IRVINE, Calif. – Despite rumors to the contrary, the internal combustion engine is far from dead. Recently we have seen many technological advances that will increase the performance of gasoline powered engines. One of these, first reported back in August 2017, was Mazda’s success with compression ignition. On Tuesday, Mazda invited us to its R&D center in California to learn more about this clever new Skyactiv-X engine, but more importantly we actually got to drive it on the road.
What is so special about this engine then?
The idea behind Skyactiv-X is to be able to run the engine with as much lean fuel-air mixture (known as λ) as possible. Because lean combustion is cooler than the stoichiometric reaction (where λ=1 and there is just enough air to burn each fuel molecule completely but no more), less energy is wasted as heat. What’s more, the exhaust gases contain some nasty nitrogen oxides, and the unused air is put to work. It absorbs combustion heat and then expands and pushes down on the piston. The result is a cleaner, more efficient, and more powerful machine. And Skyactiv-X uses it abundant partial pressure: a λ up to 2.5.
That’s like the grail of car cleaning, so you might wonder why everyone doesn’t do it. As is always the case, the real world is not so simple. The problem with very lean air/fuel mixtures is that their combustion is not particularly stable; because the fuel molecules are not very concentrated, it is easy for the combustion event to take place. The solution, then, is to mix things up to a much larger scale than usual. And if you pour enough fuel and air, a wonderful thing happens: it ignites without the need for a spark.
This is known as cost compression integration, or HCCI, a concept that Kyle Niemeyer covered in depth for us back in 2012. HCCI has some other benefits, too. On top of the combustion process and with less impurities, the combustion event happens faster, with a higher pressure, so you get more work from the same energy. All of this sounds amazing, so you’re probably asking yourself why every gasoline engine on the road doesn’t just use HCCI.
Unfortunately, it has been one of those ideas that worked in the lab but could never translate to a production machine. The biggest problem is always managing correctly when the engine’s compression ignition occurs, something you want as close to dead center as possible.
HCCI, but with a spark plug?
Mazda’s success lies in knowing that the spark plug can still have a role. The Skyactiv-X engine is designed to have a very high ratio—16:1 in fact—and to use a very lean air:fuel ratio, but both of these are below the threshold required for HCCI to occur. Instead, the Mazda engine uses fire to get the party started; The resulting fireball then adds more heat and pressure to the combustion chamber, and voila! Compression stress is induced. Mazda calls it Spark Control Compression Ignition, or SPCCI.
Obviously, this is not without challenges. Fuel: the air mixture needs to be more rich near the spark for it to ignite than you want it to be throughout the rest of the cylinder. These need to be separate areas to avoid dropping to 2 or below (which will not undergo compression fire). That is achieved by rotating the air inside the cylinder and generating a vortex effect, where the cooling medium is small enough λ to ignite through the spark, surrounded by a high λ region located at the compression junction.
Mazda’s next challenge is to stop the spark, or crash. Higher compression ratios increase the potential for knocking, which is why higher compression ratio engines often also require more expensive, higher octane fuel that is resistant to knocking. Now, technically, the compression in attack, but if it occurs before it blows – at the top dead in the middle – bad things can happen, because the combustion event will cause downward pressure on the piston as it moves up on the stroke.
The solution here is to spend less time heating the oil: air mixing. The first small injection of fuel comes first, then most of the fuel is injected into the cylinder as late as possible during the compression stroke. This is done using multiple orifice injectors to increase atomization and mixing of fuel and air.
If all that wasn’t enough, there’s the added problem of keeping track of the compression ratio. In the past, this has been one of the most difficult problems for HCCI engineers to solve. Ideally, you want combustion to occur at the same point in the engine’s system each time—about four degrees after dead center. But as the ambient conditions change—a cold day in Denver versus a hot one in Houston—the time required for the fireball to reach sufficient pressure also changes. So the engine should be able to change the spark timing to keep the peak pressure in the right place.
Skyactiv-X does this by monitoring the pressure in each cylinder, so you know the pressure rise trace of every combustion event. If it deviates, the spark timing is adjusted to compensate. Jay Chen, one of Mazda’s powertrain engineers, explained that this is something the company has been thinking about for a while, but it has been made possible recently now that the engine control systems are fast enough to control event by event.
The Skyactiv-X has a few other features that differentiate it from Mazda’s current Skyactiv-G engine family. It has an advanced direct injection fuel system to boost atomization that runs somewhere between a current gasoline direct injection system and a DI diesel system, although I wasn’t able to get a specific fuel pressure figure. There are in-cylinder pressure sensors, which are necessary to feed the engine’s digital brains the data needed to control the timing. There is a low-boost Roots-Type supercharger of sorts-Mazda calls this a high response air supply, as its job is not to give power, and it only really contributes under high loads. There’s also an air-to-water intercooler, an exhaust gas regulator (EGR) that helps prevent premature combustion, electronic valve timing adjusters (that use stepper motors) for faster valve timing, and, finally, 48v hybrid starter mild-stop system.