2012 Mazda3 Skyactiv achieves 40 mpg without stop/start

  • 28-Oct-2011 09:44 EDT
aei- SKYACTIV-G Powertrain1.JPG

The Skyactiv-G engine for the U.S. launches with 12:1 compression ratio. A 13:1 version will follow in the upcoming CX-5 crossover. A 14:1 engine is aimed at the Europe and Japan markets. An electric cam phaser was deemed essential for valve control.

Mazda’s zoom-zoom has met the 2016 U.S. federal Corporate Average Fuel Economy (CAFE) standards, and the result is a happy marriage called Skyactiv-G (gasoline). For many years the automaker has sacrificed some fuel economy to produce its performance-tuned line of vehicles. But with the forthcoming 35.5 mpg (6.6 L/100 km) fleet average regulation, and no hybrids for the U.S. market in the near term, Mazda needed an engineering approach that increases fuel economy without sacrificing its signature driving experience.

The 2012 Mazda3 with the optional new powertrain will carry U.S. EPA fuel economy ratings of 40 mpg (5.88 L/100 km) highway, and 28 mpg city (8.4 L/100 km). These are improvements of 21% and 16%, respectively, compared with the car's current powertrain. Horsepower and torque are also increased, from 148 hp (110 kW) to 155 hp (116 kW), and from 135 to 148 lb·ft (184 to 201 N·m). The Skyactive-G is calibrated for regular-octane fuel.

The company can do some things others cannot. Because of its relatively small size and tightly focused model line (it doesn't make large trucks, for example), Mazda currently doesn't need hybrids to meet U.S. CAFE. It has a hybrid technology agreement with Toyota and reportedly will have a Japanese-market hybrid in 2013. The company is thus freer to engineer very specific vehicles and follow the unique-technology path that has defined much of its history.

Skyactiv-G appears conventional: a long-stroke engine with high compression and direct injection bolted to an all-new six-speed automatic transmission with a converter clutch. But a look inside both parts of the powertrain for U.S. models shows some interesting new engineering. And with careful attention to the cost-value equation, this version of Skyactiv avoids the start/stop system, which some observers had considered the engine's most innovative aspect. [See Skyactiv development feature, AEI Feb. 1, 2011 p.28-29.]

Going long stroke 

The gasoline engine still is 2.0 L, but it has been changed from an oversquare 87.5 x 83.1 mm bore and stroke to a long stroke 83.5 x 91.2 mm. Compression ratio is increased from 10:1 to 12:1 in the U.S. and 13:1 in Japan. Both engines are compatible with regular-octane fuel. A premium-octane, 14:1 compression version is available in Europe and Japan.

All Skyactiv-G engines have high-pressure (2900 psi/200 bar) direct injection with six-hole injectors that can be pulsed twice per cycle—once at the bottom of the intake stroke and once at the top of the compression stroke.

The smaller bore and smaller combustion chamber contribute to faster combustion, but there also are additional features to make the engine knock-resistant. They include an evaporative cooling effect from the first injection pulse; a piston crown with a diesel-inspired combustion cavity to help maintain fast combustion; and a 4-into-2-into-1 exhaust manifold to purge hot exhaust.

The 4-2-1 exhaust manifold will not be used on a U.S. market model until next year, when the larger CX-5 crossover is imported, as the manifold will not package into the Mazda3. Like the Japanese market model, the 4-2-1 manifold will enable the CX-5 version of Skyactiv to have the 13:1 compression ratio and run on regular-octane fuel.

The variable intake and exhaust valve timing contribute to the torque improvement, and the Skyactiv engine has an all-electric intake camshaft phaser—an expensive but precision piece of hardware that was introduced by Lexus.

The valve timing switches into late intake valve closing instead of closing the throttle on deceleration, when the vehicle is at near-zero load. This not only reduces engine pumping losses but also effectively lengthens the stroke and wrings the last bit of power from the power stroke (basically a Miller/Atkinson cycle effect). Engines with hybrid electric or supercharger assist, such as the Mazda Millenia produced from 1993-2003, get the fill-in power that is missing at the end of the Miller/Atkinson power stroke.

However, because there is no meaningful load to overcome on deceleration, the Skyactiv engine just keeps running until the electric cam phaser instantly re-establishes the valve timing needed for loaded operation.

A long-stroke engine is subject to increased piston friction as rpm increases, hence its usual tuning for low-end torque. However, the Skyactiv-G was subjected to Mazda’s “gram strategy” approach (make every directionally correct step, even if very small), re-engineering to take out weight and improve manufacturing of components to reduce friction and drag throughout the engine.

Mazda cites a 74% reduction in oil pump drag; 31% in water pump drag; 25% in reciprocating drag (from lighter pistons and lower-tension piston rings, which compensate for the longer stroke); and 50% reduction in valvetrain friction.

Start/stop and diesel

The Skyactiv start/stop system had appeared outwardly to be far less expensive than the alternatives, because it uses the alternator load and a special crank-angle sensor to help stop the engine in a position so precise that a hot restart would occur solely with the enhanced fuel injection, not a special starter. However, it still requires a heavy-duty starter that if necessary could “bump” the engine if it stops just short of that precise position.

The start/stop system also needs the backup of a second battery, so battery failure would not leave the car unable to restart in a critical situation. Therefore, overall cost is not insignificant, a Mazda engineer said, estimating it at $200 per unit.

At this time, however, start/stop has virtually no effect on CAFE because the EPA driving cycles include few stops long enough to make a difference. Any company using the system on U.S.-market cars is trying to improve “real world” fuel economy, estimated at up to 5-10% in “city” driving, depending on the amount of stop-and-go operation.

As detailed in the Feb. 1 AEI article, Mazda also has a Skyactiv approach to diesels, featuring a unique “low-compression” design (just 14:1). However, the Skyactiv-D is not yet ready for production. The company's existing 2.2-L diesel available outside the U.S. is so close in BTU-compensated fuel economy to the Skyactiv-G that it likely would not be imported.

All-new six-speed automatic

The six-speed automatic, Mazda’s first all-new transmission since 1983, is a planetary type with a torque converter. However, it was designed to match a dual-clutch transmission (DCT) in key aspects, including fast, crisp shifts. The five-speed automatic remains available on non-Skyactiv models.

The Skyactiv torque converter operates between 0 to 5 mph (0-8 km/h), which makes parking and other low-speed operations easier for the driver to modulate. Then, a multi-disc clutch in the converter engages, and the transmission has almost the efficiency of a DCT or a manual. Under higher load, the low-speed converter clutch engagement can result in more vibration, and Mazda has incorporated a large damper to absorb it.

Mazda also has been able to design the Skyactiv transmission so it achieves very fast shift speeds that on downshifts match or even exceed a DCT. This was done with short hydraulic circuits, a “gram strategy” approach to minimizing the tolerances that affect shift consistency, and connections to a hydraulic-electric-electronic module called the “Mechatronic.”

Each solenoid valve is individually calibrated to compensate for any production variations that remain. Mazda tests showed that all downshift times were within the engineering targets and with one close exception, actually faster than a competitive DCT. Of course a DCT does maintain an upshift advantage, but it’s the downshifts that take longer.

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