What is the purpose of the start-stop system

In order to reduce fuel consumption and pollutant emissions into the atmosphere, car manufacturers are constantly working on innovative technologies to optimize engine operation and the associated systems. One of the most successful technical solutions is the start-stop system, which switches off the engine during short stops, e.g. B. in traffic jams or at traffic lights. This article explains how the system works and what its advantages and disadvantages are.

History of this technology

The first Attempts to create a system that controls engine operation and avoids idling were made by Toyota as early as the 1970s. The Toyota Crown sedan had a system that turned off the engine after 1.5 seconds of idling.

In the first half of the 80s, similar technologies were installed in the Audi 100 and Volkswagen Santana. In this case, the engine had to be switched off and restarted manually at the push of a button. The same solution was used for the Fiat Regatta ES, but the accelerator pedal had to be pressed to restart the engine. After some time, the Opel Ascona, the Volkswagen Lupo and the Audi A2 also joined this system.

Nowadays, most of the well-known automobile manufacturers issue their cars with a start-stop system. These include: Peugeot, Citroën, Smart, as well as Volvo, Land Rover, Volkswagen, Seat, Nissan as well as Fiat, Bentley, BMW, Ford, Chevrolet, Hyundai, Honda, Jaguar, Kia, Mazda, Toyota and many others.

functional principle and types

Each car manufacturer has its own system for shutting down and automatically restarting the power unit. The most well-known technological solutions include the following companies:

  • Bosch: The system is called Start/Stop and is considered the simplest and most reliable. It enables the pollutant content in the exhaust gases to be reduced by 8% and fuel consumption to be reduced by 15%. It is installed in Audi, BMW, Volkswagen, Fiat and other brands of cars. This system requires a strong starter that can handle multiple engine starts. Furthermore, this system is controlled by the ECU motor. The operation takes into account the values ​​of the following measuring elements: vehicle speed sensor, as well as clutch and brake pedal position sensors and crankshaft position sensor, battery charge sensor and others.

When the vehicle comes to a standstill, it automatically switches off the engine. In vehicles with manual transmission, the engine starts as soon as the clutch pedal is pressed or the brake pedal is released in vehicles with automatic transmission. In order to save more fuel, in later system generations the engine can also be switched off while driving, but only if the ECU (the engine control unit) gives the appropriate command after calculating the speed of the car and the execution of any maneuvers and road gradients etc. has determined.

  • Kia Motors: The system developed by this company is known as the ISG (Idle Stop & Go). Although it is based on the same principle as the Bosch start/stop system, it not only switches off the engine but also starts the alternator. During the acceleration phase, the system disconnects the alternator and the entire load is transferred to the battery. During the braking phase, the generator is activated again and the battery is charged. This approach also results in significant fuel savings: around 6% in the combined cycle and up to 15% in urban driving.
  • Valeo: In this case, the system is called STARS (Starter Alternator Reversible System) and is used in Citroën cars, as well as Mercedes-Benz and other cars. Thanks to this innovation, fuel consumption can be reduced by around 10%. Equipped with its own control unit, the system uses a reversible alternator that combines the functions of starter and alternator. Thanks to a special drive belt and a specially designed tensioner, the device can transfer effort in one of two directions: either to start the engine or to charge the battery. The main advantage of this approach is the high response speed: it takes only half the time that a conventional starter takes to start the engine. Braking energy is also used efficiently.
  • Mazda: This system is called SISS (Smart Idle Stop System) or i-Stop and is only compatible with vehicles with automatic transmission and direct petrol injection. It guarantees up to 9% fuel savings. Repeated starting of the engine occurs thanks to forced injection into the cylinders and ignition of the air-fuel mixture. For this purpose, the pistons are brought into the most favorable position for a quick start during the braking phase. The starter is only used as a backup energy source.

Advantages and disadvantages

Traffic jams are one of the main problems of today's metropolis. Based on the latest studies by the US company INRIX Roadway Analytics, in 2017 people in Moscow spent a good 91 hours in traffic jams, in London 73, in Paris 65, in Istanbul 59, in Zurich 54, in Monaco 49 and in Oslo 47. And this statistic refers only to 240 working days and does not take weekends into account. Driving in traffic jams significantly increases fuel consumption. The more fuel that is burned, the more pollutants are released into the atmosphere.

The main purpose of the start-stop system is to provide a solution to these problems. Since this system switches off the engine when it is not needed, it contributes significantly to fuel savings.

There is also another undeniable benefit of using this system. By minimizing idle hours and putting the extra strain on the engine at low revs, it helps to optimize the performance of the catalytic converter, extend its life and prevent carbon deposits forming on the spark plugs. In diesel engines, the formation of deposits on the EGR valve is slowed down, which reduces urea consumption and contributes to more efficient regeneration of the diesel particulate filter.

On the other hand, the start-stop system also has some disadvantages since it requires a more powerful device, especially in relation to the starter and the battery. These components cost about twice as much as their traditional counterparts. In addition, repeated starting of the engine increases the stress on the crankshaft bearings. Interviewed by British magazine Autocar, Gerhard Arnold, head of one of Federal Mogul's departments, remarked that a conventional engine can easily withstand over 50,000 starts and stops. However, with the start-stop system, the number of cycles can easily exceed 500,000 runs. In this case, the standard bearings wear out after 100,000 occasions.

Another rather annoying factor is the start-up delay. Although this is rather small (for systems with a boosted starter – 0.8 seconds, for a reversible alternator only 0.4 seconds and for injection control up to 0.35 seconds), this is an inconvenience for many people.

Moreover, the system only works if certain conditions are met.

The 10 prerequisites for the correct operation of the start-stop-system

  1. The hood and the driver's door must be closed.
  2. The driver must be wearing a seat belt.
  3. The speed of the car must be at least 4 km/h.
  4. The engine is sufficiently warmed up.
  5. The number of engine revolutions does not exceed the rated idle speed.
  6. The window defroster is switched off.
  7. The interior temperature of the passenger compartment does not differ by more than 8 degrees from the set value. This aspect is important in the cold season.
  8. The battery charge is not lower than the value preset for the system. In the case of the ISG system, for example, this must be more than 75%.
  9. The generator is fully functional.
  10. The steering wheel angle is slight.


Today, the efforts of many companies are aimed at improving the start-stop system. For example, the British company Millers Oils has already launched motor oils that reduce engine element friction and wear by as much as 50%. According to Martyn Mann, the company's technical director, these and other similar products should actively contribute to the advancement of start-stop technologies. In addition, there are other manufacturers working on creating stronger bearings, while the engineers at Bosch, Valeo, Denso and many other car manufacturers have made it their goal to reduce the engine's start-up time.

Although While this system still has some gaps and the savings achieved may seem insignificant to some – given the relative cost of the equipment – experts say this technology has a bright future. In fact, the environmental benefits are very clear and cannot be denied.

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