Solving for X: Inside the New Mazda Skyactiv-X Engine

Combining the Best of What Diesel and Gas Engines Offer

Here's a quick thought experiment: Imagine trying to devise a solution to a puzzle that's never been solved, but you've been given creative liberty to use methods and tools that don't yet exist. Working in the hypothetical can be liberating but also challenging, and for Mazda engineers, it's something they actually put into practice for developing a remarkable new technology called Skyactiv-X.

What Is Skyactiv-X?
Skyactiv-X is a new prototype engine in Mazda's Skyactiv family, joining the automaker's current Skyactiv-G (gasoline) and Skyactiv-D (diesel) engine ranks. At its crux, it's a clever solution to an engine puzzle many have tried to piece together but no one fully solved. You can think of Skyactiv-X as the cross between a gasoline and diesel engine — hence the "X" denotation. Like offspring winning the genetic lottery, it takes the best traits from both: torque and efficiency from the diesel parent; cleaner emissions and top-end legs from the gasoline side of the family.

Mazda has announced that Skyactiv-X will see production in 2019, but it's not ready to specify in which vehicle. The first Skyactiv-X engine will be a 2.0-liter inline four-cylinder, which will run on good ol' regular octane gasoline. Power figures aren't final, but as of last contact they hover in the ballpark of 178 horsepower and 170 pound-feet of torque. This is a pretty healthy bump from the current Skyactiv-G 2.0-liter's 155 hp and 150 lb-ft, but the kicker is the Skyactiv-X is estimated to be 20 percent more fuel-efficient overall and as much as 30 percent better in certain low-speed conditions.


So what does that look like in numbers that consumers can relate to? Take, for example, the current Mazda 3 that returns a respectable combined 31 mpg from its 2.0-liter engine. An otherwise identical Mazda 3 equipped with the new Skyactiv-X engine, and its significant power advantage, would be expected to return a little over 37 mpg.

If you're thinking you could achieve the same effect by going to a smaller turbocharged engine, you'd only be partially correct. Often times, to realize the economical advantage of a downsized turbocharged engine, you have to refrain from dipping too deep into the delicious boost because its efficiency drops sharply during such vigorous operation. With Skyactiv-X, you don't have to be so careful. Its efficient operating range is far broader because the way it makes power with the fuel it uses — its fundamental combustion process — is different.


How Does Skyactiv-X Work?
Warning: The following section gets a little technical, so if you're someone who doesn't care how the sausage is made, feel free to skip down. We mentioned that developing Skyactiv-X involved a little of the hypothetical, and we weren't joking. Mazda says it has had the concept for a couple years now, but it had to wait for processing power technology to further develop to handle the amount of data it's pushing through the engine's brain.

For Skyactiv-X to achieve the power, efficiency and clean emission gains it has, Mazda needed to crack a nut called homogeneous charge compression ignition (HCCI). HCCI occurs when a homogeneous or uniform mixture of fuel and air is compressed so much that it heats to the point of spontaneous ignition rather than by the command of a spark plug. Unlike a spark-ignition engine in which the flame propagates outward from the spark plug, the mixture in the cylinder of an HCCI engine combusts essentially simultaneously everywhere in the chamber.

As in a spark-ignition engine, the uniform mixing of fuel and air in an HCCI engine helps ensure a clean, even burn, while HCCI's higher compression ratio and lean operation boost efficiency, much like with a diesel engine. This sounds simple enough, but managing the timing of when HCCI occurs is not only critical but quite difficult due to variables such as air temperature, engine load, rpm, etc. If HCCI combustion occurs too late, engine power is reduced. If it occurs too early, the resulting spike in combustion pressure is like playing whack-a-mole with your engine pistons, but not in a fun way.

To deal with the timing control issue, Mazda came up with something it calls spark-controlled compression ignition (SPCCI). In Skyactiv-X, there's a small zone of air-fuel mixture that can be ignited readily by a spark plug, and the rest of the cylinder's contents is too lean to ignite without some help. The help comes in the form of a tiny fireball initiated by the spark plug that increases the cylinder pressure, causing the rest of the lean air-fuel mixture to spontaneously ignite under compression. Among engine nerds, this strategy is generically referred to as spark-assisted compression ignition. But since Mazda put it in production first, it gets to call it whatever it wants.

A useful analogy comparing these two processes is lighting a charcoal fire using very little lighter fluid (your lean fuel mixture). Trying to light it with just spark ignition, in this case, is like trying to light your mound of charcoal from one corner. One brick lights the adjacent one and so on. SPCCI is like lighting a heat source that in turn heats everything and burns the bricks all at once — and now we're barbecueing.

To keep enough air moving into the engine for an appropriately lean air-fuel ratio — in some cases twice the amount of air to fuel (30:1) compared to typical spark-ignition combustion — Skyactiv-X employs the help of an engine-driven Roots-type blower. Mazda hesitates to call it a "supercharger" (even though that's what it is) because of that word's typical association with the strategy of adding more fuel to go with the additional air. There is a clutch that engages and disengages the blower, so it's less of a parasitic drag on the engine when not in use.

The modern brain of Skyactiv-X monitors and analyzes the conditions of combustion in each cylinder of every cycle. It's so capable that it can make individual adjustments to spark and valve timing, plus the number and volume of the fuel injections. This allows it to avoid unwanted mistimed combustion as well as switch seamlessly between SPCCI and conventional spark-ignition modes.


What's It Like to Drive Skyactiv-X?
We had the chance to drive a prototype version of the Skyactiv-X engine in Mazda's next-generation small vehicle platform, disguised as a current-generation Mazda 3 hatchback. A digital tablet mounted on the dash just to the right of the steering wheel flashed three different green circles, each indicating in real time what was happening within the engine's cylinder walls: conventional spark ignition, SPCCI, or an ultra-efficient super lean-burn version of SPCCI.

We got in the manual-transmission car first, which provided the clearest picture of how the engine builds power. There was a healthy amount of torque off the line, making it feel characteristically diesel-ish. Unlike a diesel in which you have to upshift almost immediately as it quickly runs out of revs, this engine felt willing and happy to rev out to a satisfying 6,000 rpm. As a bonus of the engine's broad efficiency envelope, Mazda says it can use lower gear ratios for better low-end drive without suffering an efficiency penalty. The engine for the most part ran in SPCCI mode unless we buried the accelerator into the firewall. And if not for the green circles, there was no way we would have been privy to switchover.


The lightest green circle, the one for super lean burn, was elusive for some reason when driving the manual-transmission car. In the automatic-equipped prototype, it proved far easier to access, and it only occured during low-speed driving on flat roads. With the exception of easier super lean-burn driving, the automatic didn't feel as smooth or responsive a match with the engine.

The tuning on this Skyactiv-X engine is still being tinkered with; there are still some occasional instances of knock (preignition whack-a-mole). I could induce the knock by being a gear too high (high load) or by purposely making abrupt changes in throttle position, which is how some folks unwittingly drive on a regular basis. If you weren't looking for it, though, you might mistake the micro-knock for ambient noise or miss it altogether. Mazda says it'll have all the kinks worked out before Skyactiv-X hits production, and based on the engine's refinement at this relatively early point, we have no reason to believe otherwise.

Why Focus on Gasoline Instead of Electrification?
You might be wondering why Mazda has decided to focus on improving its gasoline engines rather than building hybrids, plug-ins, fuel cells, or battery electrics like the rest of the industry. And the reason it gives makes a lot of sense after looking at the bigger picture. Though the popularity of hybrids and EVs has expanded in recent years, their impact on fuel savings relies on proliferation.

Technology like Skyactiv-X can be more readily applied across Mazda's entire lineup of vehicles, resulting in greater net fuel savings for the near future. Plus, as Mazda eventually moves to hybridization of these models, Skyactiv-X's fuel-saving measures will be realized there as well.

When you're smart about it, you can zoom-zoom your way to a cleaner, better future.