Stop-start engines are nothing new, but the unit fitted to the Smart car takes it to extremes.

The electronics on board do not even wait for the car to come to a stop before pulling the plug on the power.

As soon as you put your foot on the brake pedal and the car’s speed drops below 5mph — say when you approach a pedestrian crossing or traffic lights — then the engine cuts out.

This, to be frank, takes some getting used to. Most drivers who take a short test drive in this little car will probably be uneasy, not to say a little anxious, about this constant loss of power.

But stick with it and have a little faith in the technology, which is much easier to manage than it sounds, and the result is a city car that is just as manoeuvrable and nippy as its predecessor and even more economical.

Smart says the so-called micro hybrid drive (mhd) system can cut the fuel consumption on the 1.0-litre petrol engine by an average of nine per cent and, in city traffic, claims it can save up to 24 per cent.

At the heart of the system is a special, belt-driven starter-generator that supplies the vehicle’s electrical system and has a secondary function as a starter.

It is able to smoothly start up the engine in a fraction of a second as soon as you release the brake pedal.

Away from the technical stuff, 11 years after its launch the Smart remains the supreme city slicker, capable of squeezing into the tightest parking spot and even allowing you to park nose in to the kerb.

The latest French-built model, with its longer wheelbase and slightly larger body, is even more comfortable with improvements in both active and passive safety.

The 19.5 cm increase in length improves rear impact protection and ride comfort and the luggage compartment can now hold 220 litres – 70 litres more than before.

Inside, features include the characteristic external-mounted air vents and dials, large fabric-covered areas on doors and instrument panel, a pair of large, surprisingly comfortable, seats with integral seatbelts and the ignition lock mounted behind the shift lever.

The cabrio’s electric soft top can be fully automatically operated and adjusted to any position. This means that the rear part, too, can be opened to the final position and closed again at the touch of a button.

Extras range from head/thorax side airbags, air conditioning (standard for passion trim cars) and electric power steering, to heated seats, leather upholstery and a voice-activated satellite navigation system.

And on the mhd version, if you still feel you cannot cope with the constant switching of power, the start/stop function can be deactivated with a press of a button on the centre console.

Auto facts Smart Fortwo cabrio passion mhd

Price: £10,426

Ins group: Three

Fuel consumption (Combined): 64.2mpg

Top speed: 90mph

Length: 269.5cm/106in

Width: 155.9cm/61.4in

Luggage space: 7.7 cu ft

Fuel tank capacity: 7.2 gallons/ 33 litres

CO2 emissions: 105g/km

Warranty: 2 years/unlim mileage