DIESEL cars from Citroen and Peugeot are in line for a major eco-boost from the end of this year.

City driving economy of 1.4-litre and 1.6-litre models is set to improve by 15 per cent thanks to the latest development in gadgetry designed to automatically stop the engine in traffic jams and start it again when power is needed.

Called e-HDi and to be launched on the Citroen C5, the new technology also promises to make cars kinder to the environment with a 15 per cent cut in city driving tailpipe emissions and a 5g/km reduction during average driving conditions.

The first equipment of its kind to be harnessed to diesel power, e-HDi is the result of a three-year joint development project between French giant PSA Peugeot Citroen and component supplier Valeo that has cost more than £300 million.

It will also feature in the all new Citroen C4 hatchback that will be previewed at the Paris Motor Show this autumn and will be a key part of the Peugeot 308 compact car range and replacement 407 line-up next year.

"We pioneered stop-start on the petrol Citroen C3 in 2004 and have drawn on all our experience to offer the best solution for diesel drivers. We believe this to be the best technology available and think it will soon become the new industry benchmark," said PSA sales and marketing vice president Jean-Marc Gales.

Speaking as the equipment was unveiled in Paris, he added: "This is a significant development because research has proved that the engine is not needed for 30 per cent of the time the average car spends in cities or busy urban traffic.

"Something needs to be done to avoid this waste of energy and we think this is an effective way to make considerable potential savings available."

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 extra current boosting its battery, the starter is 70 per cent more powerful and fires up the engine in just 0.4 seconds.

Covered by 30 patents, it 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 oils and will be fitted with low rolling resistance tyres," said powertrain and chassis engineering director Christian Chapelle.

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

According to company officials, the cost of the equipment adds up to several hundred euros but just how much customers will be charged has not yet been decided. "This will be a matter for the individual brands," said M. Gales.

The benefits of e-HDi soon showed up out on the busy streets of rush-hour Paris  - in the 20 minutes it took to drive a four-mile circuit, test car engines were out of action for more than seven minutes.

The equipment 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 need to pass before stops begin to represent meaningful savings and it takes only five seconds to replace the battery energy used in restarting.