If you’ve read the full tech breakdown of Mazda’s new SkyActiv-X petrol engine , with its claims of diesel economy and well-to-wheel CO2 emissions that better an electric car’s, then you’ll probably be wondering the following: ‘Great. But does it actually work?’

Sportingly, Mazda let Top Gear go for a drive in a Mazda 3 mule (new platform, new ultra-comfy seats, current body on top) fitted with the new 2.0-litre SkyActiv-X engine.

Now, this is a proper prototype. Mazda was happy enough with the driveability to let us have a go, but said the targeted power of 189bhp and 170lb ft isn’t quite there yet.

And it’s rare. Mazda shipped five of its six prototype mules to Portugal for this. They are rough and ready. The cars have no traction control, ESP, autonomous emergency braking or any airbags.

So if Top Gear and assorted journalists nerf them, it kills off the programme. Ladies and gents, it is no exaggeration to say the fate of the petrol engine, and perhaps of the car itself, rests on the next 90 minutes driving.

Okay, maybe that’s a slight exaggeration. Usefully, though, the starter button works. The engine starts with the briefest of diesel-like chugs, before settling into a more distant whir. It idles happily at 800rpm. The iPad fixed to the dashboard shows I’m in the regular spark ignition programme.

When the engine switches to Spark Controlled Compression Ignition (SPCCI) at a cruise, a yellow circle graphic will pop up. It’s joined by a green circle when I’m light enough on the throttle that the engine automatically moves into lean fuelling mode, using its little supercharger to bias the air fuel mixture towards more air, while slightly starving the combustion chambers of fuel. The idea is that unless I study the iPad, I’ll never know what’s going on under the bonnet.

Off we go. The engine immediately feels torquier than the 163bhp Mazda 3 the engineers have brought along for comparison. It’s still obviously naturally aspirated – there’s no Jabba the Hutt (a fat lazy slug) of boost at low revs, but the delivery is linear and smooth and crucially it delivers more punch than the current engine.

And what about the fuel economy? Ah. Can’t say. The trip computer doesn’t work in the prototype.

Happily, as we motor through the outskirts of Porto, the iPad graphics say the engine is spending a good 75 per cent of its time in SPCCI mode. Only under maximum throttle does it resort back to regular ignition. The engineers say a gentle cruise of about 80kph (50mph) is optimum for graduating into lean burn mode, but I managed to maintain a steady 110kph (68mph) on the motorway with the engine apparently running its cleanest, most parsimonious cycle.

When you’re below 2,000rpm, there is a slight diesely chug to the engine note. It’s that faint dugga-dugga grumble, only without the vibration. Worth mentioning, but as this is an infant prototype, it’s something Mazda can likely iron out long before this engine goes into the next Mazda 3. Which, incidentally, will hopefully look a lot like the stunning Kai concept car.

Mazda’s not bringing the tech to market until next year. It says the current prototypes are still hand-made, too heavy (though still all-aluminium) and it’ll make refinements. The engine on Mazda’s test bed is currently returning 20 per cent better economy than a non-SkyActiv-X engine. The target is to make that closer to 30 per cent this year.

Imagine, an engine good for 190-odd horsepower, with petrol response, diesel economy, mated to Mazda’s snickety little manual ‘box, in say, an MX-5. It’s very promising real-world tech, this.

Mazda says because these new engines operate at higher revs far more efficiently than today’s engines, they can get away with higher engine speeds, and shorter gearing. All the engineers I spoke to agreed that making these things fun to drive is a priority, not a lip service afterthought.

And Mazda’s European boss Jeff Guyton, when asked about how that might influence a new Mazda sports car, or perhaps a sorely missing line of hot hatches, smiled and said “you can expect different flavours of SkyActiv-X engine”.

Sounds yummy.