Environmental Engineering Reference
In-Depth Information
Table 3.4 presents some interesting data. Again, performance on grade is much
more demanding than adding occupants as can be seen from the passing manoeuvre
times. The change from one to four occupants for the same manoeuvre makes only
a 17% increase in acceleration times, but climbing just a 3% grade results in a very
noticeable 90% and 256% increase in times.
3.3 Braking and energy recuperation
The performance of vehicle hybrid propulsion systems is strongly dependent on the
type of brake system used. The energy recuperation component of fuel economy
gain depends on (1) the hybrid M/G rating and (2) the types of regenerative brake
systems employed.
It is far more important to implement active brake controls on rear wheel drive
versus front wheel drive hybrid vehicles when regenerative braking is employed.
Vehicle rear brakes tend to lock and skid far easier than front wheels due to normal
weight balance allocation and dynamic weight shifting during braking. Once the
rear wheels lock up, the vehicle anti-lock braking system (ABS) will engage and
start to modulate the brake line pressure at approximately 15 Hz. Engagement of
ABS pre-empts regenerative brakes in a hybrid vehicle regardless of architecture.
There are several versions of regenerative brakes available depending on the
level of energy recuperation anticipated. Series regenerative braking systems
(RBS), as the name implies, engage the electric M/G first in generating mode.
Then, if the brake pedal is depressed further, and/or faster, the brake controller adds
in the vehicle's service brakes to gain a more rapid deceleration. Lastly, if the brake
pedal is depressed hard, the ABS controller engages and controls braking using the
service brakes only.
Parallel RBS is the system most commonly employed in mild hybrids. Parallel
RBS does not require electrohydraulic braking (EHB) or electromechanical braking
(EMB) systems, but uses both M/G braking and service brakes in tandem. An
algorithm in the M/G controller proportions braking effort between regenerative
and service brakes.
Split parallel RBS is another transitional system wherein the service brakes are
not engaged for low effort braking but the hybrid M/G is. Again, ABS pre-empts
any RBS actions.
Interactions with vehicle longitudinal stability programmes, such as interactive
vehicle dynamics (IVD) (as used in NA), electronic stability programmes (ESP)
(as used in Europe) and vehicle stability controls (VSC) (as used in Asia-Pacific),
are all coordinated by the vehicle system controller.
The following subsections elaborate on each of the RBS discussed above.
3.3.1 Series RBS
Series RBS introduces electrical regeneration sequentially with the vehicle's ser-
vice brakes in proportion to the brake pedal position. Figure 3.6 illustrates how the
