Billed as the Next Big Thing for the gasoline engine, Homogeneous Charge Compression Ignition (HCCI) allows for diesel-like operation wherein compression, not spark, combusts lean air/gasoline mixtures for big fuel-economy gains. At least, that?s the theory. Volatile and complex to manage, HCCI?s useful operating window?and efficiency improvements?is limited to low loads: idle or cruising on the highway. Higher rpm and higher engine loads require HCCI engines to revert to spark ignition, an unpredictable, complex process.
For years, HCCI?s challenges have meant that production applications have eluded industry juggernauts such as Hyundai, General Motors, Ford, and Mercedes-Benz. Now, tiny Mazda, the Japanese automaker that has just 2 percent of the global car market, says it will put a compression-ignition gas engine on sale by 2019. Mazda?s Skyactiv-X engine technically doesn?t use HCCI, though, trading that acronym for SPCCI, or Spark Plug Controlled Compression Ignition. Verbiage aside, the end result is the same, compression ignition of a lean air/fuel mixture. Here?s how it works. Nerd alert!?it?s complicated.
To Recap, This Is How Engines Work
Understanding Skyactiv-X is easier with a background in the ?suck, squeeze, bang, blow? of the four-stroke gasoline engine?s Otto combustion cycle, a diesel?s operation, and finally HCCI. Let?s begin with the gas engine, which mixes air and fuel during the intake stroke (piston moving down in the cylinder) before igniting it a...