BMW Announces Production Versions of ActiveHybrid X6 and ActiveHybrid 7; Part 2: The ActiveHybrid 7 with Li-ion Pack
|The BMW ActiveHybrid 7. Click to enlarge.|
BMW announced the production versions of the BMW ActiveHybrid X6 and the BMW ActiveHybrid 7. Both will make their world premieres at the Frankfurt Motor Show (IAA) in September. The ActiveHybrid X6 will reach US BMW Centers in fourth-quarter 2009, and the ActiveHybrid 7 will arrive in the US in Spring 2010.
The ActiveHybrid 7 is a mild-hybrid application, and offers approximately a 15% reduction in fuel consumption. (Earlier post.) The combination of an upgraded twin-turbo, High Precision Direct Injection, V8 engine with a 3-phase synchronous electric motor gives the BMW ActiveHybrid 7 a combined output of 455 hp (339 kW) and maximum torque of 516 lb-ft (700 N·m) (preliminary figures).
|Components of the BMW ActiveHybrid 7. Click to enlarge.|
Power is transmitted via an 8-speed automatic transmission with the electric motor positioned between the engine and the torque converter. The electric motor receives its supply of energy from a compact, 400 Wh lithium-ion battery.
The BMW ActiveHybrid 7 accelerates to 60 mph in 4.8 seconds (preliminary), while delivering fuel economy approximately 15% better than the 750i. Official EPA ratings will be available closer to the US market launch.
|Powertrain of the BMW ActiveHybrid 7. Click to enlarge.|
The BMW ActiveHybrid 7 features Auto Start Stop to maximize efficiency by avoiding engine idling when stopped. Air conditioning and ventilation system remains fully operational when using Auto Start Stop. In this case, power is supplied directly through the 120-volt on-board network powered by the lithium-ion battery.
The ActiveHybrid 7 stores energy in a compact, high-performance lithium-ion battery. The processes of charging the battery, supplying power to the electric motor, and electric ancillaries are managed by a control unit developed specifically for the BMW ActiveHybrid 7.
Braking and Energy Regeneration. The BMW ActiveHybrid 7 features an enhanced version of Brake Energy Regeneration already used in some current BMW models running on a combustion engine alone. Brake Energy Regeneration converts some of the vehicle’s kinetic energy into electric power for the on-board network whenever the driver lifts off the accelerator or applies the brakes. On acceleration, electricity is fed back into the on-board network from the lithium-ion battery.
When the driver requires only a small amount of brake force, the system uses only the drag of the electric generator (motor). The mechanical brakes are activated only when the driver presses down the brake pedal harder for more stopping power.
Two interconnected on-board networks for flexible management of energy.
The BMW ActiveHybrid 7 features both a conventional 12-volt on-board network fed by an absorbent glass mat (AGM) battery and an additional battery featuring lithium-ion technology operating at 120V.
The lithium-ion battery supplies power to the electric motor as required and serves to start the engine. Electric power generated by Brake Energy Regeneration is fed into both on-board networks. The air conditioner compressor on BMW ActiveHybrid 7 is integrated in the 120-volt network. All other electrically operated functions of the car are supplied by the 12-volt network. The two on-board networks are connected by a voltage converter featuring flexible energy management. Both batteries are charged and maintained through Brake Energy Regeneration. The 12-volt battery is fed with energy from the high-voltage network.
|The battery pack. Click to enlarge.|
Battery pack. The BMW ActiveHybrid 7 gives up almost no luggage capacity to its conventional siblings (750i and 750Li), because the Li-ion battery pack is small enough to be mounted underneath the trunk floor. Furthermore, the weight penalty incurred by this battery is minimal, meaning less energy is consumed to carry it. Using this technology enables the ActiveHybrid 7 to maximize storage capacity and acceleration in the hybrid luxury sedan segment. BMW claims it is the fastest-accelerating hybrid sedan in the world.
The battery has a capacity of 400 Wh (0.4 kWh). The pack comprises 35 cells and an integrated control unit which continuously analyzes the charge level and ensures appropriate dependability under all conditions. The lithium-ion battery measures just 14.6 x 8.7 x 9.1 inches (37 x 22 x 23 centimeters) and weighs 59.5 lb (27 kg).
Hence, it is only slightly larger than a conventional car battery and is housed conveniently in the trunk floor between the wheel arches of BMW ActiveHybrid 7, safely surrounded by a high-strength special casing. This location provides protection for the battery and helps to preserve the 7 Series’ ideal 50-50 weight distribution.
Twin-turbocharged V8 gasoline engine with High Precision Direct Injection. The combustion engine is the innovative reverse-flow V8 introduced in the BMW X6. The world’s first V8 gasoline engine with two turbochargers in the V-section between the two rows of cylinders develops power throughout the entire engine speed range. The spontaneous and direct response of this engine results from its compact configuration, which allows the shortest exhaust manifolds.
Displacing 4.4 liters, this outstanding engine delivers its maximum output of 440 hp between 5,500 and 6,000 rpm. Peak torque of 475 lb-ft is maintained from 2,000 to 4,500 rpm, with High Precision Direct Injection ensuring precise supply of fuel at all times. Piezo-injectors positioned in the combustion chambers between the valves ensure a smooth, efficient and clean combustion process. The engine fulfils the European EU5 standard as well as the ULEV II standards in the US.
Compared with the engine featured in the BMW 750i/Li, this version has been modified in numerous respects to the specific requirements of the BMW ActiveHybrid 7. There is no starter, alternator, or belt drive for the air-conditioning compressor and hydraulic pump. The cooling system has been modified for hybrid operation.
The response of the engine is enhanced by the electric motor with its additional torque. The direct response to the accelerator pedal is impressive when compared either with conventional 7 Series models or existing hybrid vehicles already in the market. Acceleration to 60 mph in 4.8 seconds (preliminary) ranks it among the fastest and most thoroughbred sports cars in the world. Top speed of BMW ActiveHybrid 7 is electronically limited to 150 mph (241 km/h).
Electric motor. The three-phase synchronous electric motor featured in BMW ActiveHybrid 7 is positioned between the engine and the torque converter, and is connected to the crankshaft. Shaped like a disc, the electric motor weighs 50.7 lb (23 kg). Its compact design ensures full integration in the transmission housing.
The electric motor generates boost of approximately 20 hp (15 kW) and peak torque of 155 lb-ft (210 N·m). When operating as a generator in Brake Energy Regeneration, the electric motor produces approximately 27 hp (20 kW) of brake force.
|Electric machine and 8-speed transmission. Click to enlarge.|
8-speed automatic transmission in combination with Auto Start Stop. To transmit the drive power developed jointly by the V8 and the electric motor, the BMW ActiveHybrid 7 features a newly developed 8-speed automatic transmission specifically tailored to the demands of hybrid technology.
The new 8-speed close-ratio automatic transmission uses an innovative gear-set configuration providing two additional gears and a larger overall gear range than the 6-speed automatic transmission used by BMW without negative effects on size, weight, or internal efficiency. With the new transmission, the V8 engine is better able to stay in the power peak while ensuring greater overall efficiency.
Overall efficiency is further enhanced with the Auto Start Stop function. Auto Start Stop automatically switches off the combustion engine at idle, eliminating fuel consumption when stopped. Once the driver’s foot comes off the brake pedal, the engine restarts instantaneously.
Starting of the V8 engine is accomplished by the electric motor, which is supplied with power from the high-voltage battery. This enables the engine to start extremely quickly and without any vibration. The automatic transmission is maintained in a “ready-to-drive” mode by a new hydraulic pressure accumulator for the valve body of the transmission.
Additionally, the engine control unit “remembers” engine crank position and uses delay-free ignition to restart the engine in less than one crank rotation. The engine is also switched off when the driver uses the Auto Hold function. The driver just has to slightly press down on the gas pedal in order to reactivate the engine.
Display. Featuring Black Panel technology, the instrument cluster in the BMW ActiveHybrid 7 is supplemented by an energy flow display in the lower section of the tachometer, a blue arrow shows the current status of Brake Energy Regeneration. When accelerating, an additional graphic display illustrates the boost effect generated by the electric motor.
Using the iDrive control system in the Central Display, the driver is able to see a detailed view of the interaction between the V8 engine and electric motor. Beyond the current charge level of the lithium-ion battery, this display also shows the real-time flow of energy. As an alternative to this graphic display, the system can analyze the current efficiency. In this case a bar graph, which is updated every minute, shows the efficiency levels achieved by the hybrid components during the last fifteen minutes.
Cooperation with Daimler. The hybrid components featured in BMW ActiveHybrid 7 were developed jointly by BMW and Daimler in a cooperation project seeking to develop and test components for hybrid drive in luxury performance cars. (Earlier post.) Apart from the electric motor and the lithium-ion battery, this joint venture also served to develop the power electronics for the high-voltage network featured in the BMW ActiveHybrid 7.
Each manufacturer subsequently integrated the hybrid components into the respective model in accordance with individual, brand-specific requirements.