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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 an 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, then the engine is 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 CO2 emissions by up to 3.5%. 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.

In hybrid vehicles, the system works very efficiently due to the availability of the large rechargeable battery used to assist in powering the engine. 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 the electric energy to the wheels. When engine power is required, the engine is quickly started and assists the electric motor in powering the vehicle.

illustration of adaptive cruise control with a car in front

The concept is simple; however numerous parameters must be monitored and controlled in order to ensure the driver's comfort whenever the engine turns off. The system must monitor the state of charge (SOC) of the battery, so that the engine can be started again. The system monitors the cabin climate to ensure the driver's thermal comfort. 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 Hybrid Electronic Control Unit for Auto Start/Stop System

The system also requires the use of a relatively new device called an integrated starter generator, which is a component that combines the starter and alternator into one unit. This system is able to draw direct power from the battery to start the vehicle and recharge the battery while the battery is running or during regenerative braking.

Start-stop systems have been popular in Europe since the 1980s, but are only available of a few non-hybrid models in the U.S. Some argue that stop-start systems are not sufficiently rewarded under U.S. fuel economy regulations. Under the driving cycle the EPA currently uses to test fuel economy, cars spend relatively little time idling long enough to take full advantage of the potential fuel savings afforded by start-stop systems.

Engine temperature, external temperature, cabin temperature, battery status, steering angle, seat belt connection, brake pedal position, accelerator pedal position, clutch pedal position (non-hybrid), gear shift (non-hybrid), wheel speed
Starter motor, valve timing
Data Communications
Bosch, Denso, Valeo
For More Information
[1] Start-Stop System, Wikipedia.
[2] Five Things You Need To Know About Stop/Start Systems,
[3] Auto Start Stop Feature | Idle Stop Feature,
[4] Kia Motors Idle Stop and Go System, YouTube, Nov. 8, 2011.
[5] Engine Stop-Start Systems on Nonhybrid Vehicles, Car and Driver, Apr. 2011.
[6] Start-Stop Technology Fact Sheet, Johnson Controls Website.
[7] AGM Battery Takes Primary Role for Idle Stop-Start in Microhybrids, Automotive Engineering, Feb. 14, 2012.
[8] Stop Idling! Stop-start Systems Have Great Promise for Saving Fuel, ConsumerReports, June 29, 2012.
[9] Engine Stop-Start Systems Save Fuel at Low Cost, John O'Dell,, Oct. 9, 2012.
[10] 2014 Chevrolet Malibu Start-Stop System..., Stephen Edelstein, Green Car Reports, Oct. 10, 2013.
[11] Fuel-Saving 'Stop-Start' Technology to Become Norm, Paul A. Eisenstein,, Dec. 31, 2013.