Start-stop system

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This article refers to the automotive technology. For the use of start-stop systems in telecommunication, see asynchronous serial communication.

A start-stop system automatically shuts down and restarts an automobile's internal combustion engine to reduce the amount of time the engine spends idling, thereby improving fuel economy. This is most advantageous for vehicles which spend significant amounts of time waiting at stop lights or frequently come to a stop in traffic jams. This feature is present in hybrid electric vehicles, but has also appeared in vehicles which lack a hybrid powertrain. For non-hybrid vehicles, fuel economy gains from this technology are typically in the range of 5 to 10 percent.

One of the earliest vehicles to use this technology was the Volkswagen Polo Formel E model of the 1980s. Volkswagen also tried it in the Golf Ecomatic and in the Lupo 3L in the 1990s, though those early implementations were considered rather disconcerting by many drivers. Citroën introduced a more refined system in its C2 and C3 models by 2006, and BMW is including the technology across many of its cars and the MINI line for 2008.

At present, BMW's implementation is probably the most advanced. Many of these systems use an integrated starter-generator (ISG), for example by Valeo or Denso, which combines the role of the starter and alternator into one unit. BMW has been interested in ways to reduce parasitic losses on engines, so the company took the technology a step further. Instead of using an integrated starter generator, BMW has used an enhanced-starter which is nothing more than a conventional starter, developed by Bosch GmbH, that can withstand the increased number of engine-starts in a stop-start vehicle. BMW has developed the system such that the alternator is not activated most of the time. This means that electrical components in the vehicle are normally running on battery power. When the battery needs to be charged or when decelerating or braking, the alternator is activated to recharge the battery (regenerative braking). Since this battery experiences very different load characteristics than a normal car battery, BMW used an absorptive glass mat type instead. Due to the use of regenerative braking, some have started calling this type of system a micro hybrid, a step below a mild hybrid system where the ISG can be used to assist the engine or even drive the vehicle.

Since automobile accessories like air conditioners and water pumps have typically been designed to run off a serpentine belt on the engine, those systems need to be redesigned to function properly when the engine is turned off. Typically, an electric motor is used to power these devices instead.