Like other stop-start equipment, the PSA package shuts down the engine in traffic hold-ups and brings it back to life again when it is needed. But, with its hybrid battery system backed by a booster providing an extra five volts of current, the starter-alternator delivers up to twice as much power as usual to fire up the motor.

At just 0.4 seconds, the time it takes e-HDi to restart is 30% less than rival systems. Covered by 30 patents, the system has been tested to 600,000 restart operations and is claimed to last the life of the car.

“When we started redesigning our engines to meet new emissions legislation it soon became obvious that we needed something better than a conventional stop-start system using a reinforced starter motor.

"Vehicles equipped with this package will have a battery booster, revised gearing and variable oil pumps. They will also use new lubricants and will be fitted with low rolling resistance tyres,” said powertrain and chassis engineering director Christian Chapelle.

In two years, one third of PSA diesel engines will come in e-HDi form and the French group aims to have sold one million examples on its compact, lower and upper mid-range models by 2013, when the equipment will become available on petrol motors and the two-litre diesel engine.

How much will the e-HDi package cost? “We are not able to say at this time – this will be a matter for the brands,” said M. Gales.

We soon discovered that e-HDi is super-quick in restarting the motor during a four-mile crawl around the traffic-choked Champs-Elysees in Paris.

And the fact that the engine of our C5 Touring was out of action for seven of the 20 minutes we spent on our drive highlighted the significant potential benefits of the equipment in both operating cost and air quality.

It saves more fuel than usual by killing the engine as speed falls below 12mph in manual models and 5mph in cars with automatic transmission rather than shutting off at standstill.

Then instantaneous, vibration free and almost inaudible restarts have the motor running again as traffic begins moving.

According to PSA boffins, only three seconds have to pass before stops begin to represent meaningful savings and it takes only five seconds to replace the battery energy used in restarting.