The Best of Both Worlds

New pistons in Mazda’s revolutionary new crossover engine

The SKYACTIV-X from Mazda is the world’s first serially produced gasoline engine to feature the possibility of compression ignition. It thus combines the advantages of gasoline and diesel engines. This groundbreaking innovation relies on pistons made by Hiroshima-based Japanese Rheinmetall subsidiary Kolbenschmidt KK (KSKK).

A crossover design, the SKYACTIV-X combines the advantages of a conventional gasoline-powered engine with the compression ignition of a diesel. Among the specific pluses of a gasoline engine are a larger useful rpm range and lower emissions, e.g. of nitrogen oxide. Compression ignition’s virtues include its sprightly responsiveness and greater fuel efficiency. Mazda’s Spark Controlled Compression Ignition (SPCCI), a special combustion process, makes this combination of the best of two worlds possible. Thanks to continuous operation with spark plugs, it enables a seamless transition from external ignition to compression ignition. This means that the engine can run on an extremely lean, homogenous fuel-air mixture.

Controlled self-ignition

The SPCCI combustion process has two major advantages over other procedures: Because the compression ignition is triggered by spark plugs, controlled self-ignition always takes place at the optimum point in time. As a result, fuel-saving self-ignition takes place in a broad load and rpm range, which significantly reduces fuel consumption. In addition, thanks to the spark plugs, the system can instantly switch over to external ignition in those rpm and load ranges where compression ignition isn’t possible. This makes sure that the compression ratio selected doesn’t have to be too high. This innovative technology enables a significant reduction in fuel consumption: compared to the current 2.0-liter SKYACTIV-G gasoline engine, SKYACTIV-X consumes 20 percent less fuel on average, and up to 30 percent less in the lower load range.

KSKK, a proven partner

For piston developers, this innovative combustion technique poses special challenges. However, Mazda quickly found the right partner, as Tadayuki Kuramoto, President and Member of the KSKK board, reports: “We weren’t picked just because Mazda appreciated our technological know-how. It was also because of our long-standing track record with regard to quality, costs, punctual delivery, service, and development. KSKK supported Mazda as early as the advanced development stage, and was ultimately selected to be the series development supplier in 2016.” One of the challenges facing the developers: Self-ignition causes vibrations akin to knocking, a problem that Mazda solved with its Natural Sound Smoother, a vibration absorption system. “Here, a so-called dynamic damper weighing about 50 grams is pressed into the piston pin and moves with the same frequency as the vibrations produced by SPCCI combustion,” explains Kuramoto.

Lighter pistons wanted

Because this special structure makes the piston pin relatively heavy, the piston itself has to be as light as possible in order to offset the piston system’s heightened inertial mass. “KSKK’s LiteKS-Plus piston turned out to be just the right system for this application. Combined with a suitable ring carrier, it provides Mazda with a robust, low-weight piston featuring high protection, which makes an important contribution to the overall success of the SPCCI system,” declares Kuramoto. Another challenge relating to an engine concept with a high compression ratio is the need to minimize variation in the compression ratio. For the piston, this meant that variation in the crown volume had to be kept to a minimum. KSKK achieved this through stringent, high-precision control of the crown shape and compression height.

Flexibility in production

Kuramoto has special praise for his colleagues on the factory floor: “Just one year before the start of production, Mazda decided to use a ring carrier in order to reduce wear and tear on the part. Despite having no experience in producing pistons with a ring carrier for gasoline engines, we managed to get ready for production in extremely short order, including the casting cell and processing line for manufacturing a piston of this type.”

A system with a future

Looking beyond the SPCCI process, Kuramoto sees further applications for the KSKK piston: “It features narrower tolerances in compression height and the crown profile resulting in lower volume vibration than conventional pistons. This makes it an ideal candidate for gasoline engines with high compression ratios that react sensitively to changes in this ratio.”