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Driving Mazda's Next Mazda 3 with Its Skyactiv-X Compression-Ignition Gas Engine


Mazda had a simple explanation for why we had flown to Germany to drive its made-in-Japan next-generation Mazda 3 hatchback in prototype guise: Such early prototypes are allowed to roam German roads, as they are not in Japan, at least not without passing rigorous inspections. Oh, and Mazda has a technical center in Germany with engineers as eager as we were to experience and wrap their heads around the new 3’s Skyactiv-X compression-ignition-capable inline-four gasoline engine. So that is how we came to pilot several matte-black Mazda 3 test mules powered by Skyactiv-X engines and wearing decoy current-generation bodywork—and lacking airbags, stability control, and fully operational air conditioning—on a humid summer day in Frankfurt.

X Marks the Spot

For an in-depth rundown of how the Skyactiv-X engine works, you can grab a shovel and dig through our explanation. The shorter version is this: Mazda wanted to build a Homogeneous Charge Compression Ignition (HCCI) gasoline engine, which can combust ultra-lean gasoline/air mixtures for big efficiency gains, but found it too volatile and difficult to manage. Understandably so, given that HCCI engines use high compression ratios to achieve ignition without spark plugs. That requires fine control over the air/fuel mixtures and temperatures inside each cylinder to work properly. Transitioning between lean compression ignition and richer spark ignition, a necessity given the former’s intolerance for higher engine speeds and high-load situations such as acceleration, only compounds these issues.

Instead, Mazda achieves compression ignition using a spark plug as a combustion control. This generated a new acronym: SPCCI, for Spark Plug Controlled Compression Ignition. It enables the Skyactiv-X to use extremely lean fuel mixtures like an HCCI engine—the sort that are so lean they can’t be combusted via spark, only by compression—but it does so over a much broader swath of driving scenarios, including under moderate load and higher engine speeds. When operating with compression ignition (CI), Skyactiv-X mixes air and fuel during the intake stroke (as it does during spark ignition, only much leaner) but then injects a second dollop of fuel just before the power stroke and ignites it using the spark plug. The flame created spreads out and down while also raising the cylinder pressure high enough—along with the compression from the piston—to combust the lean primary air/fuel mixture.

A small Roots-type supercharger further expands the CI operating window by feeding more air into the engine, leaning out the mixture sufficiently for compression ignition even at higher engine speeds. Mazda borrows the turbocharged CX-9’s cooled exhaust-gas recirculation (EGR) setup to keep combustion temperatures in check, and uses a high-pressure fuel injection system to help each combustion cycle’s second injection of fuel to atomize more thoroughly, avoiding soot buildup from incomplete burn.

Most surprisingly, at least in prototype form, the Skyactiv-X is pretty much a modified Skyactiv-G engine like that in the current Mazda 3. Only the pistons are notably different, and where the G has variable intake-valve timing, the X adds it to the exhaust side to enable finer control over valve events. This should keep costs nearer to the Skyactiv-G than to Mazda’s pricier 2.2-liter Skyactiv-D diesel engine, which requires expensive emissions equipment and a sturdier block design.

Mazda is predicting about 188 horsepower and 170 lb-ft of torque from the 2.0-liter Skyactiv-X. Today’s 2.0-liter Skyactiv-G engine, as fitted to the 3, makes 155 horsepower and 150 lb-ft of torque, putting the X on a similar plane as the uplevel 3’s 184-hp 2.5-liter four. Mazda further claims the X will be up to 30 percent more efficient than the smaller Skyactiv-G. We’ll try to confirm those claims when we eventually test a production-spec Skyactiv-X in about two years when the next-gen 3 reaches dealerships.

It’s . . . Alive!

In its current state of tune, the prototype engine is functional. That’s noteworthy only because the SPCCI tech is so unique that its working didn’t seem like a given until we had keyed the ignition for ourselves. Mazda’s engineers are operating on a more ambitious plane, however, and are adamant that they didn’t want the X’s efficiency to come with any compromises that would threaten the brand’s zoom-zoom image. They’re on the right track, because the X is plenty responsive and feels about as powerful as the current Mazda 3’s larger 2.5-liter four.

We had assumed the X’s zeal might sag during CI operation, with deadened responses like those felt in hybrids that can briefly run solely on their relatively weak electric motors. That does not seem to be the case, and the engine is relatively tractable running under compression ignition. Should your acceleration needs outpace CI’s operating parameters, the X switches to conventional spark ignition, the faintest hint of supercharger whine dancing among engine noises familiar to anyone who has driven a four-cylinder Mazda recently. Even under compression ignition, the engine is smooth and quiet; the engine behaves so similarly in each ignition state that it takes concentration to detect which one is in play. Traverse a smooth road at lower speeds in CI mode and listen closely enough, and you can just make out a diesel-like prattle. At higher speeds or on coarse road surfaces, you’ll never detect it.

Because the X is more likely to enter CI during lighter-load use and can stay in that ultraefficient mode even at higher rpm, we’re told selecting lower gear ratios has little effect on fuel economy. During our drive in the manual-transmission prototype, we could leave the shift lever in fourth or fifth gear instead of sixth and stay in CI while taking advantage of the engine’s sharper responses to dice through city traffic.

So far, the switchover from spark ignition to compression ignition is the only issue needing attention outside of final, production-ready engine calibration. The process is seamless, with zero vibrational cues that the X has chosen an ignition type and fully committed to it. However, it is audible, thanks to copious knock (detonation) that hangs around any time the engine isn’t fully in a single ignition setting. During clean transitions from spark to compression ignition or back, the X emits its stuttered knocking soundtrack for about a second or two. However, whenever the computers move close to one ignition state from the other, the knocking drags on. This condition, in which fuel mixtures aren’t quite set for the desired state of ignition, occurred whenever the engine was spinning at low rpm—a CI-favorable condition—but loads were moderate, favoring spark ignition.

According to Mazda, there’s nothing inherently bad about this knocking phenomenon. It’s a necessary concession to the complex, on-the-fly switchover between ignition types. The automaker plans to mask the sound, likely using insulation around the engine to make it less noticeable.

Stifling the X’s weirder noises will be critical, because like the latest CX-5, the new 3—even in preproduction guise—is noticeably more hushed than its predecessor. A steady 115 mph on the autobahn sounded like 80 mph in our recently departed 2015 Mazda 3 hatchback. The interiors of the cars we drove were entirely unfinished and lacked trim, so no judgments can be made about quality or design. Our limited exposure to fast, curvy roads during our drive hinted that the new 3 is just as satisfying to drive as the current model, with the benefit of a more composed ride and more refined wheel control on bumpy surfaces. The steering is still a joy to use, although final calibration of the chassis systems is ongoing. In short, the 3 feels every bit like an evolution of today’s 10Best Cars–winning model. Only now, in addition to being fun to drive, the next 3 promises better fuel economy, power, and an available working compression-ignition gasoline engine. We only have two years to decide which feat is more impressive.