In automobiles, a start-stop system automatically shuts down and restarts the internal combustion engine to reduce the amount of time the engine spends idling, thereby reducing fuel consumption and emissions. This is most advantageous for vehicles which spend significant amounts of time waiting at traffic lights or frequently come to a stop in traffic jams. Start-stop technology may become more common with more stringent government fuel economy and emissions regulations.
For non-electric vehicles, fuel economy gains from this technology are typically in the range of 3-10 percent. Hybrid/electric assist vehicles experience almost no delay in power from a stop, due to the instant availability of power from the traction battery to the electric motor(s). Gasoline powered cars on the other hand generally experience slight delays (albeit fractions of a second).
In the United States, idling wastes approximately 3.9 billion gallons of gasoline per year. Since automobile accessories like compressors and water pumps have typically been designed to run on a serpentine belt on the engine, those systems must be redesigned to function properly when the engine is turned off. Typically, an electric motor is used to power these devices instead. Start/stop activation depends on specific driver inputs as well as operating conditions. The engine must have reached proper temperature to get adequate light-off of its catalytic converter and also to ensure proper lubrication and as effortless a restart as possible.
The first vehicles to use this technology were the Fiat Regata ES with the City-matic system and Volkswagen Polo Formel E models of the 1980s. The Volkswagen Group also adopted it in the Golf Ecomatic in 1994 and in the Volkswagen Lupo 3L and the Audi A2 3L in 1999. Though these early implementations were considered rather disconcerting by many drivers and were commercial failures, both the Volkswagen Lupo and the Audi A2 were more efficient than any production car available in the US at the time of their release.
Start-stop systems require technological solutions to avoid premature bearing wear. Engines for start-stop applications need to be designed for 250,000 to 300,000 starts. Traditional bearing shells with aluminum or copper lining show severe wear after only 100,000 cycles. In a start stop system the short stop times keep the engine and oil warm, retaining lubrication. Some implementations do not use a starter motor, eliminating concerns of starter motor wear. Mazda i-stop used in their Mazda3/Axela line (in Europe and JDM) uses combustion to restart their engine by sensing the position of the piston in the cylinder. They claim quieter and quicker engine restart within 0.35 seconds.
Start-stop systems are also heavily reliant on the car’s battery. Testing indicates that AGM batteries diminish in their ability to support start-stop functionality over time. While alternatives exist (NiZn, Lithium-Ion, supercapacitors, PbC), virtually all automakers continue to use conventional AGM lead acid batteries.