Mercedes-Benz 9-Speed Automatic Transmission Oil Supply and Transmission Cooling

The ratios for the gear ranges are implemented by planetary gear sets. In this automatic transmission, all transmission functions and control components are combined into one assembly module.

M42 Electric transmission oil pump

Oil supply with stopped combustion engine
If the oil supply fails with a stopped combustion engine, all control elements and actuators change to the basic state under no load and the automatic transmission is no longer force-fit.

When the engine is started and the oil supply has been built up, the gear position has to be changed from “N” to “D”. The resulting delay between the intent to start off and the time at which the vehicle does start off is noticeably minimized thanks to the electric transmission oil pump. The electric transmission oil pump is actuated in start/stop mode with the combustion engine stopped and takes on the basic supply of the control elements and actuators here. A defined basic pressure is thus maintained.

The electric transmission oil pump also supports the primary pump when the combustion engine is running at low rpm. The volumetric flow rate delivered by the electric transmission oil pump as required is requested in the low-rpm range of the combustion engine during shift operations (filling of the actuator) or with increased cooling requirements.

Support of the working pressure supply (boosting)
With an increased volumetric flow requirement (e.g. during shift operations), the electric transmission oil pump supports the mechanically driven primary pump as required. The volumetric flow rate for the electric transmission oil pump additionally provided here counteracts drops in working pressure due to insufficient supply in these situations.

Highly efficient vane cell pump in off-axis configuration
13 oil pump (primary pump)

Support of cooling and lubrication
The electric transmission oil pump is actuated as required with high cooling and lubrication requirements in order to provide an additional volumetric flow rate. Thanks to the option of providing an additional volumetric flow rate as required, the primary oil pump could be considerably downsized and the CO₂ output thus decreased.