Why would you guess that? The Mazda engine appears to be an SI gasoline fuel engine that has HCCI capability under certain operating conditions. The HCCI combustion process is notoriously difficult to control even under the best circumstances, so I'd speculate this Mazda engine will only use HCCI under light load operation. Fortunately, production auto engines spend much of their time operating at light load. In general, the very rapid heat release rate characteristics of HCCI combustion results in higher peak cycle pressures. But limiting HCCI combustion to light load operating conditions should keep cylinder pressure levels within the capability of modern automotive aluminum blocks/heads.
The current Skyactiv-G engine block is aluminum. Honda diesels (not sold in North America) use aluminum blocks. I don't think this will have peak cylinder pressure as high as a turbodiesel does.
Wonder what it's going to sound like? The nature of HCCI combustion ought to make it sound like the worst case of detonation you've ever heard ... unless they've managed to control the combustion rate somehow.
Detonation is combustion front travelling at mach 1, hence the pressure front builds as combustion progresses - forming a shock wave which is capable of applying a large and rapid spike of force to the chamber wall.
HCCI produces a myriad of tiny shock waves originating from all points in the chamber - there is no chance they will arrive simultaneously at the chamber wall.
The summary I read said they don't run HCCI at full load and retain spark plugs to aid combustion when lean, so it's clearly a partial load operating mode. So is every other HCCI I've seen mentioned by the majors in the DOE HCCI Consortium. Kudos to Mazda. They are small but have one of the best profit margins in the business while sustaining a very productive engine R&D effort. They were also issued a new patent on the Wankel and have said it will be HCCI capable.
