For drivers of new Audi diesels, the fear of filling the tank with the wrong fuel is safely in the past. The German car maker has fitted new type of fuel filler to its A4, A5 and Q5 that promises to be fool-proof.

The filler neck has an internal flap that opens only when you use a diesel fuel nozzle, which is broader than that for the unleaded pump. Pick the wrong fuel type and it stays closed, making it impossible to add fuel.

A new generation of diesel engines is now almost as quiet as the best petrol units, while producing lower CO2 emissions and promising far superior fuel economy. As a result, four out of 10 new cars purchased in the UK has a diesel engine.

And many of these go to first-time diesel drivers who are most likely to make a mistake. Some 150,000 drivers put the wrong fuel in their tanks, it is estimated, and repairs cost between £300 and £5000.

Audi isn’t the first car maker to introduce such a device – Ford has installed its Easyfuel system on a range of models for the last couple of years. But Audi intends in time to install its system across all of its diesel cars.