FORD has finally given the world-class chassis found in its Focus a world-class engine.

Yes, the five-pot turbo-petrol Volvo unit that so titillates Focus XR5 drivers is a great thing in the highly-charged hot-hatch niche that the hottest Focus operates within, but only a fraction of new-car buyers would consider one of these $40,000 machines.

And – sure – the 107kW 2.0-litre four-cylinder Duratec is one of the better petrol engines in its segment – as anybody who drives the sluggish 1.8 or 2.0 units found in the original old-shape Focus will agree – but you could never call it truly remarkable.

No, that accolade goes to the TDCi – or 2.0-litre Duratorq four-cylinder engine.

Co-developed by PSA Peugeot Citroen and found (in various shapes and sizes) in a number of products ranging from the recently released Citroen C4 Picasso to the Europe-only Mini Cooper D, this powerplant is a blast.

Not only because it delivers maximum torque from really low revs that it breaks below the 150g/km carbon dioxide emissions barrier while returning less than 6L/100km that – almost unbelievably – the test cars that we drove over a series of rural Northern NSW roads were really, really quiet or that the six-speed manual gearbox has a satisfying feel to it.

No, the greatest and most fabulous thing about the engine in the blink-it-and-you’ll-miss-it Focus facelift is that – finally – there is an affordable (ie not $35,990 XR5 Turbo) engine that works with its beautifully measured and resolved chassis.

Tickle the throttle in third gear at about 50km/h and you are launched – not lurched – into motion instantly and effortlessly. The engine revs eagerly all the way to its red line in a very much un-diesel way, and you are swept away on a wave of constant and instant forward thrust.

Remembering that the Focus is renown for its steering alacrity and supple ride quality, you can squirt up a winding road by simply squeezing the ball of your right foot, all the time going precisely where you aim with no fuss, no lag and no strain.

Drive a diesel back to back with the regular petrol unit, and you will notice two things.

Firstly, the latter feels lethargic by comparison, no matter how good it really is and, secondly, the former has slightly nose-heavier handling.

The TDCi does feel a tad less willing to change direction (that would be the 120kg-odd weight penalty for going diesel talking), but this Focus is like any Focus in that it is measurably and undoubtedly ahead of all the competition – this side of a BMW 1 Series anyway.

It should hardly matter, because the $27,990 TDCi – we must insist on the $1200 DSC stability control and curtain airbag safety pack – is a riot on any interesting road.

In fact, so invigorating is the diesel/Focus combo that we urge all small-car buyers to ignore the un-ignorable fact that Ford does not (yet) offer an automatic gearbox with this powerplant, because the six-speed manual helps make this motor come alive in your hand.

Once again, because the Focus is so well engineered, the gearbox and clutch are as light and as progressive to use as the steering is, involving you in the driving experience as only Ford does at this price level.

If you truly want a small car that feels and drives almost like a hot-hatch, while returning some truly remarkable fuel economy numbers, Ford’s Focus TDCi shoots straight to the top of the class.

Indeed, people considering the BMW 120d, Audi A3 2.0 TDI or Alfa Romeo 147 JTD – costing $19,810, $19,760 and $12,000 more respectively – should drive the Focus first if they have any sense.