Homogeneous charge compression ignition (HCCI) engines combine a diesel's torque and fuel efficiency with a gas engine's cleaner emissions. The HCCI technique has been a goal of automakers since the hot-bulb compression engine was first prototyped in the late 1800s. There have been many hurdles to jump, including getting the timing, temperature and mixture right, but Mazda thinks it has done just that with its new Skyactiv-X four cylinder. The company also predicts, and we agree, that the internal-combustion engine is going to be around for a few decades before it all goes to nuclear or unobtainium or whatever is next. Here’s what you need to know.

Mazda’s aim with the new tech is a holistic approach to greening the earth. That means not just looking at tank-to-wheel pollution, but (oil) well-to-wheel pollution. Skyactiv-D and Skyactiv-G were just steps in the plan. Skyactiv-X is the goal. Mazda is also looking at renewable liquid biofuel made from microalgae. Remember, we’re still talking combustion engines here.

Skyactiv-G and D were just waypoints to X.

About those hurdles: Gasoline engines do some things better and diesels are better at others. With compression ratio, diesels have the edge; specific heat ratio, diesels again; combustion period (gas); combustion timing (gas); heat transfer to cylinder walls (neither are great); air pressure difference between exterior and interior (diesels); and finally mechanical friction (gas). The next generation Skyactiv-D and G engines make improvements in all those areas but the Skyactiv-X, going into production in 2019, advances them even further.

Flame propagation at 750 rpm.

With ordinary spark ignition combustion, average engines use an air/fuel ratio of about 14.7:1. With lean spark ignition combustion, the ratio can go up to 29.4:1, but the flame won’t propagate. Combustion ignition enables a super lean burn at twice the ideal air/fuel ratio. Mazda saw 36.8:1. Even lean air/fuel mixtures will ignite and burn if highly compressed.

Lean combustion is what makes for efficiency. That makes sense. Less fuel in the same amount of air means less fuel burned. Lean burn also reduces combustion temperatures, which reduces heat transfer and cooling loss. It also increases the amount of air for a given level of torque, reducing loss from throttle closure, called pumping loss.

The expanding fireball acts as another piston to compress the lean mixture.

Mazda isn’t calling this an HCCI engine because it’s not homogeneous. It calls it an SPCCI, spark-controlled compression ignition, because a spark plug is still necessary and the spark ignition is the catalyst for the compression ignition. When the plug sparks, the expanding fireball acts as an “air piston” to compress the super lean fuel below. That fuel is so lean that the flame doesn’t even ignite it; it only ignites when the pressure wave compresses it.

The Skyactiv-X engines will come with a supercharger, but not for power. The exterior vs. interior air pressure problem is one of those hoops that Mazda needs to jump through for compression ignition and a small supercharger solves that problem over different elevations, temperatures and pressures. So don’t expect Hellcat-like whining noises.

The Mazda 3 prototype has a 190-hp four, though it didn't feel like it, and returned about 15 percent better fuel mileage.

As for improvements, Mazda says the SkyActiv-X will have better responsiveness than both the SkyActiv-G and D engines with help from a low-resistance throttle valve. Acceleration continues to a higher rpm range and fuel consumption drops. Mazda claims about 25 percent more power in an engine that’s 25 percent more efficient. We test drove a few prototypes and got a 15 percent increase in mileage without really trying, and Mazda's still in the prototype phase. The X technology also has a flat fuel-consumption curve, which means better and more predictable real-world numbers.

Check back in a week or so for our test-drive review of a Mazda 3 with Skyactiv-X technology.

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