Mazda’s New SkyActiv Engine Works Like A Diesel

In a time when hybrid cars seem to be the way of the future, Mazda is thinking outside of the box with a different kind of hybrid engine. A recent report by Nikkei reveals that Mazda’s next generation of SkyActiv motors will be powered by gasoline, but ignite it by compression alone, not spark plugs.

Mazda’s next generation of SkyActiv motors will go into production at the end of next year. They will use a system called homogenous charge compression ignition (HCCI) to make the fuel/air mixture go boom rather than traditional spark plugs. This is the same way diesel motors ignite their fuel, but no gasoline production motors have used this method until now.

The popular YouTube channel Engineering Explained provides a great explanation of how HCCI motors work, both gas and diesel.

Interestingly, EE cites Nissan rather than Mazda as his information source. And, indeed, Nissan has articles about HCCI and its future in gas powered cars on their web site. But it seems that Mazda has jumped the gun on Nissan as far as actually putting this technology into production.

Why haven’t we seen HCCI gasoline motors until now? The answer is that the technology has only recently advanced to the point where it’s possible. Diesel engines are much more forgiving. It’s a little known fact (look, I’m from Boston, I can quote Cliff Clavin) that Rudolf Diesel originally designed his engine to run on vegetable oil rather than dead dinosaurs. The petroleum based “diesel fuel” as we know it today was only supposed to be a temporary measure until a vegetable oil distribution system was put into place. Instead, Rudolf Diesel died under highly suspicious circumstances, and petroleum based diesel fuel remained in use rather than the promised vegetable oil substitute.

But I digress. You can run many different fuels in diesel engines – standard diesel fuel, various types of vegetable oil, and, with some preheating and filtration, even waste vegetable oil from your local restaurant’s deep fryers. As a result, getting the conditions right for spontaneous ignition is pretty simple compared to gasoline, which is far more picky and prone to pre-ignition, also known as knock, which can damage the engine if it’s severe enough. But modern technology, such as direct injection and variable valve timing, provide far more precise control over conditions in the cylinder than in the past. As a result, it is now possible to create the proper conditions for spontaneous ignition in a gasoline motor.

Mazda 3

As EE describes, the primary benefit of HCCI over traditional spark plug ignition is spontaneous combustion. All fuel in the cylinder ignites simultaneously, rather than starting at the spark plug and expanding outward. This increases efficiency as well as reduces NOx emissions. Mazda claims a 30% increase in efficiency with the new motor. Though Mazda did not give any more precise numbers than that, AutoEvolution reported that “the second-generation SkyActiv engine ‘would give the current Mazda3 mileage approaching 30 km per liter, according to estimates.’ That’d be 3.3 l/100 km and 70.5 miles per gallon, figures that would put the 2018 Mazda3 at the very top of the compact segment.” Never mind the compact segment – this also annihilates the current king of hybrid economy, the Toyota Prius, which gets 58mpg city / 53mpg highway. Yes, the Prius is considered midsize, not a compact like the Mazda3, but strangely the subcompact Prius C is even worse at 48/43.

Mazda MX-5

What does this mean for enthusiasts? Is the SkyActiv powered MX-5 doomed to keep its economy car horsepower figures forever, with the complexities of HCCI putting it far outside the reach of tuners? Making more power isn’t as easy as just dumping more fuel into an HCCI motor, since spontaneous combustion happens in a particular sweet spot combination of compression, temperature, and other factors. But it’s certainly not impossible, either. Diesel engines have been using HCCI for well over 100 years, and are often tuned and/or turbocharged to improve their performance.

We’ve enjoyed a recent proliferation of turbos in cars you wouldn’t previously have expected them in to provide the fuel economy numbers of a small engine without sacrificing the peak performance of a large engine. Ford’s EcoBoost motors, in particular, feel to me like they have two more cylinders than they actually do. This has also been a huge benefit to enthusiasts. A simple $500 tune from APR can boost a VW GTI to 316hp. That’s around 100 more than stock (depending on whether you got the GTI Performance Package and its extra 10hp or not), and more than the Golf R’s stock 292hp, with no additional modifications!

Will the introduction of HCCI gasoline motors be a similar game changer for enthusiasts? I don’t think so. Its primary benefits appear to be efficiency and emissions rather than performance. I have no problem with that, just as long as enthusiasts like us (a dying breed) can still get the performance we want. And the end is not nigh. The Wikipedia article on HCCI suggests that an engine could be designed to operate both HCCI as well as traditional spark ignition. That way, similar to cylinder deactivation, the engine runs as efficiently as possible when you don’t need the power – which, if we’re honest, is the vast majority of the time in normal driving – but switch back to good old fashioned spark ignition when you put your right foot down for maximum power. It would be more complicated than either system alone, but so is an engine that uses both traditional port and new fangled direct fuel injection. Such an engine works just fine in my Subaru BRZ.

Subaru BRZ / Toyota 86 / Scion FR-S motor
Photo credit: Motor Trend

As long as emissions and fuel economy technology don’t get in the way of my fun, I welcome it.

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(Thanks to Phil Geraghty for the tip!)