Standstill decoupling
When the vehicle is at standstill and drive position D is engaged, the impeller rotates and the turbine wheel is at standstill. The vehicle drives off if the brake is not applied. Torque is transmitted. If the vehicle is held back by the brake, the driving power is transformed into heat by the inner friction of the gearbox oil in the converter. This driving power is generated by idle speed control of the engine that counteracts the resistance in the converter. For this reason, the 8HP automatic transmission is equipped with a so-called standstill decoupling, already used in the 6HP model. Standstill decoupling is realized by opening multidisc brake B. This reduces engine performance. If the vehicle is at standstill and the gear selector is in D, you can reduce fuel consumption.
The following input dimensions are used for controlling the standstill decoupling:
| Input dimension | Explanation |
|---|---|
| Brake light switch | Standstill decoupling is only active if the service brake is applied. Once the service brake is released, power transmission of multidisc brake B is closed immediately even before an accelerator pedal value is detected. In the vast majority of cases, the vehicle can be prevented from rolling backwards on gradients. |
| Accelerator pedal value | If a defined accelerator pedal value is exceeded, standstill decoupling is deactivated. |
| Transmission output speed | If a transmission output speed is detected, standstill decoupling is deactivated. |
| Engine speed | The converter torque is calculated from the engine speed and the turbine speed (RPM differential) and the gearbox oil temperature. |
| Turbine speed | The converter torque is calculated from the engine speed and the turbine speed (RPM differential) and the gearbox oil temperature. |
| Gearbox oil temperature | The gearbox oil temperature is used to calculate the converter torque. In addition, standstill decoupling is only active within a gearbox oil temperature range of approx. 13 °C and 120 °C. |
| Slope angle of the vehicle | The vehicle's slope angle is used to adapt re-engaging of the clutch to the slopes you drive off from. |
To enable driving off free of deceleration and load alteration, multidisc brake B is not completely opened. A slight slip of approximately 20 % remains so that some torque is transmitted to the torque converter. If multidisc brake B was completely open, no torque would need to be transmitted to the converter (except the inner friction in the gearbox). Jerking could result when driving off. The following example shows the difference in the gearbox between a vehicle with/without standstill decoupling. Values are approximate because inner friction was not taken into account.
| Without standstill decoupling | With standstill decoupling (multidisc brake B completely open) | With standstill decoupling (multidisc brake B not completely open) | |
|---|---|---|---|
| Vehicle condition | Vehicle at standstill, service brake applied | Vehicle at standstill, service brake applied | Vehicle at standstill, service brake applied |
| Engine status | Idle speed | Idle speed | Idle speed |
| Gearbox status | Gearbox input shaft | Gearbox input shaft rotates at engine speed | Gearbox input shaft rotates at defined speed |
| RPM differential | Corresponds to engine speed | 0 | Approx.120 RPM |
| Slip in torque converter | 100 % | 0 % | 20 % |