Spark of Genius

Mazda’s Skyactiv-X—the nexus of gasoline and diesel tech—remains on track for 2019 production. We test-drive recent prototypes to check development status.

March 1, 2018

Don Sherman

Mules are logging development miles in Europe and the U.S. (Image: MAZDA)

In 2016, after six frustrating years of R&D invested in an ultra-lean-burn, high-compression gasoline engine, Mazda’s managing executive officer, Mitsuo Hitomi, 63, had this brainstorm: Adding spark ignition might help stabilize the new Skyactiv-X engine’s fickle compression-ignition combustion process. Like at least four other OEMs, Mazda considers the marriage of gasoline and diesel attributes to be the next and possibly last opportunity to reap significant efficiency gains from internal combustion.

What’s most commonly known as Homogeneous Charge Compression Ignition (HCCI) is the IC engine’s holy grail. Unlike conventional IC engines, where a flame front initiated by a spark spreads throughout the gasoline-air mixture, ignition in an HCCI engine is instantaneous — like all of a city’s street lights illuminating at once.

Exhaust side of an SpCCI engine shows belt drive to Eaton Roots-type supercharger at upper right. (Image: MAZDA)

While other makers have slowed further development of the ICE to devote greater resources to electrification, Mazda is hesitant to cease development of propulsion technology that has served it well. From the Skyactiv efficiency initiative launched in 2007, Mazda engineers set two ambitious goals: a 50% reduction in CO₂ (from the 2010 baseline) by 2030 with combustion engines shouldering most of the load. And, by combining more efficient engines with electrification, a 90% CO₂ reduction by 2050.

Fuel-consumption-rate plot showing Skyactiv-X vs. similar displacement Mazda gas and diesel engines. (Image: MAZDA)

For the small but feisty Hiroshima-based automaker, revolutionizing 19th-century inventions by Nikolaus Otto and Rudolf Diesel would be a monumental feat. Accentuating that goal, the X in the Skyactiv-X name coined last year stands for the intersection of gasoline and diesel attributes. The program remains on track for 2019 production and Mazda has kept Automotive Engineering apprised of development progress at two dedicated events since summer 2017, including mule-vehicle test drives (see sidebar). Hitomi-san’s development team has also published SAE Tech Papers on their work (https://www.sae.org/publications/technical-papers).

Like every other 21st-century attempt to ignite a homogeneous mixture of gasoline and air without a spark, Mazda’s approach creates a lean, cool blend during the intake stroke. A higher-than-normal (currently 16.0:1) compression ratio raises the charge’s temperature and pressure to the point of auto ignition as the piston nears the top of its stroke.

Then the trouble begins.

If the air-fuel mixture lights prematurely, energy in the form of pressure against a rising piston and heat transferred to surrounding engine components is wasted. When ignition is tardy, with the piston well past top-dead center, a significant portion of the expansion (power) stroke is squandered, also greatly diminishing efficiency.

Mitsuo Hitomi leads the Skyactiv-X engineering team. (Image: MAZDA)

Hitomi-san realized that a well-timed spark could be perfect for triggering compression ignition. What he and his engineering team have developed (they call it SpCCI—Spark-Controlled Compression Ignition) during the past two years in hopes of a production start sometime in 2019 is three engines in one. In high-load and -rpm circumstances, Skyactiv-X operates like a conventional high-output spark-ignition gasoline engine with a stoichiometric (14.7:1) air-fuel ratio. The second mode also is stoichiometric, but with compression- ignition triggered at the right instant by a pressure rise in the combustion chamber resulting from the spark ignition of a locally rich air-fuel mix. Mode three also uses a spark to trigger compression ignition, but now with ultra-lean (λ up to 2.5) mixtures.

Although the Skyactiv-X’s basic architecture continues Mazda’s 2.0-L engine fundamentals, key features have been added. The direct fuel-injection system currently operates “between gasoline and diesel levels” according to Mazda engineers; AE has previously reported a 500 bar (7250 psi) operating pressure (see October 2017 issue). One squirt is delivered during the intake stroke and a second injection occurs just before the sparkplug fires. The engine is significantly under-square (bore and stroke dimensions remain proprietary) to diminish combustion-chamber volume.

Electric actuators provide variable intake and exhaust valve timing. The intake system is enhanced with the addition of an Eaton-supplied Roots-type super-charger with a clutched belt drive to allow switching between normal and pressurized aspiration. An engine coolant heat exchanger diminishes the temperature of recirculated exhaust gas (EGR). A belt-driven motor-generator wired to a 24-V battery powers this engine’s stop-start feature and its ECM. A three-way catalytic converter and a particulate filter treat the emissions leaving the combustion chamber.

Swirl motion within the Skyactiv-X cylinder assists fuel vaporization and creates a consistent air-fuel mixture. The piston crown has a volcano-shaped bowl to tumble the rich mixture created near the spark plug.

While these features have been seen before, Mazda’s Skyactiv-X breaks ground with the addition of a pressure sensor in each cylinder and the use of a powerful, high-speed processor to control each and every combustion event. Details of that equipment’s speed and cost have yet to be revealed. Exact mpg and CO₂ details also are pending.

What is known is that current prototype engines deliver 178 hp (133 kW) and 170 lb·ft (230 N·m) with 6000 rpm within easy reach on 87-octane regular gasoline. To support the three combustion modes, Mazda uses valve overlap at the end of the exhaust stroke to scavenge the cylinder of hot gasses. When necessary, closing the intake valves well after BDC on the compression stroke inhibits full-load detonation (Miller cycle). Heavy doses of EGR reduce throttling losses and the supercharger aids full-throttle performance.

Mazda engineers also are exploiting a ‘gram strategy approach’ to curb the new engine’s mechanical losses. Claims thus far are 31% lower water-circulating loss; a 74% reduction in oil-pumping loss; 25% less reciprocating loss; a 54% more-efficient valvetrain (using roller-finger followers and hydraulic lash control), and 27% lower accessory drive belt losses.

Achieving the program’s near-term goals of more power with a 30% gain in efficiency indicates that genius is indeed present under the hood.