BMW unveiled its Active Hybrid 7 in August 2009. In a format known as a ‘mild hybrid’, the BMW combines a turbocharged V8 petrol engine with a disc-type electric motor. The main purpose of the motor is to boost engine output for short periods, giving stronger acceleration. This is not an economy car.
The hybrid components used in the Active Hybrid 7 were developed jointly by BMW and Daimler in a project to develop and test components for hybrid drive in luxury performance cars. Apart from the electric motor and lithium-ion battery, the joint venture also produced the power electronics used to control the car’s high-voltage network.
BMW Active Hybrid 7.
The combination of a directly-fuelled 4.4-litre V8 with a three-phase synchronous electric motor gives this Sebener an overall system output of 465PS and 700Nm. Because of the way the system operates, the outputs from the petrol engine and electric motor don’t actually add up: 449PS and 650Nm from the V8, 20PS and 210Nm from the electric motor. These figures are very substantial, even for a car weighing over two tonnes.
Power is transmitted by way of an eight-speed automatic transmission, with the electric motor located on the crankshaft between the combustion engine and the torque-converter. Power for the electric motor comes from a lithium-ion battery pack, which — apart from helping power the car — can feed power into the 12V system when necessary and operate the climate control system without the engine running. Consisting of 35 cells and weighing 27kg, the battery’s capacity is a modest 0.8kWh, confirming that this system was designed to operate in short bursts.
The battery pack lives in a high-strength steel housing at the front of the luggage compartment. In the event of a malfunction or a collision, the entire high-voltage system is automatically shut down.
Power electronic are used to get the best use of the hybrid system by balancing the power sources according to driving conditions and power requirements. An automatic stop-start system is fitted, and the motor works as a generator during overrun and braking to capture unwanted kinetic energy. Working as a generator, the motor is actually more powerful than it is when it’s driving the car, developing 27PS as a generator against 20PS as a drive motor. The motor also acts as the petrol engine’s starter.
A separate absorbent glass mat 12V battery is fitted to power the on-board systems. The Li-ion pack powers high-tension systems, with a nominal voltage of 120V. During acceleration, the Li-ion battery is the sole source of energy for the on-board network. The V8 is exempted from charging duties when loads are high, liberating more power for the process of accelerating the vehicle.
The two networks — 12V and 120V — are connected together by a voltage converter. This provides for flexible energy management, while also allowing all electrical functions on the car to be run from regenerated energy. The 12V battery can be fed with energy from the high-voltage network, keeping the 12V system running when the engine isn’t, or for underwriting the 12V battery when the V8 is started from cold.
When light braking forces are demanded, the car’s brake energy regeneration system operates by itself to provide the necessary retardation. The mechanical brakes are deployed when pedal pressures increase. So initially the 7 has rear-wheel braking. The balance between electromagnetic and mechanical braking is controlled by the car’s Dynamic Stability Control system.
Displays in the instrument cluster and the central control screen provide the driver with live information about the hybrid system’s operating efficiency and energy flows. We’re not convinced that these displays are anything more than a distraction for the driver, though they might impress the passengers.
Using both power sources in parallel, standing start acceleration to 100km/h is achieved in a laughable 4.9 seconds. Overall fuel consumption in the EU test cycle is 9.4l/100km (29.1mpg), while the CO2 yield is 219g/km.
We have to ask, what point is BMW trying to make? The company’s three-litre diesel turns out 306PS and 600Nm in its liveliest form. According to BMW’s figures, that’s enough to get the 740d it’s fitted to up to 100km/h in 6.3s. This car’s consumption and CO2 figures are 6.9l/100km (40.9mpg) and 181g/km respectively.
The Active Hybrid 7 — at least in its present form — is a pointless car. It is complex, heavy, fast beyond all possible utility, and not at all economical. The purpose of technology is to make things better, to improve our lives, not to pander to the vanity of wealthy technophiles. It’s a shame to see BMW wasting its considerable talents.