Mazda is working hard on its new SkyActiv-X engine, using technology that many people think is the Holy Grail of gasoline engines.
The SkyActiv-X engine promises the power and clean emissions of a gasoline engine with fuel economy approaching that of a diesel. What is this new technology, and how does it work?
For years, engineers have been trying to design a gasoline-powered compression-ignition engine. Known by the technical term HCCI (Homogenous Charge Compression Ignition), this engine would, in theory, offer the fuel efficiency of a diesel with the low emissions of a gasoline engine. There’s just one problem: No one has been able to build one that would work in the real world.
Except that now, Mazda has. Sort of.
The new SkyActiv-X engine isn’t exactly the Holy Grail, but it’s close enough that we’re sure any self-respecting deity would be happy to drink from it. If all goes according to plan, you’ll be able to buy a SkyActiv-X-powered Mazda in just a couple of years.
Normal gasoline engines work by pulling in a mixture of gasoline and air, compressing it, and igniting it with a spark. The resulting explosion drives the engine’s pistons downward, creating power. (The motion of the pistons also provide the power to pull in the mixture, compress it, and pump the spent exhaust out of the engine.)
What we’ve just described is a spark-ignition engine. The HCCI engine uses compression ignition, in which the fuel-air mixture is compressed to such a degree that it spontaneously combusts. If you’re an engine-savvy type, you’re probably thinking, "Hey, that sounds like a diesel," and you’re not far off the mark. Diesels work by compressing air and then spraying in fuel during the power stroke, which burns, expands, and pushes the piston downward. (Diesels are known as stratified charge compression ignition engines, or SCCI.)
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Getting gasoline to burn in the same way is a little trickier, as gasoline is much more volatile than diesel fuel. It requires careful monitoring and control, something that wasn’t possible in the past but can now be achieved using computerized controls.
Unlike Indiana Jones, Mazda wasn’t seeking the Holy Grail for its own sake. It wanted to create a gasoline-fueled lean burn engine, one that uses less fuel for a given amount of air. Gasoline engines tend to be happiest when the ratio of air to fuel is about 14.7 to 1. Make the mixture too lean, and the engine won’t run well because the flame caused by the spark doesn’t have time to spread throughout the entire cylinder. A compression-ignition engine solves this problem because when the fuel-air mixture ignites through compression, there is no single ignition point. The fuel combusts all at once, so flame propagation isn’t an issue — hence Mazda’s interest in HCCI.
Like other manufacturers, Mazda was able to get HCCI to work in the lab, but only in a very narrow range of engine speed and power. At very low and moderately high speeds, HCCI engines misfire due to either low temperature or insufficient time for combustion. And at high loads, the fuel tends to explode too early. The envelope in which HCCI would work simply wasn’t large enough to be useful, and Mazda’s engineers realized they’d need to find a way to expand it.
Mazda came up with a new ignition, which they call SPCCI — SPark Controlled Compression Ignition. The engine works something like this: During the intake stroke, a super-lean charge of fuel, air, and previously burned exhaust gases (using exhaust gas recirculation, or EGR, an existing emissions technology) are brought into the cylinder. This mixture is then compressed, just as in a regular gasoline engine.
The magic happens during the compression stroke: The fuel injector sprays in a small additional charge of fuel, and then immediately ignites it with the spark plug. This "bubble" of fuel explodes and creates a pressure wave that compresses the lean mixture, causing it to combust on its own — not through flame propagation, but through compression, like a diesel.
In a way, the operation of the engine is a bit like an implosion-type nuclear bomb, in which conventional explosives compress a mass of plutonium, causing it to detonate. The SPCCI engine fires off a small conventional charge of gasoline, which compresses the lean air-fuel mixture and causes it to explode on its own.
SPCCI isn’t quite HCCI, but it’s pretty darn close — and it solves another problem.
Though the SPCCI process greatly expands the envelope over which the engine can operate, like HCCI, there are still situations where it doesn’t work well — specifically, high loads and high engine speeds. Mazda realized it would have to use conventional spark ignition (SI) for these situations. One of the major challenges for an HCCI engine is managing the change between HCCI and SI. Here, the SPCCI process pretty much solves the problem itself: Since the spark ignition system is always active, switching to SI mode is simply a matter of changing the fuel injection and spark timing.
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From a hardware perspective, the engine is rather similar to a gasoline engine. The aluminum block must be beefed up to compensate for the higher compression ratios (which start at about 15-16:1 and rises higher when the engine is in SPCCI mode). The engine uses a small supercharger to ensure even airflow at mid-to-high engine speeds and a high-pressure fuel injection system. It relies on sophisticated pressure sensors that detect early (or late) combustion so that the timing of both fuel injection and spark can be fine-tuned. Though the costs to build the engine are higher than that of the current SkyActiv-G gasoline engine, it’s still less expensive to manufacture than a diesel.
We got a chance to drive prototypes of the upcoming Mazda3 equipped with a 2.0-liter gasoline engine producing 190 horsepower (about 187 using U.S. measurements) and 170 lb-ft of torque. Aside from a change in sound (and a display on the engineer’s test display) there was no indication that the engine was changing from SI to SPCCI modes. Our drive near Mazda’s European headquarters in the suburbs of Frankfurt, Germany, included urban, suburban, and some autobahn driving (where we kept our speeds at 85 to 95 mph). We sampled both manual and automatic cars, taking a 45-or-so-minute test drive in each, and according to the car’s data loggers, the engine was able to operate in SPCCI mode more than 95 percent of the time.
The chief benefit is fuel economy. Mazda claims a 20 to 30 percent increase in fuel economy over the already-efficient SkyActiv-G engine, though the increase will lower in North American-spec cars owing to their emissions equipment. During our test drive, we averaged 37.9 mpg in the automatic car, which Mazda estimates to be a 13.1 percent improvement over a car with the current SkyActiv-G engine. In the manual car, we averaged 33.6 mpg, a 14 percent improvement over the SkyActiv-G engine. (We took an extra high-speed Autobahn run in the manual car, which accounts in part for the lower fuel economy.)
Mazda is still developing the SkyActiv-X engine, but it expects to have it ready for sale in 2019; our best guess is that it will make its debut in the next-generation Mazda3. Given the excellent real-world fuel economy of the current SkyActiv-G engines, we expect SkyActiv-X will deliver a great mix of good power and outstanding fuel economy.