Overrun mode, basic function

Overview

This document contains information on:

 General

 Function requirements

 Regenerative braking levels in drive program EL

 Limits of regenerative braking

 Deceleration mode

 Regenerative braking in deceleration mode

 Cruising mode without use of the accelerator pedal

General

If the brake and accelerator pedal is not actuated for a rolling vehicle, then:

 Kinetic energy is absorbed via the electric machine and converted into electrical energy (regenerative braking) when the high-voltage battery is not fully charged.

 A deceleration fuel shutoff of the internal combustion engine can also take place.

Function requirements

 Hybrid drive system operational

 Accelerator not operated

 Service brakes not operated

 Vehicle rolling

 Gear range "D"

Regenerative braking levels in drive program EL

Additional function requirements:

 Drive program EL active

The driver can raise or lower regenerative braking in deceleration mode by upshifting the steering wheel gearshift button and downshifting the steering wheel downshift button. Alternatively, the driver can select automatic regulation of regenerative braking.

After the engine has started or gear range D has been set, regenerative braking level DAUTO is automatically active.

Limits of regenerative braking

The regenerative braking in deceleration mode is only reduced or not even present at all in the following situation:

 as the charge level of the high-voltage battery increases

 if the high-voltage battery is not yet at operating temperature

 when driving close to vehicle standstill

 during and after an intervention by the vehicle dynamics control

In these cases, the required braking effect is set via the service brake.

Deceleration mode

If the brake pedal is actuated regenerative braking occurs.

The powertrain control unit directly reads in the following signals in overrun mode:

 Accelerator pedal position

 Gear range status

 Engine clutch, status

 Transmission oil temperature signal

 Status of electric auxiliary oil pump

 Wheel speed

 Service brake, status

 High-voltage battery voltage

 Temperature of high-voltage battery

 Status of electric machine

 Torque of electric machine

 Engine speed of electric machine

 Tolerable charging voltage/current signal

Depending on the charge level of the high-voltage battery and the selected drive program, the powertrain control unit calculates a nominal deceleration torque, which the internal combustion engine control unit implements with deceleration fuel shutoff and/or regenerative braking in deceleration mode.

Regenerative braking in deceleration mode

Kinetic energy is taken up over the electric machine and converted into electrical energy (regenerative braking) for a low charge level of the high-voltage battery. To do this, the powertrain control unit requests a generative torque from the power electronics control unit.

The energy gained in deceleration mode is fed into the high-voltage on-board electrical system via the power inverter integrated in the power electronics control unit.

In overrun mode > 180 km/h and/or in the Sport (S) drive program, the engine separator clutch remains closed and the combustion engine goes into overrun mode fuel shutoff and thus generates an overrun torque. The electric machine supports this deceleration torque if necessary by a generative torque.

In deceleration mode and active deceleration fuel shutoff the internal combustion engine generates a boost torque which can be greater together with the recuperative boost torque than the calculated specified boost torque. In this case the deceleration fuel shutoff is not activated and the internal combustion engine generates a minimum actuatable torque. This concept ensures that the drive always remains the same for the driver as much as possible, with and without deceleration fuel shutoff.

 The decel mode with deceleration fuel shutoff is more effective from an energy usage point of view. At a low charge level for the high-voltage battery and a low nominal deceleration torque the regenerative braking in deceleration mode without deceleration fuel shutoff is actually more meaningful.

Cruising mode without use of the accelerator pedal

If the brake and accelerator pedals are not actuated for a rolling vehicle and recuperation is possible, the combustion engine in the Eco (E) drive program is disconnected from the drive up to a speed < 180 km/h via the opening of the engine separator clutch and switched off. If it is not possible for regenerative braking to take then the deceleration fuel shutoff of the internal combustion engine can also be activated.

For a slightly actuated accelerator pedal, the electric machine supports the drive for an adequately high charge level of the high-voltage battery in order to counteract the falling vehicle speed.