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. 

Ford, GM, Nissan, Honda, Mercedes-Benz, and Hyundai all developed prototype HCCI engines, and even the US Department of Energy funded research into the technology. While most efforts seemingly petered out, Mazda made compression-ignition with gasoline a reality. 

Skyactiv-X Engine

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. 

“A diesel engine has a higher efficiency than a gasoline engine because we can run it at a higher compression ratio,” says Margaret Wooldridge, a professor at the University of Michigan’s Mechanical Engineering program. “But the Otto cycle is intrinsically more efficient than the Diesel cycle, so if we can run a spark-ignition engine at a higher compression ratio, we’ll get more efficiency out of it,” Wooldridge says. “So essentially, it’s like ‘How can you make gasoline spark ignition – where it’s the fuel properties that limit you – more like a diesel engine?’” 

Skyactiv-X prototpye

A Mazda3 prototype equipped with a Skyactiv-X engine.

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. A June 2001 article from Scientific American explains it well: 

“Because the amount of burning fuel is low in comparison to the volume of air inside an HCCI engine, the combustion temperatures stay relatively low. That means the engine produces only small quantities of nitrogen oxide and dioxide (collectively, NOx). Also, because the charge is well mixed and does not contain excess fuel, the combustion generates only small amounts of sooty particulates. Engine efficiencies are high because the HCCI combustion process allows the use of high, diesel-like compression ratios (generating more power per unit of fuel burned) and because, like diesels, HCCI engines can meet load demands without the use of intake throttling, thus eliminating so-called breathing losses.” 

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. An August 2007 New York Times 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. 

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. 

SKYactiv-x
Skyactiv-x hardware

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-liter 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. 

Skyactiv-X infotainment

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. 

Mazda declined to provide Motor1 with a sales mix of Skyactiv-X vs Skyactiv-G, but SPCCI is only available on Mazda’s 2.0-liter. What’s most telling is that Mazda hasn’t brought Skyactiv-X to the U.S. in any capacity. It’s the automaker’s largest market by far, accounting for around 30 percent of its sales last year. Instead, it’s pushed its 2.5-liter turbo, a plug-in hybrid drivetrain, and a new 3.3-liter inline-six.  

Mazda confirmed to Motor1 that it is working on a Skyactiv-X six-cylinder, but when asked if this or any other SPCCI engines would come to America it simply said “[w]e’ll continue to make our best efforts to offer US customers the products/technologies that meet their needs.” 

Mazda3 Skyactiv-X

The six-cylinder may be our best hope. In an early 2020 interview with Automotive News, Skyactiv-X engineers said the current 2.0-liter engine wasn’t powerful enough for the US market. They also said that Americans generally don’t prioritize fuel economy to the same extent that buyers in other markets do. A larger Skyactiv-X engine could work here. 

Wooldridge says that while the automotive industry is still working toward improving internal combustion, there’s not a lot of work going on with HCCI. “You can control HCCI if you're going to put in some pretty significant technology... there were a lot of advances through variable valve timing. There were some cool ideas with variable compression ratio,” she says. “You can brute force it. There’s no doubt that you could make this very successful, but it was going to be so costly from what I’ve seen.” 

For Mazda, getting SPCCI to work requires a lot of this costly hardware. Mazda is also a very small automaker. It doesn’t have the resources of, say, GM, yet it’s facing the same pressure to commit huge resources to electrification. And is Skyactiv-X itself profitable as it currently stands? Will it ever be? 

Skyactiv-X assembly line

In a recent interview with Yahoo Finance, Mazda North America CEO Tom Donnelly said that the brand’s focus is set on hybrids, plug-in hybrids, and battery EVs. SPCCI may be well suited to a hybrid powertrain, where the car keeps the engine in its most efficient operating window and manages the interaction between internal-combustion and electric motor(s). But so too do the Atkinson and Miller combustion cycles employed by today’s hybrid cars, and neither requires all the fancy hardware of SPCCI.  

Ultimately, it’s a little too early to render a verdict on Skyactiv-X, but it feels like the deck is stacked against it. Yet, we live in an ever-changing world. Mazda may yet prove us wrong. 

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