Environmental Engineering Reference
In-Depth Information
In Figure 2.29 the M/G rotates in reverse direction during generating mode so
that constant speed ICE operation is possible. The generating mode continues until
the M/G speed crosses through zero, at which point it enters motoring mode. This is
an inefficient operating point requiring M/G stall torque. Once through zero speed
the M/G is motoring with positive rotation until the L/U clutch engages, pinning the
ring (ICE) and carrier (AT) gears together.
2.4 Post-transmission parallel configurations
The second option for locating the ac drive in a hybrid vehicle is to insert the
M/G at the transmission output shaft, but ahead of the final drive. In this post-
transmission configuration the M/G does not have the benefit of gear ratio changes;
therefore, it must operate over the very broad vehicle speed range. This demands a
high torque ac drive that can function over wide CPSR.
The disadvantages of post-transmission hybrids are the high torque levels,
impact of continuous engagement spin losses on fuel consumption and package
difficulty. Higher torque M/Gs are always physically larger since more rotor sur-
face area is needed to develop surface traction. Larger moment arms to this surface
traction are more restricted because the package diameters are usually constrained
to fit within transmission bell housing diameters (200-350 mm OD).
An example of a post-transmission hybrid would be an in-wheel motor or hub
motor hybrid. The GM Autonomy, for example, could be classified as a post-
transmission hybrid because the hub motor is separated from the wheel by a non-
shifting epicyclic gear.
The Autonomy (Figure 2.30) is a concept automotive chassis designed for
wide-ranging body style flexibility and cross-segment application. All propulsion,
energy storage, chassis functions and wiring for power distribution and commu-
nications are packaged within the skateboard-like chassis. Communications is via
controller area networks (CAN). Power for propulsion is at high voltage, 300 V
typical or higher when fuel cells are used. Chassis and passenger amenities are
powered by 42 or 12 V for lighting.
A concern with hub motors is their higher unsprung mass, a tendency for
torque steering and durability. Because of lower speeds and high torques, a hub
motor will be inherently heavier than its higher speed axle or pre-transmission
equivalent. Torque steer is a phenomenon due to steering and suspension geometry
design. Durability is a persistent issue with hub motors because of simultaneous
vibration, temperature, water/salt spray ingress, and sand, dust and gravel
impingement.
Torque steer can be understood by recognizing that vehicle steering geometry
will generally have non-zero scrub radius. When the suspension king pin axis
intercepts the tyre-road patch inside the plane of the wheel, the distance from
the wheel plane to the king pin axis is referred to as the tyre scrub radius. If the
intercept point is inside the wheel plane the scrub is positive and if outside it will
be negative. A negative scrub radius puts the wheel turning axis outside the wheel
