Mazda Skyactiv X: A new way for more power with less fuel
The end of the gasoline engine is nigh, or so we're led to believe. Electricity is the future king, and gas- powered cars will soon be curiosities we whisk past as we drive our noiseless EVs through a futurescape straight out of the most optimistic vision Hollywood could dream up.
Or not. The reality is that while EVs are gaining prominence, the vast majority of cars now and over the next several decades will be at least partly powered by gasoline. Because of higher fuel economy standards, hybrids and plug-ins will proliferate and dominate until it's just no longer possible to go solo with a gasoline engine anymore. Still, with more than a century of use and infrastructure, plus ongoing development, gasoline engines are far from the grave.
For example, there's Mazda's new Skyactiv-X technology. By combining principles used in gasoline and diesel engines, Mazda is making big promises: Torque increases of between 10 and 30 percent; fuel economy improvements of up to 20-30 percent, equaling the company's diesel engine; better efficiency over a wider range of engine speeds and loads, meaning fuel-friendly gear selection won't come at the expense of driving performance.
We got a look through Mazda's crystal ball at this new technology near the town of Bad Homburg outside of Frankfurt, Germany. There, nestled among the cornfields, is Mazda's European research and development division, and four cars equipped with prototype versions of this potentially revolutionary change to the gasoline-powered internal combustion engine. Spoiler alert: We drove the cars, and aside from some to-be-expected prototype quirks, they were surprisingly well refined and for the most part felt perfectly normal. What makes Skyactiv-X different from other engines gets a little complicated, and requires a quick primer on exactly how modern gasoline and diesel engines work.
Let's start with a basic gasoline piston engine. Pistons move up and down cylinders in what's called a four-stroke cycle, constituting intake, compression, combustion and exhaust. The intake stroke is downward, where the intake valves open, allowing a mixture of air and fuel to enter the cylinder at a ratio of 14.7:1, an ideal mix that ensures all the air and fuel will be used most efficiently (side note: too much fuel is called "rich," and it results in soot and other bad emissions; too little is called "lean" and it causes knocking and pinging, which damage an engine). The intake valves close to trap the fuel and air inside, and then the piston moves back up the cylinder, compressing that mixture. When it reaches the top, a spark plug ignites the fuel and air, a process known as spark ignition. The resulting expansion of gases forces the piston back down, and it's that downward motion that creates the power to move the car. After that, as the piston goes back up the cylinder, the exhaust valves open to expel the burnt gases, and the cycle repeats itself.
Modern diesel engines operate similarly, with key differences. First, during the intake stroke only air is taken in; fuel is added during the compression cycle. Second, there's no spark plug, instead, diesels compress the air-fuel mixture much more tightly, so much that it spontaneously ignites. This is called compression ignition, and among other benefits it allows for a leaner mixture, one of the reasons diesels get better fuel economy.
The Skyactiv-X difference
Mazda's Skyactiv-X process combines these two types of engines into something it calls SPCCI, for SPark Controlled Compression Ignition. As the name implies, SPCCI utilizes both spark and compression ignition. In Skyactiv-X, a very lean mixture enters the engine during the intake stroke; how lean varies, but it can be more than twice as lean as the 14.7:1 in a standard engine. Near the end of the compression stroke, additional fuel gets squirted right near the spark plug. The plug then ignites this denser mixture, and the pressure wave from that ignition then raises the effective internal pressure of the cylinder high enough to cause compression ignition of the rest of the mixture. This is worth repeating: The spark ignites part of the mixture, but it's the pressure wave from that combustion that causes compression ignition of the rest of the charge, not the spark itself.
Mazda of course has taken additional steps to precisely control this complicated system. A sensor in each cylinder monitors internal pressures, and correspondingly adjusts fuel delivery to keep it optimal. To squirt fuel into the cylinder while it's compressing means very high pressure must be used, so Skyactiv-X uses a special fuel injection system similar to the kind used on modern diesel engines. And finally, in order to ensure that a constant flow of air is delivered to the engine, Skyactiv-X employs a supercharger to stabilize airflow when the engine is above half-load, gradually adding more boost as needed. There are also certain circumstances where the engine does run purely on spark, mainly at cold starts, and at the very highest revs.
Does It Work?
We drove prototypes of the Skyactiv-X engines, equipped with both manual and automatic transmissions. The engines sounded similar to the Skyactiv-G you currently see in a Mazda3. At speed on the Autobahn, we noticed especially in the manual transmission model there was good low-end torque, good enough to accelerate the Skyactiv-X car nicely from 100 km/h to 160 km/h (about 62-100 mph) without relying on a downshift. Throttle response while at speed and from a standstill felt better than the standard engine as well. However, we also noticed a lot of pinging and knocking noises from underhood when transitioning from idle or very light throttle to more fuel. Mazda says that it's working on smoothing out this part of the engine's response. Also, at full throttle power didn't always feel very smooth, and again, this is due to the prototype nature of the engines. At the end of our test drives we were provided with fuel economy comparisons. While the numbers between the drivers varied, the average fuel economy gain on these prototype engines was about 13.7 mpg for the manual transmission cars, and 14.1 for the automatics. That improvement comes with more power and torque, mind you.
This engine will undoubtedly be more expensive to build because of the addition of the supercharger, high-pressure fuel system, and cylinder pressure sensors, not to mention the additional computing power needed to make it run right; when Skyactiv-X makes its way to passenger cars sometime in 2019, expect those cars to cost more. We also have to take Mazda at its word with those fuel economy numbers, since we can't independently verify them. While our seat-of-the-pants power and torque impressions were positive, Mazda isn't backing them up with any hard numbers yet.
Caveats aside, this is some pretty promising technology. Overall it felt more refined and finished than we would've expected for prototype versions of fairly advanced technology. Pinging aside, it also felt and sounded perfectly normal most of the time. Mazda says it will have no problem adapting the engine to hybrid powertrains in the future; Skyactiv-X is already a mild hybrid, with a motor-generator in lieu of a starter. It's too early to say that this is the engine of the future, but it's an example of how, thanks to technology, that futurescape will have more gasoline engines than you might expect.