Mazda's SkyActiv X... uses spark-controlled knock! (maybe)
 

Figured some here would get a kick out of this article:

Nerd Alert! Mazda

The real meat, however, is on the final page:

In other words, instead of creating enough pressure via compression to cause auto-ignition, the engine can create *almost* enough pressure via compression, and use traditional spark ignition of a very tiny amount of air-fuel mixture concentrated around the plug (basically, the way all direct injection engines do now to lean-burn) to push the mixture over the edge.

Very speculative, but I like the idea. Sort of the combustion equivalent of how we race in the rain... don't wait for the car to slide, push it over. Makes it more controllable.

Betcha this makes the calculations much simpler than in other HCCI systems.

That might be safer to operate with a wider range of octane ratings, or with ethanol, and also gaseous fuels such as CNG and biomethane. Honestly, even though gasoline and regular Diesel fuel are eventually going to be outdated at all, it doesn't seem so likely that the internal-combustion engine would have the same fate in a mid to long term.

Very interesting article... He gives a good explanation of a lot of their existing technology (I hadn't heard about their SPCCI engine before) and how it is likely leveraged with the HCCI engine.

Unfortunately, the real interesting bit is just a lot of educated speculation. Anyone closely following this engine would have already made most/all of the same assumptions. I'd be very interested to get more details directly from someone at Mazda.

They're very tight-lipped about it. Very frustrating!

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As Dave Coleman works at Mazda and is a good friend of the people at MotoIQ, I reckon they've probably run these guesses past him, so they may be very good guesses.

Engineering explained on the tube provided some very good insights into the process. Controlled knock and works best with 80 octane with 16:1 compression.

I wonder if they'd lose their ability to do this if you filled them with E30 or 93 octane.

That’s where my thoughts went to as well. Then I wondered if a little desiel to lower the 87 octane down would be possible.

I smell Honda's CVCC engine from the 1980's.

If you guys watch engineering explained videos he has a good contact with mazda. Hes commented multiple times saying the engine will have a peak efficiency of 43% vs the current engine at around 35%, but average thermal efficiency should be way up. And as all of us racing types know its avhp and avtq over the operating range that wins, not peak horsepower with no meat. Should be interesting. I forsee a couple numbers up on the highway but a nice jump in the city mileage. They also noted that it may not sound like a huge change but when you actually hit the gas it should have a huge advantage in efficiency over your typical prius which has a low of 2% thermal efficiency.
Next gen engine after the X is rumored to hit 56% from Mazdas very optimistic mouth so fingers crossed. This is more beneficial to saving energy than electrification if more manufacuters can get their lineups to start utilizing 50%+ efficient engines.

As you mention, average efficiency is more relevant than peak efficiency. I'd be curious what the average efficiency is over a standardized test cycle, and how it compares with other engines.