BMW has showcased two innovative new systems in the guise BMW Concept X6 Sport Activity Coupé at the International Motor Show (IAA). As well as being the first vehicle to be fitted with a unique stabilization system known as Dynamic Performance Control, the concept incorporates BMW’s ActiveHybrid system – a combination of a combustion engine and two high-performance electric motors aimed at delivering greater efficiency over a wider speed range than conventional hybrid designs.
The system uses a two-mode active transmission that optimizes performance and efficiency (which is 20% better than for a combustion engine alone according to BMW) – over the full operating range of the car. The BMW ActiveHybrid system has combined the driving dynamics of a combustion engine with the efficiency of an electric drive system via a two-mode transmission, with one setting for step-off performance and the other for higher speed driving. The transmission is based on an ECVT electrical continuously variable transmission. The two operating modes, one of which has been optimized for low, the other for high speeds, are supplemented by fixed transmission ratios.
These drive systems are connected to one another by three planetary gearsets allowing a power split covering the drive forces of the combustion engine and the electric motors in two transmission ranges. The two power-split ECVT sections allow the drivetrain to run at continuously variable speeds ensuring efficient hybrid functions throughout the full operating range of the car. The BMW Concept X6 ActiveHybrid is able to reduce fuel consumption by up to 20 percent compared with a comparable BMW running on a combustion engine alone. The compact two-mode system allows easy integration into the overall vehicle with only a minimal weight increase. The car can be driven on electric power only, on the combustion engine alone, or with a combination of both power units.
The electric motors may also be used for both accelerating and power regeneration under braking. When maximum acceleration is called on, one of the two electric motors acts as a generator, converting some of the engine’s power into electrical current that is subsequently fed to the battery or the second electric motor. The second electric motor then converts the power coming from the first electric motor or from the battery back into mechanical power for the output drive shaft on the transmission. A conventional hybrid drive system does not come with fixed mechanical transmission ratios, instead, it uses a continuously variable transmission. The disadvantage is that the actual requirement for performance and load feedback from the car are separated, the result being less than dynamic driving.
The two-mode active transmission in BMW’s ActiveHybrid system is designed to reduce the share of power flowing through the electrical branch of the transmission and therefore provides the energy of both electric motors in a more efficient way. Dr Norbert Reithofer, Chairman of the BMW Group said “It is important to discern the difference between BMW technology and other hybrids. We are combining a high-efficiency and high-performance combustion engine with two high-performance electric motors. This is truly unique. It also allows us to make use of the hybrid’s efficiency advantage across a much wider speed range.” As a part of the BMW Efficient Dynamics development strategy, the ActiveHybrid technology has been designed to be applied throughout the range of BMW vehicles in the future. “This is just a preview of how BMW mobility could look in years and decades to come – lower fuel consumption, lower emissions, and with a lot more driving pleasure”. Dr Reithofer said. Another first for the concept car is the fitting of “Dynamic Performance Control”, a system which spreads drive forces according to a given situation, feeding a varying degree to the two wheels at the rear. When over- or understeer is detected the system will automatically vary the distribution of drive forces between the wheels to enhance stability, dynamic traction, and forward motion.
In a bend this means transferring additional wheel speed to the outer rear wheel to push the car round the corner (as opposed to applying brake force to the inner wheels as currently happens with traditional stability control systems). BMW Group has been conducting research on hybrid technology and its various options for more than 15 years, and has recently joined forces with General Motors and DaimlerChrysler to form the Global Hybrid Cooperation, to develop the next generation of hybrid drive systems. BMW Group has presented several hybrid vehicle concepts, starting with a hybrid car in 1995 based on the BMW 518i.