BMW introduced all-wheel drive more than 35 years ago; for almost two decades the intelligent power distribution between the front and rear axles has been known as BMW xDrive. Now the first all-electric BMW xDrive system can also be experienced on the road. It premieres in the BMW iX and the BMW i4 M50 (electricity consumption combined: 22.5 - 18.0 kWh/100 km according to WLTP).
Like all model variants of the BMW iX, the all-electric Gran Coupé of BMW M GmbH is driven by one motor on the front and one motor on the rear axles, creating all-wheel drive without mechanical connection between the front and rear wheels. The interaction of the two motors is optimized by an innovative drive torque control.
Intelligent networking of driving stability control, wheel slip limitation near the actuator 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 power of up to 400 kW/544 hp and a maximum torque of 795 N·m, into 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 design of the actuator-related wheel slip limitation and the Combined Charging Unit (CCU), the highly integrated range of functions 0f which includes both charging and drive electronics for the first time.
The intelligent networking of all systems makes it possible to divide the drive torque requested by the driver between the two motors 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 framework 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 as well as the yaw rate of the vehicle.
On the basis of these parameters, a speed corridor is determined in which the two drive units may be active in order to ensure a stable driving condition. This also takes into account the My Mode chosen by the driver and a possible activation of the DTC (Dynamic Traction Control) mode.
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 as snow, sand or rubble, maximum propulsion is realized.
From the data provided 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 noticeably faster and with a significantly higher control quality to any change in the driving situation than would be possible with a transfer case between the axles familiar from BMW models with internal combustion engines.
The drive control of the CCU, the wheel slip limit near the actuator and the driving stability control act flexibly and precisely with each other. 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.
Optimal traction, perfect directional stability, high efficiency. For starting and accelerating, the desired drive torque is divided from the CCU between both motors to ensure optimized traction. In addition to the road friction values and the driver’s desire, the electric BMW xDrive and the wheel slip limit close to the actuator also take into account environmental parameters such as wind, inclines or gradients, the vehicle settings of the My Modes and the DTC as well as the dynamic axle load shift, which ensures more weight and better power transmission on the rear axle when accelerating.
Due to the direct integration of the wheel slip limitation near the actuator into the drive torque control of the CCU, long signal paths for driving stability control are eliminated, so that the control interventions can take place at up to 10 times higher speed and in a particularly precise dosage. This ensures that the vehicle stays in a straight line, even during extremely dynamic acceleration maneuvers.
If the accelerator pedal is operated moderately, the rear electric drive unit delivers a continuously higher share of the drive torque as the speed increases. In phases of constant driving, the drive is ultimately 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.
The full all-wheel drive functionality is available at all times. For example, even when driving in My Mode Efficient, any spontaneous load request is answered immediately with a significant increase in the drive torque on the front axle. In dynamic cornering, on the other hand, the proportion of rear wheel drive is increased.
The front wheels can absorb higher cornering forces due to the reduced drive torque. In this way, driving stability and agility of the vehicle are increased equally. Depending on the driving situation, the fully variable electric BMW xDrive system can realize any form of power transmission: the 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, the recuperation torque can be quickly and precisely divided 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 due to 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 a constant pedal feel at all times.