It runs on 87-octane gasoline but operates like a diesel – and it’ll be in your next Mazda.

 – Irvine, California

These chopped-up, matte-painted prototype cars wear the bodies of today’s Mazda 3, but underneath the surface is what Mazda believes is the future of gasoline propulsion. The hatchbacks are running Mazda’s SkyActiv-X powertrain, set to be the first production compression-ignition gasoline engine. They are not, however, just for show: Mazda is confident enough in its still-under-development technology to let us drive the cars on public roads.

SkyActiv-X (pronounced “ex” not “ten”) is the powertrain side of the next iteration of Mazda’s continual push to cut its corporate average emissions ratings while boosting fuel economy. Today’s engine families are called SkyActiv-G and SkyActiv-D, for gasoline and diesel, respectively. Compared to SkyActiv-G, X is intended to improve fuel efficiency by 20-30 percent while also boosting torque 10-20 percent, relative to today’s 2.0-liter engine.

What might those numbers turn out to be? Today’s Mazda 3 2.0-liter automatic hatchback, for instance, produces 155 horsepower and 150 pound-feet of torque, and returns EPA figures of 28 miles per gallon city and 37 mpg highway. So extrapolating, a version with the SkyActiv-X engine could boast something like 180 lb-ft and 36/48 mpg. Those are just educated guesses, of course; exact numbers won’t be confirmed for some time.

Mazda SkyActiv-X Prototype First Drive

The secret to this new performance is gasoline compression ignition, which harnesses many of the benefits that diesel engines have enjoyed for years. Normal gasoline engines set fire to a mixture of fuel and air, which expands throughout the cylinder and forces the piston down. But by squeezing a very lean mix of air and gasoline until it explodes spontaneously, like in a diesel engine, Mazda engineers say the SkyActiv-X engine can produce more torque, waste less heat, and use less fuel overall. Many automakers have explored the idea, but no one has been able to get it ready for a production model – until now.

Essentially, more power is produced over a shorter period of time each time, because compression ignition is much more violent than simply lighting an air-fuel mixture. Mazda research and development engineer Jay Chen likens it to popping an inflated balloon instead of simply letting the air out through the neck.

Mazda SkyActiv-X Prototype First Drive

The prototype engine is a 2.0-liter inline-four with a supercharger, cooled exhaust-gas recirculation, and electrically controlled variable valve timing. It has a 16.0:1 compression ratio, which is sky-high in terms of gasoline road cars, and a higher-pressure direct-injection system. Counter to most gasoline direct-injection cars, the fuel injectors are at the center of the cylinder head and the spark plug is off to the side.

Most of the time, the engine runs in what Mazda calls Spark Controlled Compression Ignition (SPCCI) mode. A very lean mix (i.e., there’s a high proportion of air to gasoline) is ingested in the compression stroke, swirling around thanks to a special piston design; the supercharger is used to add extra air if needed to thin out the mixture. Then right as the piston reaches top dead center, with that mix almost ready to ignite of its own accord, the engine sprays in another small amount of fuel and ignites it with a spark plug. That explosion pushes the remaining air-fuel mix over the edge into an explosion, producing lots of power very quickly.

X is intended to improve fuel efficiency by 20-30 percent while also boosting torque 10-20 percent.

That was Mazda’s innovation: using a spark plug to kick-start compression ignition. Otherwise, engineers say, it’s very difficult to get the explosion to happen at precisely the right time. To keep tabs on the whole process, pressure sensors in each cylinder give feedback to the engine computer: “Only now are our engine control processors fast enough to control this event by event by event,” says Chen.

Other times, however, the Mazda engine runs like a regular spark-ignition engine. It switches to this mode primarily in high-load, high-RPM running; for performance, in other words, compression ignition still won’t do the job. But because the engine always uses a spark plug all the time, engineers say the engine can “seamlessly” alternate between the two running modes.

That Mazda is confident in the engine is underlined by the fact that we’re able to drive the prototypes on public roads in normal traffic – city streets, highways, and everything in between – and with both a six-speed manual and a six-speed automatic transmission. Befitting early prototypes, the cars are missing some interior parts (no radio, no center console, etc) and have plenty of warning lights on display, plus an iPad that indicates whether the engine is running in compression- or spark-ignition mode.

Mazda SkyActiv-X Prototype First Drive

But what’s most impressive from the driver’s seat is that the 2.0-liter feels… normal. Responsive and calm, it revs easily, with strong low-end power and a very direct-feeling torque curve as revs rise. In most situations, there are no real giveaways that anything is different.

The cars certainly feel quicker, primarily at low engine speeds, than today’s 2.0-liter Mazda 3. At idle, there are no uncouth vibrations or rattles like you might expect in some direct-injection cars or diesels. And under acceleration, the engine mostly sounds like any other Mazda 2.0-liter.

Yet, the unfinished engines will knock – that tapping or rattling sound caused by pre-ignition – quite readily. The stick-shift car does so almost every time you lift the throttle; it’s tricky for the engine to cleanly transition from on- to off-throttle running, explains an engineer. Both engines will also do so occasionally under brisk acceleration, or when strained, like passing uphill. An engineer, riding shotgun, winces when the car lets out a volley of knocking noises as I pull away from a stop sign.

The cars certainly feel quicker, primarily at low engine speeds, than today’s 2.0-liter Mazda 3.

Then again, these cars are still in the testing phase, and Mazda promises these issues will be ironed out by the time production begins. Recall that unwanted engine knock is essentially compression ignition happening; Dave Coleman, Mazda’s manager of vehicle dynamics engineering, says that more software work will help prevent those situations. The problem is that the engine has many, many control strategies, and the company’s engineers are still learning how to best combine them all.

“It’s tuning,” he says. “We’re dealing with a massive multivariable equation.”

The prototype cars also have some other upgrades that will be part of the next generation of SkyActiv vehicles. For instance, the front suspension bushings are redesigned so as not to allow the front wheels to move back and forth as much when hitting bumps, which is said to make the ride more comfortable by reducing head-toss. “Our balance can react to that better,” says Coleman.

The seats have been completely redesigned to more naturally hold the driver’s spine, reducing driver fatigue. The entire chassis has been strategically stiffened, with special dampening parts added to further quell vibrations.

Mazda SkyActiv-X Prototype First Drive
Mazda SkyActiv-X Prototype First Drive

Frankly, it’s hard to feel out how much effect these changes have on our test drive. The SkyActiv-X testers mostly feel like the Mazda 3 hatchback I drove earlier in the day: sweet in all their primary controls, with a firm ride, and extremely natural steering action. In other words, continuing the class-leading dynamics and feel of today’s Mazdas.

Interestingly, Mazda is still using a six-speed automatic transmission when rivals have adopted 8, 9, or 10-speed units, or continuously variable transmissions. Because SPCCI is so efficient in low-load scenarios, Coleman says that running the engine at higher revs doesn’t really hurt fuel economy. In fact, he believes Mazda’s fun-to-drive character is better served by keeping higher cruising revs so the car can be more responsive to throttle inputs without needing to downshift so often.

“We try to build around as few gears as possible to keep that more direct feel,” he says.

However, the new SkyActiv-X engine is not without its downsides. For starters, the explosive nature of compression ignition, plus the higher-pressure fuel system, creates a whole lot of noise.

Mazda SkyActiv-X Prototype First Drive
Mazda SkyActiv-X Prototype First Drive

“The NVH [noise, vibration, and harshness] is very challenging,” says Coleman. “We’ve had to encapsulate the engine,” with special foam all around the engine bay, plus a big plastic cover obscuring the oily bits when you open the hood. He says Mazda drew inspiration from the way European luxury manufacturers keep their diesel engines quiet.

Those explosions also put much more strain on engine internal components, so they must be beefed up, almost to the strength required for Mazda’s diesel engines. As a result, not a single part is shared with today’s 2.0-liter SkyActiv-G.

“A lot of stuff looks like it’s halfway between a gas and a diesel engine,” Coleman says.

The engine is also more expensive to build – though cheaper than a hybrid, engineers note – and heavier than SkyActiv-G, though still lighter than would be an equivalent diesel engine. The aforementioned chassis reinforcements add weight, too; Mazda has traditionally tried to cut mass wherever possible from its cars.

“We’re still very weight-focused, but with a practical focus,” says Coleman.

The SkyActiv-X engine is also built with a mild-hybrid system, similar to the ones offered on cars like the Audi A8, Ram 1500, and Jeep Wrangler. Coleman had very few details about the system, other than to say it’s intended to help with engine stop-start situations. It was disabled on our prototype cars. But it is an important technology for future Mazda models: “All SkyActiv engines will feature mild-hybrid technology,” says Chen.

Mazda SkyActiv-X Prototype First Drive

Of course, Mazda’s reinvestment comes as most automakers are beginning to announce grandiose plans for electrifying all their vehicles in coming years. Mazda will do that, too – the company’s timeline shows a battery-electric car due in 2020 and a plug-in hybrid by 2021 – but the company believes there’s still more to be gained from developing internal combustion engines for electrified vehicles. The idea is that if Mazda must still equip a hybrid or plug-in hybrid car with a gasoline engine, it may as well be the most efficient gas engine possible. So SkyActiv-X is a building block toward those future hybrid models.

Mazda won’t be drawn on exactly which vehicle will introduce the SkyActiv-X engine or chassis, though it’s certain that the powertrain will only launch in an all-new car with the chassis upgrades, not simply a refreshed existing model. “I don’t think we have any chance to put it [the engine] in a sixth-generation car,” says Coleman. Mazda refers to the new chassis and engine as its seventh-generation technologies.

The company will confirm that the new car reaches the U.S. in late 2019. For now, all signs point to the model in question being a next-gen Mazda 3; a prognosticator might guess that the recent Kai concept was a pointer in that direction.

We’ll have to wait some time to see how well Mazda’s new engine can deliver on its promises of boosting both power and efficiency by sizeable amounts. Based on driving the prototypes, though, the technology is sound and – especially after another year or so of refinement – does work in real-world driving. Mazda has solved a tricky engineering problem because it delivers actual real-world results, not just because it’s a headline-grabbing one. Naysayers take note: there may still be life left yet in the internal-combustion engine.

Gallery: Mazda SkyActiv-X Prototype: First Drive