CARS.COM — It's been a long time since I've sat in a science lecture, but I recently experienced what felt like one at Mazda's Research and Development facility in Irvine, Calif. In a converted garage, several engineers took turns dropping phrases like "stoichiometric air-fuel mixture" and "ideal combustion profile" while walking reporters through Mazda's latest innovation: the SkyActiv-X, a new type of gasoline engine that Mazda says pushes the envelope — and possibly the shelf life — of the internal combustion engine further ahead. After that, it was time to take Mazda's science experiment for a spin to see how it does in the real world.
Mazda has never been afraid to be different, and though most companies think that the automotive future lies in electrification, Mazda disagrees (for the time being). There are no hybrids in the Mazda lineup; it's all gas (or gas and diesel abroad). And as stated during the presentation by Senior Vice President Special Assignments Robert Davis, Mazda's efforts are focused on making gas engines more efficient because "our goal is to make all of our cars efficient, not just a few of them hyper-efficient."
The primary innovation of the SkyActiv-X engine is that it is still a gasoline engine, but it uses compression ignition — or something very close to it. If compression ignition sounds familiar, that's because it's used in diesel engines, and it's partly what makes them more efficient. Diesel engines lack spark plugs and rely on the spontaneous ignition that results naturally from the heat generated as an air-fuel mixture is compressed.
The simplified explanation of how SkyActiv-X works is this: The engine has a much higher compression ratio than a regular gasoline engine — roughly 16 to 1 in the prototypes — that allows it to create the conditions for compression ignition in each cylinder. The higher compression allows for a much leaner air-fuel mixture, meaning that the ratio of fuel to air is lower. (Leaner mixtures represent greater efficiency but notoriously resist combusting, even in spark ignition engines.) As the piston comes up, the air and fuel are compressed while a sensor in the cylinder monitors the temperature and pressure in real time. See the engine in action below.
As you can see in the video, the SkyActiv-X is not a true compression ignition engine all the time. The engine can't run efficiently in what Mazda calls Homogenous Charge Compression Ignition operation under some conditions, such as high rpm and high load, at which point the engine switches to spark-plug-assisted compression ignition (or Spark Controlled Compression Ignition).
What the video doesn't depict is that the SPCCI engine relies on a combination of early and late fuel injector pulses as the piston rises through its compression stroke, a provision that achieves the required fuel ratio just in time for combustion but keeps the conditions in the cylinder lean and cool enough that it doesn't preignite and cause knock. Along with the efficiencies associated with high compression and lean-burn, Mazda says the combustion is more rapid than in lower-compression engines and can be timed to occur optimally, converting more of the energy into piston motion rather than being absorbed as heat by the cylinder walls and piston head.
What has held back other companies that have tried to develop gasoline compression engines in the past is switching between compression and spark ignition as needed, which is hard to do smoothly. But Mazda claims to have engineered a way to iron out the kinks in the transition.
That was a lot of technical jargon, and it's OK if you didn't get all of it because the bottom line is easier to understand: Mazda is targeting a 20 percent improvement in fuel economy and emissions for its gas engines while maintaining or exceeding the same level of performance.
An example of this is found in how it drives. Conventional cars use tall gearing in their top gears to keep engine rpm lower and efficiency higher at cruising speeds, but this makes the engine less responsive — it has to ramp up or change gears to get into the power. But because the SkyActiv-X engine design runs so much more efficiently at higher rpm, it can be paired with shorter gearing that allows cruising at higher engine speeds that put it closer to the power and makes it more responsive. Mazda also says the SkyActiv-X has a flatter torque curve, with smoother power delivery. And thankfully, they had a few prototypes on hand for us to test ourselves.
Now, this may look like a Mazda3, but don't be fooled — it's just a Mazda3 body strapped onto a few new goodies from Mazda, including a next-generation small-car platform, new seats, updated transmissions and, of course, the SkyActiv-X engine. The inside of the car reflected this, with lots of tape and plastic coverings about. There weren't even real air vents, just tubes sticking out of the dashboard.
The engine in the prototype cars was a 2.0-liter four-cylinder, producing approximately 178 horsepower and 170 pounds-feet of torque and running, like production SkyActiv-G engines, on regular-octane gas. (Mazda said the output specs are likely to change on the final engine.) I drove about an hour in both the six-speed manual and six-speed automatic prototypes, both of which had displays that showed when the engine switched between its compression-ignition and spark-assisted modes in real time.
How did it drive? Pretty normally, which Mazda engineers excitedly said was the point. Acceleration from a standstill was smooth and felt very close to driving a current-gen Mazda3, which means that the steering was dialed in and the backseat didn't have much headroom.
At highway speeds, the engine runs at higher rpm than I'm used to, but it wasn't jarring. When trying to pass, the transmission does try to do so without kicking down a gear, and it was moderately successful at this. Acceleration at speed is not as dramatic as in a normal car where there's usually a pause followed by hard acceleration; instead, the SkyActiv-X Prototype kind of eases its way into speed. It's not necessarily bad, just different than I was expecting from an engine starting from around 4,000 rpm.
There were a few indicators along the way that showed the SkyActiv-X engine isn't ready for primetime. When I drove the automatic version, a fairly prominent knocking noise came from the engine. The engine bay was wrapped in extra sound deadening and I had a stuffy head cold at the time, so I had trouble hearing exactly how loud it was, but it was still easily heard. Other folks there reported more knocking when driving the manual version, so it seems to be a symptom of the engine rather than either transmission.
Mazda representatives noted that the compression ignition threshold is highly sensitive to the temperature, humidity and pressure of the intake air, so cars that were flawless in Japan exhibited some preignition here but weren't programmed to adapt to all the conditions.
Though the SkyActiv-X Prototype may have looked like something from the future, driving it was an experience totally out of the present. Mazda says that the knocking will go away as they tune the engine more. At this point, it isn't announcing if the SkyActiv-X will become a standard engine or an optional one for future cars. Its partnership with Toyota means that Toyota could use the engine in some applications, but beyond that, Mazda says the technology won't be licensed.
The SkyActiv-X engine will begin to arrive in Mazda vehicles in late 2019.
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