Cracking the code on petrol: why Mazda's SkyActiv-X tech spurs excitement
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It seems a little low key for an automotive revolution.
Not much about the stylish hatchback appearance hints at a world-first engine technology and even lifting the bonnet doesn’t reveal many clues — although you might notice the extensive shrouding that effectively places a noise insulating capsule around the engine.
The name on the Mazda3 engine cover is SkyActiv-X. The technology that lies within is arguably the most significant efficiency advance for internal combustion engines in a generation.
It’s Mazda’s much-anticipated crossover engine – hence the use of X – combining high-revving gasoline characteristics and the high torque and fuel efficiency of a diesel.
Mazda has cracked the code for what engine designers have long-considered the "dream" engine – a gasoline engine that achieves Homogeneous Charge Compression Ignition (HCCI). And it’s the next chapter in a long Mazda history of pursuing ideal internal combustion engines.
What Mazda had done is develop proprietary control systems that enable a very high compression gasoline combustion process similar to a diesel engine using super lean air:fuel ratios. That means one spontaneous bang — prompted by the heat and pressure of compression — to push down a piston rather than a fast burn process initiated by a spark plug.
Compression ignition of super lean mixtures has been possible but considered impractical for production petrol engines. They’ve worked in a narrow range of operating conditions with air pressure, temperature and fuel quality variances providing challenges — most notably destructive pre-ignition and detonation.
Mazda has solved this by using — of all things — a spark plug aligned with other control systems, most notably a world-first petrol engine application of in-cylinder pressure sensors.
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The process is called Spark Plug Controlled Compression Ignition (SPCCI) and the result is a petrol engine operating at an unheard-of 16.3:1 compression ratio and at extremely lean air:fuel ratios.
Gasoline engines have operated most efficiently at a 14.7:1 air:fuel ratio, or stoichiometric — which means all fuel is burnt without excess air. SkyActiv-X will operate at mixtures that as more than twice as lean (above 30:1) and at which a spark plug is unable to ignite.
In the SkyActiv-X application the rising piston compresses the super lean mixture to a point just short of compression ignition taking place while the control systems are monitoring cylinder pressure.
Using a super high-pressure injector, a small, precisely timed atomised fuel charge is delivered close to the spark plug and ignited to create an expanding fireball. That raises the pressure and temperature in the combustion chamber to the point where the super lean mixture spontaneously ignites.
The majority of the air and fuel in the cylinder is combusted through compression ignition. The size and timing of the fireball is being constantly adjusted so compression ignition can take place over a wide operating range which overcomes the traditional limitation of HCCI engines.
Mazda estimates the SkyActiv-X engine operates in SPCCI phase in about 90 per cent of driving situations. Because the spark plug is constantly in operation the engine can seamlessly switch to spark ignition as needed — primarily for cold starting and under high rpm/high load situations.
The much leaner mixture burns cooler and also reduces temperature differences in the cylinder head, piston and cylinder walls. Cooler combustion also significantly reduces the formation of nitrogen oxides (NOx).
An engine operating so leanly requires twice the amount of air. The SkyActiv-X intake solution is a small displacement, belt-driven "Roots" type supercharger to rapidly supply the volume of air required without the response delay of a turbocharger.
Read more: Mazda's pioneering SkyActiv-X engine — What we know so far
Like the SkyActiv-G 2.0-litre engine, the SkyActiv-X is 1998cc displacement and shares much of block and bore/stroke architecture but with a new cylinder head design.
Maximum power is 132kW at 6000rpm (the current 2.0-litre SkyActiv-G produces 114kW) with 224Nm of torque at 3000rpm. Most notably the peak torque arrives 1000rpm earlier than in current SkyActiv-G engines.
SkyActiv-X will appear in the New Zealand market in the first quarter of 2020 powering selected Mazda3 and new Mazda CX-30 models. Mazda isn’t revealing further plans just yet, but additional derivatives will be unveiled and added to other model lines.
First impressions gained on German roads at the global press launch showed the SkyActiv-X driving experience is notably smooth. The new widescreen MZD Connect display on the dashtop signals when SPCCI mode is operating and that was the majority of the time any amount of throttle is applied.
With a relatively short drive programme it was the manual transmission Mazda3 SkyActiv-X that was perhaps the most immediately revealing about the performance of the new engine.
The linear power delivery required fewer gear changes than I was expecting and I left it under load in a tall gear on an uphill run and it responded cleanly with part throttle acceleration while remaining in SPCCI mode.
Joining the autobahn traffic flow, I revved it harder to gain pace which reveals a rich engine note and a natural feel as the acceleration builds. It was particularly flexible at low engine speeds on some steep suburban streets. Some of this comes from the supercharging and also the mild-hybrid electric assistance which is the other important part of the SkyActiv-X programme and Mazda’s first venture into electrification.
A belt-driven 24-volt integrated starter/generator assists with engine starting, the first moments of movement away from standstill and fills the lower portion of the torque curve.
Mazda engineers told me the hybrid contribution is 4.8kW of power at 1000rpm and much more importantly 60.5Nm of torque at just 200rpm. The electric assistance also helps to synchronize the gear changes and Mazda confirmed it is studying more powerful 48-volt hybrid systems for the future.
To gain further benefit from the new engine characteristics Mazda has optimised the gear ratios. Because the engine doesn’t follow a normal pattern of increased fuel usage with higher engine revs, the SkyActiv-X has a revised final drive providing shorter gear ratios and allowing the engine to rev a little higher for a sportier feel and response.
On a "blind" sampling I think most drivers would guess the SkyActiv-X Mazda3 has a displacement of more than 2.0-litres.
With 18-inch alloy wheels on a Mazda3 hatchback the fuel consumption is rated at 6.3L/100km with manual transmission and 5.8L/100km (auto) under the new WLTP "real world" test procedures.
Mazda supplied back-to-back (against Mazda3 SkyActiv-G models) fuel consumption figures for the drive loop in Germany which showed a consumption improvement in the region of 15 to 17 per cent.
The SkyActiv-X story is more than one new engine we’ll see in Mazda3 and CX-30 [pictured above] next year.
Mazda says SkyActiv-X has "opened up a new door" and further gains in reduced mechanical friction, improved thermal management, reduced pumping losses and combustion control will carry into all future Mazda engines.
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In spite of this petrol engine breakthrough Mazda says diesel powered vehicles will continue and the next phase in the company’s engine development programme is expected to be a second generation SkyActiv-D diesel.
A new in-line six-cylinder engine family is also being developed for future rear-drive and all-wheel-drive Mazda models and there are suggestions the lean-burn and compression ignition know-how gained during SkyActiv-X development could enable the highly-anticipated SkyActiv-R rotary engine as a viable production car option.