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J Engine and Engine Aggregates > Stop/Start System > Description and Operation > Description and function of start/stop system

Description and function of start/stop system

The start/stop system (ECO Start/Stop system) reduces fuel consumption in stop-and-go traffic or when driving with frequent stops and starts. Under appropriate conditions, for example at a red light, the vehicle automatically switches off the engine. This avoids emissions and saves fuel which would otherwise be used in idling. When the driver is ready to move off, the engine is started again immediately.

As soon as the driver prepares to move off (presses the clutch pedal), the engine starts quietly and almost immediately. The engine starts in just 0.3 s. This process is almost imperceptible and is completed even before a gear is fully engaged.

To process the higher number of starts, the starter motor is fitted with a heavy duty electric motor and a stronger gear mechanism for reduced noise.

As well as the uprated starter motor, highly sophisticated battery technology is required to guarantee that the vehicle battery can handle the frequent charge/discharge cycles of the stop/start process. An intelligent battery sensor (IBS) on the battery continuously monitors the charge state and general condition of the battery. The engine control module (ECM) uses the information from the IBS to establish whether the charge state and general condition of the battery are sufficient for use of the automatic start/stop system.

The ECO start/stop system reduces fuel consumption and carbon dioxide (CO2) emissions by up to 5% in urban mixed driving cycles and by up to 10% in dense traffic with frequent stops and starts.

A technically highly sophisticated control system ensures that the start/stop system meets both the driver's needs and those of the vehicle. The engine is switched off only if the vehicle speed is below 5 km/h (3 mph), the gear lever is in neutral and the clutch pedal is fully released. To start again, the driver presses the clutch pedal. The uprated starter motor starts the engine so it is ready when a gear is engaged. If the engine is switched off via the ECO start/stop system. The rev counter needle remains on the AUTOSTOP symbol (500 rpm) to indicate that the start/stop system has switched off the engine.

To guarantee that both the driver's needs and those of the vehicle can be fulfilled, the engine cannot be switched off under the following conditions:

  • The ratio between ambient and coolant temperature lies outside the specified values.
  • The ambient temperature is below -10 °C (14 °F).
  • The battery temperature is below 0 °C (32 °F) or above 55 °C (131 °F).
  • The driver's seatbelt is not engaged or the driver's door is not completely closed.
  • The clutch or accelerator pedal is pressed.
  • High demand on HVAC system.
  • The defrost function of the HVAC system is activated.
  • Low battery charge state.

Similarly, the engine starts automatically under the following conditions:

  • Driver's door opened or driver's seatbelt released
  • Bonnet opened
  • Low battery charge state
  • Increased demand on HVAC system.
  • Increased vehicle speed
  • Reduced brake servo vacuum
  • Coolant temperature above 125 °C (257 °F)
  • ECO mode disabled by driver
  • AUTOSTOP already active for over 5 min.

If the ECO start/stop system has switched off the engine and the ambient temperature is below 15 °C (59 °F), the ECM activates the additional relay of the start/stop system. Also the auxiliary coolant pump motor continues to circulate coolant while the engine is switched off. As soon as the ECO start/stop system has restarted the engine, the ECM switches the auxiliary coolant pump motor off again. The internal coolant pump in the vehicle engine takes over coolant circulation. The ECO start/stop system is activated automatically when the ignition is turned on. The driver can however disable the system at any time via the ECO switch in the instrument panel.

AutoStop conditions

The ECM sends an AutoStop message to the body control module (BCM) and switches off the engine when all the conditions listed below are fulfilled. The BCM transmits the AutoStop message to the instrument unit in which the AUTOSTOP display is then displayed in the rev counter.

  • ECO mode is activated.
  • The vehicle speed during the drive cycle was above 3 km/h (2 mph).
  • The ratio between ambient and coolant temperature corresponds to the specified values.
  • The bonnet switch status is "closed".
  • The driver's door status is "closed".
  • The driver's seatbelt status is "closed".
  • Brake servo vacuum greater than 45 kPa (7 psi).
  • The clutch switch is fully released.
  • The gearbox idle position switch shows "idle".
  • The vehicle speed is below 5 km/h (3 mph)
  • The engine speed is below 1500 rpm.
  • The coolant temperature is below 120 °C (248 °F).
  • No A/C compressor request from HVAC system (A/C or defrost mode).
  • The battery voltage is more than 12 V.
  • The battery charge state is over 75% (depending on vehicle condition).
  • The diesel particle filter regeneration is not active.

Ambient and coolant temperature to activate AutoStop function

Ambient temperature

Coolant temperature

AutoStop activated

below -10 °C (14 °F)

max.

no

below -10 °C (14 °F)

60 °C (140 °F)

Yes

0 °C (32 °F)

50 °C (122 °F)

Yes

6 °C (43 °F)

40 °C (104 °F)

Yes

12 °C (54 °F)

30 °C (86 °F)

Yes

20 °C (68 °F)

18 °C (64 °F)

Yes

30 °C (86 °F)

18 °C (64 °F)

Yes

>30°C (86 °F)

18 °C (64 °F)

Yes


AutoStart conditions

The ECM sends and AutoStart message to the BCM. When all the conditions below are fulfilled, the ECM and BCM start the vehicle.

Conditions authorised by the driver to activate AutoStart.

  • The transmission is in idle, the clutch pedal partly or fully pressed.
  • If a gear is engaged, the clutch pedal must be fully pressed to activate AutoStart.

Conditions authorised by the system to activate AutoStart (provided no gear is engaged):

  • Brake servo vacuum less than 40 kPa (7 psi).
  • The vehicle speed is above 10 km/h (4 mph)
  • The HVAC interior temperature varies by more than 3 °C (5 °F) (with automatic air conditioning).
  • There is an A/C compressor request from HVAC system (A/C or defrost mode).
  • The battery voltage is less than 11 V.
  • The battery charge state is under 73% (depending on vehicle condition).
  • Diesel particle filter regeneration is requested
  • ECO mode has been disabled.
  • The driver's door status changes to "open" and the driver's seatbelt status changes to "open".
  • The bonnet switch status changes to "open".
  • AUTOSTOP already active for over 5 min.

On cranking time over 2 seconds, manual switching on via the ignition switch is required.

System components

Engine Control Module (ECM)

The Engine Control Module (ECM) monitors the inputs from the coolant temperature sensor, the vehicle speed sensor, the "bonnet ajar" switch, the brake servo vacuum sensor, the clutch pedal position sensor, the gearbox neutral sensor and the engine speed to detect conditions for AutoStart and AutoStop. The ECM also controls the auxiliary coolant pump motor.

Coolant temperature sensor

The coolant temperature sensor (ECT sensor) indicates the engine operating temperature.

Intake air temperature sensor

This sensor is used by the ECM to monitor ambient temperature. If the temperature is too low, no AutoStop process is performed.

Interior temperature sensor

The heating/ventilation/air conditioning control unit monitors the passenger compartment temperature sensor to detect the temperature in the passenger compartment. The heating/ventilation/air conditioning control unit sends this temperature value via the data communication link to the ECM. From the temperature value received, the ECM establishes whether it is necessary to start the engine due to the temperature in the passenger compartment.

Vehicle speed sensor

The vehicle speed sensor detects the vehicle speed. If the speed detected during an AutoStop state exceeds a theoretical value, the ECM starts the engine.

"Bonnet ajar" switch

If the bonnet is in the "open" position, no AutoStop is performed. If the bonnet is opened during an AutoStop state, the engine is started automatically.

Brake servo vacuum sensor

The ECM monitors the vacuum to guarantee the proper brake force amplification for the brake pedal. If the ECM detects insufficient vacuum, it starts the engine.

Clutch pedal position sensor

The ECM monitors the clutch pedal position sensor for partly or fully pressed state. If the clutch pedal is partly pressed during an AutoStop state, the ECM starts the engine if the transmission is in neutral. If a gear is engaged, the ECM only starts the engine when the clutch pedal has been fully pressed.

Neutral position switch

The neutral position switch detects whether a gear is engaged. The ECM does not allow an AutoStop process until the clutch is pressed, the transmission brought to neutral position and the clutch pedal fully released again.

Coolant pump motor

The ECM switches on the auxiliary coolant pump motor during an AutoStop state in order to maintain the operating temperature and HVAC temperature. When the engine is started, the ECM switches off the auxiliary coolant pump motor.

Body Control Module (BCM)

The Body Control Module (BCM) monitors the ECO switch to activate or de-activate the Start-Stop system. It also controls the low speed communication bus and transmits the corresponding messages to the instrument unit and the HVAC system.

Intelligent battery sensor

The BCM, via the data communication bus, monitors the intelligent battery sensor for battery current, general battery condition and battery charge state. If the BCM detects a poor general battery condition or a low battery charge state, it does not allow an AutoStop process.

ECO switch

The ECO switch can be used to disable the AutoStart system if required.

DC converter

The DC converter monitors the battery voltage during an AutoStop state and maintains the battery voltage for the radio, the instrument cluster and the instrument panel display. On an AutoStop process, the battery voltage falls slightly. The DC converter maintains a constant voltage to guarantee the proper operation of the vehicle information displays.

Driver's door switch

The BCM constantly monitors the driver's door switch. If the door is open/ajar, the BCM does not allow an AutoStop process. If the driver's door and driver's seatbelt are opened during an AutoStop process, the BCM requests an AutoStart.

Driver's seatbelt switch

The BCM constantly monitors the driver's seatbelt switch. The BCM requests an AutoStart if the seatbelt and the driver's door are opened during an AutoStop state.

Instrument Cluster

To distinguish between a normal engine switch off (engine speed = 0 rpm) and switch off by the ECO Start/Stop system, the rev counter needle points to the AUTOSTOP symbol (500 rpm) to show that the engine has been switched off by the ECO Start/Stop system. The rev counter returns to normal mode as soon as the engine has been started or the ECO switch pressed to disable the system.

J Engine and Engine Aggregates > Stop/Start System > Description and Operation > Description and function of start/stop system

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