SkyActiv-X: the future of internal combustion engines
At the recently concluded Mazda Pan Pacific Tech Forum, journalists from all over the Asean region were given a briefing of Mazda’s latest development, the SkyActiv-X engine, featuring the company’s SPCCI or spark controlled compression ignition.
In layman’s terms, SkyActiv-X takes the best of diesel and gasoline internal combustion engines to deliver more power and better fuel efficiency, making driving enjoyment very real well into the future, as well as being more environmentally responsible in the face of climate change.
Mazda’s long-term goal is to reduce CO2 emissions levels down to 90 percent by 2050 using 2010 levels as a basis. This is Mazda’s sustainable zoom-zoom strategy for the long-term.
Going back to SkyActiv-X, how does it work?
In theory, an engine is most efficient when it has an air/fuel ratio of 14.7:1. That is, 14.7 parts air to 1 part fuel.
If one can increase the amount of air, the mixture would be leaner, thus in theory, making the mixture run cooler, with less losses caused by heat and smoke. That’s in theory, though.
In reality, anything above 18:1 air fuel ratio delivers a highly unstable situation inside the combustion chamber, causing pre-ignition/detonation.
Direct-fuel injected engines can run slightly higher air/fuel rations, but it will still not be much higher before detonation/knock or an uncontrollable combustion occurs.
The result is the dreaded ‘pinging’ you hear, which when left unattended, results in often catastrophic engine failure.
And yet, Mazda claims to be able to deliver an air fuel ratio mixture as high as 36.1:1, more than double that of a regular engine? What witchcraft is this?
A specially designed combustion chamber, a supercharger designed to deliver a precise amount of air to deliver the right amount of power, and a continuously variable ignition timing are some of the key ingredients to deliver this type of combustion event.
In normal idle mode, fuel and air combust by compressing the air/fuel mixture at a far higher rate than typical gasoline engines, much closer to how diesel engines combust diesel fuel and air, at a ratio of 16:1.
A very small spark is introduced to help kick-start things. This is the compression-ignition aspect of the SkyActiv-X.
As engine load or effort increases, combustion switches to a typical spark-controlled ignition-combustion process where spark delivered is larger, just like a regular gasoline engine.
The combustion chamber, specifically the cylinders and piston, are designed to create a vortex, making the center much like an eye-of-the-storm: very fuel-dense, focusing the fuel there so that once the spark is introduced, the mixture of air and fuel can combust with a big bang utilizing less fuel because, well basically, the fuel is right where it needs to be for an ideal combustion event rather than all-over the combustion chamber, thus also minimizing losses.
On very light loads or trailing throttle/coasting downnhill, air/fuel ratios can soar to an aforementioned 36.1:1.
A supercharger helps supply the right volume and pressure of air at all times, rather than relying on an atmospheric, normally aspirated engine to draw-in air itself.
The continuously variable ignition timing ensures spark is delivered at exactly the right time to prevent detonation.
Humidity, air-temperature and air density caused by elevation change will also throw in a spanner for the SPCCI engine were it not for the supercharger.
Ultimately, Mazda’s goal is to deliver a more powerful engine with better fuel economy.
SkyActiv-X will debut first in a 2.0-liter displacement engine because this engine category is globally important for Mazda.
The 2.0-liter displacement benefits from many tax cuts and lower-tier tax brackets for vehicles worldwide.
Currently, the 2.0 SkyActiv-G delivers 160 hp and 200 Newton meters of torque.
The SkyActiv-X is promised to deliver a peak of ~188hp and 230 Newton meters of torque (more or less the same as the bigger 2.5-liter SkyActiv-G engine found in the Mazda 6) while delivering fuel consumption figures comparable to an even smaller 1.5-liter gasoline engine.
In terms of overall efficiency, we’re talking in the region of over 35 percent of input (fuel) versus output (horsepower and torque) over the current 2.0 SkyActiv-G. Ground-breaking indeed. Let that sink in for a moment.
Emissions will depend largely on fuel quality used, but have the potential to exceed current Euro VI emissions standards.
The SPCCI will offer a far larger powerband, and thus deliver a far-wider area of efficiency, allowing Mazda to use shorter gearing, which in turn will deliver better response and acceleration, making your drive much more enjoyable.
Unlike race cars and high-performance engines, SkyActiv-X can run on as low as 91 RON octane rating gasoline.
We were able to try the SkyActiv-X as previewed on the upcoming 2019 Mazda 3 hatchback in both manual and automatic.
The new 3 is an evolution of the current model, looks wise, with a much lighter chassis (~90 kg less) thanks to the use of high-tensile strength steel in key areas, and using lower-density steel on non-crucial/structural sections, which also makes the Mazda 3 stiffer and safer.
Of note is the Mazda 3’s switch from a multi-link rear suspension to a twist-beam axle.
Spec-wise, it seems like a step back, but we’ll hold judgment on that until we really get to drive proper production cars.
Today, at Mine Circuit in Japan, we’re driving pre-production mules that look like they have led a very hard life, but still feel tight and solid.
Immediately, the SkyActiv-X feels more powerful off the line, tapering off past 4,000 rpm or so. But you can keep it at a lower gear as compared to the current-gen Mazda 3 when going uphill, and the exhaust note is markedly different between the two.
Despite the small supercharger, the SkyActiv-X engine revs and the throttle blips much more willingly than the current SkyActiv-G engine.
There are some foibbles: knock is very audible when shifting gears and when the engine transitions from compression ignition to spark ignition.
But to be fair, final calibration and mapping of the engines are far from over, and the powertrain engineers said that a bit of noise is normal in this type of engine.
The future (still) looks bright for the internal combustion engine, thanks to Mazda.
Hopefully, these theoretical enhancements and almost bleeding-edge technology will deliver its promise: lower fuel consumption, lower emissions, greater range, and greater driving pleasure.
Mazda is after all, a company that values driving enjoyment and pleasure greatly. Jinba ittai!