Environmental Engineering Reference
In-Depth Information
0.45
0.30
Service brakes
engaged
0.15
M/G regeneration
0
Compression braking
0
25
50
75
100
Brake pedal position (%)
Figure 3.6 Series RBS at vehicle speed, V
total brake force is proportioned in the series RBS configuration. When the brake
pedal is depressed, the vehicle experiences velocity retardation in proportion to
engine compression braking. This is a very mild deceleration effect and one that
drivers expect.
As the brake pedal is further depressed, an algorithm computes the M/G
braking torque (regen) so that the vehicle kinetic energy recuperated and sent to the
traction battery is controlled to mimic normal foundation brake feel. If the brake
pedal is depressed further, the deceleration effect is more noticeable, and at some
pre-defined point, the service brakes are blended in with M/G torque. Had a third
axis been included in Figure 3.6, it would show how overall brake effort is
apportioned between M/G torque and service brakes as the vehicle decelerates. As
vehicle speed decreases, and accounting for transmission down shifts, there will
come a point when M/G efficiency is too low for regeneration so that it will be
commanded off and only service brakes left in effect. The strictly linear, and
snapshot, nature of Figure 3.6 does not convey this information.
Series RBS requires active brake management on all four wheels so that the
total braking effort is coordinated. For example, the M/G may impose braking force
on the front axle wheels only and none on the rear axle. Therefore, a hydraulic
system would be necessary to actuate the rear brakes in the proper proportion to
front axle brakes so that vehicle longitudinal stability is maintained.
Brake coordination is a complex function of brake pedal position, rate of pedal
application and vehicle speed. Properly coordinated front-rear braking force opti-
mizes stopping distance without loss of tyre adhesion.
Series RBS is typically implemented with EHB hardware. EHB consists of the
hydraulic control unit that interfaces to the driver foot. The second component is
the electronic control unit that manages brake cylinder pressures and front-rear
axle brake balance.
It is insightful to examine how vehicle speed of a hybrid changes when com-
manded to generate at some fixed power level into the RESS (i.e. the battery pack).
The capability for constant power deceleration is unique to hybrid electric vehicles
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