Start/Stop [2021-2022, B420T; B420T2]: Operation

The start/stop function is activated with a normal engine start. The function can be deactivated and reactivated on the Center Console Display (CCD).

Vehicles with starter motor (12 V) 

During normal engine start and autostart, the electrical system is connected via two switches located in the battery control switch module BCSM. One switch is for the main battery (12 V) supply and the other for the converted voltage from the Mid Voltage Battery Module (MVBM) through the Mid Voltage Conversion Module (MVCM).

The Engine Control Module (ECM) decides if the combustion engine should be started by the conventional starter motor (12 V), or by the Integrated Starter Generator Module (ISGM), i.e. 12 V start or 48 V start. ISGM start is prioritized, but depending on conditions such as SoC levels of the two batteries (12 V and 48 V), the temperature of the mid voltage battery and the combustion engine oil temperature, the starter motor can be preferred.

The battery control switch module BCSM enables the start strategy by connecting the circuits with both power to the starter motor and ensuring correct power supply to the 12 V system during engine crank.

• If ISGM (48 V) start is preferred: The ECM sends a request to the Mid Voltage Battery Module (MVBM) to close the relay. The ECM also sends a positive torque request to the ISGM until a certain engine speed is reached. The ISGM is powered by 48 V from the MVBM. The relays in the BCSM are set to enable that the 12 V system in the vehicle is powered by converted voltage from the Mid Voltage Battery Module (MVBM) during engine crank.
• If starter motor (12 V) start is preferred: The ECM activates the starter motor solenoid by grounding and powering the starter motor relay coil in the engine fuse box. The starter motor is powered by 12 V from the main battery. The relays in the BCSM are set to enable that the 12 V system in the vehicle is powered by converted voltage from the Mid Voltage Battery Module (MVBM) during engine crank.

When autostop is requested, the engine is shut off by stopping the fuel supply. On diesel engines, the throttle valve first closes to create an underpressure to minimize hard compressions and vibrations. The ignition is still on, enabling other systems to continue being active.

The Engine Control Module (ECM) and the Central Electronic Module (CEM) detect and store information about what can block autostart and autostop, as well as about what can cause the system to request an autostart if the engine is autostopped.

Vehicles without starter motor (12 V) 

In vehicles without starter motor, the electrical system is connected via a switch located in the battery control switch module BCSM. The BCSM primarily supplies the 12 V system with voltage and current from the main battery. The BCSM does not have any specific role in the start process, except ensuring the control modules are power supplied at all times, including during the start process.

During crank, the Engine Control Module (ECM) signals the beginning of crank to the BCSM through the crank signal pin. The BCSM will go into crank state when the signal is received, which means that switch S1B is opened in most cases. The ECM sends a request to the Mid Voltage Battery Module (MVBM) to close the relay. The ECM also keeps sending positive torque requests to the Integrated Starter Generator Module (ISGM) until a certain engine speed is reached. The ISGM is powered by 48 V from the MVBM.

If the Mid Voltage Battery Module (MVBM) for some reason is not supplying voltage (converted to 12 V by the Mid Voltage Conversion Module (MVCM)) to the 12 V network during crank, the voltage and current are supplied by the main battery through the S1B switch in BCSM. The Engine Control Module (ECM) and the Central Electronic Module (CEM) detect and store information about what can block autostart and autostop, as well as about what can cause the system to request an autostart if the engine is autostopped.

When autostop is requested, the engine is shut off by stopping the fuel supply. The ignition is still on, enabling other systems to continue being active.

Preconditions 

The basic preconditions for the function start/stop to be activated are:

• The climate system does not request a too high blower motor unit speed.
• No risk of fog on the windshield.
• The ambient temperature is -5 - 30 °C (up to 37 °C in certain models and calibrations).
• The engine temperature is normal.
• Soot particle regeneration is not activated (diesel engines only).
• The main battery temperature is -5 - 55 °C.
• The main battery is charged.
• The Battery Monitoring Sensor (BMS) calculation of the main battery status is reliable.
• The mid voltage battery included the Mid Voltage Battery Module (MVBM) is charged.
• No trailer is connected.

Autostop 

For vehicles with an automatic transmission, autostop is requested when the vehicle is braked at low speed towards standstill. For vehicles with a manual transmission, autostop is requested when the transmission is in neutral, the clutch pedal is released and the vehicle speed is reduced towards standstill.

The autostop process:

• For vehicles with an automatic transmission the Transmission Control Module (TCM) is requested to disengage the gearbox. The autostop does not proceed until the correct response is received. The disengagement process can start slightly above 7 km/h to prepare for engine stop at 7 km/h.
• The switches in the battery control switch module BCSM stay in their normal position, i.e. both the main battery (12 V) and the converted voltage from the Mid Voltage Battery Module (MVBM) are connected.
• The electrical system is powered from both the main battery and the mid voltage battery included in the Mid Voltage Battery Module (MVBM) through the Mid Voltage Conversion Module (MVCM).
• The engine is stopped by shutting off the fuel supply.
• The ignition is still on, i.e. other systems can continue to be active.

Most vehicle systems, such as lighting or radio, work as usual during autostop. Some equipment may have a temporarily reduced functionality, such as the blower motor unit and the sound systems volume.

Multiple stops (only for vehicles with an automatic transmission) 

The engine can autostop and autostart several times consecutively, for example in a queue. However, there is a limitation to the number of autostops and autostarts allowed during a multiple stop situation. When this limitation has been reached, the vehicle must obtain a speed above a certain threshold value to reset it. The calibration of the number of allowed autostops and autostarts and the speed that needs to be reached to reset the limitation varies for different markets and engine types.

The figure below illustrates an example of a multiple stops event where three autostops and autostarts are allowed:

Courtesy of VOLVO CARS CORPORATION

1. 1.Autostop at a speed below 7 km/h.
2. 2.Autostart by the driver.
3. 3.Autostop and autostart, previous speed greater than 1 km/h.
4. 4.The vehicle does not reach a speed of 1 km/h.
5. 5.No autostop, previous speed under 1 km/h.
6. 6.Autostop and autostart, previous speed greater than 1 km/h.
7. No more multiple stops are allowed after three autostops. Reactivation requires that the vehicle must reach a speed of 10 km/h.

Autostart 

When the driver is to continue driving and the start/stop function is active, the system requests an autostart of the engine. If the vehicle has an automatic transmission, the request is sent when the brake pedal is released. If the vehicle has a manual transmission, the request is sent when the clutch pedal is depressed. The system will also request an autostart if the reverse gear is engaged. The autostart process:

• The switches in the battery control switch module BCSM stays in the same position, i.e. the electrical system is powered by both the main battery (12 V) and the converted voltage from the Mid Voltage Battery Module (MVBM).

  Exception (only vehicles with starter motor (12 V)): If the Engine Control Module (ECM) decides that the conventional starter motor (12 V) should be used to start the combustion engine, then the switches in the battery control switch module BCSM ensure that the starter circuit is disconnected from the rest of the electrical system while the main battery remains connected, and the electrical system is therefore powered by both the main battery and the converted voltage from the Mid Voltage Battery Module (MVBM).

• The automatic gearbox is requested to engage.
• When the engine has been started, the switches in the battery control switch module BCSM are set back to normal, i.e. both batteries are connected to the electrical system.