Tokio Ishino, staff manager for powertrain development at Mazda, would rather the power plant not even be called a diesel, referring to it as a “new combustion mechanism”.
The engine meets stringent US exhaust emissions standards – traditionally a sore spot for European and Asian automakers hoping to offer their diesels in the US. It passes EU emissions restrictions that will not be enacted for another few years. It will have other manufacturers looking at their own diesels and wondering why they have to plug up the exhaust with pricey, energy-sapping equipment to be street-legal in the US.
Mazda has achieved this diesel landmark thanks to what Masaki Kodama, deputy programme manager for the vehicle development division, called a “fundamental change in engine design”. The unit achieves a remarkably low compression ratio – what Mazda said is the world’s lowest for a passenger-car diesel engine.
Compression ratio is the difference between the highest volume at any given time in a combustion chamber and the lowest. Essentially, it is a function of how much room the chamber’s piston occupies at the height of its stroke. High compression is good, but only to a point. Too high and the detonations fall out of line. This leads to “knocking” in a gasoline engine. In a diesel, it means smoky, toxic exhaust gases.
In the Mazda 6, Ishino explained, a low compression ratio allows perfect ignition timing. Combustion only happens when fuel and air are fully mixed in the chamber before they ignite. There are consequently no oxygen deficiencies and no fuel hotspots – the factors that, respectively, cause NOx and soot. Optimised combustion means the Mazda 6 would meet US particulate-emissions regulations without filters. The engine is, quite simply, clean.
In normal diesel engines, Ishino said, a higher compression ratio is needed for cold-weather starts and to ensure smooth operation as they warm up. Mazda’s Skyactiv-D cures this with ceramic glow plugs to start it up and variable exhaust valve lift, which allows hot exhaust gases to circulate and warm the engine more quickly.
Already on sale in Europe, the motor is a marvel. With a two-stage turbocharger and high-accuracy Piezo fuel injectors, it delivers either 150 horsepower or 175 horsepower, depending on specification, with knock-free, rattle-free refinement. It also revs to 5,200rpm, higher than most diesels.
The difficulty, and frankly, the delay for US sales relates to the quality of the fuel. “The cetane number is low, which means the fuel itself has less ability to auto-ignite,” Ishino said. The challenge for Mazda is to ensure the engine starts up without difficulty and still controls soot and NOx in such conditions, because the diesel formulation in the US will not change for the sake of accommodating a clever engine.
“This has required additional development,” Ishino added. That is why Americans can currently buy a 2.5-litre gasoline Mazda 6, but not a 2.2-litre diesel. But it is on the way, scheduled to arrive in the second half of 2013 for the 2014 model year – again, with no additional NOx aftertreatment. It will burn fuel as cleanly in the US as it does in Europe.
Mazda has even taken the technology racing, bringing it to the Rolex Grand-Am series. Putting out 400 horsepower, more than half of the racing engine is comprised of components lifted directly from the passenger-car variant. And the racing sedans, which saw action on at the Rolex 24 Hours of Daytona on 26 and 27 January, do not require exhaust aftertreatment, either.
To be fair, all three sedans retired on their debut in the Rolex 24 with varying degrees of engine trouble. John Doonan, director of Mazda Motorsports, said in a statement that the race was “a learning experience, a data point if you will”. Mazda has since expressed that this is part of a long-term commitment to diesel in the US, hurdles and all.
In creating Skyactiv-D, Mazda has put diesel on as level a playing field with gasoline as it has been in quite some time, all while retaining the technology’s advantages in the areas of low-end grunt, durability and fuel economy. You do not have to say you are driving diesel, and nobody will guess, either.
Whether US consumers tune in to the revolution, of course, remains to be seen.
Already on sale in Europe, the motor is a marvel.