Simanaitis Says

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MAZDA HAS announced a production breakthrough coming in 2019 with its Skyactiv-X, hitherto a research goal known as Homogenous Charge Compression Ignition that combines the advantages of gasoline and diesel combustion.

To appreciate HCCI, let’s backtrack to the two conventional forms of internal combustion, gasoline and diesel. Gasoline IC ignites its homogenous mixture (that is, a fixed proportion of fuel and air) with a spark plug. Diesel IC ignites its stratified (variable and layered) charge by compression alone. Thus, their respective names, spark-ignition IC and compression-ignition IC.

Each technology has tradeoffs of engine design and cost, operational constraints, combustion efficiency, and emissions output. For instance, a typical gasoline engine’s compression ratio is around 10:1. By contrast, a diesel’s compression ignition (and its efficiency) depends upon a very high compression ratio; 20:1 is not uncommon. One consequence of a diesel’s combustion efficiency is its high emissions of NOX, oxides of nitrogen. (Volkswagen’s diesel scam was NOX-related.)

Gasoline IC compression ratio is fuel-dependent; higher-octane gasoline allows a higher compression ratio and greater efficiency. However, design a compression ratio too high (or use a fuel of too low an octane) and the result is knock, aka detonation or premature ignition. It’s a harmful spontaneous ignition of the mixture at the wrong time and wrong place within the combustion chamber.

As the name HCCI implies, this innovative combustion is gasoline-like (in having a homogenous charge of fuel and air) and diesel-like (in igniting it solely through compression). Loosely, but not absurdly non-technically, HCCI combustion is sort of like a high-compression-ratio gasoline engine beneficially and controllably knocking, only this time the knocking is occurring throughout the combustion chamber at an appropriate time.

Finessing HCCI operation has been a research goal for decades. Theoretical benefits include 30-percent higher efficiency of gasoline combustion, negligible NOX emissions or diesel soot, and enhanced fuel economy. Disadvantages stem from HCCI’s inherent “smart knock” nature: Controlling the combustion isn’t as simple as timing gasoline IC’s spark or diesel IC’s fuel injection.

On August 8, 2017, Mazda Motor president Masamichi Kogai discussed a family of Skyactiv engines displaying advanced technology. Image from

Computerized engine controls and other subtleties are at the heart of Mazda’s Skyactiv-X technology entering production in 2019. Full details haven’t been released, but Mazda claims such an engine is 20 to 30 percent more efficient than a comparable gasoline sibling, at least as fuel-efficient as its diesel counterpart, and with none of the diesel’s soot and NOX shortcomings.

One detail known is the Skyactiv-X’s supercharging. This variable boost gives another tool in finessing what Mazda calls “spark controlled compression ignition.” Skyactiv-X could monitor temperature and pressure in each cylinder and vary boost or valve timing accordingly. It might vary EGR (exhaust gas recirculation) to optimize combustion-chamber temperatures for best HCCI operation.

Mazda’s SPCCI, spark controlled compression ignition, can be tailored to engine load. Image from, one of the more detailed current descriptions of Skyactiv-X.

Mazda’s retention of spark ignition is beneficial in cold starts and also gives the option of dual-mode HCCI operation. In lean-burn light-load cruising, for instance, compression ignition gives optimal fuel economy. Under heavy load, a seamless transition to spark ignition responds with more power.

Internal combustion isn’t dead yet, despite plenty of efforts around the world to eliminate it. However, be it battery electric, fuel-cell electric, or whatever else, any replacement must be demonstratively better than existing technology. And existing IC tech is a moving target. ds

© Dennis Simanaitis,, 2017


  1. carmacarcounselor
    August 10, 2017

    I have been following this technology since Daimler was reported to be working on it some time back. I am sure it was an engineering holy grail long before that.
    One question: They claim a certain percentage improvement in efficiency, supposedly approaching that of a traditional diesel.
    A part of a diesel’s better fuel mileage derives from the greater amount of carbon in a unit of diesel fuel, while a significant disadvantage of the spark ignition engine is that it farts out somewhere around 25% of its available energy as noise and heat.
    Does their claim mean that the HCCI extracts a proportion of gasoline’s available fuel value comparable to a diesel’s extraction from diesel fuel, or that its energy output per unit volume of fuel (mpg) is comparable to a diesel?

    • simanaitissays
      August 10, 2017

      A question to you: Doesn’t the diesel also produce heat and noise (indeed, higher combustion temperature and more clatter than gasoline IC)?

      I appreciate your question as well: Just where do HCCI benefits come from? i seem to recall a lot was made of HCCI’s dual-mode nature: lean-burn CI mode under light load, transitioning to spark-ignited HC mode when power is demanded. And, of course, the higher cr than with traditional (knock-limited) gasoline IC.

      There were plenty of HCCI SAE papers about 15 years ago when its holy grail was being quested. Alas, if I saved them, I have no idea where they are now. A literature search at SAE would be appropriate.

      Or we’ll have to wait for a Mazda paper on the topic. Or a press kit.

      By the way, I agree about diesel fuel’s more Cs; thus its more stringent emissions targets in Euro regs. There’s also the higher compression ratio, Carnot Cycle diagrams and all that jazz I’ve forgotten. If, indeed, I ever knew it very well…

  2. sabresoftware
    August 11, 2017

    And I believe that the energy density of gasoline is still much higher than the best current battery technology (46.4 MJ/kg for gasoline, 48 for diesel, versus 0.36 to 0.875 for Lithium-Ion batteries according to Wikipedia). To get equivalent travel distance would require some very heavy batteries, which would require significantly more energy to move all that additional mass, hence yet more heavy batteries, etc.

    In our jurisdiction 83% of our electricity comes from combustion of coal, natural gas and biofuels. The Government has mandated that coal will be gone by 2040. We currently have 752 windmills. To replace the coal we’d need another 3700 or so more. To eliminate natural gas, another 2700. Not all areas are amenable to wind installations. Solar isn’t even a blip, other than residential systems, maybe a few commercial installations.

    Adding hydro isn’t really feasible. The last dam built in our area (and that was for irrigation, not power) was probably 30 years ago, and there was so much opposition at that time.

    Nuclear, well don’t even go there.

    And that is before we start adding demand for electric cars.

    Scary. One easy conclusion is that our power rates are going to skyrocket.

    Of course 2040 is 23 years away, or in political speak, 5-6 governments later. A lot can change in that time.

    Of course all that could change in a few days when we might in the early stages of WWIII if two belligerents have their way. And the environmental hit from WWIII will make the CO2 threat seem like a tempest in a teapot.

    OK on that happy note I’m off to bed!

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