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BMW Active Hybrid: X3 EfficientDynamics

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BMW Active Hybrid: X3 EfficientDynamics
Higher Performance, Lower Emissions
The Engine
Bundled Energy
Gearbox and Battery
Broadside Power
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BMW Active Hybrid: X3 EfficientDynamics
Source: BMW Group

Presenting the Active Hybrid Technology Concept at the 2005 Frankfurt Motor Show BMW is unveiling trendsetting drive technology for efficient dynamics based on a very special concept car: The BMW Concept X3 EfficientDynamics combines intelligent technological solutions based on superior drivetrain, transmission and energy storage components.
The transparent side-sill trim, for example, offers the beholder a clear view of the copper-coloured electrical energy storage units forming part of the BMW Active Hybrid Concept.

The big advantage of this concept is that it enhances both the spontaneity and the dynamics of a car on even less fuel than before thanks to the regeneration of energy for an additional electrical drive system.
The BMW Concept X3 EfficientDynamics is a breakthrough in technology clearly showing that the dedicated motorist may by all means still enjoy sheer driving pleasure also in future, combining supreme dynamics with equally supreme fuel economy.

Energy Management BMW-Style: Supreme Dynamics on Optimum Fuel Economy.
The BMW Group defines “hybridisation“ as the intelligent management of the flow of energy within the car. A hybrid concept BMW-style must naturally retain features typical of BMW such as dynamic performance combined with the optimum use of fuel, it must be suitable for all purposes, and it must be available for the widest possible range of use within BMW’s model series. Precisely this is the philosophy borne out by the technology featured in the BMW Concept X3 EfficientDynamics standing out clearly from all other hybrid systems already in place or currently being developed: The core element in the case of BMW’s concept is the Active Transmission operating in conjunction with the most efficient combustion engine and, without requiring any additional space, accommodating both the electric motor and the electronic control unit. And as a further highlight, the entire system is supplied with energy primarily not by a conventional battery, but rather by high- performance capacitors referred to as Super Caps.
Higher Performance, Lower Emissions.
Driving calculations based on the BMW Concept X3 EfficientDynamics serving as a high-tech development vehicle confirm the great advantages BMW’s concept offers the driver: These calculations show that the car accelerates much faster than a comparable production vehicle of the same kind with conventional drive, the BMW Concept X3 EfficientDynamics accelerating according to calculations in approximately 6.7 seconds from 0–100 km/h and reaching a top speed of approximately 235 km/h or 146 mph.
This superior performance comes with a further reduction of fuel consumption and, accordingly, emissions in the European driving cycle by approximately 20 per cent. Intelligent management of the flow of energy, therefore, lifts those virtues so typical of BMW to a new, even more fuel-efficient pinnacle.


Engine Technology Both Dynamic and Economical All in One: Six-Cylinders with Jet-Guided Direct Gasoline Injection.
This synthesis of economy, ecology and driving pleasure is made possible by the intelligent combination of the combustion engine and the electric motor. The main source of power, reflecting an old and proven BMW tradition, is of course the straight-six BMW engine in its most advanced and sophisticated configuration: a gasoline power unit with High Precision Injection, that is jet-guided direct gasoline injection ensuring a significant increase in efficiency on higher torque and power over the current generation of power units. And indeed, jet-guided direct gasoline injection is not the only special feature, since the six-cylinder will be the first engine completely
without V-belts, ancillaries such as the steering servo pump, the brake servo and the air conditioning compressor being driven by their own electric motors. The electrical power generator in the Active Transmission, finally, takes over the conventional functions of the alternator.

The Big Advantage of the Electric Motor: Superior Torque Right from the Start.

Additional use of an electric motor allows the utilisation of specific physical benefits: Although an all-electric drive system is inferior to a combustion drive principle, the electric motor nevertheless offers interesting features in conjunction with the combustion engine: While the combustion engine, due to its inherent system and underlying technology, is able to develop torque only as of a certain minimum speed, the electric motor generates its full torque right from the start, that is from a standstill. A further advantage is that the electric motor develops a relatively high level of torque even at low power, making the electric motor the right kind of drive technology for setting off smoothly and directly, while the combustion engine always requires a clutch to bridge the difference in engine speed to the wheels initially at a standstill before setting off.
With increasing engine speed, the output of both drive systems increases a long and almost linear curve. The only drawback is that the electric motor required to develop more power also requires a correspondingly greater energy supply, where the weight of batteries or fuel cells quickly reaches practical limits and restrictions. So with BMW’s system the torque generated by the electric motor decreases on an over-proportional curve as soon as the electric motor has reached maximum output when accelerating.
A Powerful Team: Combustion Engine Complete With a Boost Effect.
With all this in mind, one thing was clear to BMW’s development specialists right from the start: The relative overall benefit of an electric motor is greatest at low running speeds, since it is here that a combustion engine develops only part of its full torque. So instead of working painstakingly on the torque of the combustion engine, BMW’s engineers were able to develop a concept using an electric motor to boost the overall level of torque. Which, in practice, ensures full power right from the start at low speeds.


BMW Know-How at Its Best: the Active Transmission.
BMW’s development specialists have examined a wide range of possible combinations bringing together the electric motor and the combustion engine to form one common drive system – and in the process finding a solution most appropriate for BMW: the Active Transmission featured in the BMW Concept X3 EfficientDynamics. Within the same compact dimensions as the automatic transmission otherwise fitted as standard, Active Transmission comprises the six driving gears, the electric motor together with two clutches, and the car’s complete control and power electronics. So BMW’s specialists have succeeded in developing a concept housing the complete additional drive system within the space available for the hydraulic torque converter and the converter lock-up clutch, without affecting or enlarging the package
in any way.
To reach this goal, BMW’s engineers have developed an extremely compact, weight-optimised electrical motor housed on the input shaft leading into the transmission. Laid out for maximum output of 30 kW or 41 bhp, this electric motor offers the advantage of being able to briefly develop maximum output of up to 60 kW or 82 bhp without requiring any additional features or equipment.
Clearly, this makes the electric motor most suitable as an additional boosting unit and at the same time allows compact dimensions. The synchronous electrical motor incidentally operates at an operating voltage of 100–200 Volt, equal to the usual bandwidth of system voltage in the integrated power electronics.

Electric Motor with Two Clutches Replacing the Torque Converter.
Positioning the electric motor in the direct flow of power between the combustion engine and the transmission offers another big advantage: First, the electric motor is able to use the usual transmission ratios available; second, it can perform a whole range of diferent drive functions with different power and torque requirements. One example is that with a conventional automatic transmission a hydraulic torque converter acts as the set-off clutch, the converter increasing engine torque on the first few metres and therefore – in simple terms – feeding more power to the wheels than the engine would actually deliver. Now the compact electric motor is able to provide exactly the same effect.
The electric motor on the Active Transmission therefore assumes the space and function of both a torque converter and a lock-up clutch, supplemented in its operation by two oil bath clutch systems: The first set of clutch plates connects the combustion engine with the electric motor, the second links the electric motor with the transmission. Again, the objective is to fit the entire configuration including the power electronics into the transmission housing in terms of both length and diameter, thus being relatively easy to fit in lieu of a conventional BMW automatic transmission. Indeed, this compact design and configuration makes the BMW Active Transmission quite unique in the
worldwide development of hybrid drive systems.
Bundled Energy: Setting off with the Electric Motor, Accelerating with the Six-Cylinder.
The integrated hardware is nevertheless just half of BMW’s Active Hybrid concept. The other half lies in the system’s sophisticated operating strategy with its particular know-how. The first point is that the driver is no longer required to start the combustion engine first. Rather, all he has to do is press down the gas pedal and the car will start accelerate immediately. In this case the first clutch between the combustion engine and the electric motor is open in the Active Transmission, while the second clutch links the motor and the transmission unit. So the car sets off under electric power, without the slightest noise. Then, once the car has started to move, the first clutch gently engages and the combustion engine will fire. At this point the electric motor also acts as the starter for the combustion engine, the intelligent operating strategy ensuring that the entire process is smooth, comfortable, and free of any jolts.


Up to 600 Newton-Metres (442 lb-ft): The Muscle of a Diesel, the Responsiveness of a Gasoline Engine.
Given all the features mentioned, the energy strategy of BMW’s trendsetting Hybrid Concept combines spontaneous power and performance with optimum use of energy: Whenever the driver applies the brakes until the car reaches a standstill, the combustion engine is automatically switched off, thus not consuming any fuel and not generating any emissions. Then, as soon as the driver presses down the gas pedal again, the engine starts again automatically. Should the driver then continue to give gas, the 400 Newton-metre (295 lb-ft) electric motor will start initially to accelerate the car with its maximum output of up to 60 kW or 82 hp. When accelerating moderately, the system intentionally does without the combustion engine, with the vehicle being driven by the electric motor alone. Hence, the combustion engine will only cut in and drive the car when accelerating faster or when reaching a higher speed on the road.
The combustion engine is started at the same time as the electric motor whenever required for accelerating quickly from a standstill. In this case the torque of the two drive systems is combined at low speeds, developing up to 600 Newton-metres or 442 lb-ft of torque at engine speeds below 1,500 rpm – certainly a new dimension for a gasoline engine even exceeding the supreme power and muscle developed by BMW’s three-litre diesel.
The electric motor then continues to support the combustion engine up to a speed of approximately 3,000 rpm, when it is either deactivated without generating any further power or switched to its charge function, depending on the car’s current driving dynamics.
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”.


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.
Broadside Power: Copper Red Super Caps in the Side-Sills.
The Super Caps are within clear sight on the BMW Concept X3 EfficientDynamics, with tubes glowing copper red integrated in transparent side-sills. This point of installation ensures optimum results in terms of the car’s lateral and vertical dynamics as well as the space available.
Together with the Active Transmission, the Super Caps represent the key elements of the Concept Car. They are able to take up a very high level of electrical energy within fractions of a second without any losses worth mentioning, delivering that energy just as quickly and efficiently. So far Super Caps have been used above all in wind power systems as maintenance-free energy storage units with an unlimited service life.
The Super Caps featured in the X3 Concept Car measure approximately 50 millimetres or not quite 2.0´´ in diameter and generate an overall capacity
of 190 kW.

Internal Resistance Reduced to a Minimum: Switching over from Charge to Discharge without the Slightest Loss of Power.
The only disadvantage of Super Caps versus the electrochemical battery is their far lower energy density. However, even this drawback is of only theoretical significance, since the energy density offered by an electrochemical battery today is generally used only to a very small extent on account of the battery’s service life: An NiMH battery generates approximately 5 watt-hours per kilogram, as opposed to roughly 4 watt-hours/kg in the case of a Super Cap.
Another factor is that batteries have a high level of interior resistance dramatically increasing even further as they are discharged. A high level of interior resistance, however, means substantial loss of energy when charging, with a lot of the energy regained through application of the brakes thus being lost and the charge process lasting longer.
The interior resistance of a Super Cap, by contrast, is virtually zero, regardless of the current charge level. In practice this means that the cycle from charging to discharging and back again may be repeated as often as required within a very short time, Super Caps thus offering all the requirements for use in stop-and-go traffic, as already described.
The overall capacity of the Super Caps of 190 kW is quite sufficient also for a car driven very fast and dynamically. And at the same time the driver is able to drive the car under electric power alone, for example when parking or manoeuvring, Super Caps again providing adequate energy.


BMW Hybrid Cars: Fifteen Years of Research and Development.
The BMW Group has been working on hybrid drive ever since the early ’90s. In particular, BMW’s specialists have focused on the development of a disc-shaped electric motor suitable for automotive use and now featured in the X3 Concept Car.
In 1991 BMW moreover registered an “Electric Transmission” for patenting and even built various prototypes. Given the high weight of the batteries and electrical drive components, the system was however not able to meet the great demands made by BMW of a automotive concept suitable for the market and regular use.
Last Updated on Friday, 16 September 2005 08:35  

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