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BMW Active Hybrid: X3 EfficientDynamics |
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Page 3 of 6
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.
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