In automobiles, a start-stop system or stop-start 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. This feature is present in hybrid electric vehicles, but has also appeared in vehicles which lack a hybrid electric powertrain. For non-electric vehicles fuel economy gains from this technology are typically in the range of 3-10 percent, potentially as high as 12 percent. In the United States, idling wastes approximately 3.9 billion gallons of gasoline per year.
On a manual transmission vehicle, stop-start is activated as follows: Stop car and press clutch - move gear lever to neutral - release clutch - then the engine stops. The engine won't stop if the car is moving, even if the aforementioned steps are followed (note that this isn't true for all cars). The engine restarts when the clutch is pressed prior to selecting a gear to move the car. The engine may also restart if there is a demand for power from, for example, the air conditioning system.
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.
This technology has also been used on Honda Motor scooters in Asian markets for the last decade. Their PCX 125cc model was released in 2010 to be sold in Europe with this technology, though their North American model does not feature it.
Start/stop technology came to Europe first, due to regulatory differences. 25 percent of the New European Driving Cycle (NEDC) is spent idling. In comparison, only an estimated 11 percent of the United States United States Environmental Protection Agency (EPA) test is spent at idle.
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.
With a manual-gearbox car, engine shutdown typically comes with braking to a complete stop, gearbox in neutral and clutch release.
Cars with automatic transmissions shut down upon braking to a full stop - the shut down is activated by the footbrake pedal being in use when the car comes to a halt. If the car is slowed initially by manual use of the automatic gear box and final stoppage is by use of the handbrake the engine will not shut down.
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 high pricing failed to yield these cars much commercial success, both the Volkswagen Lupo and the Audi A2 (in their "3 litre" leverage) were more efficient than any production car available in the US at the time of their release.
BMW is including the technology across many of its cars and the MINI line for 2008 as part of its Efficient Dynamics brand. 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 (ISG), BMW has used an enhanced starter which is conventional starter, developed by Robert 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.
Citroën introduced a more refined system in its C2 and C3 models by 2006, named "Stop and Start". The Citroën implementation combines a SensoDrive automated gearbox and an electronically controlled reversible alternator or integrated starter-generator (ISG). An ISG, also known as "integrated starter-alternator", combines the role of the starter and alternator into one unit; manufacturers include Valeo and Denso.
Fiat introduced the Robert Bosch GmbH made system in the end of 2008 in the Fiat 500. Starting September 2009, Alfa Romeo also introduced this system in its Alfa Romeo Mito series utilising Fiat Powertrain Technologies (FPT) 1.4 L Turbo petrol MultiAir engines.
Late in 2010, Ford announced that its start-stop system, already used in its hybrids as well as several mainstream models in Europe, would be introduced in North America with the 2012 models, initially with 4-cylinder engines and later proliferating into V6s and V8s. Eventually the system will be available in all Ford vehicles.
In 2013, it announced that start-stop technology would be brought into the second generation Ford Fusion models, and it built start-stop systems into the Ford F150 2015 model for the first time as a standard feature rather than an option. Formerly, only the 2.7-liter turbocharged V-6 version came with stop-start, which requires a more robust (and expensive) absorbent-glass-mat (AGM) battery that can better handle the constant cycling and the heavier draw from vehicle accessories with the engine off.
In 2014 General Motors announced that the Chevrolet Impala and Malibu would have a (non-eAssist) start-stop system across all models.
While both the Kia Rio and Rio5 share the same new direct-injected four-cylinder 1.6L engine with its cousin Hyundai Accent/Verna (also newly redesigned for 2012), Hyundai made no mention of this feature in their Accent line. Hyundai 1.4 manual has the ISG (Intelligent Stop & Go) system (in Israel).
Honda has been using the start-stop function for over a decade via their IMA mild hybrid system in first generation Insight models in the Japanese domestic market since 1999. and more recently on the Civic Hybrid.
The redesigned 2012 Kia Rio and Rio5 (hatchback) debuted at the 2011 New York Auto Show was announced with their Intelligent Stop and Go (ISG) feature, or stop-start technology.
Kia claims the combination of direct fuel injection and stop-start will offer 30MPG City/40MPG on the highway in both 6 speed manual and automatic transmissions.
In 2008 Land Rover fitted its Freelander with Stop/Start which could reduce fuel consumption and emissions by up to 10%. As of 2016 all Land Rover and Range Rover models are fitted with Intelligent Eco Stop/Start.
Mahindra's were the first in India to launch stop-start based Micro Hybrid system in May 2000. This involved home-grown technology and first of its kind component development like Hall sensor based neutral and clutch pedal sensors, and later a joint venture with Bosch for common rail based vehicles.
The i-Stop system, Mazda’s first start stop system, detects which piston is in the best position to restart quickest, which is the one in the combustion stroke phase, where air and fuel are in the cylinder, ready to be ignited. The mixture in this cylinder is ignited by the spark plug, forcing that piston down, and with partial-assistance from the starter motor, results in a near instantaneous engine restart time of 0.35 seconds.
In 2011, Mazda announced i-ELOOP, a system which uses a variable-voltage alternator to convert kinetic energy to electric power during deceleration. The energy stored in a double-layer capacitor, is used to supply power needed by vehicle electrical systems. When used in conjunction with Mazda’s start-stop system, i-Stop, the company claims fuel savings of up to 10%.
Renault introduced the technology in all of its European models in 2010.
Toyota has been selling cars with start-stop system on their internal combustion engine vehicles since 2009, and since 1997 in their Prius hybrid line. Both Toyota and Mazda introduced stop-start technology, available also outside of Japan, in some of their 2009 model year vehicles.
Volkswagen began using start-stop systems with the Polo Formel E with SSA around 1983. Later the LUPO 3L, and after that in the Polo, Golf and Passat BlueMotion, which also include weight and aerodynamic improvements. The system is now commonplace in the Volkswagen range, fitted to all vehicles with the Bluemotion Technology package, though certain other models have the technology too. For the Lupo 3L, with an automated manual gearbox and clutch, the engine stops four seconds after the car becomes stationary with the driver's foot on the brake pedal. No other action is necessary. Removing the foot from the brake pedal initiates engine start and the car can be driven away. The gear lever remains in the drive D position throughout. The same applies to the Audi A2 1.2TDi, which is almost identical mechanically.
The US National Highway Traffic Safety Administration (NHTSA) raised questions about non-hybrid Honda vehicles equipped with the company's 'Idle Stop' transmissions in February 2001 due to concerns over the "sudden lurching forward of a vehicle in an automatic restart" – rather than the "gradual creeping forward found in current transmission designs." 
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/microhybrids on the other hand generally experience slight delays (albeit fractions of a second).
Many people think that long-term use may induce additional wear due to lack of oil lubrication. For the crankshaft bearing half shells and the big end bearings this can translate into frequent high-speed rotary movement before a hydrodynamic film is established. During this phase of boundary lubrication, metal-to-metal contact can occur between the crankshaft surface and the bearing’s sliding surface. This was not an issue while the number of engine restarts totaled at what was generally understood to be a normal magnitude. However, in a vehicle with start-stop system this effect can necessitate new technological solutions to avoid premature bearing wear, depending on the driving cycle. Consequently, future 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 heavily reliant on the 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.