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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.
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 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, SimanaitisSays.com, 2017