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Driven: Mazda3 SkyActiv-X. Image by Mazda.

Driven: Mazda3 SkyActiv-X
Mazda's clever 'hybrid' combustion engine tested in early prototype guise.


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Mazda 3 SkyActiv-X

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Mazda is pinning its hopes in the near- to mid-term on internal combustion engines remaining in the ascendancy - at least until electric cars can be largely recharged via sustainable energy sources, rather than coal-fired national power grids - so it has 'reinvented the combustion engine without reinventing the hardware'. This new SkyActiv-X 2.0-litre gasoline engine combines diesel's compression and petrol's spark combustion in one powerplant, with the aim of increasing fuel consumption on its existing SkyActiv-G unit by up to 20 per cent. So what's the SkyActiv-X like to drive?

Test Car Specifications

Model tested: Mazda3 SkyActiv-X prototype
Pricing: N/A
Engine: 2.0-litre four-cylinder SPCCI petrol (see copy)
Transmission: front-wheel drive, six-speed manual
Body style: five-door hatchback
CO2 emissions: TBC (expected 20 per cent increase in efficiency over current SkyActiv-G petrol)
Combined economy: TBC (expected 20 per cent increase in efficiency over current SkyActiv-G petrol)
Top speed: TBC
0-62mph: TBC
Power: c.190hp (estimated)
Torque: 230Nm (estimated)

What's this?

A Mazda3. However, great though that hatchback is, you can largely forget about the visuals of the car driven here, because it's what lies beneath that's of interest. First up, this car is sitting on next-generation SkyActiv-Vehicle Architecture underpinnings that are going to form the basis of the next-generation 3. This includes a stiffer body-in-white that's reinforced by three 'hoop' structures along its length, and revised suspension arms with spherical bushes aiming to improve the ride quality. It also has some clever new ergonomic seats that support your pelvis properly and keep your spine in a natural, slightly S-shaped profile that you'd employ when walking. All of the above is supposed to make the car smoother, quieter and more comfortable in every regard.

All noble stuff, but it's under the bonnet where the really intriguing work is to be found. Housed in this prototype Mazda3's conk is a 2.0-litre, four-cylinder naturally aspirated petrol engine the company is branding SkyActiv-X, to fit in with its existing SkyActiv-G (petrol) and SkyActiv-D (diesel) engine family. SkyActiv-X, however, straddles both fuel camps. It's a Spark Controlled Compression Ignition (SPCCI) engine that has spark plugs like any conventional petrol engine, but which runs a whopping 16:1 compression ratio (SkyActiv-G is at an already-pretty-manic 14:1 level) to run extremely lean, using sheer compression of the air-fuel ratio, as in a diesel motor, for the combustion process.

If it sounds weirdly improbable, it sort of is. When Mazda brings SkyActiv-X to market in 2019, most likely in the next-generation 3 for Europe, but soon followed by the 6 and all other models, it will be the first compression ignition (CI) petrol engine ever sold by a major motor manufacturer. There is a huge amount of engineering tech involved in this unit, all absolutely necessary to combine these two fuel-burning technologies together and overcome the huge challenges involved. If we went into it in great detail, it would probably sound like gobbledegook, so we'll try and boil it down as best we can for you.

As the piston is travelling up the cylinder bore to top-dead centre (TDC) on the combustion stroke, it squeezes the petrol-air mix to a level juuuuust shy of the point that compression alone would make the mixture spark. However, at a mammoth compression ratio of 16:1, you run the risk of 'knock' - the pre-detonation of the fuel-air mix ahead of TDC that is potentially catastrophic to an engine's health. Instead, a spark plug introduces a small fireball into the mix to kick things off and ensure that the chain reaction begins at exactly the right time, namely shortly after TDC, in the extremely lean mixture to totally combust the fuel.

It's doubly clever in that SkyActiv-X can run in spark ignition (SI) mode like a 'true' petrol, normally when you're asking the most of the engine with heavy acceleration or climbing hills, but it switches to CI during lighter throttle loads. Mazda is claiming that the lean-burn process is thermally more efficient, as the engine gets up to optimum operating temperature much quicker, and it's just more efficient in general, fuel returns said to be up to 20 per cent better than the SkyActiv-G unit's current numbers. It should also deliver around 190hp in final tune and 230Nm to go with it, these representing increases of 35hp and 20Nm over the existing Mazda four-pot. But as this engine is still in the prototype developmental stage, all final figures - fuel economy, emissions, power, torque and performance - are yet to be homologated, so we can't give you any concrete data as yet.

How does it drive?

What we can do is tell you how the SkyActiv-X feels from behind the wheel, when driven back-to-back with a late model of Mazda3 fitted with the 165hp SkyActiv-G engine - the sort of Mazda 3 you could buy in showrooms right now. With just six vehicles in the entire world fitted with prototype SkyActiv-X engines, Mazda decided to bring five of them to a launch event for journalists to have a drive. Now, this was purely to try and feel the additional torque the SkyActiv-X presents, to sample its added refinement and to experience some of the SkyActiv-Vehicle Architecture technology, such as the stiffer shell and comfier seats. So we're not about to talk about handling feel or any of that stuff, because it's simply not relevant.

What we can say is that a Mazda3 fitted with the 165hp SkyActiv-G engine is a lovely C-segment hatchback, even as it approaches the end of its life. But it's clear the SkyActiv-X drivetrain is a marked step on again. It remains much more hushed in operation up to 6,000rpm, delivering that lovely, linear flow of power that is the hallmark of a normally aspirated engine, and you'll feel little in the way of vibrations through either the base of the seat, the pedals or the steering wheel as it revs out. Furthermore, in-gear at 2,000rpm and above, it feels noticeably torquier, as if it employs forced induction when compared to the SkyActiv-G. However, like a small-capacity turbodiesel, ask it to accelerate from 30mph in sixth and the SkyActiv-X labours, where the G in the same situation will accelerate cleanly forward, albeit not massively quickly. More mapping work will soon solve the X's propensity to do this, though.

With rudimentary interiors, we can't say much about the cabin, but Mazda had helpfully attached a screen to the centre dashboard on which three differently shaded green rings would illuminate (providing you weren't totally off the throttle, in which case they'd all extinguish). If just one was lit up, then the car was in SI mode and at its least efficient. Two rings meant the SkyActiv-X had shifted to CI combustion and if all three were on, then it was running at its most frugal, lean-burn best. On the route laid on by Mazda, a mix of semi-urban routes, rural roads and some motorway, for the vast majority of the time the throttle was open to a degree, the car was at least in 'two rings CI' running - with the third circle appearing more often than was expected. Impressive stuff.

However, we can't confirm the actual fuel savings, so we'll just do a bit of rough extrapolation. This is because Mazda taped over the trip computers on the SkyActiv-X demonstrator cars, so we couldn't compare to the 3 with the current petrol engine. Nevertheless, that car returned an indicated 40.4mpg on a 75mph motorway cruise as the middle car in a SkyActiv-X sandwich convoy. So if Mazda's claims are to be believed, the newer CI cars should have been giving back around 48.5mpg indicated in the same conditions. That's an exceptional increase for a normally aspirated petrol engine, no doubt about it. All the signs are very good indeed for Mazda's SkyActiv-X future.


There's still a lot of calibration work to be done on SkyActiv-X before it hits the market in 2019, but even on this early showing it's clear to see this engine shows lots of promise. It's turbine-smooth, cathedral-quiet and yet punchy enough to make you forget about light-pressure turbocharging entirely. If the gorgeous Mazda Kai Concept, which was shown to us at the same event, manages to make it to showrooms largely unchanged as the next-generation Mazda3, with the improved new chassis and this fancy SPCCI engine providing the underpinnings, then we could genuinely be looking at a compact hatchback that's capable of deposing the almighty Volkswagen Golf...

4 4 4 4 4 Driving Dynamics

4 4 4 4 4 Powertrain

Matt Robinson - 21 Feb 2018    - Mazda road tests
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2018 Mazda 3 SkyactivX prototype. Image by Mazda.2018 Mazda 3 SkyactivX prototype. Image by Mazda.2018 Mazda 3 SkyactivX prototype. Image by Mazda.2018 Mazda 3 SkyactivX prototype. Image by Mazda.2018 Mazda 3 SkyactivX prototype. Image by Mazda.

2018 Mazda 3 SkyactivX prototype. Image by Mazda.2018 Mazda 3 SkyactivX prototype. Image by Mazda.2018 Mazda 3 SkyactivX prototype. Image by Mazda.2018 Mazda 3 SkyactivX prototype. Image by Mazda.2018 Mazda 3 SkyactivX prototype. Image by Mazda.


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