Hybrid architectures - Propulsion Systems for Hybrid Vehicles - page 109

is switched off, the OWC prevents it from being back driven by the flywheel and
ensures that the stator/flywheel can only rotate in the same direction as the rotor.
For its part, the rotor is connected to the wheels through reduction gears, and a
differential, but no clutch.
Operation of this PEDT is in either all electric or dual mode. In electric-only
mode with w r > w s , the battery is discharged into the stator via the power inverter.
Reaction torque on the stator extracts energy from the flywheel, adding to the
battery supplied energy. Both battery and flywheel energy accelerate the vehicle,
delivering rather brisk performance. When the flywheel energy is bled off, the
stator is clamped to the chassis by the OWC. Power transfer in this mode is limited
by the peak coupling torque existing between stator and rotor. Battery current
enables the power transfer according to the action of the power electronic control
strategy.
The second condition presents itself when w r < w s and the stator/flywheel
rotates faster than the rotor. In this mode, the machine operates as a generator
sending power to the battery. During vehicle braking the same set of conditions
apply, depending on the relative speed between the stator and rotor. Vehicle kinetic
energy is delivered to either the flywheel, the battery or both during braking.
In dual mode the ICE remains coupled to the PEDT via the drive clutch. The
ICE operates in thermostat mode when engaged, sending its power to the battery/
flywheel when not needed to meet road load. When the flywheel is charged, the
ICE is shut down and the clutch deactivated.
2.6.3 Swiss Federal Institute flywheel concept
A flywheel storage system based on CVT transmission technology has been
described by the Swiss Federal Institute of Technology (ETHZ) based on a small
gasoline engine rated 50 kW, a 6 kW synchronous M/G, a 0.075 kWh flywheel
and 5 kWh of batteries in a CVT architecture [27,28]. The architecture shown in
Figure 2.38 has four modes of operation:
Mode 1 . Normal drive : The ICE and the CVT are used for propulsion. Clutches
C1 and C2 are closed. Clutch C3 to the flywheel is open and the electric M/G is
coasting.
Mode 2 . Flywheel assist : Clutches C2 and C3 remain closed so that the flywheel
and CVT maintain propulsion. When the flywheel speed drops below 1,800 rpm, the
engine is started and C1 engaged to spin up the flywheel to 3,800 rpm and to deliver
propulsion power to the wheels. When the flywheel speed exceeds 3,800 rpm, the
engine is cut off and clutch C1 opened.
Mode 3 . Electric machine and flywheel assist : This is the zero emissions
operating mode. Power is delivered to the wheels by the M/G, augmented by
decelerating the flywheel if necessary.
Mode 4 . M/G and CVT only : This is the zero emission operating mode of a
conventional EV. Because of limited rating of the M/G and battery, vehicle
acceleration performance is limited. Without a higher rated M/G, this mode will be
used only for low speed operation.