Compression ignition, key to Skyactiv-X, has eluded engineers for decades

Tiny Mazda Motor Corp. is betting that its out-of-the-box approach to powertrain engineering has achieved what rivals 10 times its size have not: A workable compression-ignited engine that combines the best traits of diesel and gasoline engines for powerful, yet ultra-clean performance.


The new engine — dubbed Skyactiv-X by Mazda and described by some experts as the "holy grail" of powertrain technology — is a risky yet necessary gambit for Mazda as it races to stay competitive on a shoestring budget as rivals invest heftily in costly electrified cars.

In unveiling the technology Tuesday, Aug. 8, Mazda said the new engine will boost torque by up to 30 percent and improve fuel efficiency by up to 30 percent over the current line of Skyactiv-G direct-injection engines Mazda began rolling out in 2011.

Mazda said it will deploy the Skyactiv-X engine in the fiscal year ending March 31, 2019, making it the first automaker to commercialize the technology. Compression ignition, which has eluded engineers for decades, is a critical next-step Skyactiv offering for Mazda as it confronts stricter emissions regulations with little help from electrification.

"People have called it the holy grail because it combines the best benefits of the gasoline and diesel worlds," said Dean Tomazic, executive vice president and chief technical officer at FEV Group, a powertrain engineering company.

"But it is not without its challenges or we would have seen it in the market already," Tomazic said. "People have been working on it for a long time. It's almost a running joke that whenever you ask when the technology will be ready for production, the answer is always in the next 10 years."


Mazda has a penchant for zigging while the industry zags. Mazda proved its aptitude in 1967 when it pioneered the revolutionary rotary engine and again more recently when it became an early adopter of lean-burning, high-compression combustion in its Skyactiv engines.

This time, competitors from Tokyo to Detroit to Wolfsburg are following conventional wisdom by investing in battery power. But Mazda is sticking to its ​ play book of improving the humble internal combustion engine.

"We will investigate every possibility in our quest to perfect the internal combustion engine and become the world's leading company in this field," Mazda CEO Masamichi Kogai said.

But Mazda has had false starts before, as the rollout of its clean diesel engine showed. Mazda finally announced last November that it would bring its Skyactiv-D diesel engines to the U.S. this year. But that came only after six years and three delays.

This time, Mazda is betting on compression ignition, which compresses gasoline to such high pressure that it ignites — as in a diesel engine — without the need of spark.

The new engine delivers torque and fuel efficiency on par with a diesel engine but without nasty nitrogen oxide or sooty emissions.

The key is cranking up the engine's compression ratio. Higher compression tends to improve fuel economy because it can achieve the same combustion temperature with less fuel.

The technology, known as homogeneous charge compression ignition, or HCCI, is a difficult nut to crack.

If the engine is revving too quickly, there is the risk of misfire because of the high number of revolutions. If it revs too slowly, it can misfire because of low temperatures.

Engine cooling is a challenge for engineers because of the HCCI's high temperature and pressure. Finally, the engines are more fickle about fuel and perform differently depending on fuel properties.

X is for crossover

Mazda's solution was a hybrid of sorts. Engineers were able to expand the range of engine speeds at which HCCI can work. But in ranges where it still has difficulty, such as at high revolutions, Skyactiv-X still employs a traditional spark plug to help.

"We see it as a crossover between diesel and gasoline engines, and have named it Skyactiv-X accordingly," Kogai said. "We can now see a clear path to the mass production of this next-generation engine."

That transition between compression ignition and spark ignition is one of the biggest challenges. The transition must be absolutely seamless and unnoticeable to the driver. But the new compression ignition mode is extremely sensitive to even the smallest changes in in-cylinder pressure (altitude) and temperature, making it hard to manage in the wide-ranging road conditions of real-life driving.

"If you see some jerky reaction during the transition, that would be unacceptable in the marketplace," Tomazic said.

Costs can also balloon because of add-ons that might be needed to make the technology work, including superchargers, direct injection systems, exhaust gas recirculation and individual cylinder controls, Tomazic said. The engine control unit will likely require more powerful semiconductors to handle the more complicated computations needed to individually manage combustion in each cylinder.

"It can get very complex very quickly," he said.

John Kirwan, chief engineer for advanced powertrain at Delphi, said conditions have changed since earlier efforts by others to perfect the technology.

"Part of it is the controls capability — the computing power — as well as the sophistication of the [electronic] controls," says Kirwan. "There's also improved hardware. Fuel injection systems are operating more precisely and with higher pressure capabilities. Variable valve control has evolved."

‘Unwavering strategy'

But recent advances in variable valve timing and direct injection have helped usher a breakthrough, said David Boggs, a technical specialist at the engineering firm Ricardo Strategic Consulting.

He also credited Mazda's corporate culture for pushing the limits. Mazda's small size and tiny budget — Toyota plans to spend nearly eight times as much as Mazda on r&d this year — mean its engineers are conditioned to eking more from less.

"They are willing to take greater risks than bigger companies," Boggs said. "They have a tradition of being more innovative."

Mazda didn't say what vehicle would get the Skyactiv-X engine or detail how fast it would spread across the lineup. The announcement comes just days after Mazda and Japanese rival Toyota Motor Corp. agreed to invest in each other through a capital alliance.

But Mazda said it has no plans to supply Skyactiv-X to other automakers.

Even as Mazda sees its future in better internal combustion engines, it isn't ignoring the electric trend.

In announcing Skyactiv-X, Kogai also outlined plans for a pure electric vehicle in 2019 and a plug-in hybrid after that. But Mazda's building-block strategy means first things first, said Executive Vice President Kiyoshi Fujiwara, Mazda's global r&d chief.

And for Mazda, the keystone block is internal combustion.

"Of course, electrification, including systems such as mild hybrids are necessary," Fujiwara said. "But the pursuit of the ideal internal combustion engine should come first. This is our unwavering strategy."

Richard Truett contributed to this report.