Mazda Skyactiv X Prototype Explained – Video
Mazda isn’t hedging its bets on electric power just yet, believing that until the worldwide electrical grid is predominantly powered by renewable energy, an electric vehicle’s tailpipe emissions are too far offset by the dirtiness and high CO2 values of the fossil-fuelled coal, oil, and gas power plants that supply their electricity.
Mazda’s SPCCI technology avoids the need to switch quite cleverly. Its spark plugs fire during every ignition phase of the four-stroke combustion cycle, but do so in different conditions depending on the driving requirements.
Most of the time, the Skyactiv-X engine operates under compression ignition. During induction, the injector fires a lean fuel mist into the combustion chamber as a belt-driven supercharger pushes air into the mix. During the compression phase, the injector fires again with a small but slightly richer mixture around the spark plug. As the piston reaches top dead centre the plug ignites this richer mixture, the expansion of which raises pressure in the combustion chamber – enough to simultaneously ignite the lean mixture around it with compression alone.
The use of the spark plug means compression ignition can be timed, and means with simple timing adjustments and by injecting a leaner or richer mixture, the transition between spark and compression ignition cycles can be constantly varied. No switching, very little complication, and the ideal ignition for a given driving scenario. The engine operates in SPCCI mode the majority of the time, can switch into an ultra-lean burn mode during low revs and low load cruising (which, because of controlled, lower combustion temperatures, doesn’t produce high levels of NOx like previous lean burn engines), and graduates to regular spark ignition under high load and high revs.