Two motors, four driven wheels and a drive torque control that is unique in the world ensure optimum traction, driving stability and dynamics in every situation.
More than 35 years ago, all-wheel drive provided a new form of driving pleasure in a BMW for the first time. The intelligent distribution of power between the front and rear axles has been known as BMW xDrive for almost two decades. Now the fascination of four driven wheels has reached an additional, particularly sustainable dimension. The first fully electric BMW xDrive system can now also be experienced on the road. It made its debut in the BMW iX and the BMW i4 M50 (combined power consumption: 22.5 – 18.0 kWh/100 km according to WLTP; combined CO2 emissions: 0 g/km).
Like all model variants of the BMW iX, the all-electric Gran Coupé from BMW M GmbH is powered by one motor each on the front and rear axles. This creates what is known as paraxial all-wheel drive without a mechanical connection between the front and rear wheels. The interaction of the two engines is optimized by an innovative drive torque control system that is unique in the world. The electric BMW xDrive system thus ensures the ideal form of power transmission in every driving situation – combined with a typical BMW driving experience.
Intelligent networking of driving stability control, actuator-related wheel slip limitation and drive control.
The all-electric BMW xDrive system makes it possible to transform outstanding drive power, which is expressed, for example, in the BMW i4 M50 in a system output of up to 400 kW/544 hp and a maximum torque of 795 Nm, into superior driving dynamics. This requires precise distribution of the drive torque. This is ensured by linking the driving stability control DSC (Dynamic Stability Control) with the all-wheel drive-specific design of the actuator-related wheel slip limitation and the Combined Charging Unit (CCU), whose highly integrated range of functions includes both the charging and the drive electronics for the first time.
The intelligent networking of all systems makes it possible to distribute the drive torque requested by the driver between the two engines in such a way that traction and driving stability, dynamics and efficiency are guaranteed to the optimum extent for the respective driving situation. The DSC defines the general conditions under which the electric BMW xDrive system can operate. It constantly analyzes the wheel speeds, the steering angle, the driving speed, the longitudinal and lateral acceleration and the yaw rate of the vehicle. Based on these parameters, a speed corridor is determined in which the two drive units can be active in order to ensure a stable driving condition. The My Mode selected by the driver and any activation of the DTC mode (Dynamic Traction Control) are also taken into account. This mode represents a special mode of the DSC in which, in conjunction with the wheel slip limitation close to the actuator and other control functions, including expanded intervention thresholds on a normal surface, an even sportier driving style is possible without any control interventions. In addition, on loose ground, such. B. snow, sand or debris, maximum propulsion is achieved.
From the data supplied by the DSC and the drive torque requested by the driver, the CCU calculates the ideal split between front and rear wheel drive. The all-electric xDrive system can react to any change in the driving situation noticeably faster and with a significantly higher level of control quality than would be possible with a transfer case between the axles familiar from BMW models with combustion engines.
The drive control of the CCU, the wheel slip limitation close to the actuator and the driving stability control interact flexibly and precisely. If necessary, the data on the driving situation provided by DSC can already proactively include an all-wheel drive request. In many driving situations, driving stability can be increased simply by changing the distribution of drive torque between the front and rear motors, without further intervention being necessary. If a single wheel threatens to lose grip on the road, the wheel slip limitation close to the actuator with its particularly fast and precise reactions is used first. Only in critical situations does DSC also intervene with wheel-specific brake interventions.
Optimum traction, perfect directional stability, maximum efficiency.
When moving off and accelerating, the CCU divides the desired drive torque between the two motors in order to ensure optimized traction. In addition to the friction values on the road and the driver’s wishes, the electric BMW xDrive and the wheel slip limitation close to the actuator also take into account environmental parameters such as wind, uphill or downhill gradients, the vehicle settings of My Modes and the DTC as well as the dynamic axle load shift, which when accelerating results in more weight and better power transmission on the rear axle cares. Due to the direct integration of the wheel slip limitation close to the actuator in the drive torque control of the CCU, there are no long signal paths to the driving stability control, so that the control interventions can take place at a speed that is up to 10 times higher and in a particularly precise dosage. This ensures that the vehicle stays in a straight line, even during extremely dynamic acceleration manoeuvres.
If the accelerator pedal is operated moderately, the rear electric drive unit generates a continuously higher share of the drive torque as the speed increases. In phases of constant driving, the drive is finally provided solely by the motor acting on the rear wheels. This creates the traditional BMW driving experience. At the same time, the efficiency of the drive system and with it the range of the vehicle is optimized.
Full all-wheel drive functionality is available at all times. For example, when driving in My Mode Efficient, any spontaneous load request is immediately responded to with a significant increase in drive torque on the front axle. In dynamic cornering, on the other hand, the share of the rear-wheel drive is increased. The front wheels can absorb higher cornering forces due to the reduced drive torque. In this way, the driving stability and agility of the vehicle are increased in equal measure. Depending on the driving situation, the fully variable electric BMW xDrive system can implement any form of power transmission: traction-optimized all-wheel drive with variable torque distribution as well as pure front or rear-wheel drive.
Precisely controlled recuperation optimizes driving stability and efficiency.
In overrun and braking phases, both motors can use the generator function to feed energy back into the high-voltage battery. This enables a significantly higher recuperation performance than just one motor. The all-electric BMW xDrive system also makes it possible to control these recuperation processes in such a way that maximum driving stability and efficiency are guaranteed at all times. The system is designed in such a way that the more powerful motor on the rear axle also plays the main role in energy recovery. On the other hand, in curves and in adverse road conditions, a higher proportion of the recuperation power is provided by the front engine.
Just like the drive torque, the recuperation torque can also be divided quickly and precisely between the two motors. In addition, the intelligently networked drive torque control ensures that a stable driving condition is guaranteed even when decelerating. The slip control determined by the DSC helps to prevent unstable driving conditions caused by excessively high brake recuperation. The transition from deceleration through recuperation to the intervention of the hydraulic braking system is imperceptible to the driver. The integrated brake system also contributes to this, giving him a constant pedal feel at all times.
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