From one explanation I've read [1], they actually maintain two AFR "zones" in the cylinder; one rich zone around the spark plug, with the rest of the cylinder m

From one explanation I've read [1], they actually maintain two AFR "zones" in the cylinder; one rich zone around the spark plug, with the rest of the cylinder maintained lean enough for compression ignition. Then by igniting the rich mixture via spark, little NOx and less heat is produced, while the remaining lean mixture probably producing negligent NOx as expansion occurs after TDC. If they can control it tightly enough, they can probably maintain the exact pressures and temperatures to just skirt under EPA/Euro requirements in a wide range of conditions, and switch to regular spark ignition outside of that range. [1]: http://www.enginelabs.com/news/mazdas-new-spark-controlled-compression-ignition-revealed/ http://www.enginelabs.com/news/mazdas-new-spark-controlled-c...