Automotive Electronics Banner

Idle Stop-Start Systems

Basic Description

The Idle Stop-Start System is a low cost method for increasing fuel economy and decreasing emissions. The system turns off the internal combustion engine when the vehicle stops at a stop light or during stop and go traffic where the vehicle would normally idle for a minimum of three to five seconds. The engine is then automatically restarted when the driver is ready to proceed. An electronic control unit determines an appropriate time to turn off the engine based on data from various sensors. An auto start/stop system can reduce greenhouse-gas emissions by 5% to 7%. The system is currently available on most hybrid vehicles and on several non-hybrids such as BMW's EfficientDynamics vehicles and Mazda vehicles with i-Stop.

The picture below demonstrates how the system works on a full hybrid 2007 Toyota Prius. When the vehicle comes to a complete stop, the engine is turned off. Notice how the energy monitoring computer shows no flow of energy. When the brake pedal is released, the electric power from the battery is used to start the engine and assist in the initial acceleration. The yellow arrows show the flow of energy from the battery to the electric motor that provides torque to the wheels. When engine power is required, the engine is quickly started and assists the electric motor in powering the vehicle.

Prius Power-Flow Display

The concept is simple; however numerous parameters must be monitored and controlled in order to deliver a seamless user experience when using the idle stop-start system. The system must monitor the state of charge (SOC) of the battery to ensure that the engine can be started again. In addition, the engine temperature is monitored to avoid cold starts. This ensures complete combustion and optimal operation of the catalytic converter, thus reducing emissions.

Model of Electronic Control System for Idle Start/Stop System

Block Diagram of Electronic Control System for Idle Start/Stop System

Various manufactures employ different methods to quickly restart the engine. Many manufactures, such as General Motors, choose to use an integrated starter generator, which essentially combines a heavier-duty starter and alternator into one unit. The unit is able to quickly and smoothly restart the engine and then begin recharging the battery. The unit can be used for regenerative braking as well to recharge the battery. Mazda's unique i-Stop system restarts the engine using combustion rather than a starter motor alone. The system precisely controls the engine shutdown procedure so that the pistons of the engine are in optimal positions. The system then selects the cylinder with optimal location in the combustion cycle to restart the engine. Fuel is injected into the cylinder and ignited to restart the engine with the assistance of a traditional starter motor. Mazda claims this system is able to restart the engine in just 0.35 seconds, twice as fast as starter motor only methods.

Start-stop systems have been popular in Europe since the 1980s, but have only recently been introduced to the U.S. market. Audi, BMW, Chrysler, Daimler, Fiat, Ford, General Motors, Honda, Hyundai/Kia, Jaguar, Land Rover, Mazda, Nissan, PSA Peugeot, Citroën, Renault, Toyota, Volkswagen, and Volvo now offer models with start/stop systems in the U.S. The popularity of these systems will likely continue to increase due to pressure to comply with corporate average fuel economy (CAFE) standards that currently target an average of 54.5 mpg by 2025.

Engine temperature, external temperature, battery status, steering angle, seat belt connection, brake pedal position, accelerator pedal position, clutch pedal position (non-hybrid), crankshaft position, gear shift (non-hybrid), wheel speed
Starter motor, fuel pump, fuel injectors, throttle body, engine oil pump, transmission oil pump
Data Communications
Bosch, Continental, Denso, Honda, Mazda, Schaeffler, Valeo, ZF
For More Information
[1] Start-Stop System, Wikipedia.
[2] Five Things You Need To Know About Stop/Start Systems,
[3] Kia Motors Idle Stop and Go System, YouTube, Nov. 8, 2011.
[4] AGM Battery Takes Primary Role for Idle Stop-Start in Microhybrids, Paul Weissler, Automotive Engineering, Feb. 14, 2012.
[5] Stop Idling! Stop-start Systems Have Great Promise for Saving Fuel, Consumer Reports, June 29, 2012.
[6] Fifth Gear-Start/Stop Technology on Renault Megane, YouTube, Mar. 17, 2013.
[7] 2014 Chevrolet Malibu Start-Stop System..., Stephen Edelstein, Green Car Reports, Oct. 10, 2013.
[8] Fuel-Saving 'Stop-Start' Technology to Become Norm, Paul A. Eisenstein,, Dec. 31, 2013.
[9] 7 Cars That Save on Gas in a Way You Won't Believe, Brad Tuttle,, July 27, 2014.
[10] Start-Stop In U.S.: 7% Now, 57% In 2020, But Drivers 'Resist': Analyst, John Voelcker, Green Car Reports, Oct. 9, 2014.