How Mazda Skyactiv-X Works
01-25-18, 09:51 AM
#1
How Mazda Skyactiv-X Works
"Take everything you know about engines and turn it around," Mazda North America Vehicle Development Engineer Dave Coleman says, patiently and with a look of benevolent pity, as he's quizzed about the particulars of the company's new engine. The Skyactiv-X engine is enigmatic — and deceptively simple in operation. And the bottom line for American consumers is that they'll be able to buy a car by late 2019 that provides diesel-like fuel economy but runs on regular old gasoline.
Coleman was right. It's hard to wrap your head around an engine that thrives just at the point when most gas engines would aggressively self-destruct. It uses a supercharger to pump additional air — but not additional fuel. It uses spark plugs to start a combustion cycle that normally doesn't need a spark. And, quixotically, it's not displacing Mazda's own American-market diesel engine, currently languishing in a seemingly endless hell of regulatory approval.
More bizarre: Mazda is a tiny automaker facing real existential headwinds, and gasoline compression ignition is a massive challenge. GM and Hyundai announced compression ignition, or HCCI, projects (full name, homogeneous charge compression ignition) to great fanfare, but they never amounted to a production hill of beans, crippled by reliability issues or horrible vibrations. Worse, they only worked at an unusably narrow range — low RPMs and low loads. HCCI research improved direct-injection gas and diesel engine technologies for these companies, but HCCI itself remains untamed.
The ideal amount of fuel for a conventional engine to burn is about a 14:1 air-to-fuel ratio. That lets every molecule burn nicely, in theory. If you want to burn less gas, you need to get more air to ignite — for Skyactiv-X, this is roughly double the amount of air a Skyactiv-G engine would be able to burn.
Master this, and you get lower combustion gas temperatures — that means less energy is wasted as heat that has to be frittered away in the cooling system. This also eliminates some efficiency losses involved with throttle operation. But it's really tricky to get a mixture with very little fuel in it to ignite properly. Imagine trying to light a fire with the kindling spread out all over the room.
Imagine a normal gasoline combustion cycle. You mix just the right amount of air with the perfect squirt of fuel, squeeze it with the piston, and then blast it with a spark to light it off. It explodes, expands and pushes the piston down. Get the timing or compression wrong and the fuel might preignite, exerting destructive pressure on the piston while it's still moving upward. That is, to put it mildly, bad.
One way to avoid preignition is to richen the mixture. All those fuel molecules help damp down the propensity for the whole volatile cocktail to go off early. But the best way to save fuel is to cut out as much as possible. Hence all of Skyactiv-G's tricks to burn less without blowing up: direct injection and relatively high compression ratios. Don't add fuel until just the right moment of exceptional pressure, and there's nothing to detonate early. The only limitation is that it takes a significant amount of time for the flame caused by a spark plug to reach every corner of the combustion chamber. With a lean mixture, it might not even get there, resulting in the flame fizzling out.
Also, the longer it takes, the further down the piston moves, reducing the overall energy transferred to the piston and more time for kinetic energy to be frittered away as heat, wasted because it's doing work heating up the cylinder walls rather than acting on the piston.
Compression ignition is so seductive because it ignites the mixture all over. Less time for flame propagation, less wasted energy. Also, a sharper and shorter pulse of energy. That's why it's also dangerous. You need to time it exactly right. And the engine load and speed ranges at which Mazda wanted SPCCI to work mean that an HCCI process simply wouldn't work.
Remember, in theory the ideal compression is just a hair below the preignition threshold. Since ambient temperature and humidity affect air density, the perfect temperature and pressure range for SPCCI is constantly changing. The ideal compression rate is around 16:1. So this engine is dancing on the edge of disaster the whole time it's operating in SPCCI mode.
As Chen puts it, "It's funny that what we tried to avoid with Skyactiv-G is now what we want with Skyactiv-X."
Using cylinder pressure monitors, ambient condition sensors, and a lot of computing power, the application of spark is timed just right. So too is a split pulse of fuel from a very high pressure, multi-orifice injector. The high pressures (somewhere between Skyactiv-G and -D, but Mazda won't say how much) and spray pattern help the small amount of fuel spread evenly. The combustion chamber and injection pulse is designed to create a swirling effect, like a little hurricane — complete with an "eye." Mazda discovered that it could make the eye a little richer than the rest of the mixture by injecting some fuel later, while keeping things overall too cool and underpressurized to actually self-combust.
Igniting that small enriched portion in the "eye" creates enough extra heat and pressure in the rest of the combustion chamber to ignite the leaner mixture around it. The result is a controlled and very quick burn of an extremely lean mixture overall. It's the puzzle piece that makes SPCCI work at around 80 percent of the operating range of the engine in normal circumstances.
In normal spark combustion mode, used at startup and very high loads, the engine uses a variant of the Atkinson cycle to lower effective compression to prevent preignition, leaving the intake valve open to allow the piston to push some charge back out. That's nothing new; a Prius uses the Atkinson cycle at low loads to save fuel, and so do Mazda's traditional Skyactiv engines. It's not as efficient as SPCCI, but it's also only using spark-ignition for a small range of operating conditions.
In a conventional supercharged engine, more fuel is added to burn with the additional air, giving a small engine the lungs of a much bigger one. In the X, it's only there to add air, further leaning out the mixture. In fact, Mazda isn't quite sure what to call it, because calling it a "supercharger" makes it sound like the 2.0-liter Skyactiv-X should act like a bigger and more powerful engine than it does. It's a low-boost, relatively small unit, so, it's better to think of it like an air pump, there to cram some extra air in when needed.
The easy confidence to which the Mazda engineers described the principles of Skyactiv-X made it sound like a foregone conclusion, but think about it more and the remarkable complexity stands out. This engine is solving a lot of very complex problems about how to burn fuel correctly thousands of times per minute. This is a technology that can only exist in our charmed world of cheap, high-speed computing — and it's also a technology that requires dismantling many preconceived notions of internal combustion to grasp.
This is an exciting technology, one that may make internal combustion engines relevant for far longer than anyone imagined a few years ago — in theory, at least. How it'll do in the marketplace depends on too many factors to predict right now. But you can't help but admire the pluck of tiny Mazda, the company that perfected the rotary so many years ago, tackling a similarly impossible challenge.
In between diesel and spark ignition, but it's neither
To truly understand it, you have to dive into the contradictions. Take that regular old gasoline: Contrary to common sense, the lower the octane, the better it works. In the lab, the Skyactiv-X engine loves 80 octane. The lowest Americans get is 87, so the engine is tuned for that octane. Go higher and you lose some low-end torque.Coleman was right. It's hard to wrap your head around an engine that thrives just at the point when most gas engines would aggressively self-destruct. It uses a supercharger to pump additional air — but not additional fuel. It uses spark plugs to start a combustion cycle that normally doesn't need a spark. And, quixotically, it's not displacing Mazda's own American-market diesel engine, currently languishing in a seemingly endless hell of regulatory approval.
More bizarre: Mazda is a tiny automaker facing real existential headwinds, and gasoline compression ignition is a massive challenge. GM and Hyundai announced compression ignition, or HCCI, projects (full name, homogeneous charge compression ignition) to great fanfare, but they never amounted to a production hill of beans, crippled by reliability issues or horrible vibrations. Worse, they only worked at an unusably narrow range — low RPMs and low loads. HCCI research improved direct-injection gas and diesel engine technologies for these companies, but HCCI itself remains untamed.
The benefits of lean combustion
Why even try to tame HCCI? The answer is much better fuel economy and lower emissions. Less burned carbon-based fuel, less carbon dioxide released. That's simple. But there are some thermodynamic reasons for the lean combustion you can achieve with compression ignition that are worth explaining.The ideal amount of fuel for a conventional engine to burn is about a 14:1 air-to-fuel ratio. That lets every molecule burn nicely, in theory. If you want to burn less gas, you need to get more air to ignite — for Skyactiv-X, this is roughly double the amount of air a Skyactiv-G engine would be able to burn.
Master this, and you get lower combustion gas temperatures — that means less energy is wasted as heat that has to be frittered away in the cooling system. This also eliminates some efficiency losses involved with throttle operation. But it's really tricky to get a mixture with very little fuel in it to ignite properly. Imagine trying to light a fire with the kindling spread out all over the room.
The challenges of burning less fuel
So Mazda's gone sideways a bit. Rather than eliminate the spark when running in compression-ignition mode, spark is always involved. Mazda claims the production version will be up to 30 percent more efficient than the already efficient Skyactiv-G engine. To understand why, Jay Chen, Mazda Powertrain Engineer, stepped in to explain how Mazda's variant of compression ignition, known as Spark Controlled Compression Ignition (SPCCI), works.Imagine a normal gasoline combustion cycle. You mix just the right amount of air with the perfect squirt of fuel, squeeze it with the piston, and then blast it with a spark to light it off. It explodes, expands and pushes the piston down. Get the timing or compression wrong and the fuel might preignite, exerting destructive pressure on the piston while it's still moving upward. That is, to put it mildly, bad.
One way to avoid preignition is to richen the mixture. All those fuel molecules help damp down the propensity for the whole volatile cocktail to go off early. But the best way to save fuel is to cut out as much as possible. Hence all of Skyactiv-G's tricks to burn less without blowing up: direct injection and relatively high compression ratios. Don't add fuel until just the right moment of exceptional pressure, and there's nothing to detonate early. The only limitation is that it takes a significant amount of time for the flame caused by a spark plug to reach every corner of the combustion chamber. With a lean mixture, it might not even get there, resulting in the flame fizzling out.
Also, the longer it takes, the further down the piston moves, reducing the overall energy transferred to the piston and more time for kinetic energy to be frittered away as heat, wasted because it's doing work heating up the cylinder walls rather than acting on the piston.
Compression ignition is so seductive because it ignites the mixture all over. Less time for flame propagation, less wasted energy. Also, a sharper and shorter pulse of energy. That's why it's also dangerous. You need to time it exactly right. And the engine load and speed ranges at which Mazda wanted SPCCI to work mean that an HCCI process simply wouldn't work.
How Skyactiv-X solves the combustion ignition puzzle
The solution is to diverge completely from the idea that compression ignition needs to be sparkless. Since there's no transition from spark to sparkless operation, many of the hiccups of HCCI are also avoided. How it works is elegant but also devilishly difficult.Remember, in theory the ideal compression is just a hair below the preignition threshold. Since ambient temperature and humidity affect air density, the perfect temperature and pressure range for SPCCI is constantly changing. The ideal compression rate is around 16:1. So this engine is dancing on the edge of disaster the whole time it's operating in SPCCI mode.
As Chen puts it, "It's funny that what we tried to avoid with Skyactiv-G is now what we want with Skyactiv-X."
Using cylinder pressure monitors, ambient condition sensors, and a lot of computing power, the application of spark is timed just right. So too is a split pulse of fuel from a very high pressure, multi-orifice injector. The high pressures (somewhere between Skyactiv-G and -D, but Mazda won't say how much) and spray pattern help the small amount of fuel spread evenly. The combustion chamber and injection pulse is designed to create a swirling effect, like a little hurricane — complete with an "eye." Mazda discovered that it could make the eye a little richer than the rest of the mixture by injecting some fuel later, while keeping things overall too cool and underpressurized to actually self-combust.
Igniting that small enriched portion in the "eye" creates enough extra heat and pressure in the rest of the combustion chamber to ignite the leaner mixture around it. The result is a controlled and very quick burn of an extremely lean mixture overall. It's the puzzle piece that makes SPCCI work at around 80 percent of the operating range of the engine in normal circumstances.
In normal spark combustion mode, used at startup and very high loads, the engine uses a variant of the Atkinson cycle to lower effective compression to prevent preignition, leaving the intake valve open to allow the piston to push some charge back out. That's nothing new; a Prius uses the Atkinson cycle at low loads to save fuel, and so do Mazda's traditional Skyactiv engines. It's not as efficient as SPCCI, but it's also only using spark-ignition for a small range of operating conditions.
How supercharging helps
Skyactiv-X needs a lot of air, so it employs an unconventional supercharger to help feed it in. It's a small Roots-type blower that is disengaged with a clutch when not needed, reducing parasitic losses.In a conventional supercharged engine, more fuel is added to burn with the additional air, giving a small engine the lungs of a much bigger one. In the X, it's only there to add air, further leaning out the mixture. In fact, Mazda isn't quite sure what to call it, because calling it a "supercharger" makes it sound like the 2.0-liter Skyactiv-X should act like a bigger and more powerful engine than it does. It's a low-boost, relatively small unit, so, it's better to think of it like an air pump, there to cram some extra air in when needed.
Horsepower and torque numbers
Mazda won't talk specifics about engine output figures or fuel economy; most of the numbers are target percentages that compare to current engines. Since the final tune is a ways off, and will also depend on what vehicle it goes into, talking about horsepower and torque is sort of a guessing game. In Frankfurt, Mazda said the prototype engine was making about 187 horsepower and 143 pound-feet of torque. At Mazda HQ in California, the numbers bandied about are closer to 190 horsepower and 180 lb-ft of torque. You'll note these numbers look more like gasoline engine output figures rather than diesel numbers, but the torque curve looks and feels more like a diesel. To read more about how it works on the road, you can pore over our coming first drive review.Taking a step back
The whole operation is extremely complex, and the confidence of the Mazda engineers surely masks a long string of failures and frustrations before a running prototype engine emerged. And it's not perfect (yet). Our prototypes, highly modified Mazda3 chassis that are a preview of the next-generation Skyactiv platform, pinged mightily at certain points — particularly when there was an abrupt change in throttle position. Snap off or onto the throttle, and a diesel-y clatter emerged. Mazda promises that's simply a matter of additional tuning to tamp down those bursts of knock, possibly completely. But the company has definitely tamed the sort of intense stumbling that plagued the HCCI engines when they transitioned from spark to compression ignition. Despite the preignition rattle, there's otherwise little indication under way as to what type of ignition is occurring, and when it switches.The easy confidence to which the Mazda engineers described the principles of Skyactiv-X made it sound like a foregone conclusion, but think about it more and the remarkable complexity stands out. This engine is solving a lot of very complex problems about how to burn fuel correctly thousands of times per minute. This is a technology that can only exist in our charmed world of cheap, high-speed computing — and it's also a technology that requires dismantling many preconceived notions of internal combustion to grasp.
This is an exciting technology, one that may make internal combustion engines relevant for far longer than anyone imagined a few years ago — in theory, at least. How it'll do in the marketplace depends on too many factors to predict right now. But you can't help but admire the pluck of tiny Mazda, the company that perfected the rotary so many years ago, tackling a similarly impossible challenge.
01-25-18, 04:08 PM
#2
Nope. I don't see it.
I've read the original article several times, and disagree with many points the author makes, though perhaps it's just semantics.
My own understanding is that the diesel process is about as good as one can get. It's the expansion ratio that delivers power, or, looking at the same thing from a different angle, it's the amount of maximum pressure differential after ignition that does the work.
So the question then is, do we atomize the fuel into the air before bringing it into the chamber, or simply inject the fuel once the chamber is sealed off? As the piston moves up the air pressure and temperature quickly rise. If the fuel is premixed then if the compression ratio is too high then the fuel self-ignites before the spark would normally set it off, and that's preignition.
In a diesel the air can be compressed to a much higher level since there's no fuel to spontaneously ignite. Then a spray of fuel comes it, and it ignites and spikes the air pressure (the rattle of a diesel). But why not inject gasoline rather than diesel fuel? It, too will ignite. But what?--too aggressively?
Anyway, I see the Atkinson engine as a sort of diesel-like version of a gas engine, in the sense that the expansion ratio (which is the measure of efficiency) can be much higher without causing the mixture to preignite on the compression stroke (because the intake valve closes late, reducing the compression ratio and thus maximum pre-spark pressure).
So how is this Mazda engine supposed to be so great? I recall from my private-piloting days that a typical Lycoming or Continental airplane engine has very detailed graphs showing efficiencies over all operating conditions. The BEST the engine can do, given its moderate compression ratio, is to consume about 0.4 lbs of gasoline per crank horsepower per hour. This would be assuming complete combustion (not running rich) and a wide-open throttle (minimized pumping losses, like a diesel). If we slowly back off the mixture control and lean out the engine, it will not only produce less power but will soon reach the state where it runs roughly, implying to me that some cylinder charges simply don't catch on fire from the spark, even though some fuel vapor is present.
What am I missing here?
I've read the original article several times, and disagree with many points the author makes, though perhaps it's just semantics.
My own understanding is that the diesel process is about as good as one can get. It's the expansion ratio that delivers power, or, looking at the same thing from a different angle, it's the amount of maximum pressure differential after ignition that does the work.
So the question then is, do we atomize the fuel into the air before bringing it into the chamber, or simply inject the fuel once the chamber is sealed off? As the piston moves up the air pressure and temperature quickly rise. If the fuel is premixed then if the compression ratio is too high then the fuel self-ignites before the spark would normally set it off, and that's preignition.
In a diesel the air can be compressed to a much higher level since there's no fuel to spontaneously ignite. Then a spray of fuel comes it, and it ignites and spikes the air pressure (the rattle of a diesel). But why not inject gasoline rather than diesel fuel? It, too will ignite. But what?--too aggressively?
Anyway, I see the Atkinson engine as a sort of diesel-like version of a gas engine, in the sense that the expansion ratio (which is the measure of efficiency) can be much higher without causing the mixture to preignite on the compression stroke (because the intake valve closes late, reducing the compression ratio and thus maximum pre-spark pressure).
So how is this Mazda engine supposed to be so great? I recall from my private-piloting days that a typical Lycoming or Continental airplane engine has very detailed graphs showing efficiencies over all operating conditions. The BEST the engine can do, given its moderate compression ratio, is to consume about 0.4 lbs of gasoline per crank horsepower per hour. This would be assuming complete combustion (not running rich) and a wide-open throttle (minimized pumping losses, like a diesel). If we slowly back off the mixture control and lean out the engine, it will not only produce less power but will soon reach the state where it runs roughly, implying to me that some cylinder charges simply don't catch on fire from the spark, even though some fuel vapor is present.
What am I missing here?
01-25-18, 04:32 PM
#3
Lexus Fanatic
This sounds like basically a 21st-century version of the old Honda CVCC system from the 1970s....using a small, richer mixture in a pre-chamber to fire off the larger, much leaner mixture in the rest of the cylinder. It worked fine on paper....but, with the carburetors and butterfly-chokes of the period, drivability with that system was all but nonexistent until the engine was warm. The new system, of course will have the benefits of precise fuel-delivery with electronic-injection systems....so we'll see if you can actually drive the car while it is warming up....which you couldn't do with the old CVCC.
01-26-18, 09:45 AM
#6
Mazda Skyactiv-X Review
The matte black Skyactiv-X prototype looks like a rough Mazda3, perhaps reconstructed after a bad wreck by an over-enthusiastic owner of a spot welder and lots of gaffers' tape. Ribbed ducts poke out of the dash sending two breaths of conditioned air to no one in particular. Even its revolutionary engine, the thing we're here to experience, is entombed in a massive, nondescript cover to mask its unseemly noises. It's a wild, strange way to meet a very unconventional vehicle that promises diesel-like fuel economy, a wide torque band, and an exotic method for burning less gas than ever before.
It takes a few hours for Mazda's engineers to explain the fundamental principles of operation. For more detail, read our Skyactiv-X Spark Controlled Compression Ignition explainer, but here's a very brief overview.
Skyactiv-X marries some traditional gasoline engine characteristics with a novel form of compression ignition called SPCCI. The key for Skyactiv-X is to use very high compression in the cylinder and an extremely lean fuel-air mixture. Squeezed right to the cusp of getting hot enough to blow up all on its own (which is very hard to predict), a squirt of extra gas and a spark interject to cross that compression-ignition threshold in a controlled and predictable manner.
That takes a few essential components to get just right. One is a massive amount of computer processing power and some pressure sensors in the individual cylinders, because the ambient conditions change how and when these things happen. Skyactiv-X uses a clutched supercharger to pump in additional air when needed to nail the mixture precisely, and high-pressure injectors to get the low ratios of fuel to disperse properly in the chamber. And since it operates like a conventional gasoline engine sometimes, it uses valve timing to lower the very high compression ratio so it doesn't reach combustion ignition in that mode.
In practice, the Skyactiv-X runs in compression ignition mode most of the time. In practical terms, that means it drives like a torquey gasoline Skyactiv engine. The torque curve is broad and flat — diesel-like in that respect. That also means it can get away with using a six-speed transmission and a lower final drive for better response. There's enough grunt and economy together that Mazda can let the engine spin faster — at 60 mph, it's running at roughly 1,000 more RPM than a similar gas engine, with greater efficiency.
Skyactiv-X is complex, and Mazda Vice President of R&D and Design Masashi Otsuka admits it will be an expensive engine to build. "But," he said to us before we drove the cars, with a big lopsided smile, "it's less expensive than a hybrid."
That speaks to Mazda's philosophy in building the Skyactiv engine range in general. Mazda has avoided electrification almost entirely; what electrification it employs is either very minimal (i-eLoop regenerative brakingand capacitor system) or niche (Demio EV, with its rotary range extender option).
Rather than build a bunch of hybrids and EVs that are dependent on incentives to sell (and still don't sell that well — Mazda does have a point here), the company decided long ago to just improve the internal combustion engine as much as possible. That means it has the lowest average EPA fuel economy of any manufacturer — although its overall volume is small, its sales have suffered recently, and it doesn't sell any large and inefficient SUVs or trucks. Mazda's strategy is technically sound, but perhaps not as effective in the real world as it might hope. Then again, as the company is eager to point out, neither is the real-world economy of the downsized, turbocharged engine trend. They're great at EPA testing but miserably thirsty in the real world.
Skyactiv-X isn't the final result of this process; there will be more electrification as that becomes necessary and/or demanded by the marketplace. There's a next-generation pure diesel engine coming, after the long-delayed Skyactiv-D engine, using lessons and perhaps technologies developed for Skyactiv-X. But for now, Skyactiv-X is the absolute bleeding edge.
Which is not how it feels on the road in the least. It's not supposed to feel bleeding edge, to be fair. There are rough edges, too. But mostly, it feels like a direct-injected, high-compression gasoline-powered car. It does not feel supercharged in the traditional sense, either. Off the line, it's definitely perkier than the anemic 2.0-liter Skyactiv-G engine, and on par with the 2.5-liter nonturbo Skyactiv-G on paper, although the -X's torque curve is fatter. The torque wave continues as revs increase, authoritative but not necessarily sporty.
If you want to talk numbers, they're understandably fuzzy. The Skyactiv-X tune isn't final yet, so horsepower and torque figures are a moving target. In Frankfurt, Mazda said the prototype engine was making about 187 horsepower and 143 pound-feet of torque. At Mazda HQ in California, the numbers bandied about are closer to 190 horsepower and 180 lb-ft of torque. That compares favorably with the 2.5-liter Skyactiv-G, which makes 184 hp and 185 lb-ft of torque in the Mazda6 and 187 hp/186 lb-ft in the CX-5. The numbers also differ from the Skyactiv-D diesel, which makes around 173 hp and 310 lb-ft in other markets where its currently sold.
But before you wonder why it's so much less powerful than the new turbocharged Skyactiv-G 2.5T that'll be out soon in the Mazda6, remember this: The Skyactiv-X engine is really intended to beat the weakling 2.0's fuel economy numbers (28 city, 38 highway) by as much as 30 percent, while seriously exceeding its power output of 155 hp and 150 lb-ft. In short, it aims to crush the 2.0 liter's economy with 2.5-liter power.
And it does so with a pleasing torque curve, which couldn't be more different from the peaky downsized turbo gas engines competitors use. Now, it's not as steep a torque curve as a modern turbodiesel, but it also doesn't run out of breath as early. There's unusual grunt all the way to redline, indicated at just shy of 6,000 rpm (where the true redline will be isn't clear at this point). While there will be a small electric motor to enable start-stop functionality on the production version of this engine, it's disabled in these prototypes, so all we're actually feeling is the extra churn enabled by the compression-ignition scheme.
That extra torque also lets Mazda get away with a shorter drive ratio and six-speed transmissions (both manual and automatic were available to test) without sacrificing economy, since the efficient operating range of the engine is so broad. That means less gears to run through when downchanging, less hunting in the automatic, and less complexity. The engine will turn about 1,000 more revs at 60 mph than a comparable Skyactiv-G, which also offers better response.
Really hammer on it at full load and it'll switch to spark ignition, seamlessly. It's only a tablet running on the dashboard that gives away which combustion type is happening, and in normal driving it's almost always in SPCCI mode. Snap off the throttle or back onto it quickly, and there's loud and obvious clatter, the most obvious sign that something's unusual.
That clatter is not supposed to be there. The Mazda engineer riding along tells us that these abrupt throttle position changes are very hard to tune for, that some preignition happens in these situations where the engine can't react quickly enough to the changed circumstances, and also that the company intends to tune these interludes out completely for production. That's going to be essential from an NVH (noise, vibration and harshness) standpoint, in our opinion, because the clatter is loud and jarring — it could certainly hinder broad acceptance.
And it's coming through a massive amount of sound deadening. Mazda refers to it as "encapsulation" — from a secondary "hood" coated with sound deadening hiding the engine from view to panels covering the oil pan and block. Essentially, the Skyactiv-X makes nasty sounds even when working ideally, and Mazda has to add some material to mask it. The price of innovation, it seems. Not that most direct-injection gasoline engines are very pleasant to listen to, either.
The issues do seem, objectively, to be manageable with tuning. The handoff from conventional to compression ignition happens without the stumbling that plagued other companies' attempts to master the art. Underneath the rough edges is an engine that behaves like a Skyactiv-G that's been secretly working out a bit, rather than a diesel engine converted with dark alchemy to run on gas.
And there's more to these prototypes than just the engine. Were it just a regular Mazda3 with a new engine, it wouldn't need the Frankenstein panel surgery it got. But it's actually more of a next-generation Mazda3 underneath, running a seventh generation of a platform that's been evolving for a while. Think all new, but not completely reimagined. Major dimensions seem basically unchanged; the Skyactiv-X would likely fit into a sixth-generation car.
The changes seem more like refinements. Coleman described how various suspension compromises resulted in undesirable lateral motions over certain types of bumps. We watched a video of a driver's head, in slow motion, rocking back and forth. Changes to the bushings turned more of that lateral motion into vertical motion, which humans tolerate better. That means Mazda can stiffen the struts up slightly compared to the current generation car without punishing the occupants, but soften the sidewalls of the tires to get the most out of their vibration absorption and damping properties.
Likewise, unseemly vibrations haven't been attacked with more heavy sound deadening, but rather with careful application of a damping adhesive between certain places in the unibody. The idea is to make the sounds less disagreeable rather than try to crowd them out entirely.
There's also more chassis stiffness overall, with added reinforcements that spread force inputs better throughout the structure. Pair the suspension, chassis, and NVH improvements with some improved seat materials, and Mazda promises the next-generation Skyactiv cars will be less fatiguing and more fun. On the road, the differences were extremely subtle, and perhaps it'd take a long roadtrip to fully reap the benefits. The suspension, however, felt stiff — a bit more than a typical Mazda3, which is exactly what we were told to expect. It didn't feel overly stiff; there was no judder or crashing over imperfections, so the new suspension geometry seems to do its job well. It'll take a winding road to fully evaluate the new chassis' handling.
Despite refinements all around and a wildly unconventional engine, the takeaway here is that it all feels like standard Mazda stuff. Eliminate the clatter and rip off all the gaffers' tape, and it's almost assured that a Mazda3 owner could be convinced it was a revised 2.5-liter engine in a next model year 3. Final tuning will polish the Skyactiv-X's attributes, which are already present if not blindingly obvious to a casual observer. So too will the improvements to the seats and suspension. That it feels evolutionary rather than revolutionary is a compliment to the integration of this nascent technology in an excellent package.
Barring hiccups, we'll see this engine in a production car in late 2019. It's way too early for fuel consumption numbers, but expect a 20-30 percent improvement over the current Skyactiv-G 2.0-liter engine. Depending on its cost, that will definitely get some consumers' attention. And if fuel prices go up, it might spark a frenzy. Either way, no one will be able to ignore Mazda's new engine. The only question is, will anyone buy one?
It takes a few hours for Mazda's engineers to explain the fundamental principles of operation. For more detail, read our Skyactiv-X Spark Controlled Compression Ignition explainer, but here's a very brief overview.
Skyactiv-X marries some traditional gasoline engine characteristics with a novel form of compression ignition called SPCCI. The key for Skyactiv-X is to use very high compression in the cylinder and an extremely lean fuel-air mixture. Squeezed right to the cusp of getting hot enough to blow up all on its own (which is very hard to predict), a squirt of extra gas and a spark interject to cross that compression-ignition threshold in a controlled and predictable manner.
That takes a few essential components to get just right. One is a massive amount of computer processing power and some pressure sensors in the individual cylinders, because the ambient conditions change how and when these things happen. Skyactiv-X uses a clutched supercharger to pump in additional air when needed to nail the mixture precisely, and high-pressure injectors to get the low ratios of fuel to disperse properly in the chamber. And since it operates like a conventional gasoline engine sometimes, it uses valve timing to lower the very high compression ratio so it doesn't reach combustion ignition in that mode.
In practice, the Skyactiv-X runs in compression ignition mode most of the time. In practical terms, that means it drives like a torquey gasoline Skyactiv engine. The torque curve is broad and flat — diesel-like in that respect. That also means it can get away with using a six-speed transmission and a lower final drive for better response. There's enough grunt and economy together that Mazda can let the engine spin faster — at 60 mph, it's running at roughly 1,000 more RPM than a similar gas engine, with greater efficiency.
Skyactiv-X is complex, and Mazda Vice President of R&D and Design Masashi Otsuka admits it will be an expensive engine to build. "But," he said to us before we drove the cars, with a big lopsided smile, "it's less expensive than a hybrid."
That speaks to Mazda's philosophy in building the Skyactiv engine range in general. Mazda has avoided electrification almost entirely; what electrification it employs is either very minimal (i-eLoop regenerative brakingand capacitor system) or niche (Demio EV, with its rotary range extender option).
Rather than build a bunch of hybrids and EVs that are dependent on incentives to sell (and still don't sell that well — Mazda does have a point here), the company decided long ago to just improve the internal combustion engine as much as possible. That means it has the lowest average EPA fuel economy of any manufacturer — although its overall volume is small, its sales have suffered recently, and it doesn't sell any large and inefficient SUVs or trucks. Mazda's strategy is technically sound, but perhaps not as effective in the real world as it might hope. Then again, as the company is eager to point out, neither is the real-world economy of the downsized, turbocharged engine trend. They're great at EPA testing but miserably thirsty in the real world.
Skyactiv-X isn't the final result of this process; there will be more electrification as that becomes necessary and/or demanded by the marketplace. There's a next-generation pure diesel engine coming, after the long-delayed Skyactiv-D engine, using lessons and perhaps technologies developed for Skyactiv-X. But for now, Skyactiv-X is the absolute bleeding edge.
Which is not how it feels on the road in the least. It's not supposed to feel bleeding edge, to be fair. There are rough edges, too. But mostly, it feels like a direct-injected, high-compression gasoline-powered car. It does not feel supercharged in the traditional sense, either. Off the line, it's definitely perkier than the anemic 2.0-liter Skyactiv-G engine, and on par with the 2.5-liter nonturbo Skyactiv-G on paper, although the -X's torque curve is fatter. The torque wave continues as revs increase, authoritative but not necessarily sporty.
If you want to talk numbers, they're understandably fuzzy. The Skyactiv-X tune isn't final yet, so horsepower and torque figures are a moving target. In Frankfurt, Mazda said the prototype engine was making about 187 horsepower and 143 pound-feet of torque. At Mazda HQ in California, the numbers bandied about are closer to 190 horsepower and 180 lb-ft of torque. That compares favorably with the 2.5-liter Skyactiv-G, which makes 184 hp and 185 lb-ft of torque in the Mazda6 and 187 hp/186 lb-ft in the CX-5. The numbers also differ from the Skyactiv-D diesel, which makes around 173 hp and 310 lb-ft in other markets where its currently sold.
But before you wonder why it's so much less powerful than the new turbocharged Skyactiv-G 2.5T that'll be out soon in the Mazda6, remember this: The Skyactiv-X engine is really intended to beat the weakling 2.0's fuel economy numbers (28 city, 38 highway) by as much as 30 percent, while seriously exceeding its power output of 155 hp and 150 lb-ft. In short, it aims to crush the 2.0 liter's economy with 2.5-liter power.
And it does so with a pleasing torque curve, which couldn't be more different from the peaky downsized turbo gas engines competitors use. Now, it's not as steep a torque curve as a modern turbodiesel, but it also doesn't run out of breath as early. There's unusual grunt all the way to redline, indicated at just shy of 6,000 rpm (where the true redline will be isn't clear at this point). While there will be a small electric motor to enable start-stop functionality on the production version of this engine, it's disabled in these prototypes, so all we're actually feeling is the extra churn enabled by the compression-ignition scheme.
That extra torque also lets Mazda get away with a shorter drive ratio and six-speed transmissions (both manual and automatic were available to test) without sacrificing economy, since the efficient operating range of the engine is so broad. That means less gears to run through when downchanging, less hunting in the automatic, and less complexity. The engine will turn about 1,000 more revs at 60 mph than a comparable Skyactiv-G, which also offers better response.
Really hammer on it at full load and it'll switch to spark ignition, seamlessly. It's only a tablet running on the dashboard that gives away which combustion type is happening, and in normal driving it's almost always in SPCCI mode. Snap off the throttle or back onto it quickly, and there's loud and obvious clatter, the most obvious sign that something's unusual.
That clatter is not supposed to be there. The Mazda engineer riding along tells us that these abrupt throttle position changes are very hard to tune for, that some preignition happens in these situations where the engine can't react quickly enough to the changed circumstances, and also that the company intends to tune these interludes out completely for production. That's going to be essential from an NVH (noise, vibration and harshness) standpoint, in our opinion, because the clatter is loud and jarring — it could certainly hinder broad acceptance.
And it's coming through a massive amount of sound deadening. Mazda refers to it as "encapsulation" — from a secondary "hood" coated with sound deadening hiding the engine from view to panels covering the oil pan and block. Essentially, the Skyactiv-X makes nasty sounds even when working ideally, and Mazda has to add some material to mask it. The price of innovation, it seems. Not that most direct-injection gasoline engines are very pleasant to listen to, either.
The issues do seem, objectively, to be manageable with tuning. The handoff from conventional to compression ignition happens without the stumbling that plagued other companies' attempts to master the art. Underneath the rough edges is an engine that behaves like a Skyactiv-G that's been secretly working out a bit, rather than a diesel engine converted with dark alchemy to run on gas.
And there's more to these prototypes than just the engine. Were it just a regular Mazda3 with a new engine, it wouldn't need the Frankenstein panel surgery it got. But it's actually more of a next-generation Mazda3 underneath, running a seventh generation of a platform that's been evolving for a while. Think all new, but not completely reimagined. Major dimensions seem basically unchanged; the Skyactiv-X would likely fit into a sixth-generation car.
The changes seem more like refinements. Coleman described how various suspension compromises resulted in undesirable lateral motions over certain types of bumps. We watched a video of a driver's head, in slow motion, rocking back and forth. Changes to the bushings turned more of that lateral motion into vertical motion, which humans tolerate better. That means Mazda can stiffen the struts up slightly compared to the current generation car without punishing the occupants, but soften the sidewalls of the tires to get the most out of their vibration absorption and damping properties.
Likewise, unseemly vibrations haven't been attacked with more heavy sound deadening, but rather with careful application of a damping adhesive between certain places in the unibody. The idea is to make the sounds less disagreeable rather than try to crowd them out entirely.
There's also more chassis stiffness overall, with added reinforcements that spread force inputs better throughout the structure. Pair the suspension, chassis, and NVH improvements with some improved seat materials, and Mazda promises the next-generation Skyactiv cars will be less fatiguing and more fun. On the road, the differences were extremely subtle, and perhaps it'd take a long roadtrip to fully reap the benefits. The suspension, however, felt stiff — a bit more than a typical Mazda3, which is exactly what we were told to expect. It didn't feel overly stiff; there was no judder or crashing over imperfections, so the new suspension geometry seems to do its job well. It'll take a winding road to fully evaluate the new chassis' handling.
Despite refinements all around and a wildly unconventional engine, the takeaway here is that it all feels like standard Mazda stuff. Eliminate the clatter and rip off all the gaffers' tape, and it's almost assured that a Mazda3 owner could be convinced it was a revised 2.5-liter engine in a next model year 3. Final tuning will polish the Skyactiv-X's attributes, which are already present if not blindingly obvious to a casual observer. So too will the improvements to the seats and suspension. That it feels evolutionary rather than revolutionary is a compliment to the integration of this nascent technology in an excellent package.
Barring hiccups, we'll see this engine in a production car in late 2019. It's way too early for fuel consumption numbers, but expect a 20-30 percent improvement over the current Skyactiv-G 2.0-liter engine. Depending on its cost, that will definitely get some consumers' attention. And if fuel prices go up, it might spark a frenzy. Either way, no one will be able to ignore Mazda's new engine. The only question is, will anyone buy one?
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