The advantages and disadvantages of rotary engines – Jalopnik

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 that rotates. The unorthodox design didn’t catch the attention of many, but Mazda fell in love with it, adding it to the RX series coupes for decades and using it to propel the Japanese brand to victory at Le Mans.

With the discontinuation of the RX-8 in 2012, we have been on the market without a rotary power for over a decade. That doesn’t mean it’s completely abandoned, as Mazda has continually experimented with a possible return to wobbly power. The company has released 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.

We’re teased time and time again, but the folks at Mazda still can’t pull the trigger on an RX-9 – or any car for that matter. But why not? What has kept this legendary engine in check for so many years, and what could possibly see its return in the near future?

So why choose the rotary? For starters, you can do a lot with a little. The RX-8’s Renesis twin-rotor engine made 238 horsepower despite being only 1.3 liters. Its 183:1 horsepower-per-liter ratio bests Honda’s S200, with that car’s F20C engine making almost exactly the same horsepower and almost twice the size. In modern comparisons, the RX-8’s ratio is further away from the Civic Type R’s 157:1 and closer to the Ferrari F8 Tributo’s 182:1. Being able to pack so much power into such a small size keeps the engine light and small, making it perfect for sports cars – as well as being easy to pack into busy engine bays.

These engines are not only small, but also simple. Compared to piston and cylinder engines, there are far fewer moving parts in a running rotary engine. At full power, only three components move in a two-rotor power source: the eccentric shaft and the pair of rotors. Fewer things moving means fewer parts to break, making diagnostics and maintenance incredibly easy.

In motion, the rotary has two important advantages: low vibration and high speed. Naturally, the reciprocating mass of pistons causes engines to vibrate as they rotate. However, the rotary one has much less vibration, because it rotates and does not go up and down like a piston. This not only makes the engine incredibly smooth in terms of power delivery, but also allows it to rev incredibly high. Without having to worry about throwing a rod or valve float, these engines can easily reach speeds of 9,000.

Of course, every engine has its setbacks, and an engine as unique as the rotary engine has its own challenges. For starters, despite delivering impressive power for their size, rotary machines are held back by poor efficiency. The eccentric shape of the combustion chamber means lower thermal efficiency compared to piston engines. This causes a lot of unburned fuel to come out of the exhaust. This results in an engine with very low fuel consumption, with the 2004 RX-8 achieving only 30 kilometers per liter.

It passes through the oil just as quickly because it is injected directly into the intake manifold and combustion chamber. This is done to ensure that the top seals on the corners of the triangles remain lubricated, which is why RX-7 and RX-8 owners constantly check their oil levels. When these seals go bad, things can break and rotary repairs are not easy or cheap. Because the engine is so specialized, it can be difficult to find the parts and personnel to repair these engines. If you can find a shop, even simple renovations can set you back more than $2,500.

Ultimately, however, emissions were the ultimate downfall when it came to the rotating machine. The constant burning of oil and bypassing of fuel is what killed the RX-8 Europe when it failed to meet Euro 5 emissions regulations in 2010, while poor sales figures halted global production the following year. The return of the dial is not impossible; we have already used it as a range extender for the Mazda MX-30. But if we want to see the next generation of rotating machines, the engine will have to evolve into something much more efficient and reliable.