Oil Supply / Auto Start Stop
Oil Supply / Auto Start Stop
The Auto Start Stop function makes a significant contribution to increasing fuel economy in an urban driving environment. When the Auto Start Stop function was developed for an automatic transmission, the main challenge was guaranteeing prompt driving readiness of the vehicle after the engine was restarted. In order to avoid delays after a stop phase, sufficient oil pressure must be available within a very short time to ensure non-slip power transmission. On automatic transmissions without an Auto Start Stop function, oil pressure is generated by a mechanically operated transmission-oil pump that can only achieve the oil pressure and volume flow the vehicle requires to drive off when the speed of the engine increases. An electrically powered auxiliary oil pump continues to supply the required quantity of oil to the shifting elements even after the engine has stopped. Only then was it possible to minimize the response time of the transmission to such an extent that the driver and passengers would not be able to tell whether the vehicle was driving off after an Auto Start Stop phase or whether the engine had already been running. In order to minimize the length of the pressure lines, the transmission-oil pump was installed directly beside the mechanical transmission-oil pump under the transmission.
The auxiliary oil pump comprises an internal gear pump and an electric motor that powers the hydraulic pump. The electric motor is a brushless DC motor with a stator and a rotor that is activated by the Tiptronic control unit whenever the Start Stop function switches off the combustion engine.
Engine running
The oil pump (3) in the transmission generates oil pressure via the combustion engine. Only the oil pump powered by the combustion engine is active.
If the vehicle stops and the Start Stop system switches off the combustion engine, the oil pump (3) in the transmission also switches off. The electric control unit (8) then activates the electric motor (7), which in turn powers the auxiliary oil pump (6).
Engine off
The oil pressure generated forces open the shut-off valve (5) and pressurized oil then flows into the slide-valve case (4).
Automatic engine restart
The electric oil pump control (8) monitors the oil pressure and activates or deactivates the electric motor accordingly.
The oil pressurized by the electric oil pump (6) and oil pump (3) reaches the required pressure almost immediately after an automatic restart. The oil pressure in pump (6) is higher initially, but after a short time the situation is reversed and the pressure in pump (3) is higher. The shut-off valve (5) closes when the system reaches this state. The electric pump switches off.
The oil pump control also takes into account the engine speed.
- If the Start Stop function switches off the engine automatically and the engine speed falls below a defined value, the electric pump switches on.
- If the Start Stop function starts the engine automatically and the engine speed exceeds a defined value, the pump switches off.
The transmission control module and the diagnosis feature monitor the functional reliability of the system.
Control unit for the electric oil pump
If the Start Stop system switches off the engine, the transmission control module activates the electric oil pump to ensure that sufficient oil pressure is maintained to enable the vehicle to move off smoothly.