Mazda’s SkyActiv X claims to marry best of diesel and petrol

In news that’ll interest you if you could build Meccano before you could walk, Mazda has announced that it’s cracked a new type of engine that marries the best of diesel and petrol, to get more bang for your buck in every sense of the word. Sounds pretty promising, no?

Mazda calls the new engine SkyActiv X, and it’ll debut in 2019, following on from the current diesel ‘SkyActiv D’ and petrol ‘SkyActiv G’ petrol engines. The new SkyActiv X engine works like a diesel engine but burns petrol. This’ll take a little bit of explaining.

You see, a petrol engine (as you well know) uses a spark plug to ignite the petrol in the cylinder. It’s a system that’s worked since we had magnetos and moustache wax. And a diesel uses the sheer heat generated by compression to ignite the diesel. And that’s been around just as long.

Now, it’s entirely possible to ignite the petrol in a petrol-engined car using compression alone, and has been for about 100 years. That said, this is usually known as ‘a big frigging problem’, or ‘pre-ignition’, depending on the calibre and candour of your mechanic. In fact, sciencey types developed leaded and high-octane petrol with the sole intent of resisting pre-ignition, due the the engine’s tendency to get a bit over-eager and ignite the fuel before it’s supposed to, breaking many important things in the process.

In the case of the Mazda’s X engine, though, compression ignition is an entirely intended part of the cycle. And it’s here that we have to get technical again, so bear with us. You see, diesel engines run very lean, by which we mean a large air-to-fuel mixture. Put simply, diesel engine = much air, not too much fuel. Too much fuel actually has a detrimental effect on combustion by quenching the burn, because fuel has a cooling effect in the engine. No, really.

Now, you try to increase the air-to-fuel ratio in a petrol-powered car and you’ll have all sorts of problems – pre-ignition being just one. Another is that the further you head away from the ideal stoichiometric ratio (fancy-pants term for perfect air-to-fuel mixture), the more energy the spark plug has to discharge to ignite what’s in the cylinder. And, when you remove the cooling effect of the petrol in the cylinder, spark plugs have a tendency to burn themselves out and drop the spark plug tip into the cylinder. This is generally considered a bad thing.

So, let’s move on to compression ratio. If that’s just a buzzword to you, it means how big the air in the cylinder started, and how big it is after a piston’s had its way with it. So, let’s say an engine has an 18:1 compression ratio. That means that the air-fuel mixture in a cylinder is now 18 times smaller than it started off. It’s also enough to ignite a lean air-fuel mixture – diesel or petrol. So, it’s a good thing in a Massey-Ferguson, less so in a Ferrari. That’s why petrol engines have compression ratios far lower than diesel.

To recap, diesel = not much fuel + much pressure, and petrol = more fuel, less pressure. However, Mazda’s X engine covers off these seemingly contradictory modi operandi by acting as both types of engine, depending on the situation.

Mazda’s already played around with the preconceived notions of compression ratios – in that diesels should be high and petrols low – with the SkyActiv D engine, which has a much lower compression ratio in order to achieve some of the benefits (such as a comparatively free-revving nature) of petrol engines. The SkyActiv X takes things a step (or, if you will, a giant leap) further.

The SkyActiv X engine burns petrol, but works as a compression-ignition engine (i.e: like a diesel) for the bulk of its operation. But, when the computers decide that the engine needs a bit of spark-ignition (cue puns about putting the spark back into a relationship), it fires up the spark plugs that have been sitting idle until then. It sounds simple when you condense the tech into a sentence or two, but developing plugs that can sit idle, then work, for instance, is a massive engineering challenge on its own. How do you make sure they don’t get sooty? Or too hot? The mind boggles.

So, as you can tell, there are more than a few obstacles to overcome when trying to marry petrol with compression ignition. But, if Mazda has cracked it (and the proof of the pudding is always in the eating), there are a number of benefits that we can expect from post-2019 Mazdas, such as a 10 to 30 per cent improvement in torque over the SkyActiv G engine, as well as a 35 to 45 per cent improvement in fuel efficiency. In fact, it’s as efficient as the SkyActiv D engine – and that’s no mean feat. Oh, and many, many fewer bad gases that give us all asthma and unhappiness.

Now, as wonderful as all this sounds, it’s clear that all the clever lads were tied up in the engineering department, because Mazda has seen fit to name the overall strategy for its groundbreaking tech ‘Sustainable Zoom-Zoom 2030’.

And the marketing onanism doesn’t stop there. “Mazda believes its mission is to bring about a beautiful earth and to enrich people’s lives as well as society,” it said. “The company will continue to seek ways to inspire people through the value found in cars.”

In any case, let’s not lose sight of the fact that the tech itself is pretty impressive and extends a rope to consumer internal combustion engines as they start to circle the drain.

So, what do you think? Is there a place for SkyActiv X dino-juice in the future? And, if so, how should it be implemented – just for commercial vehicles, in a series hybrid or in the old-school, engine-and-gearbox arrangement that’s been doing the business for more than a century?

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