2014 Mazda6 will offer low-compression diesel

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  • Image: aei-mazda6-diesel-12-12.jpg
Image: aei-mazda6-12-12.jpg

2014 Mazda6 has a swooping hood and sculpted silhouette.

The 2014 Mazda6, featured first at the Moscow Auto Show then at the recent Los Angeles Auto Show, has more than a shapely body. Technologically, it's the first U.S. example of compression ratios “coming together” on gasoline and diesel engines, as both powerplants in the new model feature interpretations of the company’s Skyactiv technology. The introduction date for the gasoline model is January and for the diesel sometime in the second half of 2013.

The gasoline engine is Mazda’s large four-cylinder, a 2.5-L with a 13.0:1 compression ratio, and is rated at 184 hp/185 lb·ft (137 kW/252 N·m). It soon will be available in the CX-5 crossover, too. Mazda’s European Skyactiv models have compression ratios as high as 14.0:1 but require premium gasoline. The 2.2-L diesel, which also has a 14.0:1 CR, is rated at 310 lb·ft (422 N·m), and although full specifications (including power) have not been released, Mazda claims the diesel will improve fuel economy by 20%.

Low-compression technology

Low-compression diesels have become recognized as viable technology with several advantages in this era of stringent emissions regulations. The slower ignition that results will allow time for better mixing of air and fuel and, therefore, more complete combustion to reduce soot and NOx. In the case of the Mazda6, there’s no need for urea fluid injection aftertreatment to further reduce NOx.

The 2.2-L diesel, which meets European and U.S. clean diesel standards, has an aluminum block and other lightweight components, thanks to the reduced pressures from the lower CR. The diesel was re-engineered to be 10% lighter than the 2.2-L that Mazda sells in other markets. U.S. market diesels from other makers have compression ratios of over 16.0:1.

As it is, some diesels have been running with ignition at effectively lower compression by delaying injection until after top dead center (ATDC). High compression pressures and temperatures at TDC adversely affect emissions, particularly NOx.

A likely issue from lower compression is worse cold-start operation. But new ceramic glow plugs, piezoelectric injectors that can change spray pattern according to operating conditions, and variable exhaust valve lift permit the engine to operate well on a cold start, Mazda claims. The variable lift feature opens the exhaust valve slightly during the intake stroke, right after an engine cold start. So hot exhaust gas is drawn in, which raises intake air temperature to assist cold operation.

A two-stage turbocharger (one small, one large) provides torque boost throughout the range up to 5200 rpm, a specification Mazda did reveal. This doesn’t mean maximum torque is at that rpm but indicates that the horsepower peak rpm could approach that of a gasoline engine vs. traditional diesels.

Both engines will be bolted to a choice of a six-speed manual or automatic.

Mazda will be racing a performance version of the 2.2-L diesel—rated at 400 hp/298 kW and 450 lb·ft (612 N·m)—in the 2013 Grand-Am Series. Although this is sure to be some technical distance from the production engine, Mazda said 60% of the parts are stock.

"Regen" with capacitor

The Mazda6 is the first production vehicle sold in the U.S. to have a regenerative braking system that uses a capacitor. Called I-ELoop (Intelligent Energy Loop), it takes advantage of the capacitor’s ability to accept a charge quickly, and discharge quickly, too, so it can use the energy to power vehicle electrical accessories and, if necessary, recharge the car battery. The system has to be simple, because the Mazda6 doesn’t have hybrid components and/or a very large battery to accept and consistently use electricity from regenerative braking in the ways they do.

Actual fuel economy benefits haven’t been quantified because they are dependent on the percentage of stop-and-go driving. The Mazda6 alternator during regenerative braking can generate up to 25 V, which is the peak storage value of the capacitor. As the capacitor discharges, the electricity is stepped down to 12 V by a DC-DC converter.

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