Set to appear first with the fourth-generation Mazda3 in 2019, the new SkyActiv-X engine's effectiveness actually depends on pre-detonation or knocking, which higher octane petrols like 94 RON E10, 105 RON E85 and 95 RON and 98 RON Premium have been designed to resist.
SkyActiv-X technical research and control system boss Mitsuo Hitomi confirmed that in its ideal guise, the engine's ability to combust petrol through compression ignition will require Regular 91 RON fuel, and any higher octane rating will force the engine to revert to pure spark plug ignition like a conventional engine.
Hitomi-san suggested a partial solution is planned for markets like Europe where 91 RON fuel is scarce or unavailable, which would be supplied with engines using a higher 16:1 compression ratio to enable both compression and spark ignition functions to operate. This spec won't deliver quite the same efficiency gains as the 15:1 compression ratio version delivered to 91 RON-using markets such as Australia, however.
Hitomi-san explained that the ability to measure near-infinite parameters through advances in computer technology is the number one factor that will enable Mazda to produce such a system, with the SkyActiv-X ECU using dual-core processing and a 24 Volt electrical system instead of the usual 12 Volt setup. Also key is the pressure sensor required to accurately measure cylinder pressure during the compression cycle and quickly respond to changing conditions in what is a highly volatile combustion process.
Despite the accuracy required for the SPCCI process, Hitomi-san explained the SkyActiv-X engine will use conventional spark plugs in lieu of expensive bespoke parts, and that recommended service intervals will be no more frequent than existing models. The suggested oil will be no more exotic than that specified for conventional turbos, and the engine will continue to use a timing chain instead of a belt requiring regular replacement.
He also assured that the system has been designed to start reliably from temperatures as low as -30 degrees Celsius, but the engine will use spark ignition until it reaches operating temperature.