GTHO! Going In Depth With Mazda’s Brilliant Skyactiv X Engine – Technologue


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A quick Google search suggests I’ve written 15-plus articles touching on the topic of homogeneous-charge compression ignition. I drove a somewhat jerky, very diesely sounding GM prototype in 2007 and expressed sincere optimism for Hyundai’s GDCI riff on the idea in 2013. I’ve written epitaphs for these engines as the big car companies shuttered their research programs upon finding the benefits too low or the costs too high.

Read about our experience behind the wheel of a 2019 Mazda3 prototype with Skyactiv X engine tech here.

Now, seemingly out of nowhere, the Driving Matters folks at tiny, independent Mazda have announced plans to produce an engine that compression-ignites gasoline by 2020 . Mazda already has a prototype and invited me drive it. It felt darned near ready for prime time and even has a cool name: Skyactiv X.

The two “secrets” to Mazda’s success: The mixture is not homogeneous, and every compression-ignition event is initiated via spark plug. Hence they call it SpCCI, or spark-controlled compression ignition. Heywaitaminute, isn’t that cheating?

Nope, it’s brilliant. One of the major bugaboos stymying prior HCCI efforts was controlling the bumpy transition from spark ignition to compression ignition. The former demands a darned near perfect 14.7:1 air-to-fuel ratio and ignition timing that gets the fuel burning while the piston is still traveling upward. That way, peak combustion pressure arrives just after the piston starts descending again.

Compression ignition is more of an all-at-once burn of a mixture with lots more air (lean) that “auto-ignites” when the temperature and pressure reach a certain critical point—hopefully when the piston is at or just past the top of the cylinder. Sparks: easy to control; temperature/pressure: super tricky to control with more volatile gasoline.

Mazda’s CI hack is to run a compression ratio that can’t quite light off a lean gas mixture all by itself (expect 15:1 or 16:1). It then generates the critical temperature and pressure needed by spark-igniting a tiny locally rich mixture around the plug. That little, easily timed explosion creates the pressure needed to ignite the lean air-fuel mixture, and the pressure rise is more gradual, so you don’t hear that pinging/clattery sound of a diesel, and the engine block doesn’t need to be reinforced as much as the diesel Skyactiv D’s block.

The hardware needed to make it all work includes a small, clutched Roots-type compressor that can quickly provide the excess air needed for compression ignition. And by blowing into only half of the intake runners, it also induces the swirl needed to provide that rich mixture by the spark plugs. To combat soot formation, the direct-injection pressure is elevated to “about halfway between typical gas and diesel pressures” (figure 7,500 to 10,000 psi).

Meanwhile, electric cam phasers with over 70 degrees of authority are fitted to both cams to quickly swap between high valve-overlap timing for stoichiometric full spark-ignition operation (typically at higher speeds and loads) and low or no overlap during SpCCI combustion. Temperature and pressure sensors inside each cylinder and mild reinforcement of the cylinder block bedplate pretty much round out the roster of new stuff.

This new technology means particulate emissions are expected to roughly equal those of a Skyactiv G gas engine, and NOx production during cool SpCCI operation is low enough not to need a lean-NOx catalyst. Oh, and a belt alternator starter mild hybrid should smooth over any potential drivability stumbles.

Mazda claims the 2.0-liter Skyactiv X will meet or exceed the efficiency of the 2.2-liter Skyactiv D diesel engine, making it 20 to 30 percent thriftier than the 2.0-liter Skyactiv G at a price that falls halfway between those engines. And of course, because it’s a Mazda, driving fun will also increase, thanks to a 10 to 30 percent torque boost and shorter axle gearing for improved acceleration enabled by the engine’s broader peak efficiency island. That’s one island I can’t wait to zoom all over.

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