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Review: Sky's the limit for Mazda's SkyActiv-X prototype engine

The future is hybrid and electric - or so governments and vehicle manufacturers would have you believe.

The Government has already announced plans for pure-combustion engines to be phased out of new cars by 2040 and replaced by electrified vehicles, but we still live in the present, and the lack of charging infrastructure means that for many people, internal combustion is still the only viable option.

Mazda knows this and, in a bid to prolong the petrol engine's lifespan, has come up with a new type of engine, promising the fuel economy of a diesel while using cleaner petrol fuel.

It's called SkyActiv-X, and we took a trip to Portugal to get behind the wheel of an early prototype and find out more about this new technology.

How does it work?

In very simple terms, SkyActiv-X engines combine characteristics of both petrol and diesel engines.

The ground-breaking technology is that of compression ignition, where fuel is injected into the engine's cylinders and ignites without the need for a spark.

Compression ignition has been done before, but it's limited in scope. It operates only in a very narrow band of engine revs, and can be thrown off easily by environmental factors.

Mazda has overcome this by adding in a spark plug, which - by means of a second tiny burst of fuel - aids and controls compression ignition.

It also allows the engine to run as a traditional spark ignition engine under high rpm or heavy load.

The process is thus: air mixes with a tiny amount of fuel during the intake stroke, filling the cylinder with a very lean mixture. Then, as the cylinder compresses, another burst of fuel is introduced at the top of the chamber. The spark plug ignites this, and the resulting fireball spreads through the cylinder, adding enough heat and pressure for the remaining fuel to combust.

The mixture in the majority of the cylinder is kept too lean for combustion to occur without the aid of the fireball, preventing knocking.

The timing of the spark itself controls when ignition happens, and it allows fine adjustment of the engine's timing for different temperatures and environmental factors.

A small supercharger feeds air into the engine, ensuring the air/fuel mixture is kept as lean as possible even at high speeds.

The air swirls on the way into the cylinder, too - ensuring the fuel is mixed in as thoroughly as possible, important for a clean burn.

What are the benefits?

Mazda claims a car powered by a SkyActiv-X engine will produce 10-30% more torque than an equivalent SkyActiv-G petrol and return an impressive 20% improvement in fuel economy.

The torque and fuel economy curves will be flatter, too, with the company promising more repeatable results than with the current trendy downsized turbocharged engines.

"We want to provide our customers good results in the real world," said CEO of Mazda Europe, Jeffrey Guyton. "Even if you go back to the beginning of SkyActiv, we said that these cars may not be the greatest on the test cycle, because the downsized turbos perform really well in tests.

"I think you'll see the displacement of some of those really small turbo engines creeping up.

"They were designed to do well on New European Driving Cycle testing, but they're not so red-hot for real world, or WLTP testing. Mazda has always tried to do well in the real world, maybe to its detriment in the European market.

"But if you go to the US, where the testing is very much more realistic, actually Mazdas often perform better than the label because they're designed to do well in the real world."

The engine's improved economy - and, crucially, its ability to return this economy across a wider range of conditions - means that Mazda has been able to add lower gearing to the top end. RPM is kept higher while cruising, and so the engine is more responsive.

Finally, emissions of toxic NOx (nitrous oxide) gases are low. These gases contribute to local air pollution, and are a big factor in the current war on diesel. But the cooler operating temperatures of the SkyActiv-X engine ensure NOx figures are kept on the right side of regulations.

What's it like to drive?

The best compliment we can pay to the SkyActiv-X engine is how ordinary it feels. We sampled the tech in a test mule, which though it had an ordinary Mazda 3 body on top, was just a testbed for the firm's latest chassis and engine tech. The brand is targeting 187bhp and 230Nm of torque from the 2.0-litre engine we tested, though the mules were not running near those figures.

Under most conditions, SkyActiv-X feels like an ordinary petrol, albeit with torque more akin to a diesel. Tested back-to-back with a standard 118bhp petrol Mazda 3, the torque difference is noticeable - the new engine requires less frequent downshifts and pulls cleanly from low RPM. Unlike a diesel though, the engine doesn't feel too strained higher up the rev range. At high RPM, the unit switches to conventional ignition, but if there wasn't a display informing us of this, we would hardly have noticed.

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