Mazda SkyActiv-R rotary could use compression ignition

Dropping spark plugs in favour of compression ignition technology could be key to the rotary engine’s comeback. 

Mazda surprised everyone at last week's Tokyo motor show by confirming the return of the rotary engine, but a conspicuous lack of technical details left several questions about how the brand will meet modern requirements with the new SkyActiv-R engine.

Until the covers were pulled off the RX-Vision concept, it looked as though we may have seen our last rotary Mazda with the RX-8 that was killed off in 2012, given the Wankel engine design's intrinsic efficiency, emissions and reliability compromises.

The only hope looked to be as a range-extender for a petrol-electric hybrid, but this wouldn't be enough to excite fans of the classic RX models that often triumphed over cars with much bigger engines on the racetrack.

Allan Moffat even won the 1983 Australian Touring Car Championship with an RX-7, but the rotary's biggest international moment came with the quad-rotor 787B's outright victory in the 1991 Le Mans 24 Hour.   

Playing the role of a range extender is still on the cards, but Mazda's global R&D boss Kyoshi Fujiwara confirmed his plan for the new SkyActiv-R engine to stand on its own after the RX-Vision's unveiling. 

"I want to introduce new rotary without electrification first," Fujiwara said.

"If I introduce with both, people will say electrification helped the rotary engine," he added.

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Mazda recognises the engineering brownie points associated with being the only brand to successfully market the rotary engine – despite significant efforts from the likes of NSU, Mercedes-Benz and General Motors – and plans to do it all again with its industry-defying SkyActiv design and efficiency philosophy.

Fujiwara-san dismissed the idea of a diesel rotary, but revealed that SkyActiv-R will likely see the return of turbocharging. "Turbocharger is one of the big contributor for future rotary engines," he said.

HCCI relies on extra-high compression to ignite fuel,eliminating the need for spark plugs altogether.

The most recent Renesis-generation rotary that powered the RX-8 until 2012 was turbo-free aside from the limited-production hydrogen models sold in Japan and Norway. However 13B and 20B engines that preceded it in the FD RX-7 and JC Cosmo both used twin-sequential turbos to overcome the engine's intrinsic torque deficit at low revs.

The SkyActiv-R's biggest gain will likely revolve around the way it burns fuel, with Mazda developing unique methods for analysing and measuring the combustion process. Fujiwara explained that regular piston engines can be analysed with standard techniques, but the lack of such tools for rotary engines has hampered their efficiency in the past.

Previous rotary engines have also been compromised by the need to locate the spark plug outside the combustion chamber to allow the rotor to sweep past, but Fujiwara confirms this has been solved for the SkyActiv-R.

Considering the high compression ratio used by rotaries in general, this seems a natural fit with the homogeneous charge compression ignition (HCCI) expected for the second-generation SkyActiv 2 technology due in 2020. Like diesel engines, HCCI relies on extra-high compression to ignite fuel,eliminating the need for spark plugs altogether and reducing consumption and emissions significantly.

Fujiwara declined to officially confirm the move to compression ignition, but his wry smile response hinted this suggestion isn't far off the mark.

Given the likelihood the production RX-Vision will arrive as part of Mazda's 100th birthday celebrations in 2020, the incorporation of such landmark technology would make a lot of sense.