IRVINE, Calif.—Despite rumors to the contrary, the inner explosion engine is distant from dead. Recently we’ve seen several technological advances that will significantly boost the potency of gasoline-powered engines. One of these, first reported back in Aug 2017, is Mazda’s breakthrough with application ignition. On Tuesday, Mazda invited us to its RD trickery in California to learn some-more about this crafty new Skyactiv-X engine, but some-more importantly we actually got to drive it on the road.
What’s so special about this engine then?
The thought behind Skyactiv-X is to be means to run the engine with as gaunt a fuel-air reduction (known as λ) as possible. Because very gaunt explosion is cooler than a stoichiometric greeting (where λ=1 and there is accurately adequate air to totally bake any proton of fuel but no more), reduction appetite is squandered as heat. What’s more, the empty gases enclose fewer nasty nitrogen oxides, and the new air gets put to work. It absorbs the explosion feverishness and then expands and pushes down on the piston. The outcome is a cleaner, some-more efficient, and some-more absolute engine. And Skyactiv-X uses a very gaunt mix: a λ up to 2.5.
That sounds like an automotive holy grail, so you competence be wondering since everybody isn’t doing it. As is so mostly the case, the genuine universe isn’t utterly so simple. The problem with very gaunt air/fuel mixtures is that their explosion isn’t utterly stable; since the fuel molecules are so much reduction concentrated, it’s easy for the explosion eventuality to hiss out. The solution, then, is to restrict things to a much larger border than normal. And if you restrict the fuel and air enough, a smashing thing happens: it ignites but the need for a spark.
This is famous as comparable charge application ignition, or HCCI, an thought that Kyle Niemeyer covered in depth for us back in 2012. HCCI has some other advantages, too. On top of blazing cooler and with fewer pollutants, the explosion eventuality happens faster, with a aloft vigour peak, so you get some-more work out of the same energy. All of that sounds flattering wonderful, so you’re substantially asking yourself since every gasoline engine on the highway doesn’t just use HCCI.
Unfortunately, it has been one of those ideas that worked in the lab but couldn’t ever utterly be translated into a prolongation engine. The biggest problem has always been determining accurately when during the engine cycle application ignition occurred, something that you wish as close to top-dead core as possible.
HCCI, but with a hint plug?
Mazda’s breakthrough was in realizing that the hint block could still have a role. The Skyactiv-X engine has been designed to have a very high application ratio—16:1 in fact—and to use a very gaunt air:fuel ratio, but both of these are just next the threshold indispensable for HCCI to occur. Instead, Mazda’s engine uses a hint to get the party started; the ensuing fireball then adds some-more feverishness and vigour to the explosion chamber, et voila! Compression ignition has been triggered. Mazda calls this Spark Controlled Compression Ignition, or SPCCI.
Obviously, this wasn’t but challenges. The fuel:air brew needs to be a little richer nearby the hint for it to light than you wish it to be via the rest of the cylinder. These need to be graphic regions to equivocate λ dropping to 2 or next (which won’t bear application ignition). That’s achieved by swirling the air inside the cylinder and generating a spiral effect, where the ease core has a low adequate λ to light by spark, surrounded by a high λ segment that then undergoes application ignition.
Mazda’s next plea was to forestall pre-ignition, or knock. Higher application ratios boost the intensity for knock, which is since aloft application ratio engines customarily also need some-more expensive, aloft octane fuel that is knock-resistant. Now, technically, application ignition is knock, but if it occurs before you wish it to—at top passed center—bad things can happen, since a explosion eventuality will strive downward vigour on the piston as it’s moving up on a application stroke.
The solution here was to use reduction time to feverishness the fuel:air mix. There’s a tiny initial injection of fuel at first, then the bulk of the fuel is introduced into the cylinder as late as probable during the application stroke. This is finished using mixed entrance injectors to boost atomization and blending of fuel and air.
If all that wasn’t enough, there’s the combined problem of gripping lane of application ignition. In the past, this has been one of the hardest problems for HCCI engines to solve. Ideally you wish explosion to occur at the same indicate in the engine cycle any time—about 4 degrees after top passed center. But as ambient conditions change—a cold day in Denver contra a prohibited one in Houston—the time indispensable for the fireball to strech sufficient vigour also changes. So the engine ought to be means to change hint timing to keep the arise vigour at the right spot.
Skyactiv-X does this by actively monitoring the vigour in any cylinder, so it knows the vigour arise snippet of every explosion event. If it deviates, the hint timing is practiced to compensate. Jay Chen, one of Mazda’s powertrain engineers, explained that this was something the company had been meditative of for a while, but it has only recently been done probable now that engine control processors are quick adequate to control things eventuality by event.
Skyactiv-X has some other facilities that compute it from Mazda’s stream Skyactiv-G engine family. It has a higher-pressure direct-injection fuel complement to promote atomization that runs somewhere between a stream gasoline approach injection complement and a diesel DI system, nonetheless we wasn’t means to get a specific fuel vigour figure. There are in-cylinder vigour sensors, which are required to feed the engine’s digital smarts the information it needs to control timing. There’s a low-boost Roots-type supercharger of sorts—Mazda calls this a high response air supply, as its pursuit isn’t to supplement power, and it only really contributes under high loads. There’s also an air-to-water intercooler, an empty gas recirculator (EGR) cooler that helps forestall beforehand combustion, electric non-static valve timing actuators (that use stepper motors) for faster valve timing, and, finally, a 48v amiable hybrid start-stop system.
Listing picture by Jonathan Gitlin