Volkswagen Golf GT TSI

This is no ordinary fast Golf. It sports two 'chargers', super and turbo, on a petrol engine that brings the advance of the diesel to a shuddering halt. John Simister hails an automotive miracle

Price: from £18,095. On sale now.
Engine: 1,390cc, four cylinders, 16 valves, supercharger and turbocharger, 170bhp at 6,000rpm, 177lb ft at 1,750-4,750rpm
Transmission: six-speed gearbox, front-wheel drive
Performance: 137mph, 0-60 in 7.7sec, 38.2mpg official average, CO2: 175g/km

The petrol engine fights back. The diesel has been in the ascendant for a while now, with its easy-driving nature, its fuel economy and often, nowadays, a greater power output than a same-size petrol engine. The fact that modern diesels are invariably turbocharged helps here, of course, but it's still a staggering state of affairs when you think how sleepy old diesels used to be.

All is not perfect in dieselworld, however. They are good on CO2 emissions but poor on other pollutants, requiring expensive particulate filters to render them acceptably clean. They have a narrow usable torque band, running out of steam at quite low engine speeds so you need lots of gears to get the best from them. And, though modern diesels can be quiet, they really don't sound very nice.

But - as petrol engines, especially small-capacity ones, feel ever more feeble as they struggle to cope with heavier cars and tighter emissions legislation - diesels have rocketed in popularity.

This, though, is set to change because Volkswagen has launched a petrol engine so capable that it rewrites the rules of what is possible. Meet the Volkswagen Golf TSI.

Its little 1.4-litre engine produces up to 170bhp. To get an engine to produce so much more than 100bhp per litre normally means manically high revs and minimal low-speed pulling ability - fine for a racetrack but hopeless on the road.

Not here, though: the engine delivers a big-muscled 177lb ft of torque all the way from 1,750rpm to 4,750rpm. It has the low-end thrust of a good diesel and the high-end eagerness of a highly tuned petrol engine. Cake is simultaneously had and eaten, and all with an official combined-cycle fuel economy figure of 38.2mpg.

So we have a reasonably hefty Golf GT TSI able to reach 60mph in 7.7 seconds and to keep accelerating right up to 137mph, given the opportunity. All from 1.4 litres. How on earth?

The T in TSI stands for Twincharger. That means two "chargers": a supercharger, belt-driven by the engine, and a turbocharger, driven by the exhaust gases. Turbocharging is used by many engines, be they petrol or diesel, but a turbo can take a while to spin quickly enough to provide meaningful boost pressure, especially in a small engine. That can make it frustrating to drive: foot down, nothingness, then a surge of energy. A supercharger gives a quicker, more consistent response, but it saps a lot of energy at high speeds, so it's bad for fuel efficiency.

Put the two technologies together, though, and you have an engine capable of miracles. Lancia did this with its Delta S4 rally cars two decades ago, but fuel efficiency wasn't uppermost in that company's mind. The Volkswagen approach is different, and driven by the commitment among major carmakers to have average CO2 emissions, across the fleet of vehicles sold, of just 140g/km by 2008.

The supercharger comes in whenever you accelerate from low engine speeds. It spins five times faster than the engine's crankshaft, but above 2,500rpm its efforts tail off and by 3,500rpm it's out of the picture, its electromagnetic clutch disengaged and its air supply bypassed. This is because from 2,500rpm the turbocharger is making a useful contribution, and by the time the engine is producing its maximum power it is relying on the turbocharger alone.

That what is by then simply a turbocharged 1.4-litre engine can produce so much power is down to several factors. The turbo runs at a high boost pressure. The engine has a high compression ratio. And, being a direct-injection engine, it can run at that high compression ratio without destroying itself. That is because injecting the fuel directly into the cylinder has the effect of reducing the temperature inside the cylinder, because the vaporisation of the fuel soaks up heat. And this is why the Golf TSI can run at high speeds on motorways without needing the extra fuel normally needed in turbo engines to keep them cool. So that's another way it gains in economy.

This is downsizing at its most intelligent. A small engine is a light engine, even when equipped with two boost devices. A light engine means a lighter car, which uses less fuel and can have lighter brakes and wheels. And this little TSI engine is cheaper to make than a 2.0-litre turbodiesel while offering similarly easy low-speed pull.

Fine. Does it work? You bet it does. As you move off in the TSI you hear a little click as the supercharger clutch engages. There's a tiny delay in the accelerator's response because the engine's air supply has such a long journey from air filter to the four cylinders (the pipework almost encircles the engine: filter, supercharger, turbocharger, intercooler, intake port, cylinder) but then you hear a deep, guttural snort like that of an old rally car on Weber carburettors as the supercharger does its stuff. That snort, never loud but pleasingly characterful, fades as the turbo takes over, but the thrust is insistent right through the speed range.

It really does feel like a 2.0-litre engine, and a very punchy one at that. The accelerator response does soften when the turbo alone is in play, but the way you can amble along in high gears and know there's effortless overtaking urge always available is a new experience with such a small engine. I found a tight uphill hairpin bend on my test route, which involved slowing to about 15mph in second gear before accelerating away. The Golf TSI powered out of the corner as if pulled by a steam engine.

Yet this is an engine that will pull past 7,000rpm. Its breadth of ability is extraordinary, and makes for a delightful driving experience. The Golf GT to which it is fitted suits the characteristics perfectly, with a ride supple enough to soak up our crumbling road infrastructure yet steering and handling precise enough to satisfy a keen driver. It's a better compromise all round than even the excellent Golf GTI, because it lacks that car's underlying firmness and tiresome tyre roar. As you might expect, the interior trim and the exterior styling are halfway between a regular Golf and a GTI: sporty-ish but not racy.

There's also a 140bhp version of this engine for lower-range Golfs and the Touran MPV. Eventually they will replace the existing 2.0-litre engines, although the turbo version will continue for the Golf GTI and other cars in the VW group.

It's not often we can hail the arrival of a new engine that does so many things so well. But we can do so here. The Golf TSI is nothing short of a miracle.

The rivals

Vauxhall Astra 2.0T SRi £18,000

Same power as the Golf from its 2.0-litre turbo engine, but a thirstier engine. Looks like a stylish coupé. This SRi is calmer than the mad VXR but it's still a keen drive.

Audi A3 2.0 FSI Sport £19,820

Under the skin it's a Golf; outside it's a better-looking Audi. This has the 2.0-litre, 150bhp engine that's been ousted from the Golf by the TSI. The better buy is obvious.

Citroën C4 VTS 180 £17,520 (less cashback)

The futuristic-looking C4 has its switchgear in the centre of the steering wheel. With 180bhp it's certainly lively, but the ride is firm and the brakes are rather snatchy.