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BMW Active Hybrid: X3 EfficientDynamics - Gearbox and Battery

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BMW Active Hybrid: X3 EfficientDynamics
Higher Performance, Lower Emissions
The Engine
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Gearbox and Battery
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More Power in All Six Gears.
The Active Transmission Concept not only offers significant advantages when setting off and accelerating, but also serves as an additional “booster” in each of the six automatic transmission gears, since the electric motor is positioned upstream of the transmission and is able to cut in whenever the combustion engine is running at low speeds. With the driver’s commands and power requirements being carefully shared out among the two drive units, the additional power coming from the electric motor is applied only very briefly when accelerating, as under normal driving conditions. Indeed, BMW’s development specialists have determined that in most cases the electric motor is required for only about three seconds to provide significant boost and save a substantial amount of fuel at the same time.

Driving Dynamics at Its Best: Accelerating Fast, Charging Quickly.
A further advantage of these generally short operating periods of the electric motor is that the electric storage system is required to provide only a small amount of energy. And the electrical power supply unit is re-charged immediately after energy has been withdrawn, that is prior to the next power supply process. This is done primarily by switching over the electric motor to the alternator mode above all under application of the brakes and with the car in overrun.
In this case the system naturally charges not only the high-voltage energy storage unit, but also – via a converter – the conventional 12 V on-board battery. To avoid drag losses, the clutch between the combustion engine and the electric motor opens up in this case, enabling the Active Transmission to immediately re-gain kinetic energy otherwise simply converted into heat and lost in a conventional car. Being fed back into the system as electrical energy, this power cycle is referred to by experts as “recuperation”.

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Truly Unique: Recharging the Electric Power Source Every Time You Press Down the Brake Pedal.
Whenever the driver presses down the gas pedal to accelerate in stop-and- go traffic, the electrical power supply units are discharged but then re-charged again just as quickly as soon as the driver applies the brakes. It is particularly under such dynamic driving conditions with ongoing, significant changes in load that the fuel consumption of a combustion engine increases significantly. BMW Active Hybrid, in turn, offers the driver two benefits in one in this respect: even greater driving dynamics on a lot less fuel.
A further option is to re-charge the energy storage unit whenever required by raising the load points in the combustion engine: The control strategy of the BMW Active Hybrid Concept is laid out consistently never to deliver more electrical energy than can be re-charged very quickly even under the most unfavourable conditions. In other words, the driver will never experience a loss of power resulting from an empty electrical storage unit. Instead, he can rely on BMW Active Hybrid at all times, knowing that driving conditions will always remain consistently smooth and superior.

Optimum Energy Storage for Dynamic Motoring: Super Caps.
Whenever looking at electrical energy storage systems, one initially always thinks of batteries. In reality, however, the self-charging battery is most unsuitable for the driving conditions and requirements typically encountered with BMW Active Hybrid, involving very dynamic performance with fast charging and discharging processes. Particularly this style of motoring and operation calls for the use of energy storage systems with the highest possible level of power density. And precisely this is where double-layer capacitors also referred to as Super Caps are absolutely unbeatable in comparison with the electrochemical battery.
A Super Cap offers specific power density of approximately 15 kilowatts per kilogram of weight, compared to about 1.3 kW/kg in the case of a nickel/metal hydride battery, meaning that a Super Cap outperforms a self-charging battery in this respect by a factor of more than ten. This superiority is then maintained throughout the usual driving mode, the Super Cap offering a power density of more than 5 kW/kg compared with not even 0.5 kW/kg provided by a battery. So given the same weight, voltage and power supply, a double-layer capacitor offers a level of efficiency of 98 per cent compared with 84 per cent provided by an NiMH battery.

Last Updated on Friday, 16 September 2005 08:35  
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