Last year, Mazda announced that it was working on a new engine technology that uses gasoline, but combusts it like a diesel. Collectively, we all scratched our heads and tried to wrap our brains around how it worked.

Recently, however, I was able to wrap my hands around a steering wheel and get first-hand experience with the new engine technology on the road near Mazda's research and development facility in Irvine, Calif. Prepare yourself; it's about to get nerdy.

Under (lots of) pressure

As we learned back when the technology was first announced, the Skyactiv-X engine makes use of an extremely high compression ratio and a ridiculously lean air-to-fuel mix to combust gasoline under pressure like a diesel engine does. This is called homogeneous charge compression ignition (HCCI). The gearheads and tuners out there will know HCCI by a different name: knock.

Under most conditions, knock is undesirable in gasoline engines because it tends to happen at the wrong point in piston's travel, causing very bad things to happen to the engine's reciprocating parts. What's more, it's notoriously hard to tune engines for HCCI because the timing of the knock is so unpredictable and heavily dependent on a variety of factors ranging from temperature, engine speed humidity, etc. However, Mazda's system uses extremely precise application of fuel via direct injection, a swirling effect with the air in the cylinder and a perfectly timed spark to kick off the HCCI at exactly the right moment.

Basically, what Mazda has done is tune the engine's compression to operate at just below the point where HCCI would occur. Then, at precisely the right moment, a spark plug kicks. The resulting explosion increases the heat and pressure in the cylinder just enough to cause HCCI to occur throughout the rest of the cylinder. Mazda calls this Spark Controlled Compression Ignition (SPCCI), basically using that perfectly timed knock to its advantage. The result is a faster, more complete burn of the gasoline throughout the cylinder.

Lean machine

In addition to running at a high compression ratio, the Skyactiv-X engine runs extra lean. This means that there's waaay more air in the cylinder than gasoline, sometimes over twice as much air as a conventional engine. Usually, running lean is a bad thing because all of that extra air can cause the explosion from the spark to travel too slowly through the cylinder, resulting in a lot of unburned fuel and lost efficiency. But because the combustion occurs spontaneously throughout the cylinder Mazda's SPCCI system completely sidesteps this problem.

The unburnt air in the cylinder brings a few positive side effects. For starters, it absorbs some of the heat generated by the combusting gasoline, preventing it from seeping into the cylinder walls and heating up the engine. Instead, the gas expands with the heat, increasing pressure within the cylinder and helping to push the piston down even more than the combustion "bang" could alone.

The result is improved efficiency because energy that would have been wasted to just heat up the metal of the engine is now used to help move the car along. So with less fuel used, the Skyactiv-X engine can generate more torque. Theoretically, it's a win-win.

Antuan Goodwin/Roadshow

The Skyactiv-X engine

Now that we all understand how the Skyactiv-X engine works, let's talk about the prototype engine itself. Under the hoods of the matte black mules are fairly pedestrian looking 2.0-liter aluminum block, four-cylinder engines.

However, these engines run at a crazy 16:1 compression ratio. For comparison, the also Skyactiv-G engine in the average Mazda3 runs at 14:1 while most four-bangers run around 10:1. Up top, the engines have been fitted with a high-pressure direct fuel injection system and inside they feature an in-cylinder pressure sensor that can adjust injection and spark timing in real time to control the delicate SPCCI cycle.

Interestingly, the Skyactiv-X engines are force-fed air by a "lean" supercharger. According to Mazda, the roots-type compressor isn't there to add performance like a sports car, but is necessary to cram the ridiculous amounts of air into the high-compression cylinder necessary for the very lean burn. To further help control cylinder temperatures, the supercharger features a small air-to-water intercooler and even the exhaust gas recirculation emissions system is temperature controlled.

The prototypes make about 178 horsepower and 170 pound-feet of torque, though those numbers are subject to change. More impressive is that it handles the high pressure combustion with regular 87 octane fuel. Mazda's system should, I'm told, compensate for any grade fuel you feed it, but doesn't really benefit from higher octanes. If you give it more expensive premium 93, its torque curve will shift to a slightly higher engine speed, but the peaks won't change. It'll be interesting to see how this odd trait affects on-road performance.

Antuan Goodwin/Roadshow

The production Skyactiv-X spec also features a very mild hybrid system. An integrated e-motor generator will be able to recapture energy to run the electrical systems and smooth over the anti-idling stop-start system, but doesn't have enough juice to move the vehicle under e-power… for now. Mazda's been very tight lipped about the specs of that e-motor, which was actually disabled on the prototypes I tested. However, we know that mazda has plans to bring hybrid and plug-in vehicles to the market over the next few years.

On the road in the prototype

"That's enough specs," I can hear you asking. "How's it drive, nerd?" We're getting there, I promise.

After an extensive briefing, I was given a chance to drive two of Mazda's Skyactiv-X prototypes: one with a manual transmission and one automatic. They look like stealthy Mazda3s, but beneath that shell the compacts feature an all-new seventh-generation platform and, of course, the new Skyactiv-X powertrains. The final product, I'm told, will look nothing like this. I imagine it'll actually look more like the Mazda Kai concept from last year's Tokyo auto show.

Each prototype was outfitted with an iPad display on the dashboard that let me know what combustion mode the engine was operating in ranging from conventional spark ignition to SPCCI to an ultra-lean mode that runs up to 2.5 times leaner than normal.

I hit the road in the manual model first and was very pleased with the power off of the line. The engine ran smoothly, though not very quietly. Mazda's engineers warned me that these were early prototypes, so I did hear some rattling and knocking from the engine bay around town. I was told the sound would be fixed before production.

Antuan Goodwin/Roadshow

Ignoring the sound, I noticed that the torque curve felt broader and meatier than the standard Mazda3 that I'd driven to the event. There was really good passing power and the hatchback could be coaxed into accelerating uphill with less downshifting than I expected. Watching the combustion display, I noticed that the engine spent nearly the entire time in the second SPCCI mode and rarely stepped into the ultra-lean mode.

One of Mazda's claims with the Skyactiv-X engine is that it's resistant to high-RPM driving. Basically, they say that you can keep the revs higher without as much of a hit to the fuel economy -- basically, it's like being in Sport mode and Eco mode at the same time. However, the instrument cluster's fuel economy gauge was blacked out, so my tested mpg was a mystery. However, I was very pleased with the performance.

With the automatic transmission, the combustion display showed that the engine changed more liberally between the three modes. The ultra-lean icon illuminated much more frequently during my identical testing cycle, leading me to believe that the automatic is the more fuel efficient of the two transmissions. However without numbers and on such a short test, it's hard to tell how much more efficient and whether my specific driving style was the deciding factor.

Skyactiv-X's future

What's unclear at this point is whether the new Skyactiv-X technology will live alongside Skyactiv-G, which Mazda is still developing, or if it will be a replacement. We also don't know what, if any, effect the new engine will have on the pricing of Mazda's next-generation vehicles or whether it'll be offered as an upgrade or eco-trim option.

Mazda

What I do know is that I was very impressed with the Skyactiv-X's performance and left the event really excited about the potential this oddball engine has. Mazda claims that Skyactiv-X has potential to improve economy up to 20 percent over its current Skyactiv-G system. Imagine the next Mazda3 delivering over 45 mpg on the highway without the need for a CVT. Add the mild hybrid savings and consider that SPCCI has a nice low-load sweet spot around town and the gains in the city could grow by 30 percent.

If Mazda can smooth out the rough edges and if it can deliver on its efficiency promises, Skyactiv-X will be a nice performance improvement over Skyactiv-G for daily driving conditions. Those are some pretty big "IFs" but Mazda's giving itself time to sort it all out. The automaker is targeting 2019 for Skyactiv-X and the seventh-generation platform to reach production, though both are still very much in development and that window is subject to change.