|The StARS micro-hybrid stop-start system. Click to enlarge.|
Valeo announced that its StARS micro-hybrid system equips new Mercedes-Benz A- and B-class vehicles offering that function. (Earlier post.) Valeo also recently signed a contract with PSA Peugeot Citroën to equip more than one million vehicles with its Stop-Start technology by 2011.
StARS (Starter Alternator Reversible System) cuts off the engine when the vehicle is at a standstill, at traffic lights for instance, and then restarts it quickly and silently when the driver releases the brake pedal. It provides average of fuel savings of 9%. Under conditions of congested city driving, fuel savings can reach up to 25%.
The starter alternator is driven by a belt that can also be used to drive other accessories, such as the air conditioning compressor, the water pump or the power steering. A reversible tensioner allows power to be transmitted in both directions, depending on whether the StARS starter alternator is working as a starter or a generator.
The system features a battery based on technology that accepts high numbers of charge/discharge cycles. The battery is also fitted with a sensor that enables the system to inhibit the Stop-Start function if the charge level is too low or to restart the engine if the charge level drops below a critical threshold.
The StARS starter alternator is a synchronous machine with a claw rotor. It is air-cooled and uses a three-phase current that is transformed to 12 V DC voltage by a separate electronic transformer.
In starter mode, it is capable of delivering a 600 A current generating enough torque to drive the internal combustion engine immediately and at higher revs than a conventional starter. The machine can start the engine in less than 0.4 seconds, before immediately switching to alternator mode.
In this configuration, the electronic transformer uses MOSFET field effect transistor technology to rectify the three phases. This is one of the reasons behind the 82% efficiency of the StARS starter alternator, which is a full 10 points higher than the best conventional alternators on the market. The machine delivers a current of up to 180 A.
Valeo’s micro-hybrid system already equips the Citroen C2 and C3 and the smart mhd.
Valeo is developing a supercapacitor-based extension to StARS (StARS + X) to support regenerative braking and thus enable a greater reduction in fuel consumption than is possible with the stop-start system alone.
The StARS + X system uses a slightly different architecture with different motor for starting and regen, and support for voltage between 14V and 30V. The floating voltage enables extended torque for diesel or large engine cold cranking, 4 kW generation for regenerative braking and extra power supplies for peak loads. (Earlier post.)