Mazda Skyactiv-G and Skyactiv-D Engines in Detail

Around the world, automakers are grappling with the switches necessary to meet escalating fuel-economy regulations. To this end, Mazda is launching a fresh family of four-cylinder engines—fours power the vast majority of Mazda’s cars—called Sky-G (gasoline) and Sky-D (diesel). We drove both in prototypes of the next-gen Mazda six and, gratefully, either engine can be paired with the latest version of Mazda’s snick-snick six-speed manual.

We got the deep dive on the Two.0-liter version of the Sky-G, which will launch next year in the U.S., likely as part of a midcycle face lift of the Mazda 6. (A fully fresh six is a few years out yet and will be about one hundred forty pounds lighter than the current car, thanks to meticulous optimization of material thicknesses and mounting points.) In the future, there also will be variations in the 1.3- to Two.5-liter field, and Mazda has already signed a deal to license Toyota’s hybrid technology for a future Sky-based hybrid. Embarking from the ground up, Mazda has impressively leapfrogged its previous gas engine, to the tune of estimated EPA fuel-economy ratings in a Sky-equipped Mazda three of thirty mpg city and thirty nine to forty highway. That’s almost on par with VW’s Golf and Jetta diesels as well as best-in-class cars one segment smaller, such as the Ford Fiesta and Chevrolet Cruze. Here’s how they did it.

The Big Squeeze

Enlargening the compression ratio—in this case, to a staggering 14:1 from 11:1 in the current Two.0-liter (the U.S. version is Ten:1)—is a classic way to squeeze more work out of the piston’s power stroke. But it creates problems, too, because compressing the air/fuel combination this much causes excess fever build-up in the cylinder, which leads to premature auto-ignition, or knock. To keep the temperatures down, Mazda employs a earnestly lengthy 4-2-1 harass header, designed so that the hot harass gases don’t get pulled back into the next cylinder’s intake stroke. As it stands today, it doesn’t show up that the Sky could fit in a longitudinal application such as the Miata—the fat header likely would poke through a front fender.

Further improvements include the addition of direct injection and a reduction of warmth loss—too much warmth can be problematic, but temperature is a necessary byproduct of searing fuel, and squishing it all is inefficient. The heat-loss reduction comes from a smaller bore and a much more sophisticated piston form that features a cavity directly in the piston’s center, the hot area where the spark buttplug fires. Friction also has been diminished in the pistons, rods, and crankshaft (which is now forged steel instead of cast metal), and roller finger followers reduce it in the valvetrain. The engine uses 0W20 oil, which looks frighteningly like colored water. The Sky also gets dual variable valve timing, electronically varied (as opposed to using oil pressure) on the intake side, so that rapid adjustments can be made even during cold starts. Overall weight has been diminished by about fifteen pounds, including ten saved by thinning out the block where extra strength wasn’t needed.

Premium Fuel, Mid-Grade Output

Premium, 91-octane fuel is required for the Sky’s not-so-staggering one hundred sixty three hp at six thousand rpm and one hundred fifty five lb-ft at 4000, but Mazda is proud of its exceptionally broad torque band for enhanced real-world drivability. To enable running on regular gas, the U.S. version will have a compression ratio of 13:1, which means fuel economy and torque will diminish by about three to five percent, according to Mazda. The premium-fueled Sky we drove was ideally adequate in the Mazda six prototypes, albeit acceleration was rather leisurely—far slower than the current Mazda six with its 168-hp, Two.5-liter—giving us slew of time to wish for a bit more smoothness during the extended time in each gear. But being in the lighter Mazda three would help, and the tradeoff for near-diesel levels of fuel economy is very likely worth it.

Remarkably, Mazda is passing on today’s popular trend of downsized, turbocharged engines—say, a 1.4-liter turbo instead of this Two.0-liter. The company says the next generation of gasoline engines, which will employ HCCI (Homogenous Charge Compression Ignition)—essentially firing a gasoline engine like a diesel, without using the spark plugs—will erode the benefits of downsized engines. Smaller engines reduce pumping losses by operating at a higher flow (the throttle is open further) more often. In the same way, HCCI engines will have to flow more air to realize the fuel-saving, lean-combustion benefits of that cycle. Mazda claims that if it downsized the Sky family of engines they wouldn’t be able to flow enough air for HCCI without upsizing once again. Plus, as Mazda rightly points out, adding a turbocharger and an intercooler is fairly a pricey proposition.

On the diesel side, Mazda has pulled off an even more amazing feat. The Two.2-liter Sky-D (again, other sizes are likely to go after) boosts fuel economy by twenty percent over the current, Two.2-liter diesel and meets Euro six and U.S. Tier two Bin five emissions standards without using any NOx aftertreatment such as urea injection. You catch that? It meets U.S. emissions standards. That’s because Mazda is planning to bring this engine here sometime in 2012.

With the diesel, Mazda moved in the opposite direction, decreasing the compression ratio from 16.Three:1 down to 14:1. That’s the same as the gas-burning Sky-G, and a value that’s the lowest in the world among diesels, according to Mazda. Doing so reduces cylinder pressures, and therefore temperatures, which reduces NOx production and also permits the fuel to mix better, avoiding locally rich areas that produce soot. Mazda claims that the lessened friction from the diminished cylinder pressure alone is worth a 4- to 5-percent build up in fuel economy. And the diminished internal coerces also permit components such as the rods and pistons to be substantially lighter. Here, too, a forged steel crankshaft substitutes a cast-iron unit. Overall weight savings is a whopping fifty five pounds.

The downside to lowering the compression ratio of a diesel is that, during warm-up, the engine temperature can be too low to support decent combustion, and misfires result. To get around this, Mazda added a two-stage variable valve-lift system on the harass side in order to be able to create extra valve overlap. This causes the hot harass gases to be drawn back into the next cylinder to warm it up.

Other fresh features are a sequential twin-turbo arrangement—one puny and one large—which outperforms the old single, variable-geometry unit; 12-hole piezo injectors that disperse fuel into the cylinder in exacting quantities during two to eight separate injections per cycle at pressures up to two thousand nine hundred psi; and an harass manifold that’s downright integrated into the block. Here, too, fuel-economy claims are astounding: thirty one to thirty three mpg city and forty three mpg highway for a Mazda six with the Two.2-liter diesel. Does an over-40-mpg family sedan sound good to anyone else?

Output hits the gas engine in both regards: one hundred seventy three hp at four thousand five hundred rpm and three hundred ten lb-ft at 2000. Redline has been raised to a screaming (for a diesel) five thousand two hundred rpm, versus its predecessor’s 4500. And it felt notably quicker than the gas-engined car, pulling strongly via the rev range and exhibiting none of the run-out-of-breath feeling that afflicts some diesels as they wind toward the upper end of the tach. It’s exceptionally responsive, and quiet, too, with very little clatter, even when accelerating from engine speeds below one thousand five hundred rpm.

In addition to the sweet-shifting six-speed manual, we drove each engine with Mazda’s fresh Sky-drive six-speed automatic, which boasts a more aggressive lock-up clutch for the torque converter, leading to a 4- to 7-percent improvement in fuel economy. Albeit the calibration was admittedly early in development, the automatic was distinctly less epic than either of the fresh engines. In terms of feel, which Mazda claims is much more direct than before, it doesn’t seem to stand out from the current crop of high-tech automatics. The wide-open-throttle upshifts struck us as a bit lazy, too, albeit the downshifts were fairly prompt. We’ll stick with the manual, thank you very much. Few buyers do, however, which could mean bad things for Mazda’s sales.

Perhaps the best thing in all of this, however, is that Mazda’s outstanding engineering work proves that the internal-combustion engine still has slew of gams in our ever-more-regulated world.

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