More than six years ago, Mazda debuted a technology that promised to revolutionize internal-combustion. Now, we’re wondering if it was a false dawn.
Skyactiv-X, as Mazda calls it, represented the culmination of decades of auto-industry research and development into a compression-ignition gasoline engine. For years, major automakers and research groups worked toward creating a homogeneous-charge compression-ignition (HCCI) engine, which held the promise of diesel-like fuel economy with far fewer emissions.
Some basics: Gasoline engines typically run the Otto cycle. Here, the engine uses a spark plug to ignite an air-fuel mixture either premixed in the intake or, as in a direct-injection engine, mixed in the combustion chamber itself. Diesel engines run – you guessed it – the Diesel cycle. Compression itself generates the necessary heat to ignite the air-fuel mixture. There are no spark plugs.
An HCCI engine uses gasoline, but operates somewhat like a diesel, using compression instead of spark to quickly and evenly ignite a very lean air-fuel mixture. That air-fuel mixture is generated much in the same way as a gasoline engine, creating a homogeneous rather than heterogeneous charge like a diesel engine.
All well and good, but HCCI combustion is a difficult process to control, so it’s only possible within a very small set of parameters. A report about GM’s HCCI engine prototype notes that compression-ignition was only available between 1,000 and 3,000 rpm. Even within that band, the driver had to be very careful with the throttle pedal. Outside of that operating range, the engine switched back to typical spark ignition.
An Otto-cycle gasoline engine uses spark to control ignition timing, while a diesel engine uses fuel injection for the same purpose. Wooldridge says that both offer super precise control over engine performance. Achieving such precision with an HCCI engine is much more difficult without these levers. Without spark, compression ignition with a lean mixture of air and gasoline is unpredictable, and at some point, the car must switch to spark ignition anyhow.
Mazda’s great innovation was using spark to control compression ignition in a process called, imaginatively, Spark Controlled Compression Ignition (SPCCI). As an article from Mazda’s UK PR department explains, a Skyactiv-X engine creates a very lean mixture of air and fuel during the intake stroke.Â
At the end of the compression stroke, a little more fuel is injected. This essentially creates two air-fuel mixtures, a rich one formed right by the spark plug, and a leaner mix throughout the combustion chamber. The spark plug ignites the richer mixture and the fireball created then ignites the lean mixture, beginning the compression-ignition process. Since the spark plug is always used, the switch between compression and spark ignition happens quickly and seamlessly.
It requires a lot of novel – and one assumes expensive – hardware to run. An in-cylinder pressure sensor monitors conditions at all times, while a super high-pressure fuel-injection system like a diesel’s precisely meters fuel. Mazda also uses a clutched Roots-type supercharger not to boost power output, but to lean out the air-fuel mixture by up to double the ideal ratio of 14.7:1.
At its launch, Mazda touted that a 2.0-litre Skyactiv-X engine brought a 20- to 30-percent boost in fuel economy compared with its spark-ignition Skyactiv-G counterpart. In its current iteration, with a 15:1 compression ratio – earlier iterations ran 16.3:1 – the Skyactiv-X engine offers 183 horsepower and 177 lb-ft of torque to the Skyactiv-G’s 120 hp and 157 lb-ft. Yet Skyactiv-X returns 43.55 mpg overall on the European WLTP test. Cycle, while Skyactiv-G manages 41.97 mpg. C02 emissions also fall from 127 g/km to 121 g/km.
So, a big power increase, with nice bumps in fuel economy and emissions performance, too. And at least in a top-trim, UK-spec Mazda3, the Skyactiv-X engine only costs around 8 percent more. Great, right? Well, Skyactiv-X doesn’t seem like it’s set the world on fire yet.
