How Gas Compression-ignition Engines Work

The Future of Gasoline Engines
A car powered by a gas compression-ignition engine can be at least as efficient as an electric car, and possibly even more so. Reimar Gaertner/UIG

Shortly after Mazda's announcement, auto industry experts began to speculate whether a mass-market compression-ignition engine could "save" gas engines. That is, as the industry moves more toward hybrid and electric technology, could this gas engine be efficient enough to be a viable contender?

Chen says Mazda is motivated by the belief that, "by squeezing every bit of efficiency out of the internal combustion engine (in conjunction with electrification once the internal combustion engine is perfected), we can deliver a method of powering the automobile well into this century that has the potential to generate the same or less 'well to wheel' CO2 emissions as pure battery electric vehicles powered from fossil fuel based power plants of various forms."

In other words, Mazda thinks that with continued innovation, a car powered by a gas engine can be at least as efficient as an electric car, and possibly even more so. Let's take a look at how this breakthrough in compression-ignition technology is different from those that came before it.

In 2007, Motor Trend drove a Saturn Aura powered by a compression-ignition engine, which achieved a 15 percent reduction in fuel consumption over a regular Aura [source: Markus]. At the time, GM was expecting to release a vehicle with a compression-ignition engine in 2015, but the Saturn brand was shut down just a few years later, and GM gradually shifted its focus to electric and plug-in hybrid vehicles such as the Chevrolet Volt.

Around the same time, Mercedes-Benz was working on a compression-ignition system called DiesOtto, and Ford had a project in development, too [source: Estrada]. However, neither of these engines achieved the green light for production, and Hyundai's experience may help explain why [source: Markus].

Aside from Mazda, Hyundai has probably made the most progress, with efforts that first came to light around 2013 [source: Markus]. The company designed its version of a compression-ignition engine without spark plugs or glow plugs, with a target release date of 2023.

Despite promising progress, Hyundai revealed in 2016 that the engine components just weren't strong enough to deal with the compression required for the process to work. Stronger engine components, namely the block, crank and bearings, can be designed, of course; that's how diesel engines work. It's just very expensive, and those stronger components add weight to the car and reduce its overall efficiency. Hyundai had planned all along to use a turbocharger to increase power and maintain the necessary compression, but they discovered they'd also need a supercharger, which further busted the budget. And finally, Hyundai was not satisfied with the amount of pollution produced by these powertrains. In the end, the project was much more expensive and not nearly as clean and efficient as planned [source: Markus].

Mazda's development efforts have been going on almost as long as its competitors.

"Skyactiv-X was always in the plans even before the first generation Skyactiv was launched," explains Mazda engineer Chen. "The first step in this roadmap was Mazda's Skyactiv Technology [which was] introduced in 2009. The key improvement at that time was the application of unconventionally high engine compression ratio to increase overall engine efficiency as well as powertrain performance. This was achieved through a synergistic combination of existing techniques applied together to achieve what was (until then) believed impossible for production engines."

In layman's terms: "Skyactiv" is the term for Mazda's strategy of boosting compression to increase efficiency, and Mazda had to tinker around a bit to get the upcoming Skyactiv-X to work. As a result of that tinkering, Mazda added a spark plug into the mix, so the engine can switch between compression and spark-ignition depending on what is the most efficient at the time. This might sound like it goes against the basics of high-compression engine technology, but Chen says it works.

"This breakthrough, which we call spark-controlled compression ignition (SPCCI), greatly expanded useable range of compression-ignition operation and control as well as provided the solution for a seamless transition between CI [compression ignition] and SI [spark-ignition] combustion modes used at high engine speeds (in the case of Skyactiv-X)," Chen says.

Put more simply, the spark plug is the magic ingredient that enables the engine to run smoothly and adjust for different conditions, and it'll only be used when absolutely necessary. Mazda's engine is designed to monitor itself and adjust its operation based on factors like current environmental conditions, the way the car is being driven, and the driver's preferences and settings [source: Estrada].

After Mazda came up with this idea, it took another two years to develop the engine, during which time another important decision was made. Vehicles equipped with Skyactiv-X engines will feature superchargers to boost horsepower specs, which will improve driving dynamics and help convince potential buyers to take a chance on this new technology [source: Estrada].

The last big question — when can drivers expect to see it? A spokesperson for Mazda says that the company can't yet disclose which vehicles will be the first equipped with the Skyactiv-X engine or when they'll be available. We also don't know if vehicles powered by compression-ignition engines will cost more than comparable vehicles with spark-ignition engines. It's safe to speculate, though, that while Mazda will be the first to market with this technology, other manufacturers are almost certain to follow.

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