Mazda SkyActiv-X, a spark-aided Homogenous Charge Compression Ignition

Source: Kickaffe via Wikimedia Commons CC BY-SA 4.0

The battle for high efficiency, low emission engine continues to gather momentum when Mazda made a revolutionary announcement earlier this month in the form of the SkyActiv-X engine technology, which Mazda predicts will find its way into production cars by 2019. What the heck is that? Another one of Mazda's trick techno wizardry? In no uncertain term, it is one heck of a piece of tech. It is Mazda's speak for Homogenous Charge Compression Ignition (HCCI) engine, which mixes air and fuel in the cylinder like a gasoline engine, but ignite by piston compression like a diesel engine.

That sounds like a win-win proposition-diesel efficiency combines with clean gasoline emission. Except with a but. A BIG but. There has never been a production HCCI engine in mainstream cars. Why? Well, it's due to this conundrum: timing. Unlike on conventional gasoline and diesel engine where ignition timing can be easily controlled and adjusted with the spark plug (gasoline) and fuel injector (diesel), HCCI has no such luxury. It relies solely on cylinder temperature which if not controlled properly, can create a whole host of nasty problems such as the dreaded knock when it's too hot or compromised combustion if it's too cold.

To remedy this problem, Mazda throws in the spark plug. The idea is that when condition is less than ideal for compression ignition (cold engine, low ambient temperature, high load/rpm), the spark plug will take care of ignition. During highway driving or low load/rpm, the engine will revert to compression ignition to increase efficiency. Speaking of efficiency, Mazda claims their SkyActiv-X tech can improve efficiency by 20-30%. Bold claims there Mazda.

Efficiency aside, this warrants some questions:

1. How will Mazda ensure seamless transition from spark ignition to compression ignition and vice versa? 
2. What approaches will Mazda adopt to control the time combustion occurs?

Let Jason Fenske of Engineering Explained offers some educated guess:

Video Credit: Engineering Explained