Mazda’s SkyActiv-X Engine Feels So Normal, You’ll Never Realize How Amazing It Is [Prototype Review]

Mazda has an ace up its sleeve that may spark a new golden age for internal combustion

With the advancement in battery technology and increasing demand for electric vehicles, you may consider the internal combustion engine done for. Well, it’s not, you know. Mazda believes our future, at least for the next couple decades, lies with the internal combustion engine. “But wait”, you may ask. “If that’s true, how come Mazda hasn’t invested in hybrid technology or downsized, like every other manufacturer has?” Well, that’s what I went to California to find out.

Mazda flew me out to its R&D headquarters in Irvine, California to learn about its next-generation engine technology. The premise: not only are we not letting go of the internal combustion engine – we’ve found a way to make it even better. It’s one thing to sit in a presentation with all the technical information, but quite another to experience it first hand. It turns out that under the body of a matte-black Mazda 3 was the heart of the company’s future.

Welcome to the SkyActiv-X prototype. How does this change the game? In short, in combines the best aspects of gas and diesel engines. The idea is to run an engine with an extremely lean fuel-air mixture to achieve lower emissions and better fuel economy without resorting to hybrid technology or forced induction.

Mazda SkyActiv-X Prototype
Mazda SkyActiv-X Prototype [Photo: TFLcar]

How It Works

Mazda realizes the only way to run an engine with such a lean fuel mixture is through a process known as homogeneous charge compression ignition (or HCCI). You take an extremely lean air-fuel mixture, where there’s more air than fuel, and squeeze it to the point where heat and pressure cause the mixture to combust without the need for a spark to start the process. The issue here is that, because there are so few fuel molecules and they’re spread so far apart, the combustion isn’t particularly stable.

The cool part of HCCI is that the air-fuel mixture burns cooler and releases fewer pollutants into the atmosphere. The combustion cycle also happens faster with higher pressures, so you get more work (i.e. more power and torque) out of the energy in the fuel. Unfortunately, the issue comes where the fuel-air mixture spontaneously combusts on its own, and you can’t control when it happens. When that happens, you get knock, and knock can severely damage the engine in short order.

SkyActiv-X Engine
Mazda’s engine utilizes Homogenous Charge Compression Ignition (HCCI), but that process alone too unstable to reliably utilize outside a narrow range of conditions. [Photo: Mazda Tech Forum presentation]

The Challenge: Controlling When Compression Ignition Occurs

There’s a certain heat and pressure threshold with knock where, once you cross it, you start getting the aforementioned unpredictable combustion. That severely limits the range in which HCCI actually works, so it isn’t terribly practical in the real world. There are so many external factors and climates out there, how do you reliably control when compression ignition occurs?

Mazda’s breakthrough came with this revelation: the spark plug should still play a role. The SkyActiv-X engine runs at a high-compression ratio – 16:1 in its current prototype stage – and a very lean fuel-air mixture, just below the threshold needed for HCCI to happen. The problem there, naturally, is now you’re not getting combustion at all. Mazda’s answer? Use a spark plug.

As with an ordinary spark ignition engine, SkyActiv-X uses a spark to kick off the combustion event, the resulting fireball creates more heat and pressure, and bang! You get compression ignition. Mazda’s dubbed this solution Spark Controlled Compression Ignition, or SPCCI. In short, using the spark plug at times where conditions aren’t ideal for compression ignition to occur on its own will widen and smooth out the range in which the engine can effectively and reliably operate. That solution came with its own set of problems however.

Marrying spark ignition with compression ignition

The whole purpose here is to improve fuel economy. Another issue Mazda faced with trying to use a spark to control compression ignition is one where, by definition, the air-fuel mixture is too lean to effectively ignite. To get around that problem, the air-fuel mixture needs to be richer near the spark to ignite, but leaner throughout the rest of the cylinder to achieve efficient compression ignition.

“How does that work?”, you might ask. Well, the engine injects a small amount of fuel during the intake stroke to provide enough fuel to spark, but also keep the mixture lean so it doesn’t auto ignite. Then, it introduces the bulk of the fuel as late as possible in the compression stroke to achieve compression ignition. SkyActiv-X uses a small supercharger to control air delivery to the cylinders and create a vortex effect inside the combustion chamber to create two distinct fuel-air regions.

Mazda SkyActiv-X Engine
An illustration of cylinder swirl, demonstrating a hotter “eye” region near the spark plug which starts the process for compression ignition to occur. [Photo: Mazda Tech Forum presentation]

Controlling fuel distribution and reliable ignition through cylinder swirl

Think of it like a hurricane. The leaner, compression ignition fuel-air mixture swirls around the cylinder, while the calm “eye” provides a prime site in which to inject a richer air-fuel mixture near the spark plug. Once the spark plug fires, the richer region ignites, generates heat and pressure, then the other “HCCI” region ignites. Ideally, you want to hit that sweet spot after top-dead center every time.

Ambient conditions create another problem, however. External conditions here in Colorado on a cold winter’s day are different than, say, a hot, sweltering summer afternoon in Florida. As a result, the timing to get that perfect heat and pressure changes due to the air coming into the cylinders. SkyActiv-X conquers this problem by actively monitoring the pressure in each cylinder and adjusting the spark timing to keep the pressure right in the butter zone for compression ignition every time.

SkyActiv-X Engine
Mazda SkyActiv-X prototype engine. [Photo: TFLcar]

So what is SkyActiv-X like to drive?

Okay, if you don’t quite understand how it works, let me explain how it feels to drive. What particularly struck me about getting behind the wheel of the SkyActiv-X prototype is just how normal it feels. There are a few distinct differences between this engine and the current SkyActiv-G engine family. There’s that low-boost Roots-type supercharger. It’s not there to add power, but rather to control air supply under higher engine loads. There’s a higher pressure direct injection system to achieve consistent, yet lean fuel delivery. There’s the in-cylinder pressure sensors, and there’s also an air-to-water intercooler and EGR recooler to keep the temperatures just right to achieve ideal combustion. Finally, there will be a mild hybrid start-stop system. Mazda didn’t disclose the exact voltage figures, nor was the system active during my test drive.

I spent two hours driving the automatic and manual prototypes up and down sunny Orange County highways near Mazda’s R&D offices, and came away with a revelation. I, as a lay person, found the experience just so…typical. The SkyActiv-X engine is a 2.0-liter unit, making about 178 horsepower and 170 lb-ft of torque in its current state. That’s actually the same amount of power as the current 2.5-liter SkyActiv-G, and Mazda also claims SkyActiv-X will get 20% better fuel economy (they wouldn’t give me exact MPG numbers, though).

SkyActiv-X Prototype
Mazda SkyActiv-X prototype on the streets of Irvine. [Photo: Mazda]
The prototype isn’t quite ready for primetime just yet – there were a few knocking moments, as a few transition spots between spark ignition and compression ignition modes need to be hammered out. However, most of the transitions were exceptionally smooth, the engine felt nice and punchy, and I didn’t detect any weak areas in the rev band.

It’s Not Just a New Engine

I feel Mazda’s big marketing point on SkyActiv-X should be just how normal it is. The technology is amazing and they’re close to perfecting it. Dave Coleman, one of Mazda’s development engineers, stated Mazda’s aiming to put SkyActiv-X into production next year. When consumers are able to buy it, they may be hard pressed to notice it even has a new engine. It feels like and sounds like a completely normal engine.

The greatest part here is that, because its so normal, you don’t have to change your lifestyle to the car. Mazda’s achieving diesel-like fuel economy without the emissions repercussions. They’re also improving range without resorting to electrification (although that is coming eventually). Mazda’s technology proves there’s still room left for improvement in the good old internal combustion engine yet.

Stay tuned to for more updates on Mazda’s next generation technology! We’ll also cover the company’s sustainability vision and the next-generation SkyActiv chassis as well. Subscribe to The Fast Lane Car and TFLnow on YouTube for more prototype news, views, and real-world reviews!

Mazda SkyActiv-X and next-generation chassis improvements
The changes to next-generation Mazdas won’t stop with the engine.
[Photo: TFLcar]