Overrun Mode, Basic Function - GF08.00-P-1004A
Electric vehicles
Overview
This document contains information on:
- General
- Function requirements
- Function
- Regenerative braking levels
- Limits of regenerative braking
- Automatic braking effect during map-based events
- Automatic braking effect during distance control
- Distributing the total braking effect to the front and rear axles
General
In vehicles with two electric machines, the following explanations apply for each machine.
In deceleration mode, the vehicle activates the electric machine. Since no clutch is available, the frictional connection in the powertrain is always present. If the accelerator pedal is not actuated for a rolling vehicle, either Glide mode is activated or the electric machine absorbs the kinetic energy and converts this into electrical energy (recuperation).
Function requirements
- Circuit 15 ON
Function
The electric machine generate a braking effect during regenerative braking. This braking effect can be set in several regenerative braking levels or automatically controlled. The driver display shows the recuperation level that is currently set.
If the driver actuates the brake pedal, the braking effect is initially generated via recuperation before the service brake deploys. The braking with the service brake is described in a different document. 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.
Regenerative braking levels
The driver has the option to increase or reduce the recuperation in overrun mode via the "Upshift" steering wheel shift button and the "Downshift" steering wheel shift button. Alternatively, the driver can select automatic regulation of regenerative braking.
After circuit 15 is activated again, recuperation level D or recuperation level D AUTO Active. The respectively activated recuperation level depends on the vehicle model and the last selected recuperation level.
| Stage | Recuperation | Properties |
|---|---|---|
| D+ | None | The vehicle rolls freely (glide mode), no regenerative braking takes place. |
| D | NORMAL | Medium settings as compromise from braking via accelerator pedal and braking via brake pedal. In order to brake, the driver must press the brake pedal more frequently than in stage D-. |
| D- | reinforced | Increased braking effect in deceleration mode. |
| D- | high | Maximum braking effect in deceleration mode. In this stage, most kinetic energy is recovered. |
| DAUTO | Automatic | The level of the regenerative braking and the braking effect are automatically adapted to the current traffic situation. Map data can also be used for this purpose. |
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.
Automatic braking effect during map-based events
Additional function requirements:
- Level DAUTO selected
- Associated drive program active (depends on the model)
Depending on map-based events, regenerative braking is increased as required. The following events influence regenerative braking:
- Speed limit
- Inclines and gradients
- Intersections and roundabouts
- Corners
The multimedia system control unit generates data about the vehicle's probable route.
The powertrain control unit evaluates this data with the aim of adapting the driving style to the route ahead in the optimum way and thereby reducing energy consumption.
This is achieved through an intelligent combination of coasting mode and higher and longer regenerative braking phases. The working area of regenerative braking covers all manually selectable regenerative braking levels.
The driver receives feedback about the optimum accelerator pedal position via the multimedia display and the haptic accelerator pedal. The powertrain control unit calculates the appropriate specified deceleration and sends the request for the braking torque to the power electronics control unit. The power electronics control unit actuates the electric machine accordingly.
Automatic braking effect during distance control
Additional function requirements:
- Level D AUTO selected
When approaching a slower or a decelerating vehicle, or during follow-up trips downhill, regenerative braking is increased as necessary. A target deceleration is calculated on the basis of the relative speed and the distance to the vehicle in front, and the electric machine is activated accordingly.
Distributing the total braking effect to the front and rear axles
In vehicles with two electric machines, the drive control distributes the entire braking torque in one braking torque for the front axle and one braking torque for the rear axle. The torque is divided according to criteria of axle load distribution and anti-slip control, for example. The maximum braking torque is achieved with a distribution of 70 % to the front axle and 30 % to the rear axle.
The powertrain control unit requests the partial torques from the respective power electronics control unit.
| Function schematics | |||
| Function schematic for overrun mode | Model 293 | PE08.00-P-2502-97A | |
| Model 243 | PE08.00-P-2502-97D | ||
| Model 295, 296, 297 | PE08.00-P-2502-97F | ||
| Additional basic functions | |||
| Power electronics control unit, basic function | GF08.20-P-9891A | ||
| Electric drive powertrain control unit, basic function | GF08.20-P-9892A | ||
| Electric machine, basic function | GF08.10-P-2000A | ||
| Accelerator pedal sensor, basic function | GF30.20-P-2000A |