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How Gas Compression-ignition Engines Work

Mazda SKYACTIV-X engine
Mazda's SKYACTIV-X engine is the world's first commercial gasoline engine to use compression ignition. Mazda

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In summer of 2017, Mazda made an announcement: The auto company found a way to make compression-ignition gasoline engines for passenger cars. Mazda claimed its new engine could improve fuel economy by 20 to 30 percent, which is a considerable achievement for a gasoline engine.

Before taking a dive into this technology, it's worth noting that the compression-ignition engine isn't a new concept. Formula 1 cars use compression-ignition engines, and several other automakers have attempted to develop a commercially viable version for passenger cars. But Mazda's engine, dubbed the Skyactiv-X, will be the first mass produced and commercially available engine of this type. Thanks to Jay Chen, a powertrain engineer with Mazda, HowStuffWorks was able to learn how this breakthrough was achieved. First, though, we have to take a look at an engine's basic functions.

An engine works by igniting fuel in two ways: heat and compression. Spark-ignition engines are found in most gasoline cars. In these types of engines, the spark plugs fire to ignite fuel in the combustion chamber, while the fuel and air mixture is also being compressed. This is a very simplified version of the process, of course, just to illustrate the main difference between the two engine types. Spark-ignition engines follow a cycle and require precise timing to work, but are generally reliable under a variety of conditions [source: Knight].

Compression-ignition engines operate more like diesel engines. Diesels are designed for much higher compression (which requires heavier components and stronger construction) and use glow plugs as a heat source rather than spark plugs. Glow plugs heat up the compression chamber, which in turn increases the compression within the chamber. When the fuel is added to the chamber, it's sprayed across the tip of the glow plug, but the process relies on the compression more than the contact of the fuel and the plug. The lack of "spark" helps diesel engines achieve higher EPA ratings than gasoline engines with otherwise similar specs [source: Stewart].

If we're focusing on gas, you might be wondering, what's the point of explaining how a diesel engine works? Simply, to illustrate the importance of compression. The best way to improve the gas engine is to figure out how to increase compression, which enables the engine to use its fuel supply more efficiently.

A compression-ignition gasoline engine combines the best parts of these processes. The engine is programmed to trap air (typically, engine exhaust) in the engine cylinder by adjusting the timing of the exhaust and intake valves. The fuel injectors add fuel to this trapped exhaust, and since the trapped mixture is under very high compression, the relatively small amount of fuel is able to ignite.

Compression-ignition engines can even be broken down into two different types [source: Lindberg].

  • Homogenous Charge Compression Ignition (HCCI): This engine mixes air and fuel, and then compresses that mixture until it ignites. Mazda's engine will be the first HCCI-type engine to be mass produced.
  • Gasoline Direct Compression Ignition (GDCI): This engine squirts the gasoline into a mixture of air and exhaust that has already been compressed.

The main difference between these two engines is the point in the process in which the fuel is added, achieved through adjustments to the engines' cycles and timing. Otherwise, the engines function similarly; the compression is the most important factor.

The SKYACTIV-G 2.0-liter has the power of a 2.5-liter engine and the efficiency of a 1.5-liter diesel engine. Mazda
The SKYACTIV-G 2.0-liter has the power of a 2.5-liter engine and the efficiency of a 1.5-liter diesel engine. Mazda

Compression-ignition engines have a few advantages and at least as many drawbacks. Among its benefits are:

  • It uses less fuel than a spark-ignition engine
  • It uses the fuel more efficiently (in other words, less power is lost to the actual ignition and to excess heat)
  • Since less fuel is used, the car pollutes less

"As a rough analogy, spark-ignition is akin to starting a fire by lighting just one edge of the newspaper kindling and letting the flame gradually climb across the paper," explains Mazda powertrain engineer Jay Chen, via email. "[Compression ignition] is more like spontaneous combustion where the fuel and air has reached critical pressure and temperature, and the entire charge changes phase at the same time thus releasing all the energy at once. By releasing all the energy nearly at once, [compression ignition] can extract more power (since it happens well before the expansion ratio is used up) from the same amount of air while using two to three times less fuel and at much cooler combustion temperatures, which further reduces wasted heat energy and emissions formation."

Sounds great, right? The problem is that these engines are really finicky — if they were easy to design and use, we'd be driving them by now. Even if you're not familiar with diesel engines, you may have heard that they can be inconvenient under sub-optimal conditions. Part of that is due to the diesel fuel itself, which has a tendency to "gel" in very cold temperatures. We don't have that problem with gasoline, which stays liquid even in sub-freezing conditions. But compression ignition can still be affected by the weather and other ambient conditions, as well as other factors like the quality of the fuel.

"Until now, compression-ignition internal combustion engines existed only in stable laboratory conditions or raw vehicle prototypes too rough to be applied in production," Chen says.

In other words, if the pressure and temperature in the cylinders isn't carefully maintained, the process won't work. Temperatures that are too cold can damage the engine's sensitive components. If the engine gets too hot, it can start to knock — a condition that occurs when the fuel-air mixture gets too hot and detonates at the wrong time, which wastes fuel and results in a poor-running engine. A spark-ignition engine can also get too cold or too hot, but has a much higher margin of error.

Making a compression-ignition engine work reliably depends on a precise combination of air, fuel and exhaust gases mixed in the perfect ratio, at the perfect compression, with just the right amount of heat applied at the correct time. As we know, no one's been able to build a car with a compression-ignition gas engine yet, so this process needed to be further refined.

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
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.

Author's Note: How Gas Compression-ignition Engines Work

Unlike a lot of my colleagues, I'm not especially worried about "saving gas engines," even though that would probably help with job security. Maybe I should be a bit more selfish about that, but I decided to write about the compression-ignition engine simply because I'm intrigued by any innovation that can help make a car more efficient.

For that reason —sustainability in general — I'm eager to test drive a vehicle with a compression-ignition engine as soon as they're available. Like hybrids and electrics, I think there will be a lot of conversation about whether or not these vehicles are powerful enough. Honestly, I suspect the average person won't be able to tell the difference. There are a lot of ways to make a car interesting to drive aside from simply making it as powerful as possible, and that's an area in which Mazda excels.

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Sources

  • Brown, Jacob. Specialist, Product Communications, Mazda. Personal correspondence. Sept. 1, 2017.
  • Chen, Jay. Powertrain Engineer, Mazda. Personal correspondence via Jacob Brown. Sept. 1, 2017.
  • Estrada, Zac. "Mazda could have the tech to save the internal combustion engine." The Verge. Aug. 8, 2017. (Aug. 30, 2017) https://www.theverge.com/2017/8/8/16099536/mazda-compression-ignition-engine-technology
  • Knight, Cheryl. "How a Car Ignition System Works." YourMechanic. Nov. 19, 2015. (Aug. 29, 2017) https://www.yourmechanic.com/article/how-a-car-ignition-system-works
  • Lindberg, Austin. "Hyundai Developing Gasoline-Burning Compression-Ignition Engine." Car and Driver. Nov. 18, 2013. (Aug. 30, 2017) http://blog.caranddriver.com/hyundai-developing-gasoline-burning-compression-ignition-engine/
  • Markus, Frank. "Technologue: Love Child - Driving the Ultimate Bastard Engine." Motor Trend. Nov. 19, 2007. (Aug. 30, 2017) http://www.motortrend.com/news/technologue-41/
  • Markus, Frank. "Whatever Happened to Hyundai's HCCI Engine?" Motor Trend. Dec. 14, 2016. (Aug. 30, 2017) http://www.motortrend.com/news/whatever-happened-hyundais-hcci-engine/
  • Mazda. "Mazda Announces Long-Term Vision for Technology Development, 'Sustainable Zoom-Zoom 2030.'" Aug. 8, 2017. (Aug. 20, 2017) http://www2.mazda.com/en/publicity/release/2017/201708/170808a.html
  • Stewart, Jack. "Mazda's Crafty New Engine Makes More Miles From Less Fuel." Wired. Aug. 9, 2017. (Aug. 30, 2017) https://www.wired.com/story/mazda-injection-compression-skyactivx-engine/
  • Szymkowski, Sean. "How Mazda's SkyActiv-X engine, based on HCCI, actually works (video)." Green Car Reports. Aug. 21, 2017. (Aug. 30, 2017) http://www.greencarreports.com/news/1112222_how-mazdas-skyactiv-x-engine-based-on-hcci-actually-works-video

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