Mazda SkyActive X Engine — More Power, Less Fuel, Lower Emissions


Mazda has always been the outlier in the auto industry when it comes to internal combustion engines. It built its reputation on the quirky Wankel rotary engine that NSU never could get right. Mazda’s amazing rotary was half the size and half the weight of a conventional piston engine, which made it the ideal choice for Mazda’s first sports car, the iconic RX-7.

Mazda Vision Coupe
Mazda Vision Coupe at Tokyo motor show 2017. Credit:

Mazda Vision CoupeThe one thing Mazda could never tame, though, was the rotary’s thirst for gasoline. As fuel prices rose and miles per gallon became important to consumers, Mazda turned its attention to developing more efficient piston engines, leading to its current lineup of so-called SkyActive engines.

Last January, Mazda announced a new generation of gasoline engines that ran without spark plugs. Called HCCI, which stands for homogeneous charge compression ignition, the new engine used compression ratios as high as 18:1 to ignite the fuel mixture. It was said to have 30% better fuel economy with lower emissions.

This week at the Tokyo motor show, Mazda has once again announced new internal combustion technology, this time known as Spark Controlled Compression Ignition or SPCCI. Dubbed SkyActive X, this latest power unit combines the HCCI technology with the tried and true spark plug. It still uses a very high compression ratio and a combustion chamber similar in shape to the those used in diesel engines.

Mazda says it can switch seamlessly between compression and spark ignition, depending on load. It uses a new split fuel injection system and an in-cylinder pressure sensor to keep stabilize combustion and control the heat of the cylinder head. Mazda claims 30% more torque from a given quantity of gasoline and 20% better fuel economy. Here’s how Mazda explains it:

“The SKYACTIV-X controls the distribution of the air-fuel mixture in order to enable lean burn using the SPCCI mechanism. First, a lean air-fuel mixture for compression ignition is distributed throughout the combustion chamber. Next, precision fuel injection and swirl is used to create a zone of richer air-fuel mixture—rich enough to be ignited with a spark and to minimize nitrous oxide production—around the spark plug. Using these techniques, SPCCI ensures stable combustion.”

But how does it work? This is a case in which a video is worth ten thousand words. Enjoy!

So, what does any of this have to do with the coming electric car revolution we are all hoping for? Not much, actually.  But despite our fondest wishes, internal combustion engines will be with us for many more decades. If they all used significantly less fuel and created fewer carbon emissions in the meantime, that would be a good thing, right?

And if Mazda were to combine its new SPCCI engine with its stunning Vision Coupe Concept on display this week at the Tokyo motor show, that would be a very good thing.

Source: Road & Track



About the Author

I have been a car nut since the days when Rob Walker and Henry N. Manney, III graced the pages of Road & Track. Today, I use my trusty Miata for TSD rallies and occasional track days at Lime Rock and Watkins Glen. If it moves on wheels, I’m interested in it. Please follow me on Google + and Twitter.

  • Syd

    But will this have any emissions problems greater than a normal ICE engine?

    • Ed


      • Steve Hanley

        Mazda says no.

        • Ed

          My response is based on a fundamental of combustion: higher efficiency means higher peak temperatures, which means higher NOX, which means more complex and costly exhaust after treatment. I would be delighted to be proven wrong….but I strongly suspect that the engine control system is VERY tightly wrapped around the EPA test cycle, not the real world.

          • Tim Jonson

            This was recently said to be true of all HCCI engines. I googled it, and now I’m reading the opposite (??) , you can apparently achieve extremely low NOx levels, but power is limited.

          • Ed

            If I am right about the tailoring to the EPA cycle is correct – all companies do it, of course – the new testing environment should highlight it before the engine reaches the market

  • Bug S Bunny

    It’s funny how whenever the Wankel rotary is mentioned, the RX-7 is the first car they think of. Most people forget that in the early 70s (prior to the RX-7), much of Mazda’s entire lineup (coupes, sedans, and pickups) was Wankel powered.

    • Steve Hanley

      I always wanted a Mazda rotary pickup. Or a Cosmo sedan. Loudest race car I ever heard had a 4 chamber Mazda racing engine. Ear splitting and grin inducing at the same time. I had 3 Series I RX7’s, including a GSL-SE. Fine car. Loved to wind it up past 7000 rpm and feel it begging for more revs. Sadly, I got tired of having it stolen. Still miss that car.

  • trackdaze

    Rotary would make a great range extender or coupled phev like the koenigsegg regara.

    Fuel economy issues would be reduced by maintaining the engine at its optimal efficiency. Low down it sucks juice like a toddler after a trampoline session. And of course it would only be required occasionally when max acceleration or you outdrove the electric only range thereby extending the life of the rotary its other disadvantage.

    Talk is they are looking at applying skyactive tech to the rotory.

    • Steve Hanley

      Yup, somewhere in a back room at Mazda people are still trying to make the rotary relevant. They better hurry up!

      As for use as a range extender, the rotary has always had an excellent power to weight ratio and it does have significantly fewer moving parts than a piston engine. Hmmmm…..

  • gregsfc

    Caution to the author and other future writers about this promising technology: HCCI, if it does succeed, is not “gas” engines or “gas” cars. It is not exactly diesel either; it’s a combination of diesel combustion and spark-ignition. Fuel does not make an engine type; it’s the process by which it ignites or combust. Some are right mostly about the concern for costs regarding HCCI and exhaust treatment. Hyundai has made the engine type work, but claims that exhaust treatment cost is too near that of diesels. It’s not right though to blame it on NOx. It is combustion of diesel fuel itself that produce NOx that is particularly hard to break down from an engineering standpoint. Not the engine out levels specifically.

  • tony D

    So much time and money to make HCCI work for high load transitions. How about we just eliminate the need to respond to that.
    1 stop turning the wheels with the ice. Serial hybrid with electric motor for propulsion.

    2 design ice with HCCI to run binary at constant rpm maybe 2000 as a genset. On at 20 SOC off at 80 SOC . This engineering team has one task. produce the most electricity with the least fuel.

    3 equip with smallest battery to run as stop start with at creep for rush hour bump and grid. have ecu keep stats and recommend the optimum size to install after 6 months.