Mazda creates the holy grail of gasoline engines - HCCI SkyActiv-X

Mazda may have solved the mystery of HCCI engines with their latest engine to be introduced in 2019, SkyActiv-X. The engine uses a spark plug to overcome the challenges generally associated with HCCI engines. The engine will be capable of running on compression only, or using a spark plug to control timing. Compression ignition gasoline engines are 20-30% more efficient, yet have much cleaner emissions than diesel engines as they burn cooler, limiting the amount of NOx.

From Mazda's Press Release regarding SkyActiv-X:
"Technological innovations
- SKYACTIV-X is the world’s first commercial gasoline engine to use compression ignition, in which the fuel-air mixture ignites spontaneously when compressed by the piston
- A proprietary combustion method called Spark Controlled Compression Ignition overcomes two issues that had impeded commercialization of compression ignition gasoline engines: maximizing the zone in which compression ignition is possible and achieving a seamless transition between compression ignition and spark ignition.

Features
- This new proprietary combustion engine combines the advantages of gasoline and diesel engines to achieve outstanding environmental performance, power and acceleration performance.
- Compression ignition and a supercharger fitted to improve fuel economy together deliver unprecedented engine response and increase torque 10–30 percent over the current SKYACTIV-G gasoline engine.3
- Compression ignition makes possible a super lean burn4 that improves engine efficiency up to 20–30 percent over the current SKYACTIV-G, and from 35–45 percent over Mazda’s 2008 gasoline engine of the same displacement. SKYACTIV-X even equals or exceeds the latest SKYACTIV-D diesel engine in fuel efficiency.
- With high efficiency across a wide range of rpms and engine loads, the engine allows much more latitude in the selection of gear ratios, providing both superior fuel economy and driving performance."