, this allows it to rev incredibly high. Without having to deal with throwing a rod or valve float, these engines can easily hit rpm numbers of 9,000.
The cons of the rotary
Of course, every engine has its setbacks, and an engine as unique as the rotary has its own challenges. For starters, despite making impressive power for its size, rotaries are set back by poor efficiency. The eccentric shape of the combustion chamber means a lower thermal efficiency compared to piston engines. This leads to a lot of unburnt fuel leaving the exhaust. This results in an engine with very poor fuel economy, with the 2004 RX-8 only getting 18 miles to the gallon.
It goes through oil just as quickly since it's directly injected into the intake manifold and combustion chamber. This is done to ensure the apex seals on the corners of the triangles stay lubricated, and it's why RX-7 and RX-8 owners are constantly checking their oil levels. When these seals get bad, things can break, and rotary repairs aren't easy or cheap. With just how specialized the engine is, it can be hard to find the parts and personnel to fix these motors. If you can find a shop, even basic rebuilds could set you back over $2,500.
Ultimately, though, emissions were the ultimate downfall when it came to the rotary. Constant oil burning and bypassed fuel is what killed the RX-8 Europe when it failed to meet Euro 5 emissions regulations in 2010, with poor sales figures ending global production the next year. The return of the rotary isn't impossible; we've already seen it used as a range extender for the Mazda MX-30. But for us to see the next generation of rotaries, the engine will need to evolve into something far more efficient and reliable.
