Mazda’s Skyactiv X is revolutionary because of the way it bangs – or why pressure makes things difficult inside a gasoline engine
Mine Proving Ground, Yamaguchi Prefecture, JAPAN-
Mazda’s new engine tech, the company claims, makes Skyactiv X the most fuel efficient gasoline engine in the world. But how did Mazda succeed when companies like Honda, Toyota, Mercedes-Benz and more failed to develop the same kind of tech?
Before delving further, we need a short recap on how engines work.
Almost all combustion engines use a four stage cycle to produce power, known as the Otto cycle, after the man who came up with it.
You can refer to our other explainer on how alternative cycle engines work, as it includes a preface on normal Otto cycle engines.
Suck, squeeze, bang, blow: Fuel/air mix is drawn into the cylinder as the piston moves down (suck), it is squashed as the piston moves back up (squeeze), it is ignited by a spark plug (spark ignition, or SI) and is pushed down, creating power (bang), and it moves backup to push out the byproducts of combustion (blow).
Both diesel and gasoline engines use the Otto cycle, but it’s how the banging happens that makes all the difference.
Diesel engines have no spark plugs, the explosion happens through compression or auto ignition. As gas is compressed, its potential for ignition increases until it reaches blowing point. Compression ignition (CI) is simultaneous and homogeneous, that is, the entire charge explodes at once.
In contrast, a gasoline engine’s charge is ignited by a spark, from a spark plug, so the explosion originates from one source and expands outward, or propagates. Watching something explode, it seems instantaneous, but if you watch stuff blowing up in slow-motion on YouTube, you’ll know it’s really not the case.
Mazda’s Skyactiv X engine uses compression ignition in combination with a spark, called Spark Controlled Compression Ignition, or SPCCI. It’s still a sub-type of Homogeneous Charge Combustion Ignition.
Compression ignition is superior to spark ignition because it burns the fuel cleanly and completely, so there is less pollution, and more work is extracted from each charge. The fact that diesels pollute more is due to nature of the fuel, which creates more soot (particulate matter) and oxides of nitrogen.
So why don’t we simply make a compression ignition gasoline engine, combining the benefits of diesel with the less-pollutive nature of gasoline?
Gasoline is a more volatile fuel than diesel – you can set gas on fire with an open flame, but not diesel. Controlling it when compressed is even more difficult. In fact, engineers have spent huge amounts of time and money getting gasoline to do exactly the opposite.
You may have heard the term ‘compression ratio’, it’s simply used to describe an engine that squeezes its fuel-air charge a lot. A high compression ratio means more power, but it also brings with it the chance of premature detonation, before the spark ignites. That’s bad, as it damages the engine, and causes power loss.
That’s where octane comes in. There’s a misconception that a higher octane rating (RON number) means a superior fuel, but it simply denotes how resistant it is to knocking. Most engines use 95RON, but when in doubt read your owner’s manual.
So if compression ignition is hard to control when you don’t want it to happen, it’s also hard to control when you do want it to happen, especially at the level of precision required for modern engines.
But Mazda has plenty of experience with this – its current Skyactiv G engines have among the highest compression ratios for modern mass market production engines, and is also a reason why they’re very efficient.
An average Japanese 1.6-litre has a compression ratio of around 10.0:1, while the 2.0 Skyactiv G has 14.0:1, with the Skyactiv X having a ratio of 16.0:1.
HCCI (homogenous charge compression ignition) has of course been experimented with by bigger car companies, and Mazda’s justifiably proud of Skyactiv X being a production ready tech.
“Larger companies like Honda, Toyota, Mercedes-Benz and Nissan have made HCCI prototype technology, but Mazda has gone ahead of them and made it real,” adds Mr Tokushige.
Mazda’s step forward was to make an engine that combined both ignition types, rather than just focusing on making a solely CI gasoline engine. It sounds almost simple when you say it like that, but Mazda did spend more than a decade and a lot of R&D money to get Skyactiv X a reality.