ECVT 1 Mode

The first mode with variable ratio (ECVT 1 mode) is designed for low speeds and maximum tractive force. In this mode, the vehicle can be powered in the following ways:

Solely by electric machine B
Solely by the combustion engine
By electric machine B and the combustion engine.

The gear ratio for powering the vehicle using the combustion engine can be calculated as follows:

i = speed of the combustion engine/speed of the transmission output shaft

This gear ratio ranges from infinite to 1.800. The value "infinite" indicates that the combustion engine can run, despite the fact that the transmission output shaft is not moving. Thus driving off is reproduced in a similar manner as with a hydrodynamic torque converter. The gear ratio can be adjusted by controlling the speeds of the two electric machines: the higher the speed of electric machine A, the greater this gear ratio will be.

Electric machine B is connected to the transmission output shaft with a ratio of approximately four.

To implement ECVT 1 mode, only the multidisc clutch 1 in the active gearbox is closed; all others are opened.

When the vehicle is powered purely electrically, the electric machine A rotates without load, at the same time and in the opposite direction as electric machine B. This allows the transmission input shaft and thus the combustion engine to remain at a stand-still.

Fig 1: Power Flow While Driving In Pure Electric Mode In ECVT 1

Courtesy of BMW OF NORTH AMERICA, INC.

When the vehicle is powered by a combination of the combustion engine and electric machine B, the power of the combustion engine is divided into two portions. One could say that the power of the combustion engine branches off or "splits." This is the origin of the term "power-split hybrid drive." The two portions are:

A mechanical portion, which is used to propel the vehicle.
An electrical portion, because electric machine A works as a generator and generates electrical energy.

Fig 2: Power Flow When Vehicle Powered By Combination Of Combustion Engine And Electric Machine In ECVT 1 Mode

Courtesy of BMW OF NORTH AMERICA, INC.

Some or all of the electrical energy generated in generator mode can be stored in the high voltage battery. Electric machine B consumes electrical energy as a motor. Some or all of this comes from electric machine A or from the high voltage battery. How large the individual amounts of energy are depends on a number of factors and is recalculated and reset by the hybrid master control unit at all times.

The characteristic feature of both ECVT modes is that in addition to the mechanical drive path of the combustion engine, there is also an electrical drive path. This electrical drive path is identified by the fact that the combustion engine generates electrical energy using a generator, some or all of which is used by an electric machine to power the vehicle. The arrangement of this electrical drive path corresponds to that of a serial hybrid drive.

If we consider the sum of the energy flows, the electric machine can support the combustion engine. However, it is also possible to charge the high voltage battery in this mode. In this case, the combustion engine must provide a higher output and uses somewhat more fuel. However, this only appears to be a disadvantage. The hybrid operational strategy carries out this "load point increase" primarily when doing so increases the efficiency of the combustion engine. For example, the efficiency is better under full load than under partial load. Thus the energy stored in this way is obtained with a relatively low extra energy input and can be used later, for example for driving in pure electric mode.