The RATP is using a Gruau Microbus fitted with a Valeo Stop-Start system on the Bièvre-Montsouris local service in Paris. The system allows the engine to be switched to standby when the bus stops, saving on fuel consumption and reducing noise levels.

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This experiment, the result of a research partnership between the RATP, Valeo and the manufacturer Gruau, was selected and approved by ANR (French National Agency for Research) and PREDIT (French Program for Transport Research) in 2006, with the financial support of ADEME as part of the national ‘clean and cost-cutting vehicles’ programme.


The Microbus is fitted with a Stop-Start starter-alternator which has been integrated into volume-produced vehicles since 2005; the Microbus, however, has an additional function, the regenerative braking system.


The StARS starter-alternator is based on the system’s ability to stop and start the engine silently and instantly. In built-up areas, where stops are extremely frequent, the Stop-Start system could generate fuel savings of up to 20%.


The advantages are many: as soon as the bus stops, the engine shuts off, eliminating all noise and pollutant emissions, as well as fuel consumption. Cutting the diesel engine also provides a significant improvement in comfort both for the passengers and the driver, who do not feel any vibrations in the seats or the floor.

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For further fuel savings, Valeo has incorporated a braking system that recuperates energy (regenerative braking). The starter-alternator converts part of the vehicle’s kinetic energy into electricity when braking and slowing down. The electricity is stored in super-condensers and can be used to power the vehicle’s electrical network without using the battery.


Thanks to this trial run, the RATP, Valeo and Gruau hope to demonstrate the technical, economic and environmental importance of the Stop-Start concept and regenerative braking system in the field of outdoor public transport.