Starting and stopping the combustion engine

When the prerequisites for driving in pure electric mode are no longer met, for example because the driver presses down harder on the accelerator, the combustion engine must be started. To accelerate it to the starting speed, electric machine A is braked and thus works as a generator. In the meantime, electric machine B continues to power the vehicle and also has to supply extra torque: this additional torque compensates for the torque generated by electric machine A to start the combustion engine. During the start while driving, the power flow in the gearbox skeleton looks like this:

Fig 1: Power Flow While Starting Combustion Engine

Courtesy of BMW OF NORTH AMERICA, INC.

If the combustion engine is to be started while the vehicle is at a standstill, electric machine A is activated as a motor. Electric machine B supports the torque. However, for this purpose, the transmission must be in the "no power transmission" state, as only then is the transmission output shaft torque-free.

As soon as the combustion engine reaches the starting speed, the ignition and fuel injection are activated. Of course, the engine then no longer absorbs any torque, but feeds torque into the active gearbox. As described earlier in the chapter on "ECVT 1 mode", both the combustion engine and electric machine B power the vehicle.

The combustion engine does not have to be started at a certain driving speed. It can be started throughout the entire speed range at which driving in electric mode is possible.

Fig 2: Starting Combustion Engine Speed Range Graph

Courtesy of BMW OF NORTH AMERICA, INC.

The combustion engine can be shut off not only when the vehicle is at a standstill, but also while driving. After the fuel injection and ignition are shut off, the combustion engine is brought to zero speed, controlled by electric machine A.

Fig 3: Stopping Combustion Engine Speed Range Graph

Courtesy of BMW OF NORTH AMERICA, INC.