Hydraulic impulse storage, automatic transmission

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2017 BMW X3, 2016 BMW X3, 2015 BMW X3, 2014 BMW X3, and 2013 BMW X3. However, it is still accessible from the selected car via links, so may be relevant.

The hydraulic impulse storage allows the automatic engine start-stop on vehicles with automatic transmission. The hydraulic impulse storage provides the required pressure to be able to activate the shift elements during an automatic engine start.

Fig 1: Identifying 8HP Automatic Transmission

Courtesy of BMW OF NORTH AMERICA, INC.

Item	Explanation
1	8HP automatic transmission
2	Hydraulic impulse storage

System pressure is applied to the hydraulic impulse storage when driving. The piston is pressed back against the spring pressure. The electromagnetic locking unit keeps the piston in this position. The hydraulic impulse storage is ready for the engine start.

At an engine start the electromagnetic locking unit is released by the automatic engine start-stop. Spring force presses the piston into its original position. This makes the pressure required for the shift elements immediately available.

Fig 2: Identifying Electromagnetic Locking Unit Components

Courtesy of BMW OF NORTH AMERICA, INC.

Item	Explanation
A	Hydraulic impulse storage inactive
B	Hydraulic impulse storage active
1	Electromagnetic locking unit
2	Spring
3	Hydraulic impulse storage
4	Pistons
5	Non-return valve

During the engine start the hydraulic impulse storage fills system pressure into the shift elements of the drive-off gear. The engine start can therefore be carried out for selector lever position D. The automatic engine start-stop is deactivated if the selector lever is in the shift gate M/S.