True fans of the rotary engine are praying that Mazda’s March 24 U.S. patent application means that the resurrection of this free-spirited alternative to pistons is not far off. While it’s been nearly four years since the final rotary engine was built for an RX-8, hope springs eternal that Mazda’s sacred powerplant will hum again.
Since the Renesis II 16X engine program was introduced at the 2007 Tokyo Motor Show, Mazda has insisted that its engineering team had been working feverishly to bring a new rotary up to modern power, fuel-consumption, exhaust-emissions, and reliability standards. Last year Mazda stole the Tokyo show with its RX-Vision two-seat coupe concept, which presented a stunning wrapper but no engine updates beyond a fresh rotary-engine name: SkyActiv-R.
What we know about the SkyActiv-R is that it features a 23-percent-larger displacement than the 232-hp 13B engine which powered the RX-8.
The larger size and internal geometry changes are aimed at solving a key rotary issue—the impression that no one’s home at low rpm. To save weight, the SkyActiv-R uses aluminum end plates where the 13B used iron pieces instead. Injecting fuel closer to the combustion chamber improves fuel efficiency.
Even though turbocharging is contrary to core SkyActiv tenets, this new rotary will definitely be turbocharged to produce the 400-plus horsepower it will need to compete against Corvettes, Jaguars, and Porsches. The most interesting insight revealed by U.S. patent 2016/0084158 is Mazda’s intent to twirl its rotary engine 180 degrees about its longitudinal axis. This is feasible because, just like in piston engines, the internal components are happy converting air and fuel to torque and power no matter what their orientation.
The main reason for this rotation is packaging. Looking at the front of this engine (the view at the top of this post), moving the exhaust ports and the turbo from the bottom-left corner to the top-right clears space for the front chassis cross member and suspension components while allowing the engine to be mounted lower in the car.
This drawing from Mazda’s patent application shows the engine from the left side; note the exhaust system now sprouts from the top of the engine. The image at the top shows the SkyActiv-R from the front.
The top location shortens the length of the exhaust system so minimal heat is squandered before flow reaches the catalyst. There’s also more space available at the higher altitude for a larger turbocharger; don’t be surprised to see a bump in the hood of a production version of the RX-Vision. This reorientation also improves life at the intake side. Moving the intake ports from the top-left to a bottom-right location enables longer runners. These provide a ram-tuning benefit, augmenting torque at low rpm before turbo boost arrives.
Unfortunately, Mazda’s patent application offers no hint as to how engineers will remedy the rotary’s poor thermal efficiency. This is caused by the combustion chamber’s high surface-to-volume ratio, which is greater than any contemporary piston engine. Excessive surface area drains heat energy into the cooling and lubrication systems, diminishing what’s available for producing power.
Nevertheless, the rotary rumor mill reports that the RX-Vision is moving from the static concept to the working-prototype stage with production intent. Our agents are standing guard at the patent office to see what’s next for Mazda’s rotary.
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